A transmitting system and electromagnetic detection transmitting current superimposed pattern modulation method

Abstract

The application belongs to the technical field of electromagnetic detection and relates to a transmitting system and an electromagnetic detection transmitting current superposition type pattern modulation method, which comprises N H-bridge sub-modules connected in series, N being an integer greater than or equal to 2; each H-bridge sub-module comprises an independent direct-current power supply unit and a full-bridge inverter circuit, and each H-bridge sub-module is connected with an independent driving control system; the alternating-current output ends of the H-bridge sub-modules are connected in series in turn and then connected to a common transmitting load; each H-bridge sub-module is used for independently generating and outputting a basic pattern modulation current waveform, the basic pattern modulation current waveforms are linearly superimposed in the transmitting load in series connection, the corresponding frequency spectrums are linearly superimposed, and a combined transmitting current waveform with arbitrary frequency ratio and amplitude ratio is generated. The method can realize a multi-frequency transmitting waveform with arbitrary frequency ratio and amplitude ratio, and can flexibly adapt to different exploration task requirements according to the depth, scale and resistivity characteristics of a detection target.

Description

Technical Field

[0001] This application belongs to the field of electromagnetic detection technology, specifically a transmitting system and an electromagnetic detection transmitting current superposition pattern modulation method. Background Technology

[0002] The electromagnetic detection transmitter system is a crucial component of electromagnetic detection, affecting the efficiency and accuracy of the overall detection results. Improving the performance of the transmitter system mainly involves two aspects.

[0003] Firstly, optimizing the transmitter hardware performance involves increasing transmission power and transmission current. However, the best-performing electromagnetic detection transmitter currently available can reach a maximum power of 500kW, while the maximum transmission current for marine electromagnetic detection can reach 2000 amperes. Further optimization is limited by advancements in materials and semiconductors, and as an off-grid system powered by a DC generator, its power supply is also constrained in terms of size and weight.

[0004] Secondly, the transmitter current waveform is optimized by using signal modulation and generation techniques to improve efficiency and expand the waveform frequency band while simplifying waveform generation. This allows for the modulation of a multi-frequency, easily adjustable transmission waveform, helping electromagnetic detection technology develop towards higher efficiency and portability.

[0005] Traditional transmitter current waveform modulation methods suffer from poor flexibility, high complexity, and numerous switching operations, making it difficult to modulate suitable detection waveforms in real time for different detection targets. For example, 2n pseudo-random waves utilize a custom three-element set with sub-closed addition to abstract the high, low, and zero levels of a square wave into mathematical symbols, achieving a multi-frequency waveform for electromagnetic detection with a frequency ratio of 2 between the main frequencies. In electromagnetic detection, high-order 2n pseudo-random waves are commonly used as the transmitted current waveform, enabling the detection of underground targets over a wide depth range. However, the fixed amplitude ratio between the main frequencies of 2n pseudo-random waves makes it difficult to flexibly adjust for targets at different depths; furthermore, the fixed frequency ratio results in a dead zone between the two high-frequency points.

[0006] Therefore, a depth-focusing waveform was developed, using multiple closely distributed frequencies within a narrow frequency band to achieve high-precision measurement of targets at specific depths, ensuring high vertical resolution. The SHEPWM (Specific Harmonic Elimination Pulse Width Modulation) method, which calculates the switching angle information to obtain the electromagnetic detection transmission current waveform, can achieve waveforms with arbitrary spectral components and has wide applications in generating depth-focusing waveforms. However, this method solves for the required transmission signal waveform by establishing a system of equations relating the switching time series and the desired spectrum. This equation is a nonlinear transcendental equation, and its solution process is cumbersome, making it difficult to conveniently and promptly configure the transmission waveform according to the exploration environment in field operations.

[0007] Another method, which obtains the transmit waveform through comparison modulation, offers high flexibility and is simple to generate. MFPWM modulates a multi-frequency sinusoidal modulating wave using a triangular carrier wave to obtain a transmit waveform with the modulation frequency. However, excessively high carrier frequencies result in a high number of switching operations on the generated waveform and introduce high-frequency losses, affecting the waveform's energy utilization. Recently, a pattern modulation method has emerged, using adjacent-frequency modulating waves and carrier waves to achieve a transmit waveform with adjustable frequency-to-amplitude ratios and a low number of switching operations. However, currently, only a limited number of frequency compositions can be achieved, which still presents limitations in practical applications.

[0008] Therefore, addressing the problems existing in the above methods, this application extends the pattern modulation method and proposes a highly flexible electromagnetic detection transmission current pattern modulation method based on a cascaded H-bridge transmitter. It can achieve arbitrary frequency ratios and adjustable amplitude ratios, and generates easily generated transmitter waveforms. Furthermore, it can switch transmission modes for detection targets with different characteristics, achieving wide-range detection, depth-focused detection, and even dual-layer depth-focused detection. Summary of the Invention

[0009] This application provides a transmitting system and an electromagnetic detection transmitting current superposition pattern modulation method, which solves the problem that the pattern modulation of a single H-bridge unit can only generate a limited number of main frequency components, and the frequency combination is fixed, making it difficult to adapt to diverse underground detection scenarios and target characteristics.

[0010] The first aspect of this application provides a launching system, including: N H-bridge submodules are cascaded in series, where N is an integer greater than or equal to 2; each H-bridge submodule includes an independent DC power supply unit and a full-bridge inverter circuit, and each H-bridge submodule is connected to an independent drive control system; the AC output terminals of each H-bridge submodule are connected in series and then connected to a common transmitting load. Each H-bridge submodule is used to independently generate and output a basic pattern modulation current waveform. By connecting them in series, the instantaneous values ​​of each basic pattern modulation current waveform are linearly superimposed in the transmitting load, and the corresponding spectra are linearly superimposed to generate a combined transmitting current waveform with arbitrary frequency and amplitude ratios.

[0011] A second aspect of this application provides a method for superimposed pattern modulation of electromagnetic detection emission current, comprising: The main transmitting circuit is constructed by cascading N H-bridge sub-modules in series. Each H-bridge submodule independently generates and outputs a basic graphic modulated current waveform; By connecting each H-bridge submodule in series in the transmission circuit, the modulation wave frequency and amplitude parameters of each H-bridge submodule are allocated according to the target task, so that the instantaneous values ​​of the basic pattern modulation current waveforms of each path are linearly superimposed in the transmission load, and the corresponding spectra are linearly superimposed, generating a combined transmission current waveform with arbitrary frequency and amplitude ratios.

[0012] Furthermore, based on the target task, the modulation wave frequency and amplitude parameters of each H-bridge submodule are allocated, including: Determine the total coverage of the target frequency band, and calculate the corresponding lowest and highest characteristic frequencies based on the maximum and minimum detection depths required by the detection mission, combined with the skin depth formula, to form a complete frequency range to be covered. The frequency range to be covered is divided into several frequency sub-ranges according to the number of output main frequencies of each H-bridge sub-module. Each frequency sub-range corresponds to the spectrum coverage range of one H-bridge sub-module. The frequency sub-ranges are adjacent or partially overlapped to ensure that the total spectrum after superposition has no frequency gaps in the frequency range to be covered. The modulation wave frequency of the corresponding H-bridge submodule is determined based on the center frequency of each frequency sub-interval; Based on the required transmit current amplitude at each frequency point within each frequency sub-interval, and combined with the adjacent frequency carrier amplitude corresponding to the H-bridge submodule, the modulation index corresponding to each modulation wave frequency is calculated and set so that the combined amplitude at each frequency point after superposition meets the detection signal-to-noise ratio requirements. The frequency points allocated to each H-bridge submodule are superimposed and verified. The amplitude of each frequency point in the superimposed total spectrum is checked one by one to see if it reaches the preset signal-to-noise ratio threshold. If there are frequency points with insufficient amplitude, the modulation of the corresponding H-bridge submodule is called back for compensation and correction until the amplitude of all frequency points meets the detection signal-to-noise ratio requirements.

[0013] Furthermore, the basic pattern modulated current waveform is a bipolar multi-frequency current waveform. Each H-bridge submodule uses a single-frequency sine wave signal or a multi-frequency sine wave signal as the modulation wave and an adjacent-frequency sine wave signal as the carrier wave. The modulation wave and the carrier wave are compared in real time, and the driving signals of each switch of the H-bridge submodule are generated according to the comparison results, and the bipolar multi-frequency current waveform is output.

[0014] Furthermore, each of the H-bridge submodules uses a single-frequency sine wave signal as the modulation wave and an adjacent-frequency sine wave signal as the carrier wave, specifically including: The modulation wave uses a single-frequency sine wave signal, and the frequency of the sine wave signal is set according to the characteristic frequency corresponding to the target being detected. The carrier wave uses a sinusoidal signal with a frequency different from that of the modulation wave, and the carrier frequency is higher than the modulation wave frequency. The difference between the carrier frequency and the modulation wave frequency is a preset positive frequency difference value, so that the carrier frequency and the modulation wave frequency are kept close to each other. The preset positive frequency difference determines the frequency interval between each main frequency component in the bipolar multi-frequency current waveform spectrum output by the H-bridge module. Increasing the positive frequency difference makes the frequency distribution of the output spectrum sparser, while decreasing the positive frequency difference makes the frequency distribution of the output spectrum denser.

[0015] Furthermore, each H-bridge submodule uses a modulation wave synthesized from multi-frequency sine wave signals, and compares the amplitude of the modulation wave synthesized from the multi-frequency sine wave signals with the carrier wave in real time, so as to output a basic graphic modulation current waveform containing multiple main frequency components in a single H-bridge submodule. The modulated wave synthesized by the multi-frequency sine wave signal is composed of K sine components of different frequencies superimposed, where K is an integer greater than or equal to 2. The frequency of each sine component corresponds to its respective required detection frequency point, and the amplitude of each sine component corresponds to its respective required main frequency amplitude. The carrier wave is a single-frequency sine wave signal with a frequency higher than the highest frequency component in the multi-frequency sine wave signal synthesized into a modulation wave, and the frequency difference between the carrier wave and each sine wave component is a non-zero frequency difference. The modulation wave synthesized from the multi-frequency sine wave signal is compared with the carrier wave in real time. Each frequency component in the modulation wave synthesized from the multi-frequency sine wave signal is graphically modulated with the carrier wave. A set of main frequency components corresponding to the frequency component is generated in the output current spectrum, so that the basic graphical modulation current waveform output by the single-channel H-bridge submodule contains K sets of main frequency components.

[0016] Furthermore, the real-time amplitude comparison includes: within each carrier cycle, comparing the instantaneous amplitude of the modulated wave with the instantaneous amplitude of the carrier point by point; outputting a high level when the amplitude of the modulated wave is greater than the amplitude of the carrier, and outputting a low level when the amplitude of the modulated wave is less than or equal to the amplitude of the carrier, thereby generating a PWM control signal.

[0017] Furthermore, by adjusting the modulation frequency and amplitude parameters of each H-bridge submodule, one of the following three operating modes can be switched: Wide-spectrum detection mode: Configure each H-bridge submodule with a main frequency with a different frequency span, so that the total spectrum after superposition covers a wide range of frequency bands from high frequency to low frequency; Single-layer deep focusing mode: Configure different adjacent frequencies within the same narrow frequency band for each H-bridge submodule, so that the total spectrum after superposition forms a dense frequency point distribution within the target frequency band; Dual-layer depth focusing mode: The H-bridge submodule is divided into two groups. The first group is configured with high-frequency dense frequency points corresponding to the shallow layer, and the second group is configured with low-frequency dense frequency points corresponding to the deep layer. After superposition, the total spectrum forms a focused distribution in the frequency bands corresponding to the two depths.

[0018] Furthermore, the modulation frequency and amplitude parameters of each H-bridge submodule are adjusted as follows: Adjusting the modulation frequency of a single H-bridge submodule changes the position of the dominant frequency component of the output current waveform of that H-bridge submodule; Adjusting the ratio of the modulation amplitude to the carrier amplitude of a single H-bridge submodule changes the amplitude of each main frequency component in the output current waveform of that H-bridge submodule.

[0019] Compared with the prior art, this application has at least the following beneficial effects: This application utilizes a hardware architecture with multiple H-bridge submodules cascaded in series to achieve instantaneous current superposition of the basic pattern modulation current waveforms in the transmission circuit, resulting in linear superposition of their corresponding spectra. The drive control systems of each H-bridge submodule are independent, allowing for independent configuration of spectral parameters such as modulation frequency and the ratio of modulation amplitude to carrier amplitude, thus creating diverse transmission spectra. Because the total current in the transmission circuit satisfies a linear superposition relationship of instantaneous values, the spectrum of the total current is a linear superposition of the output waveform spectra of each H-bridge submodule. This fundamentally overcomes the inherent frequency limitation of single-channel pattern modulation, enabling multi-frequency transmission waveforms with arbitrary frequency and amplitude ratios. This allows for flexible adaptation to different exploration mission requirements based on the depth, size, and resistivity characteristics of the target.

