Controllable coded waveform electrical source transmitting system and control method

Abstract

The application belongs to the field of geophysical exploration, and is a controllable coded waveform electric source emission system and a control method, which comprises: an inverter circuit and an impedance matching circuit connected in parallel, and a direct current power supply is used to provide a direct current voltage; a controller is used to form a target current waveform; the inverter circuit and the impedance matching circuit are switched to be connected to an emission loop according to a current code element value; when the inverter circuit is connected, the target voltage waveform is compared with a fixed frequency triangular carrier in amplitude to generate a feedforward duty cycle; an actual emission current is obtained and compared with the target current to obtain an error signal; according to the error signal, a duty cycle correction amount is calculated and superimposed on the feedforward duty cycle to form a final duty cycle, which is distributed to the control end of a switching device in the inverter circuit, so that the coded current waveform of the actual emission current tracks the target current waveform. The application is beneficial to improve the signal-to-noise ratio of electric shock response exploration and realize the targeted excitation of electric shock linear and nonlinear response.

Description

Technical Field

[0001] This application belongs to the field of geophysical exploration, specifically a controllable coded waveform electrical source transmission system and control method. Background Technology

[0002] Electroseismic effect is a geophysical effect based on electrodynamic coupling mechanism, originating from the double-layer structure of the solid-liquid interface in porous media. When an external electric field is applied to a porous medium, it pulls the directional movement of charged ions in the pore fluid; due to the viscosity between the charged ions and the fluid, it induces relative movement between the pore fluid and the rock skeleton, thereby generating seismic waves; this conversion process is called electroseismic conversion. In oil and gas reservoir and groundwater exploration, electromagnetic exploration methods can identify electrical changes at the interface, but their spatial resolution is low. Seismic exploration has high spatial resolution, but it is only sensitive to differences in elastic wave impedance at the interface. Electroseismic conversion is sensitive to both elastic and electrical differences at the interface, and therefore is suitable for high-resolution detection of oil and gas reservoirs and groundwater. It should be clarified that exploration methods based on the electroseismic effect achieve underground exploration through the physical process of electroseismic conversion. This method is different from combined electroseismic exploration, which is equivalent to a combination of seismic and electromagnetic exploration. In actual field exploration scenarios, the signal-to-noise ratio of electroseismic conversion is low, and the effective signal is easily submerged in environmental noise. By transmitting coded current waveforms and performing cross-correlation operations on the received electroseismic response and the coded transmitted waveforms, it is possible to extract signals from low signal-to-noise ratio data. In addition, the coded current waveforms can be used to excite linear and nonlinear responses of electroseismic data, including nonlinear mechanisms such as second-order electrostriction. Existing methods employ an electromagnetic emission module to transmit a single electromagnetic pulse signal to the detection area, combined with an electromagnetic acquisition module and a seismic acquisition module to acquire the detection signal. However, transmitting only a conventional electromagnetic pulse cannot distinguish or separate the linear and nonlinear responses of the electroseismic response; furthermore, in low signal-to-noise ratio environments, it is difficult to extract the electroseismic response signal.

[0003] For the problems of low signal-to-noise ratio in field measurements of electro-vibration effects and the inability to separately excite linear and nonlinear electro-vibration responses, no dedicated instrument system or control method has been designed domestically or internationally. Therefore, a controllable coded waveform electrical source transmission system and control method can effectively improve the signal-to-noise ratio of electro-vibration response and realize the separate excitation of linear and nonlinear electro-vibration responses. Summary of the Invention

[0004] This application provides a controllable coded waveform electrical source transmission system and control method to solve the problems of low signal-to-noise ratio in field measurements of electro-vibration effects and the inability to independently excite linear and nonlinear electro-vibration responses.

[0005] The first aspect of this application provides a controllable coded waveform electrical source transmission system, comprising: The system includes a DC power supply, an inverter circuit, an impedance matching circuit, and a controller. The inverter circuit and the impedance matching circuit are connected in parallel, and the DC power supply provides DC voltage. The controller is used to determine the transmission frequency based on the target detection depth, determine the coding sequence type based on the detection mode, determine the symbol period based on the transmission frequency, and determine the output state of each symbol according to the value of each symbol in the coding sequence, thereby forming the target current waveform. The inverter circuit and the impedance matching circuit are switched to the transmission circuit according to the current symbol value. When the symbol is positive or negative, the inverter circuit is turned on and the impedance matching circuit is turned off; when the symbol is 0, the inverter circuit is turned off and the impedance matching circuit is turned on. When the inverter circuit is turned on, the target voltage waveform is calculated based on the target current waveform and the resistance and inductance values ​​of the transmitting circuit. The target voltage waveform is then compared with the amplitude of a fixed-frequency triangular carrier wave to generate the feedforward duty cycle. The actual transmitted current is obtained and compared with the target current to obtain the error signal; Based on the error signal, the duty cycle correction is calculated according to the proportional-integral control algorithm and superimposed on the feedforward duty cycle to form the final duty cycle, which is then distributed to the control terminals of the switching devices in the inverter circuit so that the encoded current waveform of the actual transmitted current tracks the target current waveform.

[0006] Furthermore, when the symbol value is positive, the controller controls the inverter circuit to output a positive sine wave segment; When the symbol value is negative, the controller controls the inverter circuit to output a reverse sine wave segment; When the code element is 0, the controller controls the inverter circuit to output 0.

[0007] Furthermore, the sinusoidal wave segments are spliced ​​together end to end on the time axis to form an coded current waveform. Each sinusoidal wave segment corresponds to a code element. All sinusoidal wave segments have the same duration and each sinusoidal wave segment starts from 0 and ends at 0.

[0008] Furthermore, the controller determines the duration of each sine wave segment based on the predetermined target detection depth, including: The required transmission frequency is determined based on the resistivity of the underground medium and the target detection depth, according to the skin depth formula. The duration of each sine wave segment is determined based on the transmission frequency, and the duration is equal to the full length of the period corresponding to the transmission frequency, ensuring that each sine wave segment is exactly one full cycle of sine wave.

[0009] Furthermore, the controller is configured to determine the encoding sequence based on the type of electro-oscillatory response excited by the target: when the target excited a linear electro-oscillatory response, the controller generates a pair of complementary bipolar Gray code sequences as the encoding sequence, and controls the inverter circuit to output two rounds of encoded current waveforms in succession, in order to achieve cross-correlation sidelobe suppression of the response; when the target excited a nonlinear electro-oscillatory response, the controller generates a unipolar pseudo-random sequence as the encoding sequence, and controls the inverter circuit to output an encoded current waveform containing a 0-level interval.

[0010] Furthermore, the controller pre-calculates the target voltage waveform required to generate the target current waveform based on the predetermined target current waveform and the resistance and inductance values ​​of the load connected to the transmitting circuit. The target voltage waveform is treated as a continuously changing modulation signal and its amplitude is compared in real time with a fixed-frequency triangular carrier signal. When the amplitude of the target voltage waveform is greater than or equal to the amplitude of the triangular carrier signal, a high level is output; otherwise, a low level is output. This generates the feedforward duty cycle for controlling the switching devices of the inverter circuit.

[0011] Furthermore, the controller acquires the actual transmission current through a Hall sensor, compares the target current with the actual transmission current, and obtains an error signal; The proportional-integral control algorithm multiplies the current error signal by a proportional coefficient, and adds the cumulative value of the error signal at all historical moments multiplied by an integral coefficient and the sampling period to obtain the duty cycle correction.

[0012] A second aspect of this application provides a control method for a controllable coded waveform electrical source transmission system, comprising: The transmission frequency is determined based on the target detection depth, the coding sequence type is determined based on the detection mode, the symbol period is determined based on the transmission frequency, and the output state of each symbol is determined according to the value of each symbol in the coding sequence to form the target current waveform. The transmission circuit connection is switched according to the current symbol value. When the symbol is positive or negative, the inverter circuit is turned on and the impedance matching circuit is turned off; when the symbol is 0, the inverter circuit is turned off and the impedance matching circuit is turned on. When the inverter circuit is turned on, the target voltage waveform is calculated based on the target current waveform and the resistance and inductance values ​​of the transmitting circuit. The target voltage waveform is then compared with the amplitude of a fixed-frequency triangular carrier wave to generate the feedforward duty cycle. The actual transmitted current is obtained by using a Hall sensor and compared with the target current to obtain an error signal. Based on the error signal, the duty cycle correction is calculated according to the proportional-integral control algorithm and superimposed on the feedforward duty cycle to form the final duty cycle, which is then distributed to the control terminals of the switching devices in the inverter circuit so that the encoded current waveform of the actual transmitted current tracks the target current waveform.

[0013] Furthermore, the type of encoded sequence is determined based on the detection mode, including: When the target excites a linear electrosonic response, a pair of complementary bipolar Gray code sequences are generated as the encoding sequence, which respectively control the inverter circuit to output two rounds of encoded current waveforms to achieve cross-correlation sidelobe suppression of the response. When the target excites a nonlinear electrosonic response, a unipolar pseudo-random sequence is generated as the encoding sequence, and the inverter circuit is controlled to output an encoded current waveform containing zero-level intervals.

[0014] Furthermore, when the symbol value is positive, the inverter circuit is controlled to output a positive sine wave segment; When the symbol value is negative, the inverter circuit is controlled to output a reverse sine wave segment. When the code element is 0, the inverter circuit output is controlled to be 0. The sine wave segments are spliced ​​together end to end on the time axis to form an coded current waveform. Each sine wave segment corresponds to a code element. All sine wave segments have the same duration and each sine wave segment starts from 0 and ends at 0.

