A towed frequency domain electromagnetic sounding depth focused transmitting system and method

Abstract

The present application belongs to the field of towed frequency domain electromagnetic detection, and is a towed frequency domain electromagnetic detection depth focusing transmission system and method, which comprises the following steps: determining the effective frequency band required by the exploration target area according to the depth range of the investigation target area by using the skin depth formula; determining the number and distribution of the required main frequencies in combination with the demand of the investigation target area for longitudinal detection resolution; setting a target function, and obtaining a switch time sequence by maximizing the target function z(S M ); and calculating the actual transmission current spectrum amplitude according to the switch time sequence. The proposed waveform has more reasonable spectrum energy distribution and less harmonic pollution. Therefore, high vertical resolution in the target detection range can be ensured. Further, a corresponding wideband resonance compensator is proposed to offset the inductance caused by the transmission coil in the wideband range, improve the current amplitude of each main frequency in the effective frequency band range, and suppress the harmonics outside the main frequencies.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of towed frequency domain electromagnetic exploration, and particularly relates to a towed frequency domain electromagnetic exploration depth focusing transmission system and method. BACKGROUND

[0002] With the rapid development of economy, resources that are easy to mine and explore are decreasing year by year. In order to improve the mining efficiency and reduce the exploration cost, the research direction of geophysical exploration gradually turns to high-precision and fine exploration. Frequency domain electromagnetic method is an important geophysical exploration method, which uses a car to tow a transmitting coil equipped with a transmitter and a receiver, and a receiving coil sensor to complete the exploration experiment. Due to its advantages of high efficiency and low cost, it has been widely used in geological survey, environmental evaluation and other fields. According to the principle of frequency domain electromagnetic method, a series of electromagnetic field signals with different frequencies are sent through the lower earth to detect the electrical information of different depths underground. In order to ensure the high vertical resolution of the target body at a specific depth, the excitation frequency should be relatively high, and the energy should be limited within the effective frequency band to realize depth focusing exploration. At the same time, the current amplitude of the effective main frequency should be higher than the noise of the system to ensure that the collected data results are meaningful. Because the inductance of the coil is usually large, it will have a significant inhibitory effect on the transmitting current, reducing the energy of the high-frequency detection frequency and the electromagnetic field strength transmitted into the ground. At the same time, the spectral energy decreases with the increase of frequency, and the detection accuracy of each main frequency is quite different. Therefore, high-precision exploration experiments put forward higher requirements for the performance of the transmission system.

[0003] In order to improve the detection efficiency, multi-frequency fusion excitation signal is widely used, that is, the frequency spectrum of the transmitted waveform contains multiple usable main frequencies, so as to detect multiple depths of strata at the same time. Constable proposed a waveform in 1996, which evenly distributes most of the spectral energy on the first and third harmonics. Srnka proposed a representative waveform, and the spectral energy is evenly distributed among the first, second and fourth harmonics. Zhou uses a pseudo-random binary sequence as the transmitted waveform, which has a fixed frequency ratio. The main frequency distribution of the signal is dispersed, and the spectral energy cannot be limited within the effective frequency band, and it is not suitable for high-precision detection of small-scale areas, which requires the main frequency distribution to be within a narrow effective frequency band to ensure high vertical resolution. In the high-precision detection experiment of the target at a specific depth, multiple transmission experiments with different base frequencies need to be carried out using the pseudo-random waveform. Gao Lihui proposed a depth focusing waveform based on the SHEPWM method, which focuses the main spectral energy on the seventh to tenth harmonics, and can achieve good detection effect in the electrical source detection experiment. However, the tenth and higher harmonics are uncontrollable, so the high-frequency harmonic pollution is serious. At the same time, affected by the inductance of the transmitting coil, the spectral energy of the traditional transmitting current is limited, which is difficult to meet the demand of high-precision detection. Liu Changsheng proposed a three-frequency resonant matching circuit, in order to offset the impedance caused by the inductance of the coil, and then improve the precision of the magnetic source detection experiment. The circuit is mainly for the main frequency dispersed transmission waveform, and the passband of each resonant point is narrow, which is not suitable for the overall improvement of the current value within the effective frequency band of the depth focusing waveform. SUMMARY

[0004] In view of the fact that the spectral energy of the transmitting current is limited in the prior art, which is difficult to meet the demand of high-precision detection, the technical problem to be solved by the present application is to provide a depth focusing transmission system and method for towed frequency domain electromagnetic detection.

[0005] The present application is implemented in the following manner,

[0006] A depth focusing transmission method for towed frequency domain electromagnetic detection, the method comprises:

[0007] According to the depth range of the investigation target area, the skin depth formula is used to determine the effective frequency band required for the exploration target area;

[0008] Combined with the demand of the investigation target area for longitudinal detection resolution, the number and distribution of the required main frequencies are determined;

[0009] The target function is set as follows:

[0010] z(S M )=w1·f1(S M )+w2·f2(S M )

[0011] wherein w1, w2 are weight coefficients, S M is a sequence of switching time, The estimated expected spectrum is I n , J n is the current amplitude and phase of the n-th harmonic, and the probe main frequency is {N1, N2,..., N i ,..., N L};

[0012] The constraint condition is

[0013] t0 M

[0014] The sequence of switching time is obtained by maximizing the objective function z(S M );

[0015] The actual spectrum amplitude of the transmitted current is calculated according to the sequence of switching time.