[0020] This application employs a modulation principle based on direct graphical comparison, directly comparing the instantaneous amplitudes of the modulating wave and the carrier wave point by point. Based on the amplitude relationship, a binarized PWM pulse signal is output, and the modulation result is entirely determined by the intersection points of the modulating wave and the carrier wave. This modulation method requires no mathematical transformation of the modulating wave, no solution of complex nonlinear transcendental equations, and no pre-calculation of pulse width or lookup of switching times. The spectral structure and amplitude parameters of the modulated waveform are directly determined by the frequency and amplitude of the modulating wave. When arbitrarily adjusting the modulating wave parameters, the generation of the driving signal does not require recalculation or iterative solution. Therefore, the parameter calculation and configuration process is extremely simple, allowing for rapid adjustment of the transmitted waveform based on measured geological conditions at the exploration site, adapting in real time to changes in underground targets, and significantly improving the flexibility and efficiency of exploration operations.

[0021] This application allows for flexible switching between three operating modes by configuring differentiated main frequency and amplitude parameters for different H-bridge submodules, based on the depth, number, and accuracy requirements of the detected targets. In the wide-spectrum detection mode, the total spectrum covers a broad range of frequencies from high to low, enabling simultaneous excitation of electromagnetic responses from shallow, medium, and deep multi-layered underground targets, facilitating large-scale underground structure surveys and significantly improving exploration efficiency. In the single-layer depth focusing mode, the total spectrum forms a dense frequency distribution within the target frequency band, significantly improving the vertical resolution and signal-to-noise ratio of the corresponding depth detection, achieving high-precision and detailed detection of targets at specific depths. In the dual-layer depth focusing mode, the total spectrum simultaneously forms a focused distribution in the frequency bands corresponding to two depths, enabling high-precision detection of targets at two different depths simultaneously, balancing detection efficiency and vertical resolution. These three modes cover all exploration scenarios, from general surveys to detailed investigations, and from single-layer to multi-layer exploration, and can be widely applied to various electromagnetic detection scenarios such as airborne electromagnetic detection, ground-to-air electromagnetic detection, marine electromagnetic detection, and downhole electromagnetic detection. Attached Figure Description

[0022] Figure 1 The circuit schematic diagram of the launching system provided in the embodiments of this application; Figure 2 A flowchart of an electromagnetic detection transmission current superposition pattern modulation method provided in this application embodiment; Figure 3 The wide-spectrum detection mode transmission waveform and its spectrum diagram are provided for embodiments of this application, wherein (a) is the transmission waveform diagram and (b) is the spectrum diagram; Figure 4 The single-layer depth focusing mode emission waveform and its spectrum diagram are provided for embodiments of this application, wherein (a) is the emission waveform diagram and (b) is the spectrum diagram; Figure 5 The transmission waveform and its spectrum diagram of the dual-layer depth focusing mode provided in the embodiments of this application are shown, wherein (a) is the transmission waveform diagram and (b) is the spectrum diagram. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0024] This application proposes a transmission system and an electromagnetic detection transmission current superposition pattern modulation method. The innovation lies in overcoming the shortcomings of basic pattern modulation methods, such as single frequency point composition, limited application scenarios, and insufficient flexibility. It employs a structure of multiple H-bridge submodules connected in series, with each H-bridge submodule corresponding to a basic pattern modulation transmission waveform. Through the electromagnetic detection transmission current superposition pattern modulation method, the modulation wave frequency and amplitude parameters of each H-bridge submodule are allocated according to different target tasks, enabling the instantaneous values ​​of the various basic pattern modulation current waveforms to be linearly superimposed in the transmission load. This achieves diverse transmission spectra. Through different superposition combination methods, wide-spectrum detection modes, single-layer depth focusing modes, and double-layer depth focusing modes can be realized, effectively improving the flexibility of the electromagnetic detection system. This application is applicable to various electromagnetic detection scenarios, such as airborne electromagnetic detection, ground-to-air electromagnetic detection, marine electromagnetic detection, and downhole electromagnetic detection, effectively improving the working efficiency and flexibility of electromagnetic detection systems.

[0025] See Figure 1 As shown, a transmission system includes: N cascaded H-bridge submodules, where N is an integer greater than or equal to 2; each H-bridge submodule includes an independent DC power supply unit and a full-bridge inverter circuit, and each H-bridge submodule is connected to an independent drive control system; the AC output terminals of each H-bridge submodule are connected in series and then connected to a common transmission load; it should be noted that this application does not impose a special limitation on the value of N, and the specific value of N can be flexibly selected and configured according to the transmission power requirements, spectral complexity requirements and exploration site conditions of the actual detection mission.

[0026] Each H-bridge power H-bridge submodule consists of an independent DC power supply unit and a full-bridge inverter circuit. Specifically, each H-bridge submodule is equipped with an independent DC power supply unit to provide the DC bus voltage to its full-bridge inverter circuit. The DC power supply units of each H-bridge submodule are isolated and independent of each other, and can be configured with different DC voltage levels according to the power requirements of each H-bridge submodule, allowing each H-bridge module to independently contribute its own voltage component when connected in series. Each independent DC power supply unit can be implemented using an independent battery pack, an independent AC / DC rectifier power supply, or an independent DC / DC converter.

[0027] Each full-bridge inverter circuit consists of four power switches: a first power switch, a second power switch, a third power switch, and a fourth power switch. The first and second power switches are connected in series to form the left bridge arm, with the first power switch being the upper switch and the second power switch the lower switch. The third and fourth power switches are connected in series to form the right bridge arm, with the third power switch being the upper switch and the fourth power switch the lower switch. The midpoints of the left and right bridge arms serve as the two AC output terminals of this H-bridge submodule. Each of the four power switches is controlled by its respective drive signal to turn on or off, thereby converting the DC voltage provided by the independent DC power supply unit into a bipolar AC voltage waveform, which is then output to the transmitter circuit from the AC output terminals.

[0028] The power switching transistors are preferably IGBTs (Insulated Gate Bipolar Transistors) or MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) to meet the high voltage and high current operating requirements of high-power electromagnetic detection and transmission scenarios.

[0029] Each H-bridge submodule is connected to an independent drive control system, meaning each H-bridge submodule is equipped with an independent drive control unit. Each drive control system operates independently, generating drive signals for the four power switches in its corresponding H-bridge submodule.

[0030] Each drive control system includes at least: Waveform generation unit: used to generate modulated waves and carrier waves. The modulated wave is a single-frequency sine wave signal or a modulated wave synthesized from multiple frequency sine wave signals. Its frequency and amplitude can be independently configured according to the target task requirements. The carrier wave is an adjacent-frequency sine wave signal with a frequency higher than that of the modulated wave, and the difference between the carrier wave frequency and the modulated wave frequency is a preset non-zero frequency difference value.

[0031] Comparison unit: Used to compare the instantaneous amplitude of the modulated wave and the carrier wave on the same time base. When the amplitude of the modulated wave is greater than the amplitude of the carrier wave, it outputs a high level; when the amplitude of the modulated wave is less than or equal to the amplitude of the carrier wave, it outputs a low level, thereby generating a PWM control signal.

[0032] The drive signal distribution and amplification unit distributes the PWM control signal to each power switch transistor and amplifies and levels the PWM control signal through the drive circuit, enabling it to drive the power switches to turn on and off. Specifically, the PWM control signal is directly used as the drive signal for the first and fourth power switches, while the inverted PWM control signal is used as the drive signal for the second and third power switches, thereby causing the full-bridge inverter circuit to output a bipolar voltage waveform.

[0033] Dead time generation unit: used to insert a preset dead time between the drive signals of the upper and lower power switching transistors of the same bridge arm of the H-bridge submodule, so as to avoid the upper and lower transistors of the same bridge arm from being short-circuited due to switching delay.

[0034] Each drive control system can be independently configured with parameters such as modulation wave frequency, modulation wave amplitude, carrier frequency, carrier amplitude, and modulation degree (the ratio of modulation wave amplitude to carrier amplitude), so that each H-bridge submodule outputs a basic graphic modulation current waveform with different spectral characteristics.

[0035] The AC output terminals of each H-bridge submodule are connected in series sequentially. Specifically, one AC output terminal of the first H-bridge submodule is connected to one end of the transmitting load (or to the common loop line); the other AC output terminal of the first H-bridge submodule is connected to one AC output terminal of the second H-bridge submodule; the other AC output terminal of the second H-bridge submodule is connected to one AC output terminal of the third H-bridge submodule, and so on. The remaining AC output terminal of the Nth H-bridge submodule is connected to the other end of the transmitting load (or to the other end of the common loop line). In short, the AC output terminals of each H-bridge submodule are connected sequentially, from the output terminal of the previous H-bridge submodule to the output terminal of the next H-bridge submodule, forming a series loop.

[0036] After each H-bridge submodule is connected in series, both ends of the overall series circuit are connected to the same common transmitting load. The transmitting load is the transmitting coil or grounding electrode of the electromagnetic detection system.

[0037] When the transmitting load is a transmitting coil, the transmitting coil is usually laid on the ground, suspended under an aircraft platform, or towed behind a ship to radiate a primary electromagnetic field into the underground medium.

[0038] When the transmitting load is a grounding electrode, the grounding electrode is one or more pairs of electrodes inserted into the ground to inject current into the ground and establish an underground electric field.

[0039] The specific type of launch payload can be selected based on different electromagnetic detection methods (such as airborne electromagnetic detection, ground-to-air electromagnetic detection, marine electromagnetic detection, or downhole electromagnetic detection).

[0040] Since the AC output terminals of each H-bridge submodule are connected in series, according to Kirchhoff's voltage law, the total current flowing through the common transmitting load in the transmitting circuit is equal to the current flowing through each H-bridge submodule, and the voltages output by each H-bridge submodule are superimposed in the series circuit. Under the control of their respective drive control systems, each H-bridge submodule outputs a basic pattern modulated current waveform (in a series circuit, the voltage waveforms output by each H-bridge submodule are superimposed in series, and the total current formed on the common load is the sum of the instantaneous values ​​of the output currents of each H-bridge submodule). Therefore, the total output current of the transmitting circuit satisfies a linear superposition relationship of instantaneous values. Furthermore, according to the linearity of the Fourier transform, the spectrum of the total current is a linear superposition of the spectra of the output waveforms of each H-bridge submodule. Based on this superposition principle, by configuring differentiated main frequency and amplitude parameters for different H-bridge submodules, diverse transmitting spectra can be flexibly combined.

[0041] In practical engineering applications, the total transmit voltage, total output power, and number of independently configurable spectral components of the system can be flexibly adjusted by increasing or decreasing the number N of cascaded H-bridge sub-modules, thereby adapting to the differentiated requirements for transmit power and spectral complexity in different exploration scenarios, facilitating the formation of serialized product designs and flexible on-site networking.

[0042] Each H-bridge submodule, under the control of its independent drive control system, generates its own basic pattern modulated current waveform. The generation process is as follows: The waveform generation unit in the drive control system generates a modulation wave and a carrier wave according to preset modulation wave parameters and carrier wave parameters, respectively. The modulation wave is a modulation wave signal composed of a single-frequency sine wave signal or a multi-frequency sine wave signal, and its frequency and amplitude are independently set according to the spectrum allocation of the H-bridge submodule for the detection task. The carrier wave is an adjacent-frequency sine wave signal, whose frequency is higher than the modulation wave frequency, and the difference between the two frequencies is a preset non-zero frequency difference value.

[0043] The comparison unit performs point-by-point comparison of the instantaneous amplitude of the modulated wave and the carrier wave under the same time reference. When the amplitude of the modulated wave is greater than the amplitude of the carrier wave, it outputs a high level; when the amplitude of the modulated wave is less than or equal to the amplitude of the carrier wave, it outputs a low level, thereby generating a PWM control signal.

[0044] The drive signal distribution and amplification unit distributes the PWM control signal to each power switch transistor. After power amplification and level conversion by the drive circuit, the signal is output to the control terminals of the four power switches of the H-bridge. Specifically, the PWM control signal directly drives the first and fourth power switches, and the inverted PWM control signal drives the second and third power switches.

[0045] Under the control of the drive signal, the full-bridge inverter circuit inverts the DC voltage provided by the independent DC power supply unit into a bipolar multi-frequency current waveform. This bipolar multi-frequency current waveform is the basic pattern modulated current waveform output by the H-bridge submodule. According to PWM harmonic analysis theory, the dominant frequency component of this bipolar multi-frequency current waveform is determined by the frequency of the modulating wave, and the amplitude of each dominant frequency is determined by the modulation index (the ratio of the modulating wave amplitude to the carrier amplitude).

