A marine controlled source audio-frequency electromagnetic method observation method

Abstract

The application belongs to the field of marine controlled source, and particularly relates to a marine controlled source frequency domain electromagnetic method observation system, which comprises a fixed position of a transmitting system, a receiving system sailing along a survey line, and a method for extracting amplitude and phase parameters of multiple effective main frequency points of an electromagnetic response signal, and obtaining abnormal position and distribution information. The amplitude and phase multi-parameter joint observation and common interpretation can improve the abnormal identification ability of the observation system under the premise of ensuring the anti-interference ability of the observation system.

Description

Technical Field

[0001] This application belongs to the field of marine controlled sources, specifically a frequency domain electromagnetic observation method for marine controlled sources. Background Technology

[0002] Controlled-Source Electromagnetic Method (CSEM) is an effective marine exploration method that has been successfully applied in the exploration of seafloor natural gas hydrates and oil and gas reservoirs. During exploration, by applying a specific excitation signal to the target area on the seafloor and extracting and observing the amplitude and phase parameters of the electromagnetic response signal, the size, depth, and spatial distribution of seafloor anomalies can be accurately evaluated. Due to its ease of implementation and strong anti-interference capability, the observation of electromagnetic response signal amplitude parameters is more common in traditional electromagnetic methods. However, in experiments detecting seafloor hydrothermal metal sulfides, because seawater, silt, and metal sulfides are all low-resistivity media with relatively similar resistivity, electromagnetic bathymetry methods based on amplitude parameter measurements are difficult to provide accurate evaluations. Compared to seawater and silt, metal sulfides exhibit a stronger polarization effect. In frequency-domain electromagnetic detection, this polarization effect manifests as a phase difference between the response and excitation signals. High polarizability anomalies can be identified through phase parameter observation. However, phase parameter measurement requires strict time synchronization between the excitation and response signals. Due to the strong electromagnetic attenuation characteristics of seawater and the complex seabed topography, traditional high-precision synchronization methods based on line synchronization and GPS systems are not feasible. Therefore, sonar systems with lower synchronization accuracy are often used as synchronization devices. Consequently, phase parameter observation is challenging to implement in marine electromagnetic detection.

[0003] Research on frequency-domain electromagnetic detection systems has a long history. JR Wait proposed the variable-frequency induced polarization (VIP) method in 1959, which involves transmitting excitation currents at two different frequencies and identifying anomalies based on amplitude parameter observations. To obtain more anomaly parameters and improve anomaly resolution, KL Zonge et al. proposed the complex spectrum method in 1975 and the amplitude-phase spectrum method in 1978, respectively, adding the observation of anomaly phase parameters. These methods, by successively transmitting excitation currents of different frequencies to observe parameters at each frequency point, not only have low detection efficiency but are also susceptible to reduced observation accuracy due to time-varying environmental noise interference. He Jishan developed the pseudo-random three-frequency induced polarization method and the pseudo-random multi-frequency induced polarization method based on the VIP method. These methods can simultaneously measure parameters at multiple frequency points through a single transmission experiment, greatly improving detection efficiency and gaining widespread application in the field of frequency-domain electromagnetic detection. However, due to the inherent characteristic of fixed waveforms in multi-frequency pseudo-random signals, the amplitude and phase parameters of the excitation current at each frequency point are uncontrollable. Due to the suppression effect of the transmitting coil inductance, the observation accuracy of high-frequency points is often low. Furthermore, since amplitude measurements are not performed under conditions of equal precision, the amplitude parameters of each frequency point of the response signal cannot be directly used; it is necessary to further calculate the amplitude ratio between the total field and the primary field at the measurement point for anomaly identification. The primary field needs to be calculated based on the relative distance and azimuth between the transmitting and receiving systems, which is not only cumbersome but also prone to inaccuracies due to complex terrain. In addition, when the number of frequency points exceeds five, the low-frequency harmonic amplitudes near the main frequency of the multi-frequency pseudo-random signal are high, interfering with parameter extraction and reducing observation accuracy. To improve observation results, Chinese patent CN202010314490.8 proposes a phase observation method based on the phase difference between the total field and the primary field. Although the phase parameters have a strong ability to identify anomalies, a GPS system is required to ensure high-precision synchronization between the excitation and response signals, making it unsuitable for seabed metal mineral detection. Summary of the Invention

[0004] The technical problem to be solved by this application is to provide a marine controllable source frequency domain electromagnetic observation method, which solves the problem that existing methods have strong anomaly identification capabilities but cannot be applied to seabed metal mineral detection.

[0005] This application is implemented as follows:

[0006] A frequency-domain electromagnetic observation method for controlled ocean sources, comprising:

[0007] The transmitting system is located in a fixed position and transmits multi-frequency excitation signals via a transmitting coil.