[0016] Further, the actual spectrum amplitude of the transmitted current is calculated according to the sequence of switching time, comprising: being calculated by the following formula:

[0017]

[0018] wherein L0 and R0 are the equivalent inductance and equivalent resistance of the transmitting coil, J n is the current amplitude and phase of the n-th harmonic, n represents the harmonic number, ω n is the angular frequency of the n-th harmonic, T0 is the period, and U(t) is the excitation voltage of the electromagnetic probe.

[0019] Further, it further comprises adjusting the main frequency current amplitude by setting a wideband resonant compensator on the transmitting circuit, the wideband resonant compensator is connected in series with the transmitting coil, and comprises a parallel inductance L1 and a capacitance C1.

[0020] Further, the impedance of the main frequency point in the main frequency band is adjusted by the wideband resonant compensator, and specifically, the total impedance of the wideband resonant compensator and the transmitting coil is represented as:

[0021]

[0022] wherein / / is a parallel operator, ω is the angular frequency of the transmitting current, L0 is the inductance of the transmitting coil, and R0 is the internal equivalent resistance of the transmitting coil and the bridge circuit;

[0023] The total impedance is minimized to make the imaginary part j to be 0, and the main frequency band range of the deep focusing waveform is f L ~ f H , and the resonance center frequency fr The center of the main frequency band is the resonance band center frequency f r is expressed as

[0024]

[0025] The following formula is obtained to calculate the parameters of the wideband resonance compensator:

[0026]

[0027] By adjusting the values of inductance L1 and capacitance C1, the impedance of the main frequency point in the main frequency band is adjusted.

[0028] A towed frequency domain electromagnetic sounding depth focusing transmission system, comprising a transmission waveform design module:

[0029] According to the depth range of the investigation target area, the skin depth formula is used to determine the effective frequency band required for the exploration target area;

[0030] Combined with the demand of the investigation target area for longitudinal detection resolution, the number and distribution of the required main frequencies are determined;

[0031] The objective function is set as follows:

[0032] z(S M )=w1·f1(S M )+w2·f2(S M )

[0033] Where w1 and w2 are weight coefficients, S M is the switch time sequence, The estimated expected spectrum is I n , J n is the current amplitude and phase of the n-th harmonic, and the detection main frequency is {N1, N2,..., N i ,...,N L};

[0034] The constraint condition is

[0035] t0<t1<....<t M

[0036] By maximizing the objective function z(S M ), the switch time sequence is obtained;

[0037] According to the switch time sequence, the actual transmission current spectrum amplitude is calculated.

[0038] Further, the actual transmission current spectrum amplitude calculated according to the switch time sequence comprises: calculated by the following formula:

[0039]

[0040] wherein L0 and R0 are the equivalent inductance and equivalent resistance of the transmitting coil, J n is the amplitude and phase of the n-th harmonic current, n represents the harmonic number, ω n is the angular frequency of the n-th harmonic, T0 is the period, and U(t) is the excitation voltage of the electromagnetic probe.

[0041] Further, the transmitting system further comprises a wideband resonant compensator arranged on the transmitting circuit to adjust the amplitude of the main frequency current, the wideband resonant compensator is connected in series with the transmitting coil and comprises an inductance L1 and a capacitance C1 connected in parallel.

[0042] Further, the impedance of the main frequency point in the main frequency band is adjusted by the wideband resonant compensator, and specifically, the total impedance of the wideband resonant compensator and the transmitting coil is represented as:

[0043]

[0044] wherein / / is a parallel operator, ω is the angular frequency of the transmitting current, L0 is the inductance of the transmitting coil, and R0 is the internal equivalent resistance of the transmitting coil and the bridge circuit;

[0045] The total impedance is minimized with the imaginary part j being 0, and the main frequency band range of the deep focusing waveform is f L ~ f H , the resonance center frequency f r is the center of the main frequency band, and according to the relationship ω = 2πf between the angular frequency and the frequency, the resonance center frequency f r is represented as

[0046]

[0047] The parameters of the wideband resonant compensator are calculated as follows:

[0048]

[0049] By adjusting the values of the inductance L1 and the capacitance C1, the impedance of the main frequency point in the main frequency band is adjusted.