[0015] Compared with the prior art, this application has at least the following beneficial effects: This application overcomes the limitation of low signal-to-noise ratio in seismic response during seismic exploration. Addressing the difficulties in observing seismic responses in the field due to low signal-to-noise ratio, and the challenge of distinguishing between linear and nonlinear seismic responses, an electrical source emission system for seismic exploration is designed based on a controllable coded current waveform. According to pre-set linear and nonlinear excitation modes, a target current waveform controlled by a corresponding controllable coded sequence is obtained. The output voltage and feedforward control signal are derived from the target current waveform. The actual emission current is acquired using a Hall sensor, and the duty cycle of the control signal is corrected through a closed-loop control algorithm, enabling the coded current waveform of the actual emission current to track the target current waveform. This application is beneficial for improving the signal-to-noise ratio in seismic response exploration and achieving targeted excitation of both linear and nonlinear seismic responses. Attached Figure Description

[0016] Figure 1 A structural diagram of the controllable coded waveform electrical source transmission system provided in the embodiments of this application; Figure 2 A flowchart illustrating the control method for a controllable coded waveform electrical source transmission system provided in this application embodiment; Figure 3 The target current waveform diagrams under complementary bipolar Gray code sequence waveforms provided in the embodiments of this application are shown in (a) and (b). (a) is the target current waveform diagram of the first Gray code sequence control output, and (b) is the target current waveform diagram of the second Gray code sequence control output. Figure 4 A waveform diagram of a unipolar pseudo-random sequence provided in an embodiment of this application; Figure 5 The waveforms of the first control signal PWMZ, the second control signal PWMA, and the third control signal PWMB of the complementary bipolar Gray code sequence provided in the embodiments of this application are shown below: (a) is the target current waveform, (b) is the waveform of the target voltage waveform and the positive carrier wave and the load wave, (c) is the first control signal PWMZ, (d) is the second control signal PWMA, and (e) is the third control signal PWMB. Figure 6 The waveforms of the first control signal PWMZ, the second control signal PWMA, and the third control signal PWMB of the unipolar pseudo-random sequence provided in the embodiments of this application are shown in the following diagrams: (a) is the target current waveform, (b) is the target voltage waveform and the waveform of the triangular carrier wave, (c) is the first control signal PWMZ, (d) is the second control signal PWMA, and (e) is the third control signal PWMB. Detailed Implementation

[0017] 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.

[0018] See Figure 1 As shown, this application provides a controllable coded waveform electrical source transmission system, including: The system comprises a DC power supply, an inverter circuit, an impedance matching circuit, and a controller. The inverter circuit and impedance matching circuit are connected in parallel, and the DC power supply provides DC voltage. The inverter circuit consists of switching devices Q1, Q2, Q3, and Q4, with the output load being the series impedance of the grounding conductor and the earth. Switching devices Q1 and Q3 are located on the upper left and upper right bridge arms of the inverter circuit, respectively; switching devices Q2 and Q4 are located on the lower left and lower right bridge arms, respectively. Switching devices Q1, Q2, Q3, and Q4 are composed of IGBT modules and their driving circuits. Freewheeling diodes VD1, VD2, VD3, and VD4 are connected in parallel to each other. The impedance matching unit consists of a switching device Q0 and a matching load R0 connected in series. The switching device Q0 is composed of an IGBT module and its driving circuit. The matching load R0 is equal to the series impedance of the grounding wire and the earth. A freewheeling diode VD0 is connected in parallel with the switching device Q0. The impedance matching unit is connected in parallel with the inverter circuit, and the DC power supply provides a stable DC voltage through the diode VD5 connected in series with the impedance matching unit or the inverter circuit.

[0019] The system includes a DC power supply, an inverter circuit, an impedance matching circuit, and a controller. The inverter circuit and the impedance matching circuit are connected in parallel, and the DC power supply provides DC voltage. The controller is used to determine the transmission frequency based on the target detection depth, determine the coding sequence type based on the detection mode, determine the symbol period based on the transmission frequency, and determine the output state of each symbol according to the value of each symbol in the coding sequence, thereby forming the target current waveform. The inverter circuit and the impedance matching circuit are switched to the transmission circuit according to the current symbol value. When the symbol is positive or negative, the inverter circuit is turned on and the impedance matching circuit is turned off; when the symbol is 0, the inverter circuit is turned off and the impedance matching circuit is turned on. When the inverter circuit is turned on, the target voltage waveform is calculated based on the target current waveform and the resistance and inductance values ​​of the transmitting circuit. The target voltage waveform is then compared with the amplitude of a fixed-frequency triangular carrier wave to generate the feedforward duty cycle. The coded current waveform is acquired in real time by a Hall sensor and compared with the target current waveform to obtain an error signal. Based on the error signal, the duty cycle correction is calculated using a proportional-integral method and superimposed on the feedforward duty cycle to form the final duty cycle. This final duty cycle is then distributed to the control terminals of the switching devices in the inverter circuit, enabling the encoded current waveform of the actual transmitted current to track the target current waveform.

[0020] Specifically, the controller controls the switching between the inverter circuit and the impedance matching circuit, as well as the on and off of the switching devices in the inverter circuit. It determines the transmission frequency based on the target detection depth, the encoding sequence type based on the detection mode, the symbol period based on the transmission frequency, and the output state of each symbol according to its value in the encoding sequence, thus forming the target current waveform. When the symbol in the encoding sequence is 0, all switching devices Q1, Q2, Q3, and Q4 in the inverter circuit are turned off, and the switching device Q0 in the impedance matching circuit is turned on. At this time, the impedance matching circuit is connected to the transmission loop. When the symbol is positive or negative, the controller collects the transmission current through a Hall sensor, generates a pulse width modulation signal according to a closed-loop feedback algorithm, and controls the on and off of switching devices Q1, Q2, Q3, and Q4, converting the DC voltage into an encoded voltage waveform with a sine wave as its waveform element, and forming an encoded current waveform with a sine wave as its waveform element at the load end.

[0021] When switching devices Q1 and Q4 are turned on, the output voltage at the load terminal is equal to the DC power supply voltage. If the emission current is positive, switching devices Q1 and Q4 form a current loop; if the emission current is negative, freewheeling diodes VD1 and VD4 form a current loop. When switching devices Q2 and Q3 are turned on, the output voltage at the load terminal is equal to the DC power supply voltage -Ud. If the emission current is negative, switching devices Q2 and Q3 form a current loop; if the emission current is positive, freewheeling diodes VD2 and VD3 form a current loop. Current loop; when switching devices Q1 and Q3 are turned on, the output voltage at the load terminal is equal to 0; if the emission current is positive, then switching device Q1 and freewheeling diode VD3 form a current loop; if the emission current is negative, then freewheeling diode VD1 and switching device Q3 form a current loop; when switching devices Q2 and Q4 are turned on, the output voltage at the load terminal is equal to 0; if the emission current is positive, then switching device Q4 and freewheeling diode VD2 form a current loop; if the emission current is negative, then freewheeling diode VD4 and switching device Q2 form a current loop.

[0022] The controller determines the type and generation method of the encoded sequence based on the exploration mode selected by the user through the human-computer interaction interface. In this embodiment, the exploration mode includes two types: linear exploration mode and nonlinear exploration mode. The two modes correspond to different encoding strategies and are used to induce linear electroseismic responses and nonlinear electroseismic responses, respectively.

[0023] When the target excites a linear electrosonic response, the controller generates a pair of complementary bipolar Gray code sequences as the encoding sequence, and controls the inverter circuit to output two rounds of encoded current waveforms in succession to achieve cross-correlation sidelobe suppression of the response; when the target excites a nonlinear electrosonic response, the controller generates a unipolar pseudo-random sequence as the encoding sequence, and controls the inverter circuit to output an encoded current waveform containing 0-level intervals.

[0024] Specifically, when the user selects the linear exploration mode, the first encoding unit inside the controller generates a pair of complementary bipolar Gray code sequences. The so-called complementary bipolar Gray code sequences refer to two sequences of the same length, where each code element in the sequence takes a value of +1 or -1, respectively representing the positive polarity output and the negative polarity output of the sine wave primitive. The two sequences satisfy a strict complementary constraint relationship: the first Gray code sequence is subjected to an autocorrelation operation with itself to obtain a first autocorrelation function, and the second Gray code sequence is subjected to an autocorrelation operation with itself to obtain a second autocorrelation function. At any non-zero time delay position, the sum of the value of the first autocorrelation function and the value of the second autocorrelation function is always equal to 0. This feature enables, in subsequent data processing, when cross-correlation is performed on the received signal using the two sequences respectively and then summation is carried out, the sidelobe noises cancel each other out, and the effective signals are superimposed in the same direction, thereby significantly improving the signal-to-noise ratio and resolution capability of the electroseismic response signal. For example, a specific example of a complementary Gray code pair with a length of 8 is: the first Gray code sequence is {-1, -1, -1, 1, -1, -1, 1, -1}, the second Gray code sequence is {-1, -1, -1, 1, 1, 1, -1, 1}, each of the two sequences contains 8 code elements, each code element takes a value of +1 or -1, and the sum of their autocorrelation functions at non-zero time delay positions is 0.

[0025] Expressed by the formula: , wherein, is the first Gray code sequence, is the second Gray code sequence, j (0≤j<N) is the autocorrelation time delay, and N is the length of the sequence; and satisfy that the sum of their autocorrelation functions is zero at non-zero time delays, so as to eliminate autocorrelation sidelobe noise and improve the resolution capability of the electroseismic response; taking a pair of bipolar Gray code sequences with a length of 8 as an example, its first Gray code sequence and the second Gray code sequence are {-1, -1, -1, 1, -1, -1, 1, -1} and {-1, -1, -1, 1, 1, 1, -1, 1} respectively, is the value obtained by multiplying the first Gray code sequence with itself term by term at the position where the autocorrelation time delay is j and then summing up, to obtain the autocorrelation function value of the first Gray code sequence ; is the value obtained by multiplying the second Gray code sequence with itself term by term at the position where the autocorrelation time delay is j and then summing up, to obtain the autocorrelation function value of the second Gray code sequence .