[0008] The receiving system travels along the survey line and collects electromagnetic response signals of the target area through receiving coils;

[0009] The amplitude and phase parameters of multiple effective main frequency points of the electromagnetic response signal are extracted to obtain the location and distribution information of anomalies.

[0010] Furthermore, obtaining information on the location and distribution of anomalies includes measuring and extracting the total field parameters of all set measurement points, and then plotting anomaly curves showing the changes in amplitude and phase parameters with the transmit / receive distance at different frequencies for anomaly observation.

[0011] Furthermore, the expression for the multi-frequency SPWM excitation signal is: u out E is the transmit voltage, E is the DC power supply, M is the modulation depth, and A is the modulation depth. i f i φ i These represent the amplitude, frequency, and phase of the i-th sine component, respectively.

[0012] Furthermore, the frequency f i =2 n f0, n=0,1,2,…,N-1, f0 is the fundamental frequency.

[0013] Furthermore, the amplitude design includes:

[0014] Frequency domain analysis of the equivalent circuit of the transmitting coil yields the following results:

[0015]

[0016] In the formula, Z tc (ω) is the expression for the equivalent impedance of the transmitting coil, expressed as:

[0017] Z tc (ω)=R tc +jωL tc

[0018] The time-domain expression of the multi-frequency excitation current signal is:

[0019]

[0020] In the formula, B i and θ i Let be the amplitude and phase of the excitation current at the i-th dominant frequency point, respectively, and be expressed as .

[0021]

[0022]

[0023] Furthermore, MEA i The calculation is as follows: Let the current amplitude B at each dominant frequency point be... i All are equal, and the relationship between the voltage amplitude at each dominant frequency point and the fundamental frequency voltage amplitude satisfies:

[0024]

[0025] Let ω be the high-frequency angular frequency. H =10R tc / L tc When ω i ≥ω H When ω is constant, the resistance Rtc is ignored; when ω is constant... i <ω H When, ignore the inductive resistance ω i L tc ,but:

[0026]

[0027] By adjusting the voltage values ​​at each main frequency point of the excitation signal, the amplitude of the excitation current at each main frequency point is controlled to be equal, and the voltage amplitude at each main frequency point satisfies the law of conservation of energy.

[0028]

[0029] Furthermore, the phase design includes: setting the initial phase of the first main frequency point to 0, with the phases of each frequency point differing by 2π / N sequentially, and the phase difference occurring after each transmission cycle T. base The phase step at each frequency point is 2π / L.

[0030] Furthermore, extracting the amplitude and phase parameters of multiple effective dominant frequency points of the electromagnetic response signal includes: assuming the sampling sequence of the total field signal is b r (n), according to the spectral analysis method of discrete Fourier transform, we have:

[0031]

[0032] In the formula, N S For the sampled sequence b r The length of (n), B r (k) is the result of the DFT transformation, then the i-th dominant frequency point f of the total field signal i Amplitude B r,i Phase difference between the excitation current and the total field They are represented as follows:

[0033]

[0034]

[0035] In the formula, φ t,i Let be the phase of the i-th dominant frequency point of the excitation current signal, and be a known quantity.

[0036] Furthermore, the total field phase During extraction, the excitation and response signals are synchronized in time. The time delay between the excitation and response signals is calculated, and a low-precision synchronization signal is used to align the excitation and response signals in the time domain. The total field phase is then extracted through the time synchronization of the excitation and response signals.

[0037] Furthermore, calculating the time delay between the excitation signal and the response signal includes: assuming the phase sequences of the excitation signal and the response signal at the i-th frequency point are φ... t,i (n) and φ r,i (n), represented as:

[0038]

[0039]

[0040] In the formula θ t,i (n) and θ r,i Let (n) represent interference noise, and d represent the time delay between the two phase sequences. Then, the cross-correlation function of the two phase sequences can be expressed as:

[0041]

[0042] Assuming the interference noise is random noise, then the phase sequence and the noise, as well as the noise itself, are all independent of each other. Therefore:

[0043] R tr (τ)=R ss (τ-d)

[0044] Based on the properties of the autocorrelation function, when τ-d=0, R ss When (τ-d) reaches its maximum value, the phase sequence φ r,i (n) and φ t,i The time delay between (n) is:

[0045]

[0046] By combining a low-precision synchronization signal, the excitation signal and the response signal are aligned on the time axis.