[0050] Compared with the prior art, the present application has the beneficial effects that:

[0051] The application is based on a nonlinear programming method, and a deep focus transmission waveform with closely distributed main frequencies is designed. Compared with the transmission waveform obtained by the traditional SHEPWM method, the proposed waveform has a more reasonable spectrum energy distribution and less harmonic pollution. Therefore, high vertical resolution in the target detection range can be ensured. Further, a corresponding wideband resonance compensator is proposed to offset the inductance caused by the transmission coil in the wideband range, improve the current amplitude of each main frequency in the effective frequency band range, and suppress the harmonics outside the main frequency. The designed resonance compensator has high flexibility in setting the passband width, which can match deep focus waveforms with different main frequency bandwidths. The feasibility of the proposed scheme is verified by simulation and experiment. BRIEF DESCRIPTION OF DRAWINGS

[0052] Figure 1 Principle of the wideband resonance compensator provided for the embodiments of the application Figure 1 (a) Circuit Figure 1 (b) Impedance characteristics

[0053] Figure 2 Structure of the transmission system provided for the embodiments of the application

[0054] Figure 3 Simulation results of the deep focus transmission method based on SHEPWM without a resonance compensator (a) current waveform (b) current spectrum

[0055] Figure 4 Parameter value curve of the capacitance C1 varying with the inductance L1 provided for the embodiments of the application

[0056] Figure 5 Impedance curve of different main frequencies varying with the inductance L1 provided for the embodiments of the application

[0057] Figure 6 Impedance curve varying with frequency provided for the embodiments of the application

[0058] Figure 7 Simulation results of the deep focus transmission scheme based on SHEPWM provided for the embodiments of the application with a resonance compensator (a) current waveform (b) current spectrum

[0059] Figure 8 Simulation results of the proposed five-frequency deep focus transmission scheme provided for the embodiments of the application without a resonance compensator (a) current waveform (b) current spectrum

[0060] Figure 9 Simulation results of the proposed five-frequency deep focus transmission scheme provided for the embodiments of the application with a resonance compensator (a) current waveform (b) current spectrum

[0061] Figure 10The five-frequency deep focusing transmission scheme provided in the embodiment of the present application has simulation results of (a) current waveform and (b) current spectrum with a resonance compensator;

[0062] Figure 11 The current gain curve with the change of frequency under different inductances L1 in the embodiment of the present application;

[0063] Figure 12 The five-frequency deep focusing transmission scheme provided in the embodiment of the present application has current spectra with different resonance compensators of (a) wide-band resonance compensator and (b) narrow-band resonance compensator. DETAILED DESCRIPTION

[0064] In order to make the objectives, technical solutions and advantages of the present application clearer and more comprehensible, the present application is further described in detail below in combination with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application.

[0065] The present application provides a deep focusing transmission method for a towed frequency domain electromagnetic probe, which comprises the following steps:

[0066] According to the depth range of the investigation target area, the effective frequency band required by the exploration target area is determined by using the skin depth formula;

[0067] The number and distribution of required main frequencies are determined in combination with the demand of the investigation target area on the longitudinal detection resolution;

[0068] The target function set is as follows:

[0069] z(S M )=w1·f1(S M )+w2·f2(S M )

[0070] wherein w1 and w2 are weight coefficients, S M is a switch time sequence, The estimated expected spectrum is I n , J n is the current amplitude and phase of the n-th harmonic, and the detection main frequency is {N1, N2,..., N i ,...,N L};

[0071] The constraint condition is

[0072] t0<t1<....<t M

[0073] The switch time sequence is obtained by maximizing the target function z(S M );

[0074] The spectrum amplitude of the actual transmitting current is calculated according to the switch time sequence.

[0075] The spectrum amplitude of the actual transmitting current is calculated according to the switch time sequence, including being calculated by the following formula:

[0076]

[0077] Wherein, L0 and R0 are the equivalent inductance and equivalent resistance of the transmitting coil respectively, J n is the current amplitude and phase of the n-th harmonic, n represents the harmonic number, ω n is the angular frequency of the n-th harmonic, T0 is the period, and U(t) is the excitation voltage of the electromagnetic detection.

[0078] Further comprising adjusting the main frequency current amplitude by setting a wideband resonant compensator on the transmitting circuit, the wideband resonant compensator being in series with the transmitting coil, and including an inductance L1 and a capacitance C1 in parallel.

[0079] The impedance of the main frequency point in the main frequency band is adjusted by the wideband resonant compensator, and specifically, the total impedance of the wideband resonant compensator and the transmitting coil is represented as:

[0080]

[0081] Wherein, / / is a parallel operator, ω is the angular frequency of the transmitting current, L0 is the inductance of the transmitting coil, and R0 is the internal equivalent resistance of the transmitting coil and the bridge circuit;

[0082] The total impedance is minimized with the imaginary part j being 0, and the main frequency band range of the deep focusing waveform is f L ~ f H , the resonance center frequency f r is the center of the main frequency band, and according to the relationship ω = 2πf between the angular frequency and the frequency, the resonance center frequency f r is represented as

[0083]

[0084] The parameters of the wideband resonant compensator are calculated by the following formula:

[0085]

[0086] The impedance of the main frequency point in the main frequency band is adjusted by adjusting the values of the inductance L1 and the capacitance C1.