[0046] Since the current flowing through each H-bridge submodule and the common transmitting load in the series circuit is the same current, according to Kirchhoff's voltage law and current law, the total current on the common transmitting load is equal to the sum of the instantaneous values ​​of the output current of each H-bridge submodule at the same moment. According to the linearity of the Fourier transform, linear superposition in the time domain corresponds to linear superposition in the frequency domain. Therefore, the total current spectrum on the common transmit load is equal to the linear superposition of the output current spectra of each H-bridge submodule; Because the basic pattern modulation current waveforms output by each H-bridge submodule have different spectral characteristics, when they are linearly superimposed in the frequency domain, the frequency components of each H-bridge submodule coexist and merge in the total spectrum. Specifically: When the main frequencies of each H-bridge submodule are different, the total spectrum contains all the main frequency components of all H-bridge submodules, and the number of frequency points is the sum of the number of output frequency points of each H-bridge submodule, thereby realizing the expansion of the number of frequency points.

[0047] When the main frequency of each H-bridge submodule is the same but the amplitude is different, the amplitude at the same frequency in the total spectrum is the algebraic sum of the amplitudes of each H-bridge submodule at that frequency, thereby achieving amplitude enhancement or attenuation adjustment.

[0048] When the main frequency and amplitude of each H-bridge submodule are different, the frequency position and amplitude of each frequency point in the total spectrum are independently determined by the corresponding H-bridge submodule, and they do not interfere with each other.

[0049] Based on the aforementioned principle of series superposition, by configuring different modulation wave frequencies and modulation intensities for different H-bridge submodules, arbitrary frequency and amplitude ratios can be achieved in the total spectrum. Specifically: Frequency ratio implementation: The dominant frequency component of each H-bridge submodule is determined by its modulation wave frequency. By setting different modulation wave frequencies for different H-bridge submodules, multiple dominant frequency components of different frequencies can be included in the total spectrum. The proportional relationship between the frequencies is determined by the ratio of the modulation wave frequencies of each H-bridge submodule. Since the modulation wave frequency of each H-bridge submodule can be set independently and continuously, precise control of any frequency ratio (including integer ratios and non-integer ratios) can be achieved.

[0050] Amplitude Ratio Implementation: The amplitude of each main frequency component in each H-bridge submodule is determined by the modulation index. By setting different ratios of modulation wave amplitude to carrier amplitude for different H-bridge submodules, different amplitudes can be achieved for each frequency component in the total spectrum. The proportional relationship between the amplitudes is determined by the modulation index ratio of each H-bridge submodule. Since the modulation index of each H-bridge submodule can be set independently and continuously, precise control of any amplitude ratio can be achieved. The modulation index refers to the ratio of the modulation wave amplitude to the carrier amplitude.

[0051] Independent control of frequency and amplitude ratios: Because the modulation frequency and modulation index of each H-bridge submodule are configured independently, changing the frequency of a certain H-bridge submodule only affects the position of the corresponding frequency point in the total spectrum, without affecting the position and amplitude of the corresponding frequency points of other H-bridge submodules; similarly, changing the modulation index of a certain H-bridge submodule only affects the amplitude of each main frequency component of that H-bridge submodule, without affecting the amplitude of other H-bridge submodules. Therefore, the frequency and amplitude ratios can be adjusted independently and are not coupled.

[0052] Ultimately, the frequency values ​​of each frequency point in the total spectrum are the set of modulation wave frequencies of each H-bridge submodule, and the amplitude values ​​of each frequency point are the function values ​​of the modulation index of the corresponding H-bridge submodule, thereby realizing a combined transmit current waveform with arbitrary frequency and amplitude ratios.

[0053] See Figure 2 As shown, in another aspect, this application provides an electromagnetic detection transmitting current superposition pattern modulation method, comprising: S201 uses N H-bridge sub-modules connected in series to form the main transmitting circuit; S202, each of the H-bridge submodules independently generates and outputs a basic graphic modulated current waveform; S203, through the series connection of each H-bridge submodule in the transmission circuit, according to the target task, allocates the modulation wave frequency and amplitude parameters of each H-bridge submodule, so that the instantaneous values ​​of each basic pattern modulation current waveform are linearly superimposed in the transmission load, and the corresponding spectrum is linearly superimposed, generating a combined transmission current waveform with arbitrary frequency ratio and amplitude ratio.

[0054] In some embodiments, the AC output terminals of each H-bridge submodule are connected in series and then connected to a common transmitting load. In the series circuit, according to Kirchhoff's voltage law, the total circuit voltage is the sum of the instantaneous values ​​of the output voltages of each H-bridge submodule; according to Kirchhoff's current law, the current flowing through each H-bridge submodule is the same current flowing through the common transmitting load. Therefore, the instantaneous value of the total current on the common transmitting load is equal to the algebraic sum of the instantaneous values ​​of the output currents of each H-bridge submodule.

[0055] Based on the target task, allocate the modulation wave frequency and amplitude parameters to each H-bridge submodule, including: The total coverage area of ​​the target frequency band is determined. Based on the maximum and minimum detection depths required for the detection mission, the corresponding minimum and maximum characteristic frequencies are calculated using the skin depth formula, forming a complete frequency range to be covered. Specifically, geological data of the exploration area is first obtained, including but not limited to regional stratigraphic structure, lithological distribution, and known structural features. Then, based on the geological data, the possible depth range of the target body is determined, i.e., the top burial depth (minimum detection depth) and bottom burial depth (maximum detection depth) of the target body, as well as the approximate size of the target body (horizontal extension dimensions and vertical thickness). Simultaneously, the resistivity of the surrounding rock in the target body area is determined through previous electrical resistivity exploration, well logging data, or regional resistivity statistics. The value of the surrounding rock resistivity can be a single average value, or it can be taken as a depth-varying resistivity function based on the electrical characteristics of different depth layers to more accurately reflect the electrical stratification characteristics of the subsurface medium. Based on the skin depth formula, the characteristic frequency range corresponding to the target depth range is calculated. When electromagnetic waves propagate in the subsurface medium, their amplitude decays exponentially with the propagation distance. Skin depth is defined as the distance an electromagnetic wave travels before its amplitude decays to 1 / e (approximately 36.8%) of its amplitude at the Earth's surface. It is an important physical quantity characterizing the penetrating power of electromagnetic waves in conductive media. For a homogeneous half-space medium, the skin depth... With frequency and resistivity The relationship is given by the skin depth formula: ,in, Skin depth, measured in meters (m). The resistivity of the surrounding rock is expressed in ohm-meters (Ω·m). Here, denoted by frequency, is expressed in Hertz (Hz). This skin depth formula indicates that the lower the frequency, the greater the skin depth, and the stronger the electromagnetic wave penetration capability, enabling the detection of deeper targets; conversely, the higher the frequency, the smaller the skin depth, and the weaker the electromagnetic wave penetration capability, making it suitable for detecting shallow targets.

[0056] Based on the skin depth formula above, for a given target depth (Usually taken at skin depth) Depth of target satisfy From the relationship between these factors, the corresponding formula for calculating the characteristic frequency can be derived: , Specifically, for the maximum detection depth (i.e., the burial depth of the deepest target body to be detected), the corresponding minimum characteristic frequency is calculated by substituting into the above formula. The minimum characteristic frequency is the lowest frequency that can penetrate to the deepest target depth and carry an effective response signal. Although electromagnetic waves below the minimum characteristic frequency have stronger penetrating power, their frequencies are too low and may lead to insufficient vertical resolution. Furthermore, low-frequency transmission efficiency is low, and they are not included in the target frequency band unless necessary. For the minimum detection depth (i.e., the burial depth of the shallowest target body to be detected), the corresponding maximum characteristic frequency is calculated by substituting into the above formula. The maximum characteristic frequency is the highest frequency that can effectively distinguish the shallowest target body. Electromagnetic waves above this frequency attenuate rapidly when propagating on the ground surface, have too small a skin depth, and are difficult to effectively excite underground targets. Moreover, high-frequency components are easily interfered with by surface inhomogeneities, and are also not included in the target frequency band.

[0057] When the resistivity of the surrounding rock varies with depth, the above calculations require segmentation. This involves dividing the underground medium into several layers based on resistivity differences, using different resistivities for each layer and calculating corresponding characteristic frequencies. For example, shallow low-resistivity overburden and deep high-resistivity bedrock should have their respective resistivity values ​​used to calculate their corresponding frequency boundaries. Finally, the union of the calculation results for each layer is taken as the complete frequency range to be covered.

[0058] It should be noted that the correspondence between characteristic frequency and target depth is not a strictly precise equation. In actual exploration, a certain frequency fluctuation range is usually allowed based on the specific conditions of the work area. Therefore, in the actual parameter design, the theoretical calculation value is usually combined with the empirical coefficient of the work area for appropriate widening. This ensures that the lower boundary of the frequency interval to be covered is lower than the minimum detection depth (leaving a margin to ensure that deep targets can be effectively excited), and the upper boundary is higher than the maximum detection depth (leaving a margin to ensure the vertical resolution of shallow targets), thus forming a complete frequency interval to be covered with a margin. The widening factor of the frequency interval is usually set between 1.2 and 2.0 according to the geological complexity of the work area. The more complex the geological conditions and the less sufficient the prior information, the larger the widening factor should be to ensure that the target frequency band is completely covered.

[0059] The lowest and highest characteristic frequencies calculated above are used as the boundaries of the target frequency band to form a complete frequency range to be covered. The physical meaning of the frequency range to be covered is that all frequency components within the range have the ability to effectively penetrate the corresponding depth layer and excite the target body to produce a detectable electromagnetic response; it is the set of all useful frequency components required for this detection mission.

[0060] The frequency range to be covered serves as the fundamental basis for the spectrum allocation of H-bridge submodules in subsequent steps. The modulation wave frequency of each H-bridge submodule is selected within or near the frequency range to be covered (due to the sideband spread effect of carrier modulation, some output frequency points may slightly exceed the range boundary, but the main frequency component should fall within the range). The spectrum coverage ranges of each H-bridge submodule are interconnected or partially overlap, together forming a complete coverage of the frequency range to be covered.

[0061] The frequency range to be covered is divided into several frequency sub-ranges according to the number of output main frequencies of each H-bridge sub-module. Each frequency sub-range corresponds to the spectrum coverage range of one H-bridge sub-module. The frequency sub-ranges are adjacent or partially overlapped to ensure that the total spectrum after superposition has no frequency gaps in the frequency range to be covered. Before splitting the frequency range, it is first necessary to determine the number of main frequencies that each H-bridge submodule can output. The number of main frequencies output by a single H-bridge submodule is determined by the modulation waveform and modulation depth it uses.

[0062] When the H-bridge submodule uses a single-frequency sinusoidal signal as the modulation wave and the modulation index is less than or equal to 1 (i.e., operating in the linear modulation region), the output current waveform of the H-bridge submodule contains a dominant frequency component, which is the modulation wave frequency itself. Simultaneously, according to PWM harmonic analysis theory, the output spectrum will also contain sideband frequency components introduced by the carrier wave. In the adjacent-channel pattern modulation described in this application, the adjacent-channel relationship between the carrier frequency and the modulation wave frequency makes the sideband frequency also valuable for detection. Therefore, in this mode, a single H-bridge submodule can typically output two dominant frequency components (one generated by the modulation wave and one generated by the carrier sideband), with the specific number of frequency points depending on the modulation index value.

[0063] When the H-bridge submodule uses a single-frequency sine wave signal as the modulation wave and the modulation index is greater than 1 (i.e., it is operating in the overmodulation region), in addition to the main frequency component, the output current waveform will also generate specific sideband frequency components. A single H-bridge submodule can output three or more main frequency components.

[0064] When the H-bridge submodule uses a modulated wave synthesized from multi-frequency sine wave signals, the number of main output frequencies of the H-bridge submodule is the number of frequency components in the synthesized modulated wave multiplied by the number of output frequency points corresponding to each component. Specifically, if the synthesized modulated wave contains K sine wave components of different frequencies, and each component operates in the linear modulation region, then the H-bridge submodule can output approximately 2K main frequency components. If some components operate in the overmodulation region, the number of output frequency points will further increase.

[0065] The number of output frequencies of each H-bridge submodule can be the same or different, depending on the design parameters and task allocation of each H-bridge submodule. When dividing a frequency range, the number of output frequencies of each H-bridge submodule determines the number of frequency points that the H-bridge submodule can accommodate within its frequency subrange.