[0047] Compared with the prior art, the advantages of this application are as follows:

[0048] This application proposes a frequency-domain electromagnetic observation system based on a multi-frequency SPWM excitation field source. Leveraging the flexibility of amplitude and phase configuration at various frequencies of the multi-frequency SPWM excitation signal, equal-precision observation of amplitude parameters at multiple frequencies is achieved by setting the excitation current amplitude to be equal at each frequency. The phase of the excitation signal at each frequency is set to change according to a specific pattern, and the time delay between the two signals is calculated based on the cross-correlation of the phase sequences at various frequencies of the excitation signal and the total field response signal, thereby achieving synchronization between the signals. Since the phase change period is set to be much larger than the transmission period, the synchronization accuracy requirements of the observation system can be reduced. This observation system can directly identify anomalies based on the detected total field amplitude and phase parameters, eliminating the tedious single-field calculation at each measurement point, and achieving higher observation accuracy and detection efficiency. By employing a joint observation and interpretation of multiple parameters (amplitude and phase), the anomaly identification capability of the observation system can be improved while ensuring its anti-interference capability. Attached Figure Description

[0049] Figure 1 Schematic diagram of the principle of frequency domain electromagnetic detection for controllable sources in the ocean;

[0050] Figure 2 Simplified model diagram for electromagnetic response parameter measurement: (a) relative positional relationship; (b) equivalent circuit model of the abnormal loop;

[0051] Figure 3 Schematic diagram of the multi-frequency SPWM excitation field source generation principle (a) Schematic diagram of the multi-frequency SPWM electromagnetic transmission system structure (b) Schematic diagram of the multi-frequency SPWM excitation signal modulation principle;

[0052] Figure 4 This is a schematic diagram of the phase configuration of each main frequency point of the multi-frequency SPWM excitation signal provided in an embodiment of the present invention;

[0053] Figure 5 The spectrum analysis diagrams of the five-frequency pseudo-random signal and the five-frequency SPWM excitation signal provided in the embodiments of the present invention are as follows: (a) voltage spectrum of the five-frequency pseudo-random signal; (b) current spectrum of the five-frequency pseudo-random signal; (c) voltage spectrum of the five-frequency SPWM signal; (d) current spectrum of the five-frequency SPWM signal.

[0054] Figure 6 Phase change curves of the five-frequency SPWM excitation signal at each frequency point: (a) Phase change curves of the five-frequency SPWM excitation signal at each frequency point (L=50); (b) Phase change curves of the five-frequency SPWM excitation signal at each frequency point (L=200). Detailed Implementation

[0055] 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.

[0056] This application presents a controlled-source frequency-domain electromagnetic detection method for the ocean. The detection system comprises three parts: a transmitting system, a receiving system, and subsequent data processing and interpretation. First, an alternating excitation current is injected into the transmitting coil via a high-voltage cable through a shipborne transmitting system. This excitation current establishes an alternating primary electromagnetic field in the ocean space. Under the influence of this alternating electromagnetic field, a seabed anomalous conductor generates induced eddy currents and a secondary induced electromagnetic field, which radiates into space. The receiving coil converts the total field signal from the superposition of the primary and secondary electromagnetic fields into an induced electromotive force signal. This signal contains geoelectric response information, which is collected and stored by the receiving system. During sea trials, the research vessel carrying the transmitting system remains stationary, while the research vessel carrying the receiving system navigates along a predetermined survey line, comprehensively recording and observing the electromagnetic response information of the target area. In the later stages of the exploration experiment, data processing methods such as forward and inverse modeling are used to further obtain electrical information such as resistivity and polarizability at different depths in the area under test, providing an interpretation of the location and distribution of seabed anomalous bodies. As the source of the frequency-domain electromagnetic detection system, the performance of the transmitting system has a crucial impact on the difficulty of data extraction and the observation results.

[0057] According to the frequency domain electromagnetic method theory, different detection frequencies correspond to different depths of electrical information, and the detection depth can be approximately given by the skin depth formula (1).

[0058]

[0059] Where δ represents the exploration depth, ρ represents the resistivity of the medium in the measurement area, and f represents the detection frequency.

[0060] To complete the detection within the target depth range, traditional frequency sweeping transmission methods require transmitting multiple frequencies one by one, resulting in low detection efficiency. Furthermore, inconsistent environmental noise at different frequencies significantly reduces observation accuracy. However, employing a multi-frequency excitation signal transmission method can solve this problem. By extracting the amplitude and phase parameters of multiple effective dominant frequencies in the response signal, rapid and high-precision exploration of strata at different depths can be achieved.

[0061] To analyze the ability of amplitude and phase parameters to identify anomalies, a simplified model for electromagnetic response parameter measurement was established to simulate the experimental process of controlled-source electromagnetic detection in the ocean. The relative positions of the transmitting coil, receiving coil, and anomaly loop are as follows: Figure 2As shown in (a), the transmitting coil and receiving coil are located in the same horizontal plane, and the anomaly loop is located below this horizontal plane. The positions of the transmitting coil and the anomaly loop are fixed, while the receiving coil moves along the measurement line to acquire the total field signal. Mtr, Mta, and Mar are the mutual inductances between the transmitting coil and the receiving coil, between the transmitting coil and the anomaly loop, and between the anomaly loop and the receiving coil, respectively. The equivalent circuit model of the anomaly loop is shown below. Figure 2 As shown in (b), it is equivalent to a closed loop formed by resistor Ra and inductor La connected in series.