[0026] When the user selects the nonlinear exploration mode, i.e., when the target excites a nonlinear electrosusceptibility response, the second encoding unit inside the controller generates a unipolar pseudo-random sequence. A unipolar pseudo-random sequence is one where each symbol takes the value 1 or 0, representing either output or no output, respectively. The unipolar pseudo-random sequence is generated by the linear feedback shift register inside the controller according to a modulo-2 addition recursive rule: each newly generated symbol is obtained by performing a modulo-2 addition (i.e., XOR logic) operation on several historical symbols at specified positions in the register. This sequence has a spectral distribution characteristic similar to white noise, meaning the power spectrum is approximately uniformly distributed over a wide frequency band. When this unipolar pseudo-random sequence is applied to the ground as an encoding sequence, the output current waveform contains rich frequency components due to the numerous random 0 / 1 switches in the sequence, effectively exciting the underground medium to produce nonlinear electrosusceptibility effects, including nonlinear physical mechanisms such as second-order electrostriction. Simultaneously, the numerous 0-level intervals in the unipolar pseudo-random sequence provide a time window for the recovery and identification of the nonlinear response.

[0027] The formula is expressed as: , in, Unipolar pseudo-random sequence, The number of stages in a linear feedback shift register; For the first Unipolar code elements with a value of 1 or 0; This is the feedback coefficient, which takes the value of 0 or 1. For the first Unipolar code elements are binary codes that take the value 1 or 0. A unipolar code element is a binary code sequence in which each code element takes only one of two states: 1 or 0.

[0028] Each symbol in the encoded sequence occupies a fixed time window on the time axis; the width of this time window is called the symbol period. The value of the symbol period is determined by the transmission frequency. The controller first calculates the transmission frequency f based on the target detection depth and the skin depth principle, expressed as: The relationship between the skin depth δ, the transmission frequency f, and the dielectric resistivity ρ satisfies the following: the deeper the detection depth, the lower the required frequency; the shallower the detection depth, the higher the required frequency. After determining the transmission frequency f, the symbol period T is determined by multiplying the reciprocal of the transmission frequency by 2π, i.e. The physical meaning of this relationship is that each symbol period contains exactly one complete sine wave period, that is, the output waveform corresponding to each symbol is a complete sine wave segment of one cycle.

[0029] After determining the type of the encoding sequence and the period of each symbol, the controller maps the value of each symbol to the specific output state within the time window to which the symbol belongs, according to the arrangement order of the symbols in the encoding sequence, thereby forming a complete target current waveform.

[0030] The specific mapping rules are as follows: When the symbol value is +1, within the corresponding time window, the controller plans to output a positive sinusoidal wave segment. The instantaneous current value rises sinusoidally from 0 amplitude to a positive peak value and then falls back to 0 amplitude over time, as expressed by: ,in For the set current amplitude, Angular frequency, The time measured from the start of this symbol window. The target current amplitude; When the symbol value is 0, the controller plans not to output current within the time window corresponding to that symbol, and the output value is always 0. The expression is: ; When the symbol value is 1 (which only exists in unipolar pseudo-random sequences in nonlinear modes), the controller plans to output a positive polarity sine wave segment within the time window corresponding to the symbol value, and the output method is the same as when the value is +1. When the symbol value is 0 (which only exists in unipolar pseudo-random sequences in nonlinear modes), the controller plans not to output current within the time window corresponding to the symbol, and the output method is the same as when the value is 0.

[0031] The output states corresponding to all the above symbols are arranged sequentially on the time axis according to the symbol number. The output of the previous symbol returns to 0 amplitude at the end of the window, and the output of the next symbol starts from 0 amplitude at the beginning of the window. Adjacent windows are all 0 at the junction, realizing a smooth transition of the current waveform and avoiding electromagnetic shock interference caused by amplitude jumps.

[0032] Taking a set of complementary Gray code sequences of length 8 in linear probe mode as an example, the first Gray code sequence is {-1, -1, -1, 1, -1, -1, 1, -1}, and each symbol period is... The controller, according to the values ​​of each symbol in the sequence, in the first symbol period... The internal output is a negative polarity sine wave segment, in the second symbol period. Then, a negative polarity sine wave segment is output in the third symbol cycle. Then, output a negative polarity sine wave segment in the fourth symbol period. The system outputs a positive polarity sine wave segment, and so on, until all 8 code segments are sequentially spliced ​​together on the time axis to form the first complete target current waveform. Subsequently, the controller controls the output of the second complete target current waveform in the same manner according to the second polarity Gray code sequence {-1, -1, -1, 1, 1, 1, -1, 1}. Both waveforms have the same amplitude, the same code period, and the same total duration.

[0033] Taking a unipolar pseudo-random sequence {0, 1, 0, 1, 1, 1, 0} of length 7 in nonlinear detection mode as an example, the controller outputs no current in the first symbol period, outputs a positive sine wave segment in the second symbol period, no current in the third symbol period, outputs a positive sine wave segment in the fourth symbol period, outputs a positive sine wave segment in the fifth symbol period, outputs a positive sine wave segment in the sixth symbol period, and no current in the seventh symbol period. Each output segment and the 0 level are alternately arranged on the time axis to form a complete target current waveform.

[0034] The target current waveform, i.e. the desired output current waveform, is expressed as: ,in, For the target current amplitude, The amplitude of the transmitting current. For the first in the encoded sequence Each code element For waveform primitives, For the symbol period, The length of the encoded sequence is given, and the waveform primitive is a sine wave segment of a complete symbol period.

[0035] The target current waveform can be uniformly described as: a time series consisting of a series of windows of equal duration connected end to end, where the time width of each window is equal to the symbol period. The waveform shape within each window is uniquely determined by the value of the corresponding symbol, and the entire waveform is arranged sequentially according to the order of symbols in the coding sequence. Using a sine wave as the basic element, the polarity (positive, negative, 0) of each basic element is controlled by the coding sequence, achieving separate excitation of linear and nonlinear electrosonic responses: in linear mode, a bipolar coding method combined with cross-correlation processing of complementary bipolar Gray code sequences is used to extract the linear response; in nonlinear mode, a unipolar pseudo-random coding method directly excites the nonlinear response.

[0036] In this application, the load end of the transmitting circuit can be selected to be connected to an inverter circuit or an impedance matching circuit. The two are controlled by three control signals output by the controller. At any given time, only one circuit is connected to the transmitting circuit, while the other is disconnected, so as to avoid energy conflict or equipment damage caused by the simultaneous connection of the inverter circuit and the impedance matching circuit.

[0037] At the start of each symbol cycle, the controller reads the value of the current symbol from the encoded sequence and generates a corresponding circuit switching decision based on this value. This decision is executed through three control signals: the first control signal PWMZ controls the on / off state of switching device Q0 in the impedance matching circuit; the second control signal PWMA controls the on / off state of switching devices Q1 and Q4 in the inverter circuit; and the third control signal PWMB controls the on / off state of switching devices Q2 and Q3 in the inverter circuit. The logical relationship between the three control signals is coordinated by the controller's internal symbol parsing and state decision-making to ensure that the switching action is executed according to the correct timing.

[0038] When the controller reads that the current symbol value is +1 or -1 (corresponding to positive or negative polarity output in linear mode, and a symbol value of 1 in nonlinear mode), it indicates that a sinusoidal current needs to be output to the load within the current symbol cycle. At this time, the controller executes the inverter circuit connection operation, specifically including the following actions: First, the controller sends a turn-off command to the switching device Q0 via the first control signal, PWMZ. Upon receiving the turn-off signal, the switching device Q0 immediately cuts off, disconnecting the impedance matching circuit from the transmitting circuit. After the matching load R0 is disconnected from the transmitting circuit, the energy output from the DC power supply no longer flows through the matching load R0, eliminating the current shunting and energy consumption caused by the matching load R0.

[0039] After switching device Q0 is turned off, the controller immediately sends ready signals to switching devices Q1-Q4 through the second control signal PWMA and the third control signal PWMB, putting the inverter circuit into a modulated output state. At this time, the conduction and turn-off of the four switching devices are controlled by the pulse width modulation signal. The conduction duration and conduction sequence of each switching device can be adjusted in real time according to the needs of the target current waveform, so that the inverter circuit operates in a sinusoidal pulse width modulation mode, inverting the DC voltage provided by the DC power supply into an AC voltage and applying it to the load terminal, driving the grounding wire to inject the coded current waveform into the ground.

[0040] It should be noted that the above switching actions are completed sequentially within a very short time. There is a strict time sequence between the turn-off of switching device Q0 and the readiness of switching devices Q1-Q4. It is essential to ensure that the impedance matching circuit is turned off first and the inverter circuit is turned on subsequently to prevent both circuits from being connected to the transmit circuit simultaneously. The timing synchronization unit inside the controller strictly controls the toggling time of each control signal through a unified system clock to ensure a safe and reliable switching process.

[0041] When the symbol value is positive, such as +1, the controller controls the inverter circuit to output a positive voltage waveform with the same polarity as the sine wave fundamental element, and the load current is a positive polarity sine wave segment. When the symbol value is negative, such as -1, the controller controls the inverter circuit to output a negative voltage waveform with the opposite polarity to the sine wave fundamental element, and the load current is a negative polarity sine wave segment. In both cases, the inverter circuit is in exactly the same connection state; only the polarity control logic of the pulse width modulation signal is different, specifically manifested in the reversal of the reference polarity of the target voltage.

[0042] When the controller reads a current symbol value of 0, it indicates that no current needs to be output to the load during the current symbol period. At this time, the controller performs an impedance matching circuit connection operation, specifically including the following actions: First, the controller sends forced shutdown commands to the switching devices Q1-Q4 via the second control signal PWMA and the third control signal PWMB. Upon receiving the shutdown signal, all four switching devices simultaneously turn off, completely severing the connection between the inverter circuit and the transmitting circuit. After the inverter circuit is disconnected, the load no longer receives excitation voltage from the DC power supply, and the output current quickly returns to zero.