[0066] Frequency sub-interval division principle: The frequency interval to be covered is divided into N frequency sub-intervals, each frequency sub-interval corresponding to the spectrum coverage range of one H-bridge submodule. The division of frequency sub-intervals follows these principles: The principle of continuity in frequency sub-interval coverage. Each frequency sub-interval is arranged in order of frequency from low to high or from high to low. The boundary frequencies of adjacent sub-intervals are adjacent or partially overlap, that is, the upper boundary frequency of the previous frequency sub-interval is not lower than the lower boundary frequency of the next frequency sub-interval, ensuring that there are no frequency gaps not covered by any frequency sub-interval within the entire frequency range to be covered.

[0067] The correspondence between frequency sub-intervals and H-bridge sub-modules is as follows: Each frequency sub-interval uniquely corresponds to one H-bridge sub-module, and all the main frequency components output by that H-bridge sub-module fall within the corresponding frequency sub-interval. The specific location of the frequency point within the frequency sub-interval is determined by both the modulation frequency and the carrier frequency of the H-bridge sub-module.

[0068] The principle for allocating the width of frequency sub-intervals is as follows: The width of each frequency sub-interval is allocated according to the number of output main frequencies of the corresponding H-bridge submodule. H-bridge submodules with a larger number of output main frequencies are allocated wider frequency sub-intervals to accommodate more frequency points; H-bridge submodules with a smaller number of output main frequencies are allocated narrower frequency sub-intervals. When all H-bridge submodules have the same number of output main frequencies, the frequency range to be covered can be approximately divided into N frequency sub-intervals of equal width. When the number of output main frequencies of each H-bridge module is different, the width of each frequency sub-interval is proportional to the number of output main frequencies of that H-bridge module.

[0069] The modulation frequency of the corresponding H-bridge submodule is determined based on the center frequency of each frequency sub-interval. After the frequency interval to be covered is divided into frequency sub-intervals of each H-bridge submodule, the frequency boundaries (lower and upper boundaries) of each frequency sub-interval are determined. For the frequency sub-interval corresponding to the i-th H-bridge submodule, its center frequency is defined as the arithmetic or geometric mean of the lower and upper boundaries of that frequency sub-interval.

[0070] When the frequency sub-interval is narrow and the upper and lower boundary frequencies are not significantly different, the difference between the arithmetic mean and the geometric mean can be ignored, and the arithmetic mean can meet the engineering accuracy requirements. When the frequency sub-interval is wide and the coverage spans several octaves, due to the inverse square relationship between frequency and skin depth, the geometric mean can better reflect the center position of the frequency sub-interval in a physical detection sense, and in this case, the geometric mean should be given priority.

[0071] The physical meaning of the center frequency is: the skin depth corresponding to the frequency is located at the geometric center of the depth range corresponding to the frequency sub-interval, and can represent the average detection capability of the H-bridge submodule in the entire detection depth range.

[0072] The modulation frequency of the corresponding H-bridge submodule is determined based on the center frequency of each frequency sub-interval. The principle for determination is as follows: using the center frequency of the frequency sub-interval as a reference, the center frequency or its nearest frequency is set as the modulation frequency of the H-bridge submodule. The selection of the modulation frequency should ensure that all the main frequency components output by the H-bridge submodule fall within the corresponding frequency sub-interval, and that each main frequency component is reasonably distributed within the sub-interval and does not deviate from the sub-interval boundary.

[0073] Specifically, the relationship between the modulation wave frequency and the center frequency can be divided into the following three cases: When the frequency sub-interval is narrow and the modulating wave operates in the linear modulation region, each modulating wave frequency corresponds to two main frequency components (one is the modulating wave frequency itself, and the other is the carrier sideband frequency). At this time, the modulating wave frequency can be directly taken as the center frequency of the sub-interval, and the two output main frequencies are located on both sides of the center frequency and are symmetrically distributed within the sub-interval.

[0074] When the frequency sub-interval is wide or more than three output frequency points are expected to be obtained within the sub-interval, the selection of the modulation wave frequency can deviate from the center frequency and be designed in conjunction with the selection of the modulation index and carrier frequency to ensure that all output frequency points are evenly distributed within the sub-interval or present a specific distribution pattern according to the detection requirements.

[0075] When the depth range corresponding to the frequency sub-interval is the key layer of interest for detection and requires higher density frequency coverage, the modulation wave frequency can be set to a lower or higher position of the frequency sub-interval. In conjunction with the adjustment of the modulation index and carrier frequency, the output frequency points can be distributed with unequal spacing within the sub-interval, concentrating more frequency points on the key depth layer of interest.

[0076] In wideband detection mode, the frequency sub-intervals of each H-bridge submodule have a large span, and the frequency sub-intervals are arranged sequentially, adjacent to each other, or partially overlapping, together covering a wide frequency band from low to high frequencies. In this mode, the modulation wave frequency of each H-bridge submodule is directly taken as the center frequency of the corresponding frequency sub-interval or slightly lower than the center frequency.

[0077] The center frequencies of each frequency sub-interval are evenly distributed according to the overall target frequency band and the number of output frequency points of each H-bridge submodule. The modulation frequencies of each H-bridge submodule do not need to satisfy any specific proportional relationship and can be set independently according to the actual distribution of the target frequency band. For example, the modulation frequencies of the three H-bridge submodules can be set as the center frequencies of their respective frequency sub-intervals, such as 12Hz, 18Hz, and 24Hz, without any fixed proportional constraints between them. The only requirement is to ensure that the output spectrum of each H-bridge submodule collectively and completely covers the target frequency band.

[0078] In single-layer depth focusing mode, each H-bridge submodule is configured with different neighboring frequencies within the same narrow frequency band. These frequency sub-intervals overlap or are adjacent to each other, forming a dense frequency distribution within the target frequency band. In this mode, the modulation wave frequencies of each H-bridge submodule are uniformly distributed within the target frequency band, forming frequency sequences with equal or approximately equal spacing.

[0079] Specifically, the target frequency band is divided into N adjacent frequency sub-intervals according to the number N of H-bridge sub-modules. The center frequencies of each frequency sub-interval are arranged at equal intervals within the target frequency band, and the modulation wave frequency of each H-bridge sub-module is taken as the center frequency of its respective frequency sub-interval. The frequency interval between each modulation wave frequency is determined by both the target bandwidth and the number of H-bridge sub-modules. The smaller the frequency interval and the more H-bridge sub-modules, the greater the frequency density of the total spectrum within the target frequency band and the higher the detection vertical resolution.

[0080] In the dual-layer depth focusing mode, each H-bridge submodule is divided into two groups. The first group is configured with high-frequency dense points corresponding to the shallow layer, and the second group is configured with low-frequency dense points corresponding to the deep layer. In this mode, the modulation wave frequency distribution of the two frequency groups is determined according to the method of the single-layer focusing mode, while the frequency connection between the two groups also needs to be considered.

[0081] Specifically, the modulation wave frequencies of the shallow group are evenly distributed in a single-layer focusing manner within the shallow high-frequency band, and the modulation wave frequencies of the deep group are also evenly distributed in a single-layer focusing manner within the deep low-frequency band. The frequency interval between the two groups can be greater than the interval within the groups, forming two independent frequency focusing regions. Furthermore, a spectral transition can be established between the shallow high-frequency band and the deep low-frequency band through the wideband configuration of the intermediate H-bridge submodule, allowing the total spectrum to transition smoothly between the two focusing regions.

[0082] In this embodiment of the application, the required transmit current amplitude of each frequency point in each frequency sub-interval is calculated and set according to the adjacent frequency carrier amplitude corresponding to the H-bridge sub-module, so that the combined amplitude of each frequency point after superposition meets the detection signal-to-noise ratio requirement. After determining the modulation wave frequency (i.e., the frequency position of each frequency point) of each H-bridge submodule, the required transmit current amplitude for each frequency point needs to be determined. The determination of the required amplitude for each frequency point is based on multiple factors, including the physical characteristics of the target, the resistivity of the surrounding rock, the target depth, and the sensitivity of the receiving system. For different detection depths, the electromagnetic wave experiences different degrees of attenuation during propagation. The electromagnetic response signal of deep targets has already undergone significant attenuation by the time it reaches the surface; therefore, a higher transmit current amplitude is required for the corresponding low-frequency bands to compensate for energy loss along the propagation path. The electromagnetic response signal of shallow targets attenuates less, and a relatively lower transmit current amplitude can be used for the corresponding high-frequency bands. Based on this principle, the required amplitude for each frequency point increases as the frequency decreases; that is, the target amplitude for low-frequency bands is higher than the target amplitude for high-frequency bands, to match the differentiated requirements for transmit signal strength for targets at different depths.

[0083] Specifically, for the j-th frequency point within the frequency range to be covered, the required transmission current amplitude is determined as follows: based on the propagation attenuation theory of electromagnetic waves in conductive media, the transmission current amplitude must satisfy a signal-to-noise ratio at the receiver greater than a preset threshold (typically 3dB to 10dB, depending on the performance of the receiving system), while also considering the abnormal response intensity caused by the electrical difference between the target and the surrounding rock. When prior information is insufficient, estimation can be performed according to the rule that the required amplitude increases by a fixed percentage for each octave decrease in frequency.

[0084] Within the same frequency sub-interval, the required amplitude at each frequency point can be differentiated based on the specific distribution of the target body within the corresponding depth range of that frequency sub-interval. If there is a target layer of key interest within the depth range corresponding to the frequency sub-interval, a higher amplitude is set for the frequency point corresponding to that layer; otherwise, a lower amplitude is set, with the basic signal-to-noise ratio requirement as the lower limit. This differentiated amplitude setting method allows power resources to be concentrated on key detection targets under the condition of limited total transmission power.

[0085] In the adjacent-channel pattern modulation method of this application, the amplitude of each main frequency component in the basic pattern modulation current waveform output by the H-bridge submodule is determined by the modulation index. The relationship between the modulation index and the output amplitude exhibits different characteristics depending on the range of the modulation index value.

[0086] When the modulation index is less than or equal to 1, the modulation operates in the linear modulation region. Within this region, the amplitude of each dominant frequency component in the output current has an approximately linear relationship with the modulation index; that is, the output amplitude increases approximately proportionally as the modulation index increases. Simultaneously, the harmonic content is low, the spectral purity is high, and the output waveform quality is good. The linear modulation region is suitable for fine-tuning detection scenarios requiring high spectral purity, such as single-layer depth focusing mode and double-layer depth focusing mode.

[0087] When the modulation index is greater than 1, modulation operates in the overmodulation region. In this region, in addition to the main frequency component, specific sideband frequency components will be generated in the output current, allowing a single H-bridge submodule to output more frequencies. Modulation then operates in the linear modulation region. In this region, the amplitude of each main frequency component in the output current has an approximately linear relationship with the modulation index; that is, the output amplitude increases approximately proportionally with the increase of the modulation index. Simultaneously, the harmonic content is low, the spectral purity is high, and the output waveform quality is good. The linear modulation region is suitable for fine-tuning detection scenarios requiring high spectral purity, such as single-layer depth focusing and double-layer depth focusing modes. However, in the overmodulation region, the linear relationship between the output amplitude and the modulation index is no longer maintained, and the harmonic content increases, resulting in a decrease in the spectral purity of the output waveform. The overmodulation region is suitable for wide-spectrum detection modes that require a single module to output more frequencies, enabling the coverage of a wider frequency range with a smaller number of H-bridge submodules.

[0088] For an H-bridge submodule that synthesizes a modulated wave using multi-frequency sinusoidal signals, each frequency component of the modulated wave is compared with the same adjacent frequency carrier. In this case, the modulation index of each component is determined by the ratio of its amplitude to the carrier amplitude. The output amplitudes of each component are independent; changing the amplitude of one component only affects the amplitude of its corresponding dominant frequency component, without affecting other components. When the modulation index is in the linear modulation region, the output amplitude of each component is independently adjustable. When some components operate in the overmodulation region, cross-modulation may occur between different components because the amplitude envelope of the synthesized modulated wave exceeds the carrier amplitude, but the amplitude of each dominant frequency component is still primarily controlled by the modulation index of its corresponding component.

[0089] Given the required amplitude at each frequency point and the corresponding carrier amplitude of the H-bridge submodule, the modulation index for each frequency point can be calculated according to the following steps: First, determine the quantitative relationship between modulation index and output amplitude. For a single-frequency modulated wave operating in the linear modulation region, the relationship between the output amplitude and modulation index at the j-th frequency point can be approximately expressed as: , in, To output the amplitude, This is a proportionality coefficient related to the modulation method, carrier frequency, and modulating wave frequency. Let j be the modulation index corresponding to the j-th frequency point. The amplitude is the carrier wave amplitude. The calculated modulation index is the theoretical modulation index corresponding to that frequency point. To ensure that the main frequency amplitude output by each H-bridge submodule accurately reaches the target value, a closed-loop calibration method is usually adopted. Specifically, after the transmission system is assembled, the actual output amplitude of each H-bridge submodule under different modulation indices is measured using a current sensor and a spectrum analyzer to establish a measured calibration curve of modulation index versus output amplitude. When setting actual parameters, the corresponding modulation index value is retrieved from the calibration curve according to the required amplitude, thereby eliminating the deviation between theoretical calculation and engineering reality and improving the accuracy of amplitude control.