[0062] Let the excitation current at a certain frequency be expressed as:

[0063] I t (ω)=A t sin(ωt) (2)

[0064] In the formula A t Let ω be the amplitude of the transmitting current, and ω be the angular frequency of the transmitting current. Then, the magnetic flux generated in the receiving coil and the fault loop are respectively expressed as ψ. tr (ω) and ψ ta (ω), then the expressions are as follows:

[0065] ψ tr (ω)=M tr I t (ω)=M tr A t sin(ωt) (3)

[0066] ψ ta (ω)=M ta I t (ω)=M ta A t sin(ωt) (4)

[0067] The induced current generated within the abnormal loop is expressed as:

[0068]

[0069] In the formula E a (ω) represents the induced electromotive force of the abnormal loop, Z a (ω) is the abnormal loop impedance, expressed as:

[0070] Z a (ω)=R a +jωL a (6)

[0071] Then the induced current I a The magnetic flux generated in the receiving coil by (ω) can be represented as:

[0072]

[0073] Then the receiving coil is filled with excitation current I t (ω) and abnormal loop induced current I a The total magnetic flux (total field) produced by (ω) is expressed as:

[0074]

[0075] The total induced electromotive force is then expressed as:

[0076]

[0077] If we express the trigonometric functions in equation (9) in complex form, then equation (9) can be rewritten as:

[0078]

[0079] In the formula, the real part and the virtual part These are the induced electromotive forces of the secondary and primary fields, respectively. The total field amplitude is expressed as:

[0080]

[0081] The phase of the total field relative to the excitation current is expressed as:

[0082]

[0083] According to equations (11) and (12), the total field amplitude, phase parameter, and anomaly loop parameter L a R a The excitation signal angular frequency ω and the mutual inductance M affected by the relative position ar M ta M tr The amplitude and phase parameter expressions both contain product coupling terms of three influencing factors. According to frequency domain electromagnetic theory, the detection depth is related to both the excitation frequency and the transmit / receive distance. Therefore, by analyzing the variation patterns of the amplitude and phase parameter curves at different transmit / receive distances at the same frequency point, as well as the differences in amplitude and phase parameters between different frequencies of the total field signal at the same measurement point, the location and distribution information of anomalies can be obtained. For the amplitude parameter, the term after the square root includes the primary field, and the amplitude of the primary field is often stronger, making the amplitudes of the total field and the primary field relatively close, resulting in small differences in amplitude parameters at each dominant frequency point. The phase parameter, however, is more affected by frequency near the anomaly, making it easier to directly obtain the location of the anomaly loop from the anomaly curve.

[0084] When using the absolute amplitude parameter of the total field for anomaly analysis, the location of anomalies can be directly identified at the same measurement point based on the amplitude differences between different frequencies of the total field. Therefore, it is required that the amplitudes of all major frequencies of the excitation signal be equal. However, due to the significant differences between the major frequencies of the current in a multi-frequency pseudo-random signal, comparing the amplitudes of different frequencies of the total field signal at the same measurement point becomes meaningless. It is necessary to calculate the amplitude ratio of the total field to the primary field for anomaly feature identification. However, the seabed environment is extremely complex, making it difficult to guarantee the accuracy of the primary field calculation.

[0085] According to Equation (12), the measurement of phase parameters requires time synchronization between the excitation signal and the response signal. Due to the strong attenuation of the signal by seawater and the severe electromagnetic interference on the seabed, the traditional synchronization method based on GPS and line synchronization cannot be applied. Only sonar systems with lower synchronization accuracy can be used. Therefore, higher requirements are put forward for the phase parameter extraction method.

[0086] After completing the measurement and extraction of the total field parameters of all set measurement points, anomaly observation can be carried out by plotting the anomalous curves of amplitude and phase parameters changing with the transmitter-receiver distance at different frequency points. This allows for the analysis and interpretation of the location and distribution of the anomalous body.

[0087] To improve the efficiency of controlled-source electromagnetic detection in the ocean, multi-parameter observation systems need to achieve high-precision measurements at multiple frequencies while minimizing the requirements for synchronization accuracy. Multi-frequency SPWM excitation sources offer the advantage of controllable amplitude and phase at each dominant frequency. By optimizing the amplitude and phase of the excitation signal at each frequency, the optimal excitation signal waveform can be obtained, thereby improving the adaptability of the multi-parameter observation system in complex seabed environments.

[0088] This application provides a frequency-domain electromagnetic observation method for controlled-source ocean observation, the method comprising:

[0089] The transmitting system is located in a fixed position and transmits multi-frequency excitation signals via a transmitting coil.