[0087] In frequency domain electromagnetic prospecting, H-bridge inverter is used as transmitting bridge to ensure high transmitting power. The transmitting voltage waveform is generally generalized bipolar square wave. At this time, the voltage is always working at peak value, which can ensure high voltage utilization rate of DC power supply and low difficulty of waveform generation. The excitation voltage waveform of electromagnetic detection is periodic, with a period of T0, which can be expressed as:

[0088] U(t) = U(t + T0) (1)

[0089] Suppose the voltage waveform switches polarity M times in a period, which is even, then the switching time sequence can be expressed as:

[0090] S M = {t0, t1,..., t m ,...,t M} (2)

[0091] In this time sequence, t m time is the switching time of voltage polarity, t0=0, t M =T0.

[0092] Suppose the voltage of DC power supply is E, then the transmitting voltage waveform in a single period can be expressed as:

[0093] U(t) = (-1) m-1 E t m-1 <t≤t m (3)

[0094] In which, the voltage waveform is selected to be positive in the first interval from t0 to t1.

[0095] In order to facilitate analysis, the polarity switching time sequence of bipolar square wave is used to represent the waveform. Then, the design of expected waveform is transformed into the design of switching time sequence. In a single period, Fourier series expansion is carried out on formula (3):

[0096]

[0097] The Fourier coefficients are:

[0098]

[0099] In which n represents the harmonic number, ω n is the angular frequency of the nth harmonic, which is given by the following formula:

[0100]

[0101] Combined with formula (3), formula (5) can be further derived as:

[0102]

[0103] The voltage amplitude of each harmonic can be expressed as:

[0104]

[0105] In the magnetic source electromagnetic detection, the influence of the coil inductance on the transmitting current cannot be ignored. Therefore, the influence of the transmitting coil needs to be included in equation (8). Then, the actual transmitting current spectrum amplitude is:

[0106]

[0107] where L0and R0are the equivalent inductance and equivalent resistance of the transmitting coil, respectively, and J n is the current amplitude and phase of the nth harmonic.

[0108] Combining equation (7) and equation (9), the spectrum amplitude of the transmitting waveform can be calculated by the switching time sequence S M , that is, the current spectrum is a function of the switching time sequence S M . The goal of the spectrum design is to maximize the energy of the required frequency with equal current amplitude. In the deep focusing transmitting scheme based on the SHEPWM method, the relationship equation group between the actual current spectrum and the expected spectrum is established, and the switching time sequence with the highest matching degree is solved. However, the maximum spectrum amplitude value of each detection frequency is usually difficult to accurately estimate, so a certain margin is left for the maximum value of each harmonic when designing the expected spectrum. Otherwise, if the expected spectrum is set unreasonably, the iterative solving process will be trapped in non-convergence. In order to improve the current amplitude of the main frequency as much as possible, the sum of the main frequency energy is added as a correction term in the solving target. The nonlinear equation solving problem is converted into a nonlinear programming problem, and the solving target is to match the expected spectrum while improving the energy of the main frequency as much as possible.

[0109] Then, assuming that the estimated expected spectrum is I n , and the detection main frequency is {N1, N2,..., Ni,..., N L}, define f1(S M ) and f2(S M ) as follows:

[0110]

[0111] The set target function is as follows:

[0112] z(S M ) = w1·f1(S M ) + w2·f2(S M ) (12)

[0113] Wherein, w1, w2 are weight coefficients respectively.

[0114] The constraint condition is

[0115] t0 < t1 <... < t M (13)

[0116] Equation (10) represents the matching degree of the actual current spectrum and the expected spectrum, and equation (11) represents the total energy of the detection main frequency. By maximizing the objective function z (S M ), the main frequency amplitude can be corrected while matching the expected spectrum, and the main frequency energy can be improved as much as possible, thereby improving the field strength of the incoming ground. Then, the traditional penalty function algorithm can be easily used to solve this problem, and details are not repeated here.

[0117] During detection, the effective frequency band required for the exploration target area is determined by using the skin depth formula according to the depth range of the investigation target area. The number and distribution of the required main frequencies can be determined in combination with the demand for longitudinal detection resolution of the investigation target area. When designing the depth focusing waveform based on the SHEPWM method, it is difficult to iteratively solve the nonlinear equation set due to the large number of equation sets, and therefore only the control equation set of the first eight harmonics is established, and the amplitudes of higher harmonics are uncontrollable. Since the tenth and eleventh harmonics are close to the spectral amplitudes of the seventh, eighth and ninth harmonics to be controlled, they can be utilized. In fact, once the required main frequency harmonics change due to the transformation of the detection target, the amplitudes of the high-frequency harmonics are difficult to predict, and even can bring serious interference to parameter extraction, reducing the detection accuracy. In order to compare with the traditional SHEPWM waveform, the seventh to eleventh harmonics are set as the main frequencies. Taking equation (12) as the solving objective, the switching time sequence of the five-frequency depth focusing waveform is shown in Table I. The spectrum comparison of the proposed depth focusing waveform and the depth focusing waveform based on the SHEPWM method is shown in Table II, and the emission current base frequency is 128 Hz.