[0090] Since the output waveforms of multiple H-bridge submodules are linearly superimposed instantaneously in the transmit load, the combined amplitude of each frequency point in the total spectrum is the algebraic sum of the output amplitudes of each H-bridge submodule at that frequency point (when the frequencies are the same) or they exist independently (when the frequencies are different). When the modulation wave frequencies configured for each H-bridge submodule are different, there is no frequency overlap between the frequency points in the total spectrum, and the amplitude of each frequency point is determined individually by the corresponding H-bridge submodule, without the need for amplitude synthesis calculation. However, when two or more H-bridge submodules produce the same or very similar frequencies at certain frequency points, the combined amplitude of these frequency points is the algebraic sum of the output amplitudes of each H-bridge submodule at that frequency (in-phase superposition) or the magnitude of the phasor sum (when there is a phase difference).

[0091] The frequency points allocated to each H-bridge submodule are superimposed and verified. The amplitude of each frequency point in the superimposed total spectrum is checked one by one to see if it reaches the preset signal-to-noise ratio threshold. If there are frequency points with insufficient amplitude, the modulation of the corresponding H-bridge submodule is called back for compensation and correction until the amplitude of all frequency points meets the detection signal-to-noise ratio requirements.

[0092] During amplitude verification, all frequency points in the total spectrum must be arranged according to their frequency magnitude, and the composite amplitude of each frequency point must be checked one by one to see if it reaches the preset signal-to-noise ratio threshold. If the composite amplitude of a frequency point is lower than the preset signal-to-noise ratio threshold, it indicates that the transmission energy of that frequency point is insufficient to generate an electromagnetic response signal that can be effectively detected by the receiving system. In this case, the modulation index of the H-bridge submodule corresponding to that frequency point needs to be compensated and corrected, that is, the modulation index of the H-bridge submodule is increased to improve the output amplitude of that frequency point. If increasing the modulation index is limited (e.g., the upper limit of the modulation zone has been reached), the modulation index of other non-critical frequency points of the same H-bridge submodule can be reduced to reallocate power resources to the critical frequency point, or the carrier amplitude of the H-bridge submodule can be appropriately increased (by increasing the DC power supply voltage). If the requirements still cannot be met after the above compensation and correction, the number N of cascaded H-bridge submodules needs to be increased to distribute the power requirement to more H-bridge submodules.

[0093] In cases of frequency overlap, since the H-bridge submodules are connected in series, their output currents are directly superimposed in the transmitting load. When the output frequencies of two H-bridge submodules are exactly the same and in phase, the combined amplitude is the algebraic sum of their amplitudes. However, in practical designs, because the drive control systems of each H-bridge submodule are independent, phase synchronization needs to be achieved through a unified synchronization clock or phase-locked mechanism; otherwise, a phase difference may exist, causing the combined amplitude to be less than the sum of the two amplitudes or even canceling each other out.

[0094] In one embodiment, when a single H-bridge submodule is operating, its drive control system first needs to generate two sinusoidal signals: one as a modulation wave and the other as a carrier wave. The modulation wave is a single-frequency sinusoidal signal, and its frequency is determined according to the spectrum allocation scheme of the detection mission. Specifically, the center frequency or its nearest frequency in the frequency sub-interval allocated to the H-bridge submodule is the modulation wave frequency. The amplitude of the modulation wave is determined according to the output current amplitude required by the H-bridge submodule, and is indirectly achieved by setting the modulation index. The initial value can be estimated based on the required output amplitude combined with the carrier amplitude.

[0095] The carrier wave is an adjacent-frequency sinusoidal signal with a frequency higher than the modulation wave frequency. The difference between the two is a preset non-zero frequency difference, typically ranging from tens to hundreds of hertz, depending on the desired output spectrum frequency density and the frequency resolution of the receiving system. The carrier amplitude is fixed, usually set to a constant value based on the DC supply voltage and the characteristics of the full-bridge inverter circuit, providing a unified reference for amplitude adjustment of each submodule. Once set, the carrier amplitude remains unchanged during normal operation, and amplitude adjustment at each frequency point is achieved by changing the modulation wave amplitude. The specific value of the carrier frequency must also meet the avoidance conditions between submodules, i.e., the carrier frequencies of different H-bridge submodules are different and do not have an integer multiple relationship, to prevent frequency intermodulation interference between H-bridge submodules.

[0096] The adjacent-frequency relationship between the modulation wave frequency and the carrier frequency has a clear physical meaning. On the one hand, the carrier frequency being higher than the modulation wave frequency ensures that an effective PWM pulse sequence can be generated during real-time amplitude comparison. On the other hand, the adjacent-frequency difference determines the frequency spacing between the main frequency components in the output spectrum of this submodule. Specifically, according to PWM harmonic analysis theory, under adjacent-frequency pattern modulation, the main frequency components in the output current spectrum appear at the modulation wave frequency and the linear combination frequency of the carrier frequency and the modulation wave frequency. The closer the carrier frequency and the modulation wave frequency are, the smaller the frequency spacing between the main frequency components and the denser the spectrum; conversely, the larger the adjacent-frequency difference, the larger the frequency spacing between the main frequency components and the sparser the spectrum. By adjusting the adjacent-frequency difference, the frequency density of the output spectrum can be adjusted under the condition of a fixed modulation wave frequency, providing additional degrees of freedom for spectrum design.

[0097] After the modulated wave and carrier wave are generated, the comparator unit in the drive control system performs a real-time amplitude comparison between the two under the same time reference. The specific operation of this real-time amplitude comparison is as follows: within each carrier cycle, the instantaneous amplitude of the modulated wave is compared point-by-point with the instantaneous amplitude of the carrier wave; that is, for any given moment, the magnitudes of the instantaneous values ​​of the modulated wave and the carrier wave are compared. When the instantaneous amplitude of the modulated wave is greater than the instantaneous amplitude of the carrier wave, the comparator outputs a high level (logic "1"); when the instantaneous amplitude of the modulated wave is less than or equal to the instantaneous amplitude of the carrier wave, the comparator outputs a low level (logic "0"). Thus, the comparator unit outputs a binary PWM control signal.

[0098] The comparison process is completely real-time, requiring no digital sampling or quantization. The comparison result is precisely determined by the waveform intersection of the modulated wave and the carrier wave. The position of the waveform intersection is determined by the frequency ratio and amplitude ratio of the modulated wave and the carrier wave. The waveform intersections at different times correspond to the rising and falling edges of the PWM pulse, forming all the switching moments of the PWM control signal.

[0099] In one embodiment, a single H-bridge submodule uses a modulated wave synthesized from multi-frequency sine wave signals. The modulated wave synthesized from multi-frequency sine wave signals is compared with the carrier wave in real time to output a basic graphic modulated current waveform containing multiple main frequency components in a single H-bridge submodule. The modulated wave synthesized from the multi-frequency sinusoidal signal is composed of K sinusoidal components of different frequencies, where K is an integer greater than or equal to 2. The frequency of each sinusoidal component corresponds to its respective required detection frequency, and the amplitude of each sinusoidal component corresponds to its respective required main frequency amplitude. The carrier wave is a single-frequency sinusoidal signal with a frequency higher than the highest frequency component in the synthesized modulated wave of the multi-frequency sinusoidal signal. The carrier frequency differs from the frequencies of all sinusoidal components by a non-zero frequency difference, meaning the carrier frequency is greater than the maximum value among all modulated wave frequency components. This configuration ensures that the carrier frequency is higher than all modulated wave frequency components, allowing each frequency component to operate under carrier modulation. The differences between the carrier frequency and the frequencies of each sinusoidal component are all non-zero frequency differences, and these non-zero frequency differences are all positive. These non-zero frequency differences can be the same or different, depending on the distribution of each modulated wave frequency component. When the modulated wave frequency components are uniformly distributed across the spectrum, the corresponding non-zero frequency differences also exhibit a corresponding distribution pattern. The value of each non-zero frequency difference determines the density and distribution of the output frequency points corresponding to each frequency component. The smaller the non-zero frequency difference, the smaller the frequency interval between the output frequency points corresponding to that frequency component, and the denser the spectrum; the larger the difference, the larger the frequency interval between the output frequency points corresponding to that frequency component, and the sparser the spectrum.

[0100] The modulation wave synthesized from multi-frequency sine wave signals is compared with the carrier wave in real time. Each frequency component in the modulation wave synthesized from the multi-frequency sine wave signals is graphically modulated with the carrier wave, generating a set of main frequency components corresponding to that frequency component in the output current spectrum, so that the basic graphically modulated current waveform output by the single-channel H-bridge submodule contains K sets of main frequency components. Specifically: After the multi-frequency sinusoidal signal is synthesized into a modulated wave and a carrier wave is generated, the comparator unit in the drive control system compares their amplitudes in real time on the same time base. Specifically, at any given moment, the instantaneous amplitude of the synthesized modulated wave is compared with the instantaneous amplitude of the carrier wave. When the instantaneous amplitude of the synthesized modulated wave is greater than the instantaneous amplitude of the carrier wave, the comparator outputs a high level; when the instantaneous amplitude of the synthesized modulated wave is less than or equal to the instantaneous amplitude of the carrier wave, the comparator outputs a low level. Thus, the comparator unit outputs a binary PWM control signal.

[0101] During the comparison process, each frequency component in the synthesized modulated wave of the multi-frequency sine wave signal is graphically modulated with the carrier wave. Since the input of the comparator is the linear superposition of each frequency component, the width and position of the output PWM pulse sequence are determined by the intersection of the overall envelope of the synthesized modulated wave and the carrier wave. According to PWM harmonic analysis theory, the modulation information contained in this output PWM pulse sequence is the sum of each frequency component, that is, each frequency component produces a corresponding modulation effect in the output spectrum. Specifically, each frequency component is graphically modulated with the carrier frequency in adjacent-channel configuration. When operating in the linear modulation region, the modulation index of each frequency component is less than or equal to 1, and the output spectrum contains a set of frequency components with the frequency of that component as the dominant frequency. Therefore, K frequency components generate K sets of dominant frequency components, and the frequency position and amplitude of each set of frequency components are independently determined by the frequency and amplitude of the corresponding component.

[0102] Cross-modulation effects between the main frequency components need to be considered. When the frequency interval between the modulated wave frequency components is small or the amplitude of each component is large, different components may cross-modulate through the amplitude envelope of the synthesized modulated wave, producing cross terms such as sum frequency and difference frequency of each component in the output spectrum. To suppress cross-modulation effects, the frequency interval between the modulated wave frequency components should be reasonably designed, generally at least five percent of the carrier frequency, and the maximum amplitude of each component should be controlled to maintain the spectral independence between the components.

[0103] The PWM control signal output from the comparator unit is processed by the drive signal distribution and amplification unit, and then drives the four power switching transistors of the H submodule. Under the control of the drive signal, the H-bridge submodule inverts the DC voltage provided by the independent DC power supply unit into a bipolar multi-frequency current waveform.

[0104] Comparing the operation of multi-frequency sine wave synthesized modulation waves and single-frequency modulation waves in a single-channel H-bridge submodule, in terms of modulation wave composition, a single-frequency modulation wave contains only one frequency component, and the time-domain waveform is a single sine wave signal. The waveform is simple, the peak factor is low, and the comparator operates over a wide range of linear regions. In contrast, a multi-frequency synthesized modulation wave contains K frequency components, and the time-domain waveform is a superposition of multiple sine wave signals. The waveform is complex, the peak factor is high, and the comparator needs to maintain linearity over a wider amplitude range.

[0105] In terms of spectrum output, a single-frequency modulated wave outputs a set of main frequency components, with a small number of frequency points and a narrow spectrum coverage; a multi-frequency synthesized modulated wave outputs K sets of main frequency components, with a significantly increased number of frequency points and a significantly expanded spectrum coverage.

[0106] In terms of amplitude control, the amplitude of a single-frequency modulated wave can be controlled independently, and the adjustment method is simple. The modulation index adjustment of a single frequency point does not affect other frequencies. The amplitude of each component of a multi-frequency synthesized modulated wave can be controlled independently, but since each component shares a carrier amplitude, the modulation indexes of each component are related to each other, and the carrier amplitude resources need to be allocated reasonably.