[0043] After all switching devices Q1-Q4 are turned off, the controller immediately sends a turn-on command to switching device Q0 via the first control signal PWM_Z. Upon receiving the turn-on signal, switching device Q0 immediately turns on, connecting the matching load R0 to the transmitting circuit. The impedance of the matching load R0 is equal to the series impedance of the grounding wire and the earth. When the matching load R0 is connected, the residual energy in the transmitting circuit is released through the matching load R0 as heat dissipation, preventing energy reflection or oscillation from adversely affecting the system. Simultaneously, because the matching load R0 absorbs the residual energy in the transmitting circuit, the voltage and current at the load end are quickly suppressed to 0, ensuring that the output current is truly zero.

[0044] It should also be noted that the above switching actions follow a strict timing sequence, ensuring that the inverter circuit is disconnected first and the impedance matching circuit is connected subsequently. This prevents the two circuits from conducting simultaneously, which could cause a short circuit between the positive and negative terminals of the DC power supply via different switching devices. The interlocking protection logic set up inside the controller prevents the above-mentioned conflict from occurring at the hardware level. When either the second control signal PWMA or the third control signal PWMB is high, the first control signal PWMZ is forcibly locked to a low level by the hardware circuit; when the first control signal PWMZ is high, the second control signal PWMA or the third control signal PWMB is forcibly locked to a low level by the hardware circuit. Therefore, under no circumstances will the inverter circuit and the impedance matching circuit be connected at the same time.

[0045] The switching control when the symbol value is 0 and the switching control when the symbol value is positive or negative are complementary: when output is needed, the inverter circuit is connected and the matching circuit is disconnected; when output is not needed, the inverter circuit is disconnected and the matching circuit is connected. The two states alternate with the changes in the symbol value in the coding sequence, so that the transmitting system is in the correct circuit state in each symbol period, ensuring the accurate output of the coded current waveform.

[0046] To ensure a smooth transition between adjacent symbols in the encoded current waveform, especially when a symbol switches from non-zero to zero or from zero to non-zero, the controller has precisely designed the timing of the switching action.

[0047] Specifically, at the end of each symbol cycle, the controller first monitors whether the current output current has dropped to near zero. Since each sine wave segment naturally returns to zero at the end of the symbol cycle, switching the connection state of the inverter circuit and the impedance matching circuit at this time will not cause a current surge. At the end of the symbol cycle (which is also the start of the next symbol cycle), the controller synchronously switches the state of the three control signals, so that the circuit connection of the transmit circuit is updated synchronously with the value of the next symbol.

[0048] When multiple consecutive symbols are non-zero, the inverter circuit remains connected and the impedance matching circuit remains disconnected. The controller only needs to adjust the modulation polarity of the inverter circuit to output alternating positive and negative sine wave segments in sequence, without the need for circuit switching. This avoids unnecessary switching actions and improves the system's efficiency and reliability.

[0049] When multiple consecutive symbols are all 0, the impedance matching circuit remains connected and the inverter circuit remains disconnected. The controller does not need to perform any switching action. The transmitting system operates stably under the matched load R0 state until the next non-zero symbol arrives, at which point it switches to the inverter output state.

[0050] When the symbol alternates between 0 and non-zero, the controller performs a complete switching operation at the beginning of each symbol cycle in the manner described above. Since the switching moment is exactly at the natural zero-crossing point of the sine wave segment, the output voltage and current are both 0, and no voltage or current surge is generated at the moment of switching, achieving a soft switching effect. This is beneficial for protecting switching devices, reducing electromagnetic interference, and improving the purity of the transmitted waveform.

[0051] When the inverter circuit is in the connected state, the controller needs to provide precise pulse width modulation signals to the switching devices Q1-Q4 so that the voltage waveform output by the inverter circuit, after being filtered by the load, forms the required coded current waveform at the load end. To achieve this goal, the controller first calculates the target voltage waveform that needs to be applied across the load to generate the coded current waveform based on the target current waveform and the electrical parameters of the load in the transmitting circuit.

[0052] The load of the transmitting circuit consists of the series impedance of the grounding conductor and the earth. In the circuit model, it can be equivalent to an RL load with a resistive component R and an inductive component L connected in series. The resistive component R represents the resistance of the grounding conductor itself and the equivalent resistive loss generated when the current flows in the earth. The inductive component L represents the self-inductance of the grounding conductor and the equivalent inductance formed in the earth loop. In the RL series load, the relationship between current and voltage is described by Kirchhoff's voltage law: the total voltage applied across the load is equal to the sum of the voltage drop across the resistive component R and the induced voltage across the inductive component L.

[0053] The role of feedforward control is to pre-calculate the approximate voltage reference value required to generate the target current based on the physical model of the load before feedback correction is involved, and then directly use this reference value to generate the feedforward duty cycle of the switching devices. Because feedforward control performs predictive calculations based on the physical model of the load and does not rely on feedback information from the actual output, it has a fast response speed, enabling the output current to approach the target value before the feedback signal arrives. This significantly reduces the burden on subsequent feedback correction stages and improves the dynamic response performance of the system.

[0054] The time-by-time calculation methods for the target voltage waveform include: Read the current value in the target current waveform at the current moment, which is the expected output current at this moment. Multiply this current value by the equivalent resistance of the transmitting circuit load to obtain the resistance component voltage value, which represents the voltage drop generated by the resistance component R under the current current.

[0055] The rate of change of the target current waveform with respect to time at the current moment is calculated, i.e., the rate of increase or decrease of the current at that moment. In practice, the rate of change is approximated by dividing the difference between the current value at the current moment and the current value at the previous moment by the time interval between the two samplings. Multiplying this rate of change by the equivalent inductance of the transmitting circuit load yields the inductance component voltage value, representing the induced voltage generated by the inductance component L under the current rate of change. The direction of the inductance component L follows Lenz's law: when the current increases, the inductor generates a reverse voltage to prevent the current from increasing; when the current decreases, the inductor generates a forward voltage to prevent the current from decreasing.

[0056] Adding the voltage values ​​of the resistive and inductive components yields the target voltage value at that moment. Performing this calculation sequentially for each sampling moment of the target current waveform yields the target voltage waveform, which changes synchronously with the target current waveform. This target voltage waveform shares the same frequency components and symbol structure as the target current waveform, but its amplitude and phase relationships are adjusted due to the characteristics of the RL load.

[0057] The target voltage is calculated based on the target emission current and the RL load impedance model: , , in, For the target voltage, For the target current, The resistive component represents the equivalent resistance of the load in the transmitting circuit. The inductive component represents the equivalent inductance of the load in the transmitting circuit.

[0058] In inverter circuits, the most common method for obtaining an AC voltage with controllable frequency and amplitude from a DC voltage is sinusoidal pulse width modulation (PWM). The basic idea is to compare the amplitude of a continuously varying target voltage waveform (called the modulating wave) with a fixed-frequency triangular carrier wave (called the carrier wave) in real time. Based on the comparison result, a series of pulse signals with varying widths are generated. These pulse signals are used to control the switching devices to turn on and off, making the equivalent output voltage obtained on the load approximately equal to the target voltage waveform.

[0059] The frequency of the triangular carrier wave is much higher than that of the target voltage waveform, typically in the range of several kilohertz to tens of kilohertz, while the target voltage waveform's frequency is in the range of tens to hundreds of kilohertz. Within each carrier cycle, the comparator compares the instantaneous amplitude of the modulating wave with the instantaneous amplitude of the triangular wave: when the instantaneous amplitude of the modulating wave is greater than that of the triangular wave, the comparator outputs a high level; when the instantaneous amplitude of the modulating wave is less than that of the triangular wave, the comparator outputs a low level. The ratio of the duration of the high level within one cycle to the carrier cycle is the duty cycle for that carrier cycle.

[0060] Represented as: , in, The feedforward signal is used to generate a PWM signal using a unipolar control method. Specifically, when the feedforward signal is 1 and the target voltage is greater than 0, switching devices Q1 and Q4 are turned on; when the feedforward signal is 0 and the target voltage is less than 0, switching devices Q2 and Q3 are turned on; when the feedforward signal is 0 and the target voltage is greater than 0, switching devices Q1 and Q3 are turned on; when the feedforward signal is 1 and the target voltage is less than 0, switching devices Q2 and Q4 are turned on.

[0061] In some embodiments, a Hall sensor is connected in series in the transmitting circuit to detect the actual transmitting current flowing through the load in real time. The Hall sensor operates based on the Hall effect principle, and its core component is the Hall element. When the transmitting current flows through the conductor inside the Hall sensor, a magnetic field proportional to the current magnitude is generated around the conductor. This magnetic field acts on the Hall element, causing a Hall voltage proportional to the magnetic flux density to be generated across the Hall element. Since the magnetic flux density is linearly related to the transmitting current, the magnitude of the Hall voltage directly reflects the instantaneous value of the transmitting current.

[0062] The Hall sensor is installed in an easily accessible location within the transmitting circuit, and its signal output is connected to the controller's signal conditioning port via a shielded cable. The analog voltage signal output by the Hall sensor is amplified, filtered, and level-shifted by the signal conditioning circuit before being sent to the controller's internal analog-to-digital converter for digital sampling. The converter samples and quantizes the conditioned current signal at a fixed sampling frequency, converting the continuous analog voltage signal into discrete digital quantities for the controller to perform numerical calculations.

[0063] The sampling frequency is selected according to the Nyquist sampling theorem and must be at least twice the highest frequency component of the transmitted signal. In typical implementations, the sampling frequency of the analog-to-digital converter is usually set to an integer multiple of the carrier frequency or synchronized with the carrier frequency to ensure a sufficient number of sampling points in each carrier cycle, guaranteeing the real-time performance and accuracy of the closed-loop feedback control. For sinusoidal symbols with low symbol frequencies that only contain fundamental frequency components, the sampling frequency is also much higher than the signal frequency, enabling the complete capture of detailed changes in the current waveform.

[0064] Corresponding to the measured transmission current, the target current waveform inside the controller is also stored and updated in digital form. The target current waveform generates a corresponding target current value at each sampling moment, according to the same sampling frequency and time base as the measured transmission current.