[0090] The receiving system travels along the survey line and collects electromagnetic response signals of the target area through receiving coils;

[0091] The amplitude and phase parameters of multiple effective main frequency points of the electromagnetic response signal are extracted to obtain the location and distribution information of anomalies.

[0092] Obtaining information on the location and distribution of anomalies includes measuring and extracting the total field parameters of all set measurement points, and then plotting anomaly curves showing the changes in amplitude and phase parameters with the transmitter-receiver distance at different frequencies for anomaly observation.

[0093] The transmitting system of this application is a multi-frequency SPWM electromagnetic transmitting system structure, as shown in the schematic diagram below. Figure 3As shown in (a), the system includes a shipboard DC power supply, a drive control circuit, an H-bridge transmitting circuit, and a towed transmitting coil. The DC power supply E is obtained by rectification and filtering from the shipboard AC generator and serves as the energy source for the transmitting system. The H-bridge transmitting circuit converts the DC power into bipolar AC power, which is applied to both ends of the transmitting coil via a high-voltage cable. The transmitting coil can be equivalently represented by an equivalent inductance L. tc and equivalent resistance R tc The series connection is as follows. Inside the drive control circuit, a multi-frequency sinusoidal modulation signal and a triangular carrier wave are compared to generate control signals s1 and s2. After passing through the drive circuit, these signals control the H-bridge transmitting circuit to output a multi-frequency SPWM excitation signal, which is applied to both ends of the transmitting coil, generating a multi-frequency excitation current in the transmitting coil. This multi-frequency excitation current, through the transmitting coil, excites an alternating primary electromagnetic field containing multiple main frequency components in space, serving as the excitation field source. Let the transmitting voltage u... out and emission current i out The positive direction is defined as from U to V.

[0094] The principle of multi-frequency SPWM excitation signal modulation is as follows: Figure 3 As shown in (b). Let N be the number of dominant frequency points required for detection, and let the dominant frequency points be f0, f1, ..., f2. N-1 Then the multi-frequency sinusoidal modulation signal u m The expression is

[0095]

[0096] In the formula, A i f i φ i ω i The amplitude, frequency, phase, and angular frequency of the i-th sine component are given by ω. i =2πf i M is the modulation depth, which ranges from 0 to 1.

[0097] triangular carrier u c It has a peak-to-peak value of 2 and a frequency of f. c A bipolar isosceles triangular wave. To ensure the waveform quality of the multi-frequency excitation current signal, the carrier angular frequency f... c f must be satisfied c ≥10f i Where i = 0, 1, ..., N-1. If f c If 10kHz is chosen, the value of the main frequency of the excitation signal should be in the range of 0 to 1kHz.

[0098] Based on the rule-based sampling method, via triangular carrier u c For multi-frequency modulated signal u mThe modulation generates control signals s1 and s2. At this time, the transmitted voltage is a PWM signal with a pulse width varying according to a multi-frequency sinusoidal law, which has the same characteristics as the multi-frequency sinusoidal modulation signal u. m Same dominant frequency. Only by modulating the multi-frequency modulation signal u... m The design can fit a multi-frequency SPWM excitation signal and a multi-frequency excitation current that contain the required main frequency information.

[0099] Based on Fourier transform analysis of the spectral characteristics of the multi-frequency SPWM excitation signal, the transmit voltage u out The Fourier series expression is:

[0100]

[0101] The first term on the right side of the equation represents the dominant frequency component, containing all desired dominant frequency points; the second term represents the high-frequency harmonic component, generated by the modulation of the triangular carrier by a multi-frequency sinusoidal modulation signal, denoted as Equation H. Expanding Equation H yields...

[0102]

[0103] According to equation H, the angular frequencies of the high-frequency harmonics generated by the multi-frequency sinusoidal modulation signal modulating the triangular carrier wave are: Amplitude When n = 1, 3, 5, ..., k i =0,2,4,…; when n=2,4,6,…, k i =1,3,5,…

[0104] According to equations (14) and (15), the multi-frequency SPWM excitation signal contains each desired main frequency point. Within the effective frequency band corresponding to the target area, except for each desired main frequency point, the amplitude of the other frequency components is 0. The low-order harmonics near each desired main frequency point are eliminated by multi-frequency sinusoidal modulation, which can effectively reduce the difficulty of data processing and parameter extraction. The high-frequency harmonics generated by the multi-frequency sinusoidal modulation signal modulating the carrier are distributed in the upper and lower sidebands near odd multiples of the carrier frequency and integer multiples of the carrier frequency. Compared with the desired main frequency point, the frequency of the high-frequency harmonic components is very high and the amplitude is very low. Since the transmitting coil is inductive, it has a good suppression effect on high-frequency harmonics. During detection, the influence of high-frequency harmonics can be further reduced by increasing the carrier frequency. When ignoring high-frequency harmonics, the expression of the multi-frequency SPWM excitation signal is:

[0105]

[0106] Generally, the DC power supply E is determined before an electromagnetic detection experiment. Therefore, by adjusting the parameters A of the multi-frequency modulation signal... i f i and φ iThe design allows the multi-frequency SPWM excitation signal to have the desired frequency spectrum, amplitude spectrum, and phase spectrum, enabling arbitrary configuration of the number, distribution, amplitude, and phase of the main frequency points within a specific frequency band.