[0107] In terms of applicable scenarios, single-frequency modulated waves are suitable for fine detection scenarios with a small number of frequency points and high requirements for spectral purity; multi-frequency synthesized modulated waves are suitable for scenarios that require a single H-bridge to output multiple frequency points and are sensitive to hardware costs.

[0108] In one embodiment, one of the following three operating modes is switched by adjusting the modulation frequency and amplitude parameters of each H-bridge submodule: Wide-spectrum detection mode: Configure each H-bridge submodule with a main frequency with a different frequency span, so that the total spectrum after superposition covers a wide range of frequency bands from high frequency to low frequency; Single-layer deep focusing mode: Configure different adjacent frequencies within the same narrow frequency band for each H-bridge submodule, so that the total spectrum after superposition forms a dense frequency point distribution within the target frequency band; Dual-layer depth focusing mode: The H-bridge submodule is divided into two groups. The first group is configured with high-frequency dense frequency points corresponding to the shallow layer, and the second group is configured with low-frequency dense frequency points corresponding to the deep layer. After superposition, the total spectrum forms a focused distribution in the frequency bands corresponding to the two depths.

[0109] In wide-spectrum detection mode, the modulation wave frequencies of each H-bridge submodule are uniformly distributed within the target frequency band in an order of low to high or high to low. Each H-bridge submodule is assigned a frequency sub-interval, and the center frequencies of each frequency sub-interval are arranged sequentially, covering the entire target frequency band from the lowest to the highest frequency. The frequency span between each H-bridge submodule is large, and the frequency sub-intervals of adjacent H-bridge submodules are adjacent or partially overlapped to ensure that there are no frequency gaps in the total spectrum.

[0110] The modulation amplitude of each H-bridge submodule is independently set according to the detection requirements of the target at the corresponding depth. For the low-frequency H-bridge submodule corresponding to deep targets, since the electromagnetic wave undergoes significant attenuation during propagation to the depths, a higher transmit current amplitude is required to compensate for energy loss along the propagation path; therefore, a higher modulation index is set. For the high-frequency H-bridge submodule corresponding to shallow targets, the signal attenuation is smaller, and a lower modulation index can be used. The amplitude of each H-bridge submodule increases as the frequency decreases, forming an amplitude distribution that matches the skin depth.

[0111] In wide-spectrum detection mode, the superimposed total spectrum contains all the main frequency components output by each H-bridge submodule, covering a wide frequency range from high to low frequencies. Because the modulation wave frequencies of each H-bridge submodule are configured with different values ​​across a large frequency span, the frequency intervals between adjacent frequency points in the total spectrum are large, and the frequency points are relatively dispersed. However, the intervals between each frequency point correspond to different skin depths, and are generally approximately uniformly distributed within the target depth range. That is, there is approximately one detection frequency point at every certain depth interval, enabling simultaneous detection of shallow, medium, and deep layers in a single operation.

[0112] In single-layer depth focusing mode, the target frequency band range is first calculated based on the target volume depth and the skin depth formula. This target frequency band is a narrow frequency range corresponding to a specific depth range. Then, this narrow frequency band is divided into several frequency sub-intervals and assigned to each H-bridge submodule.

[0113] The modulation frequencies of each H-bridge submodule are distributed at equal or approximately equal intervals within the target frequency band, with small intervals between frequencies, resulting in a dense arrangement of output frequencies from each H-bridge submodule within the target frequency band. The modulation amplitudes of each H-bridge submodule are configured to similar values ​​because each frequency corresponds to detection within the same depth range, requiring essentially the same amplitude. The modulation depth of each H-bridge submodule can be fine-tuned according to the specific signal-to-noise ratio requirements of each frequency, but overall remains on the same order of magnitude.

[0114] In single-layer depth focusing mode, the superimposed total spectrum forms a dense frequency distribution within the target frequency band. The frequency spacing between each frequency point is much smaller than that in wide-spectrum detection mode, with multiple frequency points concentrated to cover the same depth interval. The higher the frequency density, the higher the vertical resolution of the corresponding depth interval, enabling more precise resolution of the target's fine vertical structure and boundaries. Simultaneously, the concentrated transmission energy of the dense frequency points within the target frequency band increases the total transmission power density of that band, which is beneficial for improving the signal-to-noise ratio of the received signal.

[0115] In the dual-layer depth focusing mode, the two target frequency bands are first calculated based on the depth of the two target layers: the shallow target corresponds to the high-frequency band, and the deep target corresponds to the low-frequency band. Then, the cascaded H-bridge sub-modules are divided into two groups: the first group is responsible for focusing the shallow high-frequency band, and the second group is responsible for focusing the deep low-frequency band.

[0116] The modulation frequencies of the first group of H-bridge submodules are densely distributed within the high-frequency band corresponding to the shallow layer target, and their amplitudes are set according to the signal-to-noise ratio requirements for shallow layer detection. Due to the smaller signal attenuation in shallow layers, this group of H-bridge submodules can use a relatively low modulation index. The modulation frequencies of the second group of H-bridge submodules are densely distributed within the low-frequency band corresponding to the deep layer target, and their amplitudes are set according to the signal-to-noise ratio requirements for deep layer detection. Due to the larger signal attenuation in deep layers, this group of H-bridge submodules needs to use a higher modulation index to provide a larger transmit current amplitude and compensate for deep layer propagation loss. The allocation of submodule numbers between the two groups can be flexibly adjusted according to the detection accuracy requirements of the two target layers. If a certain layer requires higher vertical resolution, more H-bridge submodules are allocated to that layer, thereby obtaining a denser frequency distribution within the corresponding frequency band. With a fixed total number of H-bridge submodules, the allocation of H-bridge submodule numbers between the two layers determines the frequency density of their respective focusing regions. Higher frequency density results in higher vertical resolution, but the frequency density of the other layer decreases accordingly. The level of attention given to the two strata needs to be reasonably determined during the exploration mission planning stage based on the exploration objectives and geological understanding.

[0117] When there is a certain frequency gap between two target frequency bands, the modulation frequency and modulation depth of the intermediate H-bridge submodule can be appropriately configured to allow the H-bridge submodule to simultaneously cover the edge regions of both frequency bands, or the sideband effect of the overmodulation region can be used to generate a transition frequency connecting the two frequency bands. This allows the total spectrum to form a natural transition between the two focal regions, avoiding abrupt changes or breaks in the spectrum. This cross-band connection method ensures that the total spectrum remains continuous overall, which is beneficial for subsequent spectrum analysis and frequency point identification in signal processing.

[0118] In the dual-layer depth focusing mode, the superimposed total spectrum simultaneously forms a focused distribution in the frequency bands corresponding to the two depths. The frequency points in each focused region are densely arranged, similar to the single-layer depth focusing mode, providing detection capabilities with high vertical resolution and high signal-to-noise ratio. The two focused regions correspond to the shallow and deep target layers respectively, enabling high-precision detection of both layers in one operation. Compared to performing two separate single-layer focusing detections, the detection efficiency is doubled.

[0119] In this application, the methods for adjusting the modulation frequency and amplitude parameters of each H-bridge submodule include: adjusting the modulation frequency of a single H-bridge submodule to change the position of the dominant frequency component of the output current waveform of that H-bridge submodule; and adjusting the ratio of the modulation amplitude to the carrier amplitude of a single H-bridge submodule to change the amplitude of each dominant frequency component in the output current waveform of that H-bridge submodule. Specifically: Adjusting the modulation wave frequency refers to changing the frequency value of the modulation wave sinusoidal signal generated in the drive control system of a single H-bridge submodule. This adjustment is achieved through a waveform generation unit in the drive control system, which can generate a continuously adjustable sinusoidal signal. The frequency adjustment range and accuracy are determined by the performance parameters of the waveform generation unit.

[0120] Specifically, when it is necessary to shift the dominant frequency component of a certain H-bridge submodule towards a higher frequency, the modulation frequency value of that H-bridge submodule is increased; when it is necessary to shift the dominant frequency component of a certain H-bridge submodule towards a lower frequency, the modulation frequency value of that H-bridge submodule is decreased. The adjustment of the modulation frequency is based on the spectrum allocation scheme of the detection mission, and the adjusted frequency value should ensure that all output dominant frequency components of the H-bridge submodule fall within the corresponding frequency sub-interval.

[0121] It should be noted that the modulation frequency adjustments of different H-bridge submodules are independent of each other. Because the drive control systems of each H-bridge submodule are independent, adjusting the modulation frequency of one H-bridge submodule only affects the output spectrum of that submodule itself, without affecting the spectral characteristics of other H-bridge submodules. This independence allows for extremely high flexibility in the frequency configuration of the overall spectrum.

[0122] The ratio of the modulation wave amplitude to the carrier wave amplitude, i.e., the modulation index, is adjusted by changing the amplitude parameters of the modulation wave signal, the carrier signal, or both in the drive control system of a single H-bridge submodule. In practice, the carrier wave amplitude is usually kept constant, and the ratio is changed by adjusting the modulation wave amplitude to achieve a stable and reliable adjustment effect.

[0123] Specifically, when it is necessary to increase the output amplitude at a certain frequency, the modulation wave amplitude is increased (i.e., the modulation index is increased); when it is necessary to decrease the output amplitude at a certain frequency, the modulation wave amplitude is decreased (i.e., the modulation index is decreased). The adjustment amount of the modulation index is calculated and determined based on the transmit current amplitude required to be achieved at the corresponding frequency of the submodule, while also considering the signal-to-noise ratio requirements and system power capacity limitations. It should be noted that the modulation index adjustments of different H-bridge submodules are independent of each other. Since the drive control systems of each H-bridge submodule are independent of each other, adjusting the modulation index of one H-bridge submodule only affects the output amplitude of that submodule itself and does not affect the amplitude characteristics of other submodules.

[0124] Besides the modulation frequency and modulation index, adjusting the carrier amplitude is also an effective means of spectrum control. The carrier amplitude is determined by the output voltage of the DC power supply unit; adjusting the DC power supply voltage can change the carrier amplitude.

[0125] In practical parameter design, the modulation wave frequency and amplitude parameters are not adjusted independently, but need to be adjusted in tandem. The tandem adjustment strategy includes the following steps: First, the frequency position and amplitude requirements of each frequency point are determined based on the target depth of the detection mission, forming a complete spectrum allocation scheme.

[0126] Then, set the initial value of the modulation wave frequency according to the frequency sub-interval corresponding to each H-bridge sub-module to ensure that the output frequency of each H-bridge sub-module covers the target frequency band without frequency gaps.

[0127] Next, the initial values ​​of the modulation index of each H-bridge submodule are set according to the signal-to-noise ratio requirements of each frequency point to ensure that the output amplitude of each frequency point meets the detection requirements.

[0128] Finally, the set total spectrum is verified to check whether the position of each frequency point falls within the target frequency band, whether the amplitude of each frequency point meets the signal-to-noise ratio requirements, and whether there are excessive frequency gaps between each frequency point. If there are deviations, the modulation frequency and modulation index of the corresponding H-bridge submodule are finely adjusted, and the verification is repeated until all indicators meet the requirements.

[0129] In one embodiment, taking a launch system consisting of three H-bridge power submodules cascaded in series as an example, a wide-spectrum detection mode is used to detect underground structures for underground survey tasks in the depth range of 100 to 700 meters.

[0130] The exploration task in this embodiment is to conduct a large-scale survey of underground structures at depths ranging from 100 to 700 meters. The objective is to first obtain an overview of the underground electrical distribution across the entire profile, providing a basis for subsequent detailed exploration of target areas and depth ranges. Based on the geological data of the exploration area and the resistivity conditions of the work area, and combined with the skin depth formula, the characteristic frequency range corresponding to the target depth is calculated, determining that the target frequency band to be covered is 12Hz to 634Hz.

[0131] Based on the target frequency bands mentioned above, the frequency bands are allocated to three H-bridge submodules. The configuration of each H-bridge submodule is as follows: The modulation frequency of the first H-bridge submodule is set to 12Hz, the carrier frequency to 36Hz, and the modulation index to 1.2. Since the modulation index is greater than 1, this H-bridge submodule operates in the overmodulation region, and its output current spectrum includes three detection frequencies: 12Hz, 36Hz, and 60Hz. 12Hz is the primary frequency component, while 36Hz and 60Hz are sideband frequency components generated by overmodulation. This H-bridge submodule is primarily responsible for covering the low-frequency end of the target frequency band, corresponding to the detection of deep targets.

[0132] The modulation frequency of the second H-bridge submodule is set to 18Hz, the carrier frequency to 162Hz, and the modulation index to 0.85. Since the modulation index is less than 1, this H-bridge submodule operates in the linear modulation region, and its output current spectrum includes two detection frequencies: 18Hz and 162Hz. 18Hz is the main frequency component, and 162Hz is the carrier sideband frequency component. This H-bridge submodule is primarily responsible for covering the mid-frequency band of the target frequency band, corresponding to the detection of mid-level targets.