[0065] The value of the target current waveform is determined by the value of the target current waveform expression at each sampling moment. This waveform expression defines the current change pattern within each symbol period in a segmented form: a symbol value of +1 or 1 represents a positive polarity sine wave, a symbol value of -1 represents a negative polarity sine wave, and a symbol value of 0 represents a 0 level. At each sampling moment, the controller obtains the target current value at that moment by looking up a table or by real-time calculation, based on the symbol number of the current moment and the phase position within that symbol period. To ensure strict time alignment between the target current and the measured current, the generation of the target current value and the sampling triggering of the analog-to-digital converter use the same system clock source and undergo the same delay compensation processing.

[0066] At each sampling moment, after the analog-to-digital converter completes the sampling and quantization of the actual transmitted current and outputs a digital value, the controller immediately performs an error calculation operation. The specific method of error calculation is as follows: subtract the actual transmitted current value (encoded current waveform) at the current moment from the target current value at the current moment, and the difference is the current error signal at that moment.

[0067] The sign and magnitude of the error signal reflect the degree and direction of deviation between the actual output and the target requirement. When the error is positive, it indicates that the actual current is less than the target current, the inverter circuit output is insufficient, and the duty cycle of the pulse width modulation signal needs to be increased to improve the output. When the error is negative, it indicates that the actual current is greater than the target current, the inverter circuit output is excessive, and the duty cycle needs to be decreased to reduce the output. When the error is 0, it indicates that the actual current is consistent with the target current, the output state is ideal, and no adjustment is required.

[0068] The absolute value of the error signal reflects the severity of the deviation. The larger the absolute value of the error, the farther the actual current deviates from the target current, and the greater the required duty cycle correction; the smaller the absolute value of the error, the closer the actual current is to the target current, and the smaller the required duty cycle correction.

[0069] To ensure that the error signal accurately reflects the deviation between the target current and the actual current at the same moment, the generation time of the target current value and the sampling time of the actual current must be strictly synchronized. In this application, the controller achieves sampling synchronization in the following way: A unified sampling trigger signal is generated by the system clock unit of the controller. This trigger signal is simultaneously transmitted to the target current waveform value generation unit and the start-up port of the analog-to-digital converter. At the beginning of each sampling cycle, the rising edge of the sampling trigger signal triggers two operations simultaneously: first, the target current generation unit calculates the target current value according to the current moment and latches it into the comparison register; second, the analog-to-digital converter starts sampling and converting the Hall sensor output signal. The two operations are triggered on the same clock edge, ensuring that the target current value and the actual transmitted current value correspond to exactly the same time point, eliminating the phase error introduced by time misalignment.

[0070] After the conversion is completed, a conversion completion interrupt signal is generated. The controller reads the conversion result in the interrupt service routine and reads the synchronously latched target current value from the comparison register. Then, it performs a subtraction operation to obtain the error signal. The entire process is completed in one go within the interrupt service routine, minimizing the delay from sampling to error output and ensuring the real-time performance of closed-loop control.

[0071] In one embodiment, the closed-loop control dynamically adjusts the duty cycle based on a PI algorithm combined with an error term: , in This is the duty cycle correction amount. The duty cycle after closed-loop feedback. For feedforward duty cycle, This is the proportionality coefficient. The integral coefficient is... The current sampling frequency, For the first Error signal at each sampling time, For the first Error signal at each sampling time.

[0072] Within each symbol period, the error signal exhibits a certain regularity in its variation over time. For an ideal sine wave output segment, at the beginning of the symbol, the target current is 0, and the actual transmitted current starts from 0 after the switch action, with the two essentially consistent, resulting in an error signal close to 0. During the rising phase of the sine wave, if the actual transmitted current lags behind the target current due to inductor delay, the error signal is positive, and the feedback control should increase the duty cycle to accelerate the current rise. Near the peak of the sine wave, the rate of change of current approaches 0, the inductor component decreases, and the actual current gradually approaches the target current, with the error approaching 0. During the falling phase of the sine wave, if the actual current decreases slowly due to inductor freewheeling and is higher than the target current, the error signal is negative, and the feedback control should decrease the duty cycle to accelerate the current fall. At the end of the symbol, the target current is 0, and the actual transmitted current quickly returns to 0 after the switching device is turned off, returning the error signal to 0.

[0073] The controller repeats the error calculation process at each sampling time, forming a continuously changing error time series. This series reflects the dynamic deviation between the actual current waveform and the target current waveform in real time, serving as the direct basis for the closed-loop feedback correction unit to perform proportional-integral control algorithm and duty cycle correction. Accurate acquisition of the error signal sequence is a crucial link in the entire closed-loop feedback control chain; its accuracy and real-time performance directly determine the tracking performance of the encoded current waveform on the target current waveform.

[0074] Feedforward control pre-calculates the approximate voltage reference value required to generate the target current based on the load's RL model and generates the feedforward duty cycle, ensuring that the actual transmitted current is close to the target current on a macroscopic level. However, due to numerous unpredictable factors during actual transmission, such as the drift of grounding resistance with soil moisture, changes in the equivalent inductance of the ground due to ambient temperature, fluctuations in the on-state voltage drop of switching devices with temperature, and drops in the output voltage of the DC power supply due to load variations, feedforward control alone cannot accurately track the target current. Therefore, this application introduces a closed-loop feedback correction stage on top of feedforward control. By detecting the deviation between the actual transmitted current and the target current in real time, the duty cycle of the pulse width modulation signal is dynamically corrected, ensuring that the coded current waveform always tracks the changes in the target current waveform.

[0075] The basic idea of ​​closed-loop feedback correction is as follows: the difference between the actual transmitted current and the desired target current is used as an error signal. This error signal is converted into a duty cycle correction value according to a certain control algorithm. This correction value is then superimposed on the feedforward duty cycle to form the final duty cycle after feedback correction. This final duty cycle is used to control the on / off state of the switching devices, thereby changing the actual output voltage and bringing the actual current closer to the target current. This process is repeated in each sampling period, forming a continuous closed-loop adjustment that ensures the actual transmitted current maintains accurate tracking of the target waveform under load changes and external interference.

[0076] In this embodiment, the closed-loop feedback correction adopts the proportional-integral (PI) control algorithm widely used in industrial control. This algorithm has a simple structure, convenient parameter tuning, and high steady-state accuracy, which can meet the requirements of the seismic exploration launch system for current waveform accuracy. The proportional element responds instantly to the error at the current moment, while the integral element eliminates long-term accumulated systematic deviations. The combination of the two ensures both the system's rapid response capability and the absence of steady-state error in the output.

[0077] The proportional-integral (PI) control algorithm receives one input and outputs one control quantity in each sampling period. The controller's input is the current error signal at the current sampling moment, which is provided by the error generation circuit described above. The controller's output is the duty cycle correction, which is added to the feedforward duty cycle.

[0078] The conversion relationship between input and output is determined by two parameters: the proportional gain and the integral gain. The proportional gain determines the controller's response to the current error; a larger proportional gain means a greater contribution of the current error to the correction, resulting in a faster system response. However, an excessively large proportional gain may lead to system oscillation or even instability. The integral gain determines the controller's response to accumulated historical errors; a larger integral gain means a greater contribution of accumulated historical errors to the correction, resulting in a stronger ability to eliminate steady-state deviations. However, an excessively large integral gain may lead to system overshoot and oscillation. In practical engineering applications, the values ​​of the proportional gain and integral gain are determined through a combination of theoretical calculations and on-site debugging to achieve a reasonable balance between system response speed and stability.

[0079] The specific calculation method for duty cycle correction is as follows: At each sampling time, the proportional-integral control algorithm calculates the duty cycle correction as follows: First, the error signal at the current sampling moment is multiplied by a scaling factor to obtain the scaling term. The scaling term reflects the need for immediate correction of the current deviation: the larger the current error, the further the actual current deviates from the target current at the current moment, and the greater the amount of immediate correction needs to be applied; when the current error is 0, the scaling term is 0, and no immediate correction is needed.

[0080] Secondly, the error values ​​at all historical sampling times are accumulated, and the accumulated result is multiplied by the product of the integral coefficient and the sampling period to obtain the integral term. The integral term reflects the need to compensate for the cumulative effect of historical deviations: if the system has a long-term positive deviation (the actual current is always less than the target current), the accumulated error will continue to increase, and the integral term will also increase accordingly, gradually increasing the duty cycle to eliminate the deviation; when the deviation is eliminated, the error value becomes 0, the accumulated value stops increasing, and the integral term remains at its current value, keeping the duty cycle at the level required to eliminate the deviation, thereby achieving zero steady-state error tracking.

[0081] Then, the proportional term and the integral term are added together to obtain the duty cycle correction at the current sampling time. This duty cycle correction can be positive or negative: a positive value indicates that the duty cycle needs to be increased to increase the output current; a negative value indicates that the duty cycle needs to be decreased to reduce the output current; a correction of 0 indicates that no adjustment is needed and the feedforward duty cycle is sufficient.

[0082] Finally, the duty cycle correction is superimposed on the feedforward duty cycle to obtain the final duty cycle after closed-loop feedback correction. This superposition operation is essentially a fine-tuning process based on the coarse adjustment provided by the feedforward control, using feedback control to ensure that the final duty cycle includes both the macroscopic control component based on the load model and the microscopic correction component based on the real-time error.

[0083] The final duty cycle obtained after closed-loop feedback correction is a numerical value representing the proportion of time the switching device should be on within the current carrier cycle. This value needs to be converted into a specific switching device drive signal and distributed to the control terminal of the corresponding switching device in the inverter circuit.