[0107] Regarding frequency, traditional 2 n The frequency ratio between each main frequency point of the multi-frequency pseudo-random signal is fixed, according to 2 n Exponential distribution. Based on the principles of electromagnetic exploration and practical experience, detection efficiency is high when the dominant frequencies of the excitation signal are distributed exponentially. The multi-frequency SPWM excitation signal is generated by modulation, allowing arbitrary configuration of the number and distribution of dominant frequencies. To ensure detection efficiency and align with 2... n Comparative analysis of multi-frequency pseudo-random signals; the distribution of the main frequency points of multi-frequency SPWM excitation signals also follows 2 n Exponential design. Let the fundamental frequency be f0, then the i-th dominant frequency of the N-frequency SPWM excitation signal is f0. i =2 n f0(n=0,1,2,…,N-1).

[0108] According to the skin depth formula, excitation signals of different frequencies can detect different depths of the target area. During the detection experiment, the required fundamental frequency and number of frequency points are determined based on the depth range of the target being investigated and the required longitudinal detection resolution. Theoretically, the more frequency points the excitation signal contains, the higher the detection efficiency. However, limited by the requirements of the receiving system's acquisition accuracy and signal-to-noise ratio, the number of main frequency points contained in a multi-frequency excitation signal generally does not exceed 11.

[0109] The amplitude of this application can be obtained by performing frequency domain analysis on the equivalent circuit of the transmitting coil.

[0110]

[0111] In the formula, Z tc (ω) is the expression for the equivalent impedance of the transmitting coil, expressed as:

[0112] Z tc (ω)=R tc +jωL tc

[0113] The time-domain expression of the multi-frequency excitation current signal is:

[0114]

[0115] In the formula, B i and θ i Let be the amplitude and phase of the excitation current at the i-th dominant frequency point, respectively, and be expressed as follows:

[0116]

[0117]

[0118] According to equation (19), since the transmitting coil is inductive, when the voltage amplitude MEA at each main frequency point in the multi-frequency SPWM excitation signal... i At the same time, the amplitude of the high-frequency component is lower. In order to achieve equal accuracy observation at each frequency point, the current amplitude B at each main frequency point is set to... i Since they are all equal, and combining with equation (17), the relationship between the voltage amplitude at each major frequency point and the fundamental frequency voltage amplitude should satisfy the following:

[0119]

[0120] Let ω be the high-frequency angular frequency. H =10R tc / L tc When ω i ≥ω H At that time, the inductive reactance ω in the transmitting coil impedance i L tc It plays a major role, and the resistance R is ignored. tc When ω i <ω H At that time, the resistance R in the transmitting coil impedance tc Plays a major role, neglecting inductive resistance ω i L tc Therefore, equation (21) can be simplified to:

[0121]

[0122] Therefore, by adjusting the voltage values ​​at each dominant frequency point of the excitation signal, the amplitude of the excitation current at each dominant frequency point can be controlled to be equal. Simultaneously, the voltage amplitude at each dominant frequency point should satisfy the law of conservation of energy.

[0123]

[0124] Under the premise of satisfying the law of conservation of energy, according to equation (16), the multi-frequency sinusoidal modulation signal parameter A can be adjusted during detection. i Modulation depth M and DC voltage E are used to improve the signal-to-noise ratio of the observation system.

[0125] For phase, the observation of phase parameters requires that the excitation signal and response signal be aligned on the time axis. Therefore, the time delay of the synchronization signal should be less than the period of the excitation signal; otherwise, errors will occur in the extracted parameters. To reduce the synchronization accuracy requirements, based on the flexibility of setting the phase of each main frequency point of the multi-frequency SPWM excitation signal, the phase of each frequency point is set to change with time according to an approximately sawtooth wave pattern, such as... Figure 4 As shown in the figure, φ t,i (i = 0, 1, 2, ..., N-1) represents the phase of the i-th dominant frequency point of the multi-frequency SPWM excitation signal. Let the period of the excitation signal be T. baseThen the phase change period T L Let T be the length of L excitation signal cycles. L =L·T base The parameter L was designed based on the synchronization accuracy requirements of the detection experiment. The initial phase of the first main frequency point was 0, and the phase difference between each frequency point was 2π / N. After each transmission cycle T... base The phase step at each frequency is 2π / L. Since the phase change period is much longer than the excitation signal period, a longer synchronization signal time delay is allowed.