[0118] Compared with the SHEPWM scheme, the proposed depth focusing waveform has better spectral characteristics. According to Table II, the amplitudes of the main frequencies are approximately equal, and the amplitudes are all greater than 0.48 A. The spectral energy is more reasonably distributed, and equal accuracy measurement is easier to achieve. At the same time, the amplitudes of the useless harmonics are low, reducing the interference to data processing and parameter extraction. However, the amplitudes of the low-frequency harmonics are still slightly high, and need to be further suppressed.

[0119] Table I Switching time sequence of the five-frequency depth focusing waveform designed

[0120]

[0121] Table II Spectrum comparison of the proposed depth focusing waveform and the depth focusing waveform based on the SHEPWM method

[0122]

[0123] The embodiment further adjusts the amplitude of the main frequency current by setting a wide-band resonant compensator on the transmitting circuit.

[0124] Affected by the transmitting magnetic dipole, the amplitude of the main frequency of the deep focusing waveform is suppressed by the inductive impedance. In order to reduce the impedance of the transmitting coil to multiple continuous main frequency bands and ensure that sufficient energy is transmitted into the ground, the embodiment proposes a wide-band resonant compensator as shown in Figure 1 (a) and the impedance characteristic as shown in Figure 1 (b). The resonant compensator is composed of inductance L1 and capacitance C1. L0 is the inductance of the transmitting coil, and R0 is the internal equivalent resistance of the transmitting coil and the bridge. The front end inputs the deep focusing waveform from the transmitter.

[0125] The total impedance of the coil and the compensator is expressed by the following formula:

[0126]

[0127] Where / / is the parallel operator, and ω is the angular frequency of the transmitting current. In order to minimize formula (14), it is necessary to make the circuit work in a resonant state, that is, to make the imaginary part j to be 0. At this time, the system impedance is the lowest, which is approximately the internal equivalent resistance R0.

[0128] Suppose the main frequency band of the deep focusing waveform ranges from f L to f H . Let the resonant center frequency f r be the center of the main frequency band. According to the relationship between the angular frequency and the frequency ω = 2πf, combined with formula (14), the resonant band center frequency f r can be expressed as

[0129]

[0130] After determining the radius and the number of turns of the transmitting coil according to engineering needs, L0 is determined. After winding the transmitting coil L0, it is difficult to fine-tune the inductance value by winding. Therefore, the design of the wide-band resonant compensator mainly designs the parameters L1 and C1, which need to follow the following relationship:

[0131]

[0132] According to formula (14) and formula (16), by adjusting the values of inductance L1 and capacitance C1, the impedance of the main frequency point in the main frequency band can be adjusted, that is, the passband of the resonant circuit can be adjusted, such as Figure 1(b) as shown.L1 affects the bandwidth of the resonant matching circuit.The larger L1 brings a wider passband, and the current amplitude of multiple main frequencies in the effective frequency band is improved. When the parameters of the compensator inductance L1 and capacitance C1 are reasonably selected, the imaginary part of the impedance in the effective frequency band is close to zero, realizing the improvement of the continuous multiple main frequency points of the deep focusing waveform, that is, providing resonance in a wide frequency band. Thus, the reactive power loss is reduced. Compared with the LC series resonant circuit, the proposed compensator has higher control flexibility for the bandwidth of the passband. When the passband is set to be narrow, the low-frequency harmonic can be well selected and filtered to match the deep focusing waveform with a narrow effective frequency band, further improving the spectral characteristics of the deep focusing waveform. Therefore, the effect of the proposed resonant compensator is similar to that of a band-pass filter, which can select the main frequency in the passband range.

[0133] The application also provides a towed frequency domain electromagnetic exploration deep focusing transmission system, comprising a transmission waveform design module:

[0134] According to the depth range of the investigation target area, the effective frequency band required for the exploration target area is determined by using the skin depth formula;

[0135] In combination with the demand of the investigation target area for longitudinal detection resolution, the number and distribution of required main frequencies are determined;

[0136] The target function is set as follows:

[0137] z(S M )=w1·f1(S M )+w2·f2(S M )

[0138] Wherein, w1 and w2 are weight coefficients, S M is a sequence of switching times, The estimated expected spectrum is I n , J n is the current amplitude and phase of the n-th harmonic, and the detection main frequency is {N1, N2,..., N i ,...,N L};

[0139] The constraint condition is

[0140] t0<t1<....<t M

[0141] By maximizing the target function z(S M ), the sequence of switching times is obtained;

[0142] The actual transmission current spectrum amplitude is calculated according to the sequence of switching times.

[0143] The spectrum amplitude of the actual transmitting current is calculated according to the switch time sequence, including: being calculated by the following formula:

[0144]

[0145] Wherein, L0 and R0 are the equivalent inductance and equivalent resistance of the transmitting coil respectively, J n is the current amplitude and phase of the n-th harmonic, n represents the harmonic number, ω n is the angular frequency of the n-th harmonic, T0 is the period, and U(t) is the excitation voltage of the electromagnetic detection.