[0084] In unipolar modulation, the controller allocates the final duty cycle to different combinations of switching devices based on the polarity of the current target voltage. The specific allocation logic is as follows: When the target voltage is positive, the final duty cycle signal is allocated to the drive terminal of switching device Q4 to control the high-frequency turn-on and turn-off of the fourth switching device. Simultaneously, the controller continuously outputs a high-level turn-on signal to the drive terminal of switching device Q1 and a low-level turn-off signal to the drive terminals of switching devices Q2 and Q3. Thus, during the high-level duty cycle signal period, switching devices Q1 and Q4 are simultaneously turned on, applying a positive voltage across the load. During the low-level duty cycle signal period, switching device Q1 remains on while switching device Q4 is off, and the load current forms a freewheeling loop through the freewheeling diodes of switching devices Q1 and Q3, resulting in a zero voltage across the load.

[0085] When the target voltage is negative, the final duty cycle signal is allocated to the drive terminal of switching device Q3 to control the high-frequency turn-on and turn-off of switching device Q3. Simultaneously, the controller continuously outputs a high-level turn-on signal to the drive terminal of switching device Q2 and a low-level turn-off signal to the drive terminals of switching devices Q1 and Q4. Thus, during the high-level duty cycle signal period, switching devices Q2 and Q3 are simultaneously turned on, applying a negative voltage across the load. During the low-level duty cycle signal period, switching device Q2 remains on while switching device Q3 is off, and the load current forms a freewheeling loop through the freewheeling diodes of switching devices Q2 and Q4, resulting in a zero voltage across the load.

[0086] The duty cycle is updated once per carrier cycle, and the corresponding drive signal changes accordingly. The switching devices perform turn-on and turn-off actions according to the updated duty cycle. Since the carrier frequency is much higher than the symbol frequency, the duty cycle changes continuously in a sinusoidal pattern within each symbol cycle, synthesizing a smooth sinusoidal current segment at the load end. The continuous calculation of the error signal within each sampling cycle and the cycle-by-cycle correction of the duty cycle form a fast and stable closed-loop adjustment process, ensuring that the encoded current waveform always tracks the target current waveform with high precision.

[0087] In another aspect, this application provides a control method for a controllable coded waveform electrical source transmission system, comprising: S201, determine the transmission frequency based on the target detection depth, determine the coding sequence type based on the detection mode, determine the symbol period based on the transmission frequency, and determine the output state of each symbol according to the value of each symbol in the coding sequence to form the target current waveform; S202: Switch the transmission circuit connection according to the current symbol value. When the symbol is positive or negative, turn on the inverter circuit and turn off the impedance matching circuit; when the symbol is 0, turn off the inverter circuit and turn on the impedance matching circuit. S204 When the inverter circuit is turned on, the target voltage waveform is calculated based on the target current waveform and the resistance and inductance values ​​of the transmitting circuit. The target voltage waveform is then compared with the amplitude of a fixed-frequency triangular carrier wave to generate the feedforward duty cycle. S204 obtains the actual transmission current through a Hall sensor and compares it with the target current to obtain an error signal; S205, based on the error signal, calculate the duty cycle correction amount according to the proportional-integral control algorithm, and superimpose it onto the feedforward duty cycle to form the final duty cycle, which is then allocated to the control terminal of the switching device in the inverter circuit so that the encoded current waveform of the actual transmitted current tracks the target current waveform.

[0088] In step S201, the primary task is to determine the transmission frequency based on the target detection depth. In seismic exploration, the frequency of the transmitted signal determines the skin depth of the electromagnetic wave in the subsurface medium, and thus the target depth that the detection signal can reach. The physical meaning of skin depth is the depth at which the amplitude of an electromagnetic wave attenuates to approximately 37% of its surface value when propagating in a conductive medium; it reflects the effective penetration capability of the electromagnetic wave. The relationship between frequency and skin depth is as follows: the lower the frequency, the greater the skin depth, the stronger the signal penetration capability, and the ability to detect deeper geological targets; the higher the frequency, the smaller the skin depth, and the signal energy is concentrated in the shallow layer, suitable for fine detection of shallow targets. The controller calculates the required transmission frequency according to the skin depth formula based on the target detection depth value input by the operator and the known or estimated subsurface resistivity value of the exploration area. For deep targets, the controller selects a lower transmission frequency to obtain sufficient penetration depth; for shallow targets, a higher transmission frequency is selected to obtain higher resolution and signal-to-noise ratio. Once the transmission frequency is determined, the time width of each symbol is also determined, which is equal to the complete duration of the period corresponding to the transmission frequency.

[0089] The second task of step S201 is to determine the type of coded sequence based on the detection mode. This application provides two selectable detection modes: a linear detection mode and a nonlinear detection mode, used to excite and extract the linear and nonlinear responses of the electrosurge, respectively. When the operator selects the linear detection mode, the controller generates a pair of complementary bipolar Gray code sequences. Bipolarity means that each symbol in the coded sequence takes a positive or negative value, corresponding to the positive and negative polarity outputs of the sine wave primitives, respectively; complementarity means that the two sequences satisfy a strict autocorrelation complementarity constraint, i.e., the sum of the autocorrelation functions of the two sequences at all non-zero offset positions is 0. This characteristic ensures that in later data processing, when the two sets of received signals are cross-correlated with their corresponding transmitted sequences and then summed, the effective signals are superimposed in the same direction while the autocorrelation sidelobe noise cancels each other out, significantly improving the signal-to-noise ratio. When the operator selects the nonlinear detection mode, the controller generates a unipolar pseudo-random sequence, where each symbol takes a value of 1 or 0, corresponding to output and no output, respectively. This sequence is generated by the linear feedback shift register inside the controller according to the modulo-2 recursive rule. It has spectral characteristics that are close to white noise and can distribute energy evenly over a wide frequency band, effectively exciting the nonlinear electro-vibration effect of the underground medium.

[0090] The third task in step S201 is to map the values ​​of each symbol in the encoded sequence to specific output states, forming a complete target current waveform. The mapping rules are as follows: when the symbol value is positive or 1, a positive polarity sine wave segment is output within the corresponding time window. This sine wave segment starts from 0 amplitude, passes through a positive peak, and returns to 0 amplitude; when the symbol value is negative, a negative polarity sine wave segment is output. This sine wave segment starts from 0 amplitude, passes through a negative peak, and returns to 0 amplitude; when the symbol value is 0, the output is 0, and there is no current output within this symbol period. All segments are spliced ​​together end-to-end on the time axis in symbol order, with adjacent segments having a value of 0 at the junction, achieving a smooth transition. At this point, the complete target current waveform is determined.

[0091] In step S202, after the target current waveform is determined, the controller processes each symbol in the encoding sequence according to the order of the symbols, and switches the circuit connection status in the transmission loop in real time according to the value of the current symbol.

[0092] When the controller reads that the current symbol value is positive or negative, it indicates that a sinusoidal current needs to be output to the load within the current symbol period. At this time, the controller executes the inverter circuit connection operation: first, it turns off the switching device Q0 in the impedance matching circuit through a control signal, disconnecting the matching load R0 from the transmitting circuit; then, it sends enable signals to the switching devices Q1-Q4 in the inverter circuit, putting the inverter circuit into a modulated state. At this point, the transmitting circuit and the inverter circuit are connected, and the electrical energy provided by the DC power supply can be delivered to the load after conversion by the inverter circuit. It is important to emphasize that there is a strict time sequence between the turn-off of switching device Q0 and the enable of switching devices Q1-Q4. The impedance matching circuit must be disconnected first before the inverter circuit can be connected, and there is interlocking protection between the two, ensuring that they will never conduct simultaneously under any circumstances.

[0093] When the controller reads that the current symbol value is 0, it indicates that no output current is needed in the current symbol period. At this time, the controller executes the impedance matching circuit connection operation: First, it sends a forced shutdown command to the first to fourth switching devices in the inverter circuit, causing all four switching devices to be turned off, and the connection between the inverter circuit and the transmitting circuit is completely severed; then, it turns on the switching device Q0 in the impedance matching circuit, connecting the matching load R0 to the transmitting circuit. The impedance of the matching load R0 is equal to the series impedance of the grounding wire and the earth. Its function is to provide a discharge path for the residual energy in the transmitting circuit when the inverter circuit is turned off, while maintaining the impedance continuity of the transmitting circuit and avoiding voltage surges due to open circuit of the load.

[0094] The two switching operations described above are performed at the beginning of each symbol cycle. Since the symbol switching time coincides precisely with the natural zero-crossing point of the sine wave segment (i.e., the moment when the current is 0), no voltage or current surge occurs during the switching instant, achieving a safe and smooth soft switching. The controller completes the above switching operation through the coordinated use of three control signals: the first control signal PWMZ, the second control signal PWMA, and the third control signal PWMB. The first control signal PWMZ controls switching device Q0, the second control signal PWMA controls switching devices Q1 and Q4, and the third control signal PWMB controls switching devices Q2 and Q3.

[0095] In step S203, when the inverter circuit is in the ON state, the controller needs to provide a pulse width modulation signal to the switching devices to drive the inverter circuit to work. The purpose of this step is to pre-calculate the voltage reference value required to generate the target current based on the physical model of the load, and generate the feedforward duty cycle accordingly.

[0096] The specific process of feedforward calculation is as follows: The controller reads the target current value at the current moment from the target current waveform determined in the previous steps, and simultaneously reads the equivalent resistance and equivalent inductance values ​​of the transmitter loop load from the system parameter storage area. The transmitter loop load can be equivalently represented as an RL model with a resistive component R and an inductant component L connected in series. The resistive component represents the resistance of the grounding conductor itself and the equivalent loss when current flows through the ground, while the inductant component represents the self-inductance of the grounding conductor and the equivalent inductance of the ground loop. Based on the voltage balance relationship of the RL series load, the controller determines the target voltage value that needs to be applied across the load at the current moment. This yields the target voltage waveform that changes synchronously with the target current.