[0126] To quickly extract amplitude and phase parameter information at multiple frequency points of the response signal, a spectral analysis method based on Discrete Fourier Transform (DFT) is employed. Let the sampling sequence of the total field signal be b. r (n), according to the DFT, we have:

[0127]

[0128] In the formula, N S For the sampled sequence b r The length of (n), B r (k) represents the result of the DFT transformation. Then, the i-th dominant frequency point f of the total field signal... i Amplitude B r,i Phase difference between the excitation current and the total field They are represented as follows:

[0129]

[0130]

[0131] In the formula, φ t,i Let be the phase of the i-th dominant frequency point of the excitation current signal, and be a known quantity.

[0132] According to equation (26), the total field phase Extraction requires ensuring the synchronization of the excitation and response signals in time. Traditional GPS synchronization uses second pulse signals sent to the transmitting and receiving systems as synchronization markers, mapping each cycle of the excitation and response signals one-to-one. Therefore, the time delay of the synchronization signal must be less than the cycle length of the excitation signal; otherwise, cycle misalignment will occur. Due to the harsh experimental conditions on the seabed, neither linear synchronization nor GPS synchronization can be achieved, and the sonar synchronization systems commonly used in marine exploration have low accuracy. By setting the phase of each frequency point of the multi-frequency SPWM excitation signal to change according to a specific pattern with the number of transmission cycles, the time delay between the excitation and response signals can be calculated by analyzing the cross-correlation of the phase sequences of each main frequency point of the excitation and response signals. This, combined with the low-precision synchronization signal, allows for the time-domain alignment of the excitation and response signals.

[0133] Let the phase sequences of the excitation signal and the response signal at the i-th frequency point be φ. t,i (n) and φ r,i (n) can be represented as:

[0134]

[0135]

[0136] In the formula θ t,i (n) and θ r,i Let (n) represent interference noise, and d represent the time delay between the two phase sequences. Then, the cross-correlation function of the two phase sequences can be expressed as:

[0137]

[0138] Assuming the interference noise is random noise, then the phase sequence and the noise, as well as the noise itself, are independent of each other. Therefore, the above equation can be rewritten as:

[0139] R tr (τ)=R ss (τ-d) (30)

[0140] Based on the properties of the autocorrelation function, when τ-d=0, R ss When (τ-d) reaches its maximum value, the phase sequence φ r,i (n) and φ t,i The time delay between (n) is:

[0141]

[0142] By combining a low-precision sonar synchronization signal, the excitation and response signals can be aligned on the time axis. This is because the sonar signal has low precision, and alignment is performed using this low-precision signal. Adding a sonar, with a phase change period of T... L =L·T base Therefore, the allowed synchronization signal time delay is amplified to L times the original. At this time, by combining equations (25) and (26), the amplitude and phase parameters of each frequency point of the electromagnetic response signal are extracted and observed, and then the location and distribution of the anomaly are explained.

[0143] To verify the feasibility and effectiveness of the proposed multi-frequency excitation field source, simulation models of a five-frequency SPWM transmitting system and a five-frequency pseudo-random transmitting system were built in Matlab / Simulink. The simulation parameters are: equivalent inductance L of the transmitting coil. tc =2mH, equivalent resistance R of the transmitting coil tc =0.1Ω, fundamental frequency f0 is 128Hz, number of main frequency points N is 5, carrier frequency f cIt is 20kHz.

[0144] like Figure 5 (a) and (b) are the spectra of the five-frequency pseudo-random excitation voltage and current signals, respectively. Figure 5 (c) and (d) are the spectra of the five-frequency SPWM excitation voltage and current signals, respectively. Figure 5 (a) It can be seen that the amplitudes of the main frequency points of the pseudo-random voltage signal are relatively close, but the amplitudes of some low-frequency harmonic voltages reach the same order of magnitude as the main frequency points, which brings great difficulties to data processing and parameter extraction. According to Figure 5 (b) The current spectrum is affected by high-frequency suppression from the coil, resulting in very low current amplitudes at 1024Hz and 2048Hz. To improve the detection accuracy of high-frequency components, the current amplitude of high-frequency components can be increased during exploration experiments by increasing the power supply voltage. However, since the pseudo-random excitation signal waveform is fixed, the current amplitude of low-frequency components will also increase accordingly, making it impossible to achieve equally accurate measurements. Figure 5 (c) To ensure equal current amplitude at each frequency, the voltage amplitude at each frequency of the five-frequency SPWM excitation signal is set to be approximately proportional to the frequency. According to... Figure 5 (d) The amplitude of the current signal is equal at all frequencies, enabling equal-precision observation. Furthermore, the harmonic content near the dominant frequency is low, and the spectral characteristics are significantly better than those of the 5-frequency pseudo-random signal. This demonstrates the flexibility of setting the amplitude of each frequency point based on the multi-frequency SPWM excitation signal, allowing for equal-precision observation at each dominant frequency point by setting the current amplitude to be equal.