[0146] The transmitting system further comprises a wide-band resonant compensator arranged on the transmitting circuit to adjust the main frequency current amplitude, wherein the wide-band resonant compensator is connected in series with the transmitting coil, and comprises an inductance L1 and a capacitance C1 connected in parallel.

[0147] Simulation and analysis

[0148] The embodiment utilizes MATLAB / Simulink to perform simulation to verify the feasibility of the proposed transmitting scheme based on the wide-band resonant compensator. Figure 2 The structure of the proposed transmitting system is shown, which mainly comprises a direct current power supply, a control signal, an H-bridge transmitting bridge circuit, a resonant compensator and a transmitting coil.

[0149] Under the conditions that the power supply voltage is 10V, the inductance of the transmitting coil is 200μH, the total internal impedance of the transmitting coil and the inverter bridge is 2Ω, and the base frequency of the transmitting current is 128Hz, the SHEPWM-based deep focusing transmitting scheme before and after adding the resonant compensator is compared and simulated. Figure 3 The simulation results of the SHEPWM-based deep focusing transmitting scheme are shown. Figure 3 (a) shows the current waveform. Due to the influence of inductance, the bipolar square wave is distorted, and the rising and falling edges of the current are obviously slow. As Figure 3 (b) shows that the frequency points are dense, and high-precision detection in a specific depth range can be achieved. Only the first nine harmonics of the waveform are controlled. The seventh, eighth and ninth harmonics are set as the detection main frequency, and the amplitudes are 2.05A, 1.97A and 1.92A respectively. Due to the influence of the inductance of the transmitting coil, the main frequency current amplitude is suppressed, and the amplitude decreases with the increase of the frequency. The tenth and eleventh harmonics are not controlled, and the amplitudes are 1.96A and 2.58A respectively. The tenth harmonic is close to the set main frequency amplitude and can be utilized. However, the eleventh harmonic amplitude is too high and cannot be used for equal-precision measurement. At the same time, due to the uncontrollability in the high-frequency region, there is serious high-frequency harmonic pollution.

[0150] So, a wideband resonant compensator is designed. When the inductance L0 of the transmitting coil is determined, the relationship between the inductance L1 and the capacitance C1 should satisfy equation (16). According to the characteristics of the deep focusing waveform, the resonant center point is selected as 1088 Hz. In order to select the optimal parameters conveniently, the inductance L2 is calculated from 400 μH to 8000 μH with a step of 10 μH, and all possible capacitance C values satisfying equation (17) are solved. The solution set is shown in Figure 4 It can be seen that the values of the inductance L1 and the capacitance C1 are approximately inversely proportional. The inductance value should not be too small, otherwise the capacitance value will be too large, which is not suitable in terms of volume and cost. At the same time, if the inductance is too large, its weight and internal resistance will also increase, which will bring higher primary frequency energy loss. Therefore, the inductance L1 should be selected moderately, and the range of 400 μH-5000 μH is reasonable.

[0151] Figure 5 The curves of the impedance of each primary frequency with different resonant compensator parameters are shown. It can be seen that as L1 increases, the impedance of each primary frequency decreases. The change of L1 has little effect on the eighth and ninth harmonics near the resonant frequency, but has a greater effect on the amplitudes of the seventh, tenth and eleventh harmonics. In order to obtain a wider passband and ensure that each primary frequency has a lower inductive reactance, a larger L1 should be selected within a reasonable range. When the inductance value is higher than 5 mH, the effect on the inductive reactance is already small. Considering the common specifications of devices, the inductance L1 is taken as 4 mH, and the capacitance C1 value is taken as 120 μF. At this time, the impedance values at each primary frequency are 2.081 Ω, 2.008 Ω, 2.007 Ω, 2.052 Ω and 2.13 Ω, respectively.

[0152] Figure 6 The gain curve with the designed resonant matching circuit is shown. It can be seen that the gains at the 6th, 7th, 8th, 9th and 10th harmonics are 0.92, 0.93, 0.94, 0.95 and 0.96, respectively. Within the effective frequency band, the gain at each frequency is higher than 0.92. Because the designed resonant matching circuit is passive, it cannot produce additional gain. However, within the effective frequency band, the output can be almost without attenuation. This shows that the designed resonant matching circuit can achieve resonance within a wide frequency band. At other frequencies, due to the high-frequency impedance of the load inductance, the output of the system will be significantly reduced. Therefore, the designed resonant matching circuit can achieve lossless output at the designed target frequencies and suppress other frequency components.

[0153] Figure 7 The simulation results after adding the wideband resonant matching circuit are shown. Figure 7 In (a), the peak-to-peak value of the current time-domain waveform is increased from 9.8 to 12.4 A. According to the frequency spectrum shown in Figure 7 (b), the primary frequency amplitudes are increased to 2.26 A, 2.33 A, 2.35 A, 2.44 A and 3.23 A, respectively, compared with Figure 3The current spectrum in (b) has a more obvious increase in the main frequency current value, which can effectively improve the detection accuracy. The low-frequency harmonic amplitude is lower, which can reduce the difficulty of data processing and parameter extraction.