[0097] After obtaining the target voltage waveform, the controller uses it as a modulation signal and compares its amplitude with that of a fixed-frequency triangular carrier signal moment by moment, generating a feedforward duty cycle signal using sinusoidal pulse width modulation (PWM). The frequency of the fixed-frequency triangular carrier is much higher than the frequency of the target voltage waveform, typically set in the range of several kilohertz to tens of kilohertz. Within each carrier cycle, the comparator compares the instantaneous amplitude of the target voltage with the instantaneous amplitude of the triangular carrier: when the target voltage amplitude is greater than or equal to the triangular wave amplitude, a high level is output, indicating that the switching device should be turned on during that period; when the target voltage amplitude is less than the triangular carrier amplitude, a low level is output, indicating that the switching device should be turned off during that period. The ratio of the high-level duration to the carrier cycle is the duty cycle for that carrier cycle. This duty cycle signal varies with the target voltage amplitude: the larger the target voltage amplitude, the larger the duty cycle, the longer the conduction time, and the higher the average voltage received by the load; the smaller the target voltage amplitude, the smaller the duty cycle, the shorter the conduction time, and the lower the average voltage received by the load. The resulting pulse sequence, whose width varies with the target voltage amplitude, is the feedforward duty cycle.

[0098] This feedforward duty cycle serves as the basis for subsequent closed-loop feedback correction, providing the main component of the duty cycle for the system and making the actual output current macroscopically close to the target waveform. Because feedforward control performs predictive calculations based on the physical model of the load, its response speed is fast, enabling the output current to approach the target value before the feedback signal intervenes, greatly reducing the burden on feedback control.

[0099] In step S204, while the feedforward control drives the inverter circuit to output current, the controller monitors the actual transmission current in real time through a Hall sensor connected in series in the transmission circuit.

[0100] The Hall sensor operates based on the Hall effect principle, and its output signal is an analog voltage proportional to the instantaneous value of the transmitted current. This analog signal is amplified and filtered by a signal conditioning circuit, and then digitized by an analog-to-digital converter at a fixed sampling frequency, converting it into a digital current value. The sampling frequency must meet the requirements of the Nyquist sampling theorem and be at least twice the highest frequency component of the transmitted signal to ensure complete capture of detailed changes in the current waveform. In a typical embodiment of this application, the sampling frequency is synchronized with or an integer multiple of the triangular carrier frequency to ensure a sufficient number of current sampling points are acquired within each carrier cycle.

[0101] At each sampling moment, after the analog-to-digital converter completes sampling and outputs the digital current value, the controller immediately performs an error calculation operation: subtracting the actual transmitted current value at the current moment from the target current value to obtain the current error signal. To ensure that the target current value and the actual current value correspond to exactly the same moment, the generation of the target current value and the sampling trigger of the analog-to-digital converter use the same system clock source, and the two are strictly synchronized.

[0102] The sign and magnitude of the error signal reflect the degree of deviation between the actual output transmitting current and the target current requirement: a positive error indicates that the actual current is less than the target current, requiring an increase in the duty cycle to improve the output; a negative error indicates that the actual current is greater than the target current, requiring a decrease in the duty cycle to reduce the output; an error signal of 0 indicates that the actual transmitting current is consistent with the target current, requiring no adjustment. This error signal is the sole input for the next closed-loop feedback correction stage, and its accuracy and real-time performance directly determine the effectiveness of the closed-loop control.

[0103] In step S205, the controller inputs the error signal generated in the previous step into the proportional-integral control algorithm, calculates the correction amount of the duty cycle according to the proportional-integral control algorithm, and adds the correction amount to the feedforward duty cycle to form the final duty cycle, which is then distributed to the control terminals of each switching device in the inverter circuit.

[0104] The proportional-integral control algorithm performs the following calculations at each sampling time: First, the error value at the current sampling time is multiplied by the proportional coefficient to obtain the proportional term, which reflects the immediate correction requirement for the current deviation; the larger the error, the larger the correction. Then, the error values ​​at all historical sampling times are accumulated, and the accumulated result is multiplied by the integral coefficient and the sampling period to obtain the integral term, which reflects the compensation requirement for the cumulative effect of historical deviations and is used to eliminate long-term systematic deviations. Finally, the proportional term and the integral term are added to obtain the duty cycle correction.

[0105] The calculated duty cycle correction is superimposed on the feedforward duty cycle generated in step S203 to obtain the final duty cycle after closed-loop feedback correction. The final duty cycle includes both the feedforward control component based on the load model (coarse adjustment) and the feedback correction component based on real-time error (fine adjustment). The two components work together to enable the control system to have both fast response and high-precision tracking performance.

[0106] The final duty cycle is expressed as a numerical value representing the proportion of time the switching devices should be on within the current carrier cycle. The controller allocates this value to different combinations of switching devices based on the polarity of the current target voltage: when the target voltage is positive, the duty cycle signal is allocated to the drive terminal of switching device Q4 to control its high-frequency on / off state, while switching device Q1 remains on, and switching devices Q2 and Q3 remain off, with the load voltage alternating between positive and 0 voltage; when the target voltage is negative, the duty cycle signal is allocated to the drive terminal of switching device Q3 to control its high-frequency on / off state, while switching device Q2 remains on, and switching devices Q1 and Q4 remain off, with the load voltage alternating between negative and 0 voltage.

[0107] The duty cycle is updated once per carrier cycle, and the switching devices perform turn-on and turn-off actions according to the updated duty cycle. Since the carrier frequency is much higher than the symbol frequency, the duty cycle changes continuously with a sinusoidal pattern within each symbol cycle, and after filtering by the load, a smooth sinusoidal current segment is synthesized. The continuous calculation of the error signal within each sampling cycle and the cycle-by-cycle correction of the duty cycle form a fast and stable closed-loop adjustment process, ensuring that the actual transmitted current always tracks the target current waveform with high precision. The proportional element ensures the system's rapid response to instantaneous deviations, while the integral element ensures that the system has no steady-state error during long-term operation. The combination of these two elements enables this application to stably and accurately output the coded current waveform in complex field exploration environments.

[0108] Figure 3 The waveform diagram of complementary bipolar Gray code sequences is shown. Figure 3 (a) in the figure shows the target current waveform output by the first Gray code sequence control. The waveform is composed of 8 sine wave segments spliced ​​end to end on the time axis. Each segment corresponds to a code element in the sequence. When the code element value is +1, a positive polarity sine wave segment is output (the waveform is above the zero line). When the code element value is -1, a negative polarity sine wave segment is output (the waveform is below the zero line). The current value is zero at the junction of adjacent segments to achieve smooth switching. Figure 3Figure (b) shows the target current waveform controlled by the second Gray code sequence. The first waveform is below the zero line, corresponding to the code symbol -1; the second waveform is below the zero line, corresponding to the code symbol -1; the third waveform is below the zero line, corresponding to the code symbol -1; the fourth waveform is above the zero line, corresponding to the code symbol +1; the fifth waveform is above the zero line, corresponding to the code symbol +1; the sixth waveform is above the zero line, corresponding to the code symbol +1; the seventh waveform is below the zero line, corresponding to the code symbol -1; and the eighth waveform is above the zero line, corresponding to the code symbol +1. The two sequences are identical in the first four code symbol positions, both being {-1, -1, -1, +1}; and completely complementary in the last four code symbol positions. The first Gray code sequence is {-1, -1, +1, -1}, and the second Gray code sequence is {+1, +1, -1, +1}, with the corresponding positions being opposites of each other. This complementary relationship ensures that the sidelobe noise in the two sets of cross-correlation results can cancel each other out during data processing.

[0109] Figure 4 The waveform of the unipolar pseudo-random sequence is shown. Figure 4 As can be seen, the waveform consists of multiple symbol periods, with each symbol period outputting a complete sine wave segment or no current output. Figure 3 Unlike the bipolar Gray code sequence shown, the symbol values ​​of the unipolar pseudo-random sequence are only 1 or 0, corresponding to the two states of having output and having no output, respectively.

[0110] Figure 5 The waveforms of the first control signal PWMZ, the second control signal PWMA, and the third control signal PWMB of the complementary bipolar Gray code sequences are shown. Figure 5 As shown in (a), in linear detection mode, the target current waveform is controlled by a complementary bipolar Gray code sequence. The horizontal axis represents time, and the vertical axis represents current. The waveform is formed by splicing a series of sine wave segments end-to-end on the time axis, with each segment corresponding to a symbol. When the symbol value is +1, a positive polarity sine wave segment is output; when the symbol value is -1, a negative polarity sine wave segment is output. The current value is zero at the junction of adjacent segments, achieving smooth switching.

[0111] Figure 5 As shown in (b), the positive triangular carrier wave is distributed within the positive voltage range, and the negative triangular carrier wave is distributed within the negative voltage range. Their frequencies are much higher than the frequency of the target voltage waveform. During the positive half-cycle, when the target voltage amplitude is greater than or equal to the amplitude of the positive triangular carrier wave, the comparator outputs a high level; otherwise, it outputs a low level. During the negative half-cycle, when the target voltage amplitude (absolute value) is greater than or equal to the amplitude of the negative triangular carrier wave, the comparator outputs a high level; otherwise, it outputs a low level. This comparison method converts the continuously changing target voltage waveform into a discrete pulse width modulation pulse sequence.

[0112] Figure 5As can be seen in (c), the first control signal PWMZ remains low throughout the entire 8 symbol cycles. The reason is that... Figure 5 (c) in the code corresponds to a bipolar Gray code sequence, where the symbol values ​​are only +1 or -1, and there are no zero symbols. Since all symbols require output current, the impedance matching circuit is always in the off state, and the switching device Q0 remains off throughout the transmission process. If a zero symbol appears in the encoded sequence, the first control signal PWMZ will go high at the corresponding position, causing the transmission circuit to switch to the impedance matching state.

[0113] Figure 5 As shown in (d), the second control signal PWMA exhibits dense high-frequency pulses within the positive polarity symbol period. The pulse width varies with the target voltage amplitude; the larger the amplitude, the wider the pulse, and the smaller the amplitude, the narrower the pulse. Specifically, within the fourth symbol period (positive sine wave), the second control signal PWMA has a series of pulses with varying widths, wider in the middle and narrower at the edges, corresponding to the sine wave's pattern of rising and then falling. The same pattern is observed within the seventh symbol period. In the remaining negative polarity symbol periods, the second control signal PWMA remains at a low level. This corresponds to the positive half-cycle operating states of switching devices Q1 and Q4: switching device Q1 (upper left bridge arm) is continuously turned on during the positive half-cycle, while switching device Q4 (lower right bridge arm) is turned on and off at high frequency, synthesizing a positive voltage waveform at the load end.