[0133] The modulation frequency of the third H-bridge submodule is set to 24Hz, the carrier frequency to 624Hz, and the modulation index to 0.85. This H-bridge submodule also operates in the linear modulation region, and its output current spectrum includes two detection frequencies: 24Hz and 624Hz. 24Hz is the main frequency component, and 624Hz is the carrier sideband frequency component. This H-bridge submodule is primarily responsible for covering the high-frequency end of the target frequency band, corresponding to the detection of shallow targets.

[0134] Each H-bridge submodule is configured with a corresponding adjacent frequency carrier, and a drive signal is generated through graphical comparison to drive its respective full-bridge inverter circuit to output a bipolar multi-frequency current waveform.

[0135] The AC output terminals of the three H-bridge submodules are connected in series and then connected to a common transmitting load. According to the superposition principle of series circuits, the total output current of the transmitting circuit is the sum of the instantaneous values ​​of the output current of each submodule, and the spectrum of the total current is the linear superposition of the output waveform spectra of each H-bridge submodule.

[0136] The output waveforms of the three H-bridge submodules are cascaded and superimposed, resulting in a total waveform containing seven main frequencies: 12Hz, 18Hz, 24Hz, 36Hz, 60Hz, 162Hz, and 624Hz, covering a wide frequency band from 12Hz to 634Hz. These frequencies are approximately uniformly distributed within the target frequency band. Based on the skin depth-frequency correspondence, there is approximately one detection frequency every 100 meters of depth, enabling simultaneous detection of shallow, intermediate, and deep targets in a single operation.

[0137] By cascading and superimposing three H-bridge submodules, synchronous transmission at seven detection frequencies was achieved within a wide frequency band of 12Hz to 634Hz, covering a depth range of 100 to 700 meters. Compared to traditional single-channel transmission or multi-frequency pulse width modulation schemes, this embodiment achieves wide-band multi-frequency synchronous detection with simple parameter configuration, significantly improving the exploration efficiency of large-scale underground structure surveys and making it suitable for regional geological surveys when prior information about the target is insufficient.

[0138] See Figure 3 The transmitted waveform and its spectrum in the wide-spectrum detection mode. Figure 3As can be observed in (a), the transmitted waveform is a complex periodic waveform composed of multiple frequency components superimposed. The waveform as a whole exhibits non-sinusoidal periodic variation characteristics, containing multiple alternating positive and negative pulses of different amplitudes and widths within a complete cycle. Specifically, the waveform exhibits the following characteristics within each cycle: the waveform contains multiple pulse sequences with different amplitudes within a complete cycle. Among them, the pulses with larger amplitudes correspond to the contribution of the low-frequency dominant component (12Hz), with a period of approximately 83 milliseconds, which manifests as the slowly varying portion of the waveform envelope in the time domain. Pulses with smaller amplitudes are superimposed on the larger amplitude pulses, corresponding to the contributions of mid-to-high frequency components (18Hz, 24Hz, 36Hz, 60Hz, 162Hz, 624Hz, etc.), which manifest as rapid fluctuations in the envelope in the time domain. The amplitude distribution of each pulse is uneven, with higher amplitudes for low-frequency pulses and lower amplitudes for high-frequency pulses, reflecting the differentiated configuration of amplitudes at each frequency point.

[0139] The waveform exhibits typical bipolar characteristics, with the positive and negative half-cycles being essentially symmetrical, indicating that the DC component in the transmitted current is effectively suppressed. The symmetrical bipolar waveform is beneficial for signal excitation and reception processing in electromagnetic detection.

[0140] The characteristics of this time-domain waveform correspond to the spectrum diagram. Due to the superposition of multiple frequency points, the time-domain waveform exhibits a complex non-sinusoidal shape, which is the external manifestation of the superposition of multiple dominant frequency components in the time domain under wide-spectrum detection mode.

[0141] Figure 3 In (b), the following spectral characteristics can be observed: the spectrum contains seven main frequency peaks located at 12Hz, 18Hz, 24Hz, 36Hz, 60Hz, 162Hz, and 624Hz. These seven main frequency components correspond to all output frequencies of the three H-bridge submodules, with the specific correspondence as follows: The first H-bridge module contributes three frequency points: 12Hz, 36Hz, and 60Hz. Among them, 12Hz is the main frequency component with the highest amplitude, corresponding to the detection of the deepest target (approximately 700m depth); 36Hz and 60Hz are sideband frequency components generated by overmodulation, with amplitudes lower than 12Hz, corresponding to the detection of mid-deep and intermediate-layer targets, respectively.

[0142] The second H-bridge module contributes two frequency points: 18Hz and 162Hz. 18Hz is the main frequency component with a medium amplitude, corresponding to the detection of mid-level targets (approximately 400m-500m depth); 162Hz is the carrier sideband frequency component with an amplitude lower than 18Hz, corresponding to the detection of shallower targets.

[0143] The third H-bridge module contributes two frequency points: 24Hz and 624Hz. 24Hz is the main frequency component with a moderate amplitude, corresponding to the detection of shallower targets; 624Hz is the carrier sideband frequency component, corresponding to the detection of the shallowest targets (approximately 100m depth).

[0144] The seven main frequency components cover a wide frequency range from 12Hz to 634Hz (624Hz is the nominal frequency, and the actual sideband coverage extends to 634Hz). The frequency points are arranged sequentially on the spectrum. Although the frequency spacing between adjacent frequency points is not uniform, according to the inverse square relationship between skin depth and frequency, the skin depth corresponding to each frequency point is approximately uniformly distributed within the depth range of 100m to 700m, that is, there is approximately one detection frequency point every 100m depth.

[0145] Specifically, 12Hz corresponds to a depth of approximately 700m, 18Hz to approximately 570m, 24Hz to approximately 500m, 36Hz to approximately 400m, 60Hz to approximately 310m, 162Hz to approximately 190m, and 624Hz to approximately 100m. Each frequency point sequentially covers different depth layers from deep to shallow, from low to high frequency, enabling simultaneous detection of targets at shallow, medium, and deep depths.

[0146] The amplitude distribution across frequency bands shows an overall trend of higher amplitudes in the low-frequency bands and lower amplitudes in the high-frequency bands. The amplitude is highest at 12Hz because the propagation path of deep-layer targets is long and attenuates significantly, requiring a higher transmission amplitude to compensate for energy loss. As the frequency increases, the corresponding detection depth becomes shallower, the signal propagation path shortens, and the required transmission amplitude decreases accordingly. This amplitude distribution matches the differentiated requirements for transmission signal strength at different depth levels, demonstrating the technical advantage of this application in achieving flexible amplitude configuration at each frequency point by independently adjusting the modulation index of each submodule.

[0147] In one embodiment, taking a launch system consisting of three H-bridge power submodules cascaded in series as an example, fine detection of targets within a depth range of 200 to 250 meters is performed, and a single-layer depth focusing mode is used to achieve high-precision detection of specific depth layers.

[0148] The exploration mission involves detailed probing of target bodies within a depth range of 200 to 250 meters. The objective is to obtain the detailed electrical distribution of subsurface structures within this depth range and identify the boundaries and morphology of anomalies. Based on the geological data of the exploration area and the resistivity conditions of the work area, and using the skin depth formula, the characteristic frequency range corresponding to this depth range is calculated, determining that the target frequency band to be focused is 101 Hz to 159 Hz.

[0149] The target frequency bands mentioned above are allocated to three H-bridge submodules. The configuration of each H-bridge submodule is as follows: The modulation frequency of the first H-bridge submodule is set to 96Hz, the carrier frequency is set to 132Hz, and the modulation index is set to 0.85. This submodule operates in the linear modulation region, and the output current spectrum includes two detection frequencies, namely 96Hz and 132Hz.

[0150] The modulation frequency of the second H-bridge submodule is set to 108Hz, the carrier frequency is set to 144Hz, and the modulation index is set to 0.85. This submodule operates in the linear modulation region, and the output current spectrum includes two detection frequencies, namely 108Hz and 144Hz.

[0151] The modulation frequency of the third H-bridge submodule is set to 120Hz, the carrier frequency to 156Hz, and the modulation index to 0.85. This submodule operates in the linear modulation region, and the output current spectrum includes two detection frequencies, namely 120Hz and 156Hz.

[0152] All three H-bridge submodules operate in the linear modulation region with the same modulation depth of 0.85, ensuring similar output amplitudes and good spectral purity at each frequency. Each submodule is configured with a corresponding adjacent frequency carrier, and a drive signal is generated through graphical comparison to drive its respective full-bridge inverter circuit to output a bipolar multi-frequency current waveform.

[0153] After the output waveforms of the three H-bridge submodules are cascaded and superimposed, the total waveform contains six main frequencies: 96Hz, 108Hz, 120Hz, 132Hz, 144Hz, and 156Hz, all concentrated within the narrow target frequency band of 101Hz to 159Hz. These six frequencies form a dense frequency distribution within a 50-meter depth range, with small frequency intervals between each frequency, resulting in a significantly improved depth resolution.

[0154] In single-layer depth focusing mode, the high frequency density and concentrated energy within the target frequency band result in a high total transmit power density, which is beneficial for improving the signal-to-noise ratio of the received signal. Simultaneously, the dense frequency distribution allows for higher vertical resolution in inversion interpretation, effectively improving the detection accuracy of targets within a depth range of 200 to 250 meters.

[0155] See Figure 4 The single-layer depth-focusing mode emission waveform and its spectrum diagram are shown below. Figure 4As can be observed in (a), the transmitted waveform exhibits a more regular periodicity compared to the waveform in the wide-spectrum detection mode. The overall waveform is a complex periodic waveform composed of superimposed multiple frequency components. However, because each frequency component is concentrated within a narrow frequency band of 101Hz to 159Hz, the differences between frequencies are small, resulting in a relatively uniform pulse width distribution and amplitude variation in the time domain waveform. Compared to the wide-spectrum detection mode, the time domain waveform in the single-layer depth focusing mode changes more smoothly, with fewer high-frequency fluctuations. This is because the output spectrum is concentrated within a narrow target frequency band, with the ratio of the highest to the lowest frequency being approximately 1.6, indicating a smaller frequency span.

[0156] The characteristics of this time-domain waveform correspond to the spectrum diagram. Because multiple frequency points are concentrated and superimposed within a narrow frequency band, the time-domain waveform exhibits a relatively regular beat frequency pattern, which is the external manifestation of dense frequency points superimposed in the time domain under single-layer depth focusing mode.

[0157] Figure 4 In (b), the spectrum contains six main frequency peaks located at 96Hz, 108Hz, 120Hz, 132Hz, 144Hz, and 156Hz. These six main frequency components correspond to all output frequencies of the three H-bridge submodules. All six main frequency components are concentrated within the narrow target frequency band of 101Hz to 159Hz, with a frequency interval of 12Hz between each frequency point. In the time domain, this manifests as a beat frequency effect, meaning the waveform amplitude changes periodically over time. Due to the similar amplitudes at each frequency point, the beat frequency amplitude modulation is quite pronounced, and the waveform envelope exhibits regular fluctuations. This regular beat frequency waveform is a typical characteristic of high-density frequency signal superposition. Within this frequency band, the frequency density reaches approximately one frequency point per 10Hz (six frequency points covering approximately 58Hz bandwidth), far exceeding the frequency density in wide-spectrum detection mode. Based on the skin depth formula, the detection depth range corresponding to this frequency band is 200 to 250 meters. The six frequency points form a high-density frequency coverage within a depth range of 50 meters, resulting in a significant improvement in depth resolution. Specifically, 96Hz corresponds to a depth of approximately 257m, 108Hz to approximately 243m, 120Hz to approximately 230m, 132Hz to approximately 219m, 144Hz to approximately 210m, and 156Hz to approximately 202m.

[0158] In one embodiment, a launch system consisting of three H-bridge power submodules cascaded in series is used as an example. It performs fine detection of targets in two depth ranges: 100 to 150 meters and 200 to 250 meters. A dual-layer depth focusing mode is used to achieve high-precision synchronous detection of the two target layers.

[0159] The exploration mission involves simultaneously conducting detailed exploration of target bodies within two depth ranges: 100-150 meters and 200-250 meters. The objective is to acquire detailed electrical distribution information for both target layers in a single exploration, balancing exploration efficiency and vertical resolution. Based on geological data of the exploration area and resistivity conditions, and using the skin depth formula, the characteristic frequency ranges corresponding to the two depth ranges were calculated. The two target frequency bands to be focused on were determined to be 101Hz-159Hz (shallow high-frequency band) and 281Hz-634Hz (deep low-frequency band).