[0154] To continue, five-frequency deep focusing waveforms are designed based on the optimization objective of formula (12), and comparative simulations before and after adding a wideband resonance compensator are carried out. In order to facilitate comparison, the simulation conditions remain the same as the above simulation. Figure 8 The proposed deep focusing waveform is shown, and the fundamental frequency is 128 Hz. Due to the effect of the inductance of the transmitting coil, the waveform is distorted. The main frequency amplitudes are 2.18 A, 2.04 A, 1.99 A, 1.92 A and 1.88 A, respectively. Compared with Figure 3 , the spectral energy is more reasonably distributed, and the main frequency amplitude is closer. The twelfth and thirteenth harmonic amplitudes are low, but the harmonic amplitudes at low frequencies are high. It basically proves the feasibility of the proposed waveform generation method. At the same time, due to the suppression effect of the coil inductance, the spectral amplitude decreases with the increase of frequency.

[0155] Figure 9 The simulation results after adding the resonance compensator designed in the previous simulation are shown. It can be seen that the current peak-to-peak value increases to 12.6 A, and the main frequency amplitudes are 2.37 A, 2.40 A, 2.44 A, 2.42 A and 2.37 A, respectively. Compared with Figure 8 , the spectral amplitude has a significant increase, and the reason is that the resonance brought by the compensator cancels out most of the coil inductance. At this time, the detection energy is stronger, which can effectively improve the vertical resolution of the detection target. The current amplitude is close, which can approximately realize equal-precision measurement. In addition, the resonance compensator has a good suppression effect on low-frequency harmonics, which can effectively reduce the interference of useless harmonics to parameter extraction.

[0156] Next, comparative simulation is done before and after adding resonance compensator to the proposed two-frequency deep focusing transmission scheme. Under the conditions of power supply voltage of 5 V, transmitting coil inductance of 200 μH, total internal impedance of transmitting coil and inverter bridge of 1 Ω, fundamental frequency of 128 Hz, the simulation results are shown in Figure 10 The seventh and eighth current amplitudes are 2.21 A and 2.05 A, respectively. Due to the suppression effect of inductance, the spectral amplitude of the eighth is significantly lower than that of the seventh, which cannot realize equal-precision measurement. And there is a more obvious low-frequency harmonic at the fifth harmonic, with an amplitude of about 0.02 A.

[0157] In order to offset the inductance of the transmitting coil, the parameters of the resonance compensator are designed based on formula (12). Take a series of current gain curves when the inductance L1 is taken, and the simulation results are shown in Figure 11It can be seen that the lower the inductance L1 is, the narrower the passband is, and the stronger the selectivity of the resonant compensator to the main frequency is, and it is easier to achieve the suppression of the useless harmonics outside the effective frequency band. Since the two-frequency deep focusing waveform band is narrow, a lower inductance L1 needs to be selected when determining the parameters of the resonant compensator. However, the inductance L1 cannot be too low, otherwise it will cause obvious suppression to the amplitude of the main frequency.

[0158] In order to further compare the influence of different resonant compensator parameters on the frequency spectrum of the emission current, the emission current frequency spectrum under the conditions of narrow-band resonant compensator and wide-band resonant compensator is compared and simulated, the inductance L1 and the capacitance C1 are(400 μH, 206 μF) and(5000 μH, 143 μF), and the simulation results are shown in Figure 12 . Figure 12 (a) shows the emission current frequency spectrum when the wide-band resonant compensator is added, and the amplitudes of the main frequency are 3.26 A and 3.27 A respectively. It can be seen that compared with not adding the resonant compensator, the amplitude of the main frequency is obviously improved, and higher detection accuracy is achieved. And the amplitudes of the seventh and eighth currents are approximately equal, and basically equal-precision measurement can be achieved. However, due to the wide passband, the current amplitude at the fifth harmonic is still high, and the amplitude is 0.07 A. According to Figure 12 (b), when the narrow-band resonant compensator is added, the amplitudes of the main frequency of the seventh and eighth times are 3.20 A and 3.24 A respectively, and the amplitude of the fifth harmonic is reduced to 0.02 A, which helps to reduce the interference of the useless harmonics on the parameter extraction. Therefore, the resonant compensator has the advantage of adjustable passband, and has higher flexibility. According to the detection requirements, the width of the passband can be adjusted to realize the frequency selection of the detection main frequency and suppress the undesired harmonics.