[0114] Figure 5 As shown in (e), the third control signal PWMB exhibits dense high-frequency pulses within the negative symbol period, with the pulse width increasing as the absolute value of the target voltage increases. Specifically, within the first three symbol periods (negative sine waves), the third control signal PWMB displays a series of pulses with varying widths, wider in the middle and narrower at the edges; the same pattern is observed in the fifth and sixth symbol periods, and again in the eighth symbol period. During the remaining positive symbol periods, the third control signal PWMB remains at a low level. This corresponds to the negative half-cycle operating states of switching devices Q2 and Q3: switching device Q2 (lower left bridge arm) is continuously turned on during the negative half-cycle, while switching device Q3 (upper right bridge arm) is turned on and off at high frequency, synthesizing a negative voltage waveform at the load end.

[0115] Figure 6 The waveforms of the first control signal PWMZ, the second control signal PWMA, and the third control signal PWMB of a unipolar pseudo-random sequence are shown. Figure 6 As can be seen from (a) in the figure, the corresponding unipolar pseudo-random sequence code values ​​are: {0, 0, 1, 0, 1, 1, 0, 0}. Figure 6In (a), each sine wave segment starts and ends at zero amplitude, and adjacent symbols are connected at zero point, achieving a smooth current transition.

[0116] and Figure 5 The difference between (b) and (c) is that Figure 6 In (b), only one set of triangular carrier waves is used, instead of two sets (positive and negative). This is because the output states of a unipolar pseudo-random sequence are only output (sine wave segment) and no output (zero level). During the non-zero output period, the target voltage is always positive, and there is no negative voltage output. Therefore, only one set of triangular carrier waves is needed to complete the modulation. When the target voltage amplitude is greater than or equal to the triangular carrier wave amplitude, the comparator outputs a high level; otherwise, it outputs a low level.

[0117] from Figure 6 As shown in (c), the first control signal PWMZ is high when the symbol value is zero and low when the symbol value is 1. Specifically, the first control signal PWMZ is high (corresponding to symbol 0, 0) in the first two symbol cycles, low (corresponding to symbol 1) in the third symbol cycle, high (corresponding to symbol 0) in the fourth symbol cycle, low (corresponding to symbol 1, 1) in the fifth and sixth symbol cycles, and high (corresponding to symbol 0, 0) in the seventh and eighth symbol cycles.

[0118] from Figure 6 As shown in (d), the second control signal PWMA exhibits a high-frequency pulse sequence only at positions where the symbol value is 1. Specifically, within the third symbol period, the second control signal PWMA displays a series of pulses with varying widths, wider in the middle and narrower at the edges, corresponding to the rising and falling pattern of a sine wave; the same pulse pattern is observed in the fifth and sixth symbol periods. At positions where the symbol value is 0, the second control signal PWMA remains at a low level.

[0119] from Figure 6 As shown in (e), under unipolar pseudo-random sequence control, since the output current is always positive and there is no negative output, the third control signal PWMB remains low throughout the entire transmission process, and switching devices Q2 and Q3 remain off throughout the entire transmission process. In nonlinear detection mode, the inverter circuit only operates in the positive half-cycle unipolar output mode, and the circuit does not participate in the negative half-cycle.

[0120] 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 controllable coded waveform electrical source transmission system, characterized in that, include: The system includes a DC power supply, an inverter circuit, an impedance matching circuit, and a controller. The inverter circuit and the impedance matching circuit are connected in parallel, and the DC power supply provides DC voltage. The controller is used to determine the transmission frequency based on the target detection depth, determine the coding sequence type based on the detection mode, determine the symbol period based on the transmission frequency, and determine the output state of each symbol according to the value of each symbol in the coding sequence, thereby forming the target current waveform. The inverter circuit and the impedance matching circuit are switched to the transmission circuit according to the current symbol value. When the symbol is positive or negative, the inverter circuit is turned on and the impedance matching circuit is turned off; when the symbol is 0, the inverter circuit is turned off and the impedance matching circuit is turned on. When the inverter circuit is turned on, the target voltage waveform is calculated based on the target current waveform and the resistance and inductance values ​​of the transmitting circuit. The target voltage waveform is then compared with the amplitude of a fixed-frequency triangular carrier wave to generate the feedforward duty cycle. The actual transmitted current is obtained and compared with the target current to obtain the error signal; Based on the error signal, the duty cycle correction is calculated according to the proportional-integral control algorithm and superimposed on the feedforward duty cycle to form the final duty cycle, which is then distributed to the control terminals of the switching devices in the inverter circuit so that the encoded current waveform of the actual transmitted current tracks the target current waveform.

2. The controllable coded waveform electrical source transmission system according to claim 1, characterized in that, When the symbol value is positive, the controller controls the inverter circuit to output a positive sine wave segment; When the symbol value is negative, the controller controls the inverter circuit to output a reverse sine wave segment; When the code element is 0, the controller controls the inverter circuit to output 0.

3. The controllable coded waveform electrical source transmission system according to claim 2, characterized in that, The sine wave segments are spliced ​​together end to end on the time axis to form an coded current waveform. Each sine wave segment corresponds to a code element. All sine wave segments have the same duration and each sine wave segment starts from 0 and ends at 0.

4. The controllable coded waveform electrical source transmission system according to claim 3, characterized in that, The controller determines the duration of each sine wave segment based on a predetermined target detection depth, including: The required transmission frequency is determined based on the resistivity of the underground medium and the target detection depth, according to the skin depth formula. The duration of each sine wave segment is determined based on the transmission frequency, and the duration is equal to the full length of the period corresponding to the transmission frequency, ensuring that each sine wave segment is exactly one full cycle of sine wave.

5. The controllable coded waveform electrical source transmission system according to claim 1, characterized in that, The controller is configured to determine the encoding sequence based on the type of electro-oscillatory response excited by the target: when the target excited a linear electro-oscillatory response, the controller generates a pair of complementary bipolar Gray code sequences as the encoding sequence, and controls the inverter circuit to output two rounds of encoded current waveforms in succession, in order to achieve cross-correlation sidelobe suppression of the response; when the target excited a nonlinear electro-oscillatory response, the controller generates a unipolar pseudo-random sequence as the encoding sequence, and controls the inverter circuit to output an encoded current waveform containing a 0-level interval.

6. The controllable coded waveform electrical source transmission system according to claim 1, characterized in that, The controller pre-calculates the target voltage waveform required to generate the target current waveform based on the predetermined target current waveform and the resistance and inductance values ​​of the load connected to the transmitting circuit. The target voltage waveform is treated as a continuously changing modulation signal and its amplitude is compared in real time with a fixed-frequency triangular carrier signal. When the amplitude of the target voltage waveform is greater than or equal to the amplitude of the triangular carrier signal, a high level is output; otherwise, a low level is output. This generates the feedforward duty cycle for controlling the switching devices of the inverter circuit.

7. The controllable coded waveform electrical source transmission system according to claim 6, characterized in that, The controller obtains the actual transmission current through a Hall sensor, compares the target current with the actual transmission current, and obtains an error signal. The proportional-integral control algorithm multiplies the current error signal by a proportional coefficient, and adds the cumulative value of the error signal at all historical moments multiplied by an integral coefficient and the sampling period to obtain the duty cycle correction.

8. A control method for a controllable coded waveform electrical source transmission system, used in the controllable coded waveform electrical source transmission system according to any one of claims 1-7, characterized in that, include: The transmission frequency is determined based on the target detection depth, the coding sequence type is determined based on the detection mode, the symbol period is determined based on the transmission frequency, and the output state of each symbol is determined according to the value of each symbol in the coding sequence to form the target current waveform. The transmission circuit connection is switched according to the current symbol value. When the symbol is positive or negative, the inverter circuit is turned on and the impedance matching circuit is turned off; when the symbol is 0, the inverter circuit is turned off and the impedance matching circuit is turned on. When the inverter circuit is turned on, the target voltage waveform is calculated based on the target current waveform and the resistance and inductance values ​​of the transmitting circuit. The target voltage waveform is then compared with the amplitude of a fixed-frequency triangular carrier wave to generate the feedforward duty cycle. The actual transmitted current is obtained by using a Hall sensor and compared with the target current to obtain an error signal. Based on the error signal, the duty cycle correction is calculated according to the proportional-integral control algorithm and superimposed on the feedforward duty cycle to form the final duty cycle, which is then distributed to the control terminals of the switching devices in the inverter circuit so that the encoded current waveform of the actual transmitted current tracks the target current waveform.

9. The control method for a controllable coded waveform electrical source transmission system according to claim 8, characterized in that, The type of encoded sequence is determined based on the detection mode, including: When the target excites a linear electrosonic response, a pair of complementary bipolar Gray code sequences are generated as the encoding sequence, which respectively control the inverter circuit to output two rounds of encoded current waveforms to achieve cross-correlation sidelobe suppression of the response. When the target excites a nonlinear electrosonic response, a unipolar pseudo-random sequence is generated as the encoding sequence, and the inverter circuit is controlled to output an encoded current waveform containing zero-level intervals.

10. The control method for a controllable coded waveform electrical source transmission system according to claim 8, characterized in that, When the symbol value is positive, the inverter circuit is controlled to output a positive sine wave segment. When the symbol value is negative, the inverter circuit is controlled to output a reverse sine wave segment. When the code element is 0, the inverter circuit output is controlled to be 0. The sine wave segments are spliced ​​together end to end on the time axis to form an coded current waveform. Each sine wave segment corresponds to a code element. All sine wave segments have the same duration and each sine wave segment starts from 0 and ends at 0.

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