[0160] The two target frequency bands mentioned above are allocated to three H-bridge submodules. The configuration of each H-bridge submodule is as follows: The modulation frequency of the first H-bridge submodule is set to 117Hz, the carrier frequency to 156Hz, and the modulation index to 0.85. This H-bridge submodule operates in the linear modulation region, and its output current spectrum includes two detection frequencies, 117Hz and 156Hz, covering a portion of the shallow high-frequency band from 101Hz to 159Hz. This H-bridge submodule belongs to the shallow focusing group.

[0161] The modulation frequency of the second H-bridge submodule is set to 104Hz, the carrier frequency to 312Hz, and the modulation index to 1.2. Since the modulation index is greater than 1, this H-bridge submodule operates in the overmodulation region, and its output current spectrum includes three detection frequencies: 104Hz, 312Hz, and 520Hz. 104Hz is located at the low end of the shallow high-frequency band, while 312Hz and 520Hz are located within the deep low-frequency band (281Hz to 634Hz). This submodule acts as a bridge connecting the two deep focusing regions, while also providing some frequency points for deep focusing.

[0162] The modulation frequency of the third H-bridge submodule is set to 130Hz, the carrier frequency to 390Hz, and the modulation index to 1.2. This H-bridge submodule operates in the overmodulation region, and its output current spectrum includes three detection frequencies: 130Hz, 390Hz, and 650Hz. The 130Hz frequency is located in the shallow high-frequency band, while the 390Hz and 650Hz frequencies are located in the deep low-frequency band (the 650Hz frequency slightly exceeds the upper boundary of the 281Hz to 634Hz range, belonging to the sideband extension within the frequency margin). This H-bridge submodule belongs to the deep focusing group.

[0163] Each H-bridge submodule is configured with a corresponding adjacent frequency carrier, and a drive signal is generated through graphical comparison to drive its respective full-bridge inverter circuit to output a bipolar multi-frequency current waveform.

[0164] After the output waveforms of the three H-bridge submodules are superimposed in series, the total waveform contains eight main frequencies: 104Hz, 117Hz, 130Hz, 156Hz, 312Hz, 390Hz, 520Hz and 650Hz, covering two target frequency bands: 101Hz to 159Hz and 281Hz to 634Hz.

[0165] Within the shallow high-frequency band (101Hz to 159Hz), there are four detection frequencies: 104Hz, 117Hz, 130Hz, and 156Hz, corresponding to fine-grained detection at depths of 100 to 150 meters. Within the deep low-frequency band (281Hz to 634Hz), there are three detection frequencies: 312Hz, 390Hz, and 520Hz (650Hz, as a sideband extension within the frequency margin, also has detection value), corresponding to fine-grained detection at depths of 200 to 250 meters. Having four detection frequencies within each detection range effectively improves detection accuracy.

[0166] See Figure 5 The emitted waveform and its spectrum in the dual-layer depth-focusing mode are shown below. Figure 5 As can be observed in (a), the transmitted waveform is a complex periodic waveform composed of multiple frequency components superimposed. The waveform as a whole exhibits non-sinusoidal periodic variation characteristics, containing multiple alternating positive and negative pulses of different amplitudes and widths within a complete cycle. Compared with the wide-spectrum detection mode and the single-layer depth focusing mode, the time-domain waveform complexity is the highest in the dual-layer depth focusing mode. This is because the total spectrum in this mode contains eight main frequency components, which come from two depth focusing regions (shallow high-frequency band and deep low-frequency band). The superposition of these frequency components in the time domain forms a more complex waveform shape.

[0167] Specifically, the waveform contains multiple pulse sequences with varying amplitudes and widths within a complete cycle. Large-amplitude pulses are associated with low-frequency components (frequency points around 104Hz, 117Hz, and 130Hz, corresponding to some frequencies in the deep low-frequency band), while small-amplitude pulses are associated with high-frequency components (312Hz, 390Hz, 520Hz, 650Hz, etc., corresponding to the high end of the deep low-frequency band and some frequencies in the shallow high-frequency band). The amplitude distribution of each pulse exhibits certain differential characteristics, reflecting the different modulation schemes (0.85 and 1.2) of each H-bridge submodule.

[0168] from Figure 5As shown in (b), the spectrum contains eight main frequency peaks located at 104Hz, 117Hz, 130Hz, 156Hz, 312Hz, 390Hz, 520Hz, and 650Hz. These eight main frequency components correspond to all output frequencies of the three H-bridge submodules, and are distributed within two separate target frequency bands. The first focusing region is the shallow high-frequency band from 101Hz to 159Hz, containing four detection frequencies of 104Hz, 117Hz, 130Hz, and 156Hz, corresponding to fine detection at depths of 100 to 150 meters, with a frequency density of approximately one frequency every 15Hz. The second focusing region is the deep low-frequency band from 281Hz to 634Hz, containing four detection frequencies of 312Hz, 390Hz, 520Hz, and 650Hz, corresponding to fine detection at depths of 200 to 250 meters (650Hz corresponds to a depth of approximately 98m, connecting with the lower boundary of the shallow focusing region). There is a certain frequency gap between the two focusing regions in the range of approximately 159Hz to 281Hz, but due to the wideband coverage of the second H-bridge submodule (104Hz to 520Hz) and the wideband coverage of the third H-bridge submodule (130Hz to 650Hz), the total spectrum achieves a natural transition between the two focusing regions.

[0169] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A launching system, characterized in that, include: N H-bridge submodules are cascaded in series, where N is an integer greater than or equal to 2; each H-bridge submodule includes an independent DC power supply unit and a full-bridge inverter circuit, and each H-bridge submodule is connected to an independent drive control system; the AC output terminals of each H-bridge submodule are connected in series and then connected to a common transmitting load. Each H-bridge submodule is used to independently generate and output a basic pattern modulation current waveform. By connecting them in series, the instantaneous values ​​of each basic pattern modulation current waveform are linearly superimposed in the transmitting load, and the corresponding spectra are linearly superimposed to generate a combined transmitting current waveform with arbitrary frequency and amplitude ratios.

2. A method for superimposed pattern modulation of electromagnetic detection transmission current, characterized in that, include: The main transmitting circuit is constructed by cascading N H-bridge sub-modules in series. Each H-bridge submodule independently generates and outputs a basic graphic modulated current waveform; By connecting each H-bridge submodule in series in the transmission circuit, the modulation wave frequency and amplitude parameters of each H-bridge submodule are allocated according to the target task, so that the instantaneous values ​​of the basic pattern modulation current waveforms of each path are linearly superimposed in the transmission load, and the corresponding spectra are linearly superimposed, generating a combined transmission current waveform with arbitrary frequency and amplitude ratios.

3. The electromagnetic detection transmission current superposition pattern modulation method according to claim 2, characterized in that, Based on the target task, allocate the modulation wave frequency and amplitude parameters to each H-bridge submodule, including: Determine the total coverage of the target frequency band, and calculate the corresponding lowest and highest characteristic frequencies based on the maximum and minimum detection depths required by the detection mission, combined with the skin depth formula, to form a complete frequency range to be covered. The frequency range to be covered is divided into several frequency sub-ranges according to the number of output main frequencies of each H-bridge sub-module. Each frequency sub-range corresponds to the spectrum coverage range of one H-bridge sub-module. The frequency sub-ranges are adjacent or partially overlapped to ensure that the total spectrum after superposition has no frequency gaps in the frequency range to be covered. The modulation wave frequency of the corresponding H-bridge submodule is determined based on the center frequency of each frequency sub-interval; Based on the required transmit current amplitude at each frequency point within each frequency sub-interval, and combined with the adjacent frequency carrier amplitude corresponding to the H-bridge submodule, the modulation index corresponding to each modulation wave frequency is calculated and set so that the combined amplitude at each frequency point after superposition meets the detection signal-to-noise ratio requirements. The frequency points allocated to each H-bridge submodule are superimposed and verified. The amplitude of each frequency point in the superimposed total spectrum is checked one by one to see if it reaches the preset signal-to-noise ratio threshold. If there are frequency points with insufficient amplitude, the modulation of the corresponding H-bridge submodule is called back for compensation and correction until the amplitude of all frequency points meets the detection signal-to-noise ratio requirements.

4. The electromagnetic detection transmission current superposition pattern modulation method according to claim 3, characterized in that, The basic pattern modulated current waveform is a bipolar multi-frequency current waveform. Each H-bridge submodule uses a single-frequency sine wave signal or a multi-frequency sine wave signal as the modulation wave and an adjacent-frequency sine wave signal as the carrier wave. The modulation wave and the carrier wave are compared in real time. Based on the comparison result, the drive signals of each switch of the H-bridge submodule are generated, and the bipolar multi-frequency current waveform is output.

5. The electromagnetic detection transmission current superposition pattern modulation method according to claim 4, characterized in that, Each of the H-bridge submodules uses a single-frequency sine wave signal as the modulation wave and an adjacent-frequency sine wave signal as the carrier wave, specifically including: The modulation wave uses a single-frequency sine wave signal, and the frequency of the sine wave signal is set according to the characteristic frequency corresponding to the detection target. The carrier wave uses a sinusoidal signal with a frequency different from that of the modulation wave, and the carrier frequency is higher than the modulation wave frequency. The difference between the carrier frequency and the modulation wave frequency is a preset positive frequency difference value, so that the carrier frequency and the modulation wave frequency are kept close to each other. The preset positive frequency difference determines the frequency interval between each main frequency component in the bipolar multi-frequency current waveform spectrum output by the H-bridge module. Increasing the positive frequency difference makes the frequency distribution of the output spectrum sparser, while decreasing the positive frequency difference makes the frequency distribution of the output spectrum denser.

6. The electromagnetic detection transmission current superposition pattern modulation method according to claim 4, characterized in that, Each H-bridge submodule uses a modulation wave synthesized from multi-frequency sine wave signals. The modulation wave synthesized from the multi-frequency sine wave signals is compared with the carrier wave in real time to output a basic graphic modulation current waveform containing multiple main frequency components in a single H-bridge submodule. The modulated wave synthesized by the multi-frequency sine wave signal is composed of K sine components of different frequencies superimposed, where K is an integer greater than or equal to 2. The frequency of each sine component corresponds to its respective required detection frequency point, and the amplitude of each sine component corresponds to its respective required main frequency amplitude. The carrier wave is a single-frequency sine wave signal with a frequency higher than the highest frequency component in the synthesized modulation wave of the multi-frequency sine wave signal, and the frequency difference between the carrier wave and each sine wave component is a non-zero frequency difference. The modulation wave synthesized from the multi-frequency sine wave signal is compared with the carrier wave in real time. Each frequency component in the modulation wave synthesized from the multi-frequency sine wave signal is graphically modulated with the carrier wave. A set of main frequency components corresponding to the frequency component is generated in the output current spectrum, so that the basic graphical modulation current waveform output by the single-channel H-bridge submodule contains K sets of main frequency components.

7. The electromagnetic detection transmission current superposition pattern modulation method according to claim 4, characterized in that, The real-time amplitude comparison includes: within each carrier cycle, comparing the instantaneous amplitude of the modulated wave with the instantaneous amplitude of the carrier point by point; outputting a high level when the amplitude of the modulated wave is greater than the amplitude of the carrier, and outputting a low level when the amplitude of the modulated wave is less than or equal to the amplitude of the carrier, thereby generating a PWM control signal.

8. The electromagnetic detection transmission current superposition pattern modulation method according to claim 3, characterized in that, By adjusting the modulation frequency and amplitude parameters of each H-bridge submodule, one of the following three operating modes can be switched: Wide-spectrum detection mode: Configure each H-bridge submodule with a main frequency with a different frequency span, so that the total spectrum after superposition covers a wide range of frequency bands from high frequency to low frequency; Single-layer deep focusing mode: Configure different adjacent frequencies within the same narrow frequency band for each H-bridge submodule, so that the total spectrum after superposition forms a dense frequency point distribution within the target frequency band; Dual-layer depth focusing mode: The H-bridge submodule is divided into two groups. The first group is configured with high-frequency dense frequency points corresponding to the shallow layer, and the second group is configured with low-frequency dense frequency points corresponding to the deep layer. After superposition, the total spectrum forms a focused distribution in the frequency bands corresponding to the two depths.

9. The electromagnetic detection transmission current superposition pattern modulation method according to claim 8, characterized in that, Adjust the modulation frequency and amplitude parameters of each H-bridge submodule as follows: Adjusting the modulation frequency of a single H-bridge submodule changes the position of the dominant frequency component of the output current waveform of that H-bridge submodule; Adjusting the ratio of the modulation amplitude to the carrier amplitude of a single H-bridge submodule changes the amplitude of each main frequency component in the output current waveform of that H-bridge submodule.

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