[0159] The above only describes the preferred embodiments of the present application and is not used to limit the present application, and any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method of depth focusing of a towed frequency domain electromagnetic survey, characterized in that, The method comprises: According to the depth range of the investigation target area, the skin depth formula is used to determine the effective frequency band required for the exploration target area; Combining the demand of the investigation target area for longitudinal detection resolution, the number and distribution of required main frequencies are determined; The target function is set as follows: z(S M ) = w1 · f1(S M ) + w2 · f2(S M ) wherein w1, w2 are weight coefficients respectively, S M is a sequence of switching time, , The estimated expected spectrum is I n , J n The current amplitude and phase of the n-th harmonic are {N1, N2,..., N i ,..., N L} The constraint condition is t0 < t1 <... < t M By maximizing the objective function z(S M ), the switch time sequence is obtained; The spectrum amplitude of the actual transmitting current is calculated according to the switch time sequence.

2. The towed frequency-domain electromagnetic depth-focused transmit method according to claim 1, characterized in that, The spectrum amplitude of the actual transmitting current is calculated according to the switch time sequence, including being calculated by the following formula: where L0and R0are the equivalent inductance and resistance of the transmitting coil, respectively, J n is the amplitude and phase of the n-th harmonic current, n represents the harmonic number, ω n is the angular frequency of the n-th harmonic, T0is the period, and U(t) is the excitation voltage of the electromagnetic probe.

3. The towed frequency-domain electromagnetic depth-focused transmit method of claim 1, wherein, Further comprising adjusting the main frequency current amplitude by setting a wideband resonant compensator on the transmitting circuit, the wideband resonant compensator being connected in series with the transmitting coil and comprising an inductance L1 and a capacitance C1 connected in parallel.

4. The towed frequency-domain electromagnetic depth-focused transmit method of claim 3, wherein, The impedance of the main frequency point in the main frequency band is adjusted by the wideband resonant compensator, and the total impedance of the wideband resonant compensator and the transmitting coil is specifically represented as follows: Wherein, / / is a parallel operator, omega is the angular frequency of the transmitting current, L0 is the inductance of the transmitting coil, and R0 is the internal equivalent resistance of the transmitting coil and the bridge circuit; Minimize total impedance, with imaginary part j = 0, to focus the main band of the waveform at f L ~ f H , with resonance center frequency f r as the center of the main band, according to the angular frequency and frequency relationship ω = 2πf, the resonance band center frequency f r is expressed as The parameters of the wideband resonant compensator are calculated by the following formula: The impedance of the main frequency point in the main frequency band is adjusted by adjusting the values of the inductance L1 and the capacitance C1.

5. A towed frequency domain electromagnetic survey depth focused transmitting system characterized by, Comprise: The transmitting waveform design module: According to the depth range of the investigation target area, the skin depth formula is used to determine the effective frequency band required for the exploration target area; Combining the demand of the investigation target area for longitudinal detection resolution, the number and distribution of required main frequencies are determined; The target function is set as follows: z(S M ) = w1 · f1(S M ) + w2 · f2(S M ) Wherein, w1, w2 are weight coefficients respectively, S M is a sequence of switching time, , The estimated expected spectrum is I n , J n The current amplitude and phase of n harmonic are, and the detection main frequency is {N1, N2,..., N i ,..., N L} The constraint condition is t0 < t1 <... < t M By maximizing the objective function z(S M ), the switch time sequence is obtained; The spectrum amplitude of the actual transmitting current is calculated according to the switch time sequence.

6. A towed frequency domain electromagnetic depth focused transmitting system according to claim 5, wherein, The spectrum amplitude of the actual transmitting current is calculated according to the switch time sequence, including being calculated by the following formula: where L0and R0are the equivalent inductance and resistance of the transmitting coil, respectively, J n is the amplitude and phase of the n-th harmonic current, n represents the harmonic number, ω n is the angular frequency of the n-th harmonic, T0is the period, and U(t) is the excitation voltage of the electromagnetic probe.

7. A towed frequency domain electromagnetic depth focusing transmitter system as claimed in claim 5, characterized in that The transmitting system further comprises a wideband resonant compensator on the transmitting circuit for adjusting the main frequency current amplitude, the wideband resonant compensator being connected in series with the transmitting coil and comprising an inductance L1 and a capacitance C1 connected in parallel.

8. A towed frequency domain electromagnetic depth focused transmitting system according to claim 7, characterized in that, The impedance of the main frequency point in the main frequency band is adjusted by the wideband resonant compensator, and the total impedance of the wideband resonant compensator and the transmitting coil is specifically represented as follows: Wherein, / / is a parallel operator, omega is the angular frequency of the transmitting current, L0 is the inductance of the transmitting coil, and R0 is the internal equivalent resistance of the transmitting coil and the bridge circuit; Minimizing the total impedance, with the imaginary part j being zero, the main frequency band of the deeply focused waveform is in the range of f L ~ f H , assuming the resonance center frequency f r is the center of the main frequency band, according to the angular frequency and frequency relationship ω = 2πf, the resonance center frequency f r is expressed as The parameters of the wideband resonant compensator are calculated by the following formula: The impedance of the main frequency point in the main frequency band is adjusted by adjusting the values of the inductance L1 and the capacitance C1.

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