[0145] Figure 6 (a) and Figure 6 (b) The phase change curves of each frequency point of the five-frequency SPWM excitation signal are shown when the L value is set to 50 and 200, respectively. In both cases, the phase of each frequency point changes according to an approximate sawtooth wave law, and the phase change period is about 0.39s and 1.56s, respectively. That is, the allowable time delay of the phase change period is 50 times and 200 times the excitation signal period, respectively. The phases of each frequency point are successively 2π / 5, which is in line with the set phase change law. This verifies the feasibility of arbitrarily setting the phase of each frequency point of the multi-frequency SPWM excitation signal, which helps to reduce the requirements of the observation system on the synchronization device.

[0146] 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 frequency-domain electromagnetic observation method for controlled ocean sources, characterized in that, The method includes: The transmitting system is located in a fixed position and transmits multi-frequency excitation signals via a transmitting coil. The receiving system travels along the survey line and collects electromagnetic response signals of the target area through receiving coils; Extract the amplitude and phase parameters of multiple effective main frequency points of the electromagnetic response signal to obtain information on the location and distribution of anomalies; Extracting the amplitude and phase parameters of multiple effective dominant frequency points of the electromagnetic response signal includes: assuming the sampling sequence of the total field signal is... According to the spectral analysis method of discrete Fourier transform, we have: ， In the formula, For sampling sequence Length, As the result of the DFT transformation, the total field signal is... One main frequency point amplitude Phase difference between the excitation current and the total field They are represented as follows: ， ， In the formula, For the excitation current signal, the first The phase of each dominant frequency point is a known quantity; Phase difference between total field and excitation current During extraction, the excitation signal and response signal are synchronized in time. The time delay between the excitation signal and response signal is calculated, and the excitation signal and response signal are aligned in the time domain by combining a low-precision synchronization signal. The phase difference between the total field and the excitation current is extracted by the time synchronization of the excitation signal and response signal. ; Calculating the time delay between the excitation signal and the response signal includes: Let the excitation signal and the response signal be of the first... The phase sequences of each frequency point are as follows and , represented as: ， ， In the formula and To interfere with noise, Given the time delay between two phase sequences, the cross-correlation function of the two phase sequences is expressed as: ， Assuming the interference noise is random noise, then the phase sequence and the noise, as well as the noise itself, are all independent of each other. Therefore: ; Based on the properties of the autocorrelation function, when When =0, When the maximum value is reached, the phase sequence and The time delay between them is: ， By combining a low-precision synchronization signal, the excitation signal and the response signal are aligned on the time axis.

2. The ocean controlled-source frequency domain electromagnetic observation method according to claim 1, characterized in that, Obtaining information on the location and distribution of anomalies includes measuring and extracting the total field parameters of all set measurement points, and then plotting anomaly curves showing the changes in amplitude and phase parameters with the transmitter-receiver distance at different frequencies for anomaly observation.

3. The ocean controlled-source frequency domain electromagnetic observation method according to claim 1, characterized in that, The expression for the multi-frequency SPWM excitation signal is: , For transmission voltage, DC power supply For modulation depth, , , The first The amplitude, frequency, and phase of each sinusoidal component.

4. The ocean controlled-source frequency domain electromagnetic observation method according to claim 3, characterized in that, The frequency , =0, 1, 2, …,N-1, This is the fundamental frequency.

5. The ocean controlled-source frequency domain electromagnetic observation method according to claim 3, characterized in that, The amplitude design includes: Frequency domain analysis of the equivalent circuit of the transmitting coil yields the following results: ， In the formula, The equivalent impedance of the transmitting coil is expressed as: ， The time-domain expression of the multi-frequency excitation current signal is: ， In the formula, and The excitation current is respectively the first The amplitude and phase of each major frequency point are expressed as follows: ， 。 6. The ocean controlled-source frequency domain electromagnetic observation method according to claim 5, characterized in that, The calculation is as follows: Let the current amplitude at each dominant frequency point be... All are equal, and the relationship between the voltage amplitude at each dominant frequency point and the fundamental frequency voltage amplitude satisfies: ， Record high frequency angular frequency ,when When, the resistance Rtc is ignored; when At that time, ignoring resistance ,but: ， By adjusting the voltage values ​​at each main frequency point of the excitation signal, the amplitude of the excitation current at each main frequency point is controlled to be equal, and the voltage amplitude at each main frequency point satisfies the law of conservation of energy. 。 7. The ocean controlled-source frequency domain electromagnetic observation method according to claim 3, characterized in that, The phase design includes setting the initial phase of the first dominant frequency to 0, and the phases of each frequency point being sequentially different. Each launch cycle Phase step at each frequency point .

Images