Full-waveform targeted detection method for time domain electromagnetic induction-polarization effect

Abstract

The invention discloses a full-waveform targeted detection method for a time domain electromagnetic induction-polarization effect. The method comprises the following steps: establishing a fractional order polarization model based on geological data; full-waveform numerical simulation of multiphase polarization medium three-dimensional electromagnetic response is carried out based on a time domain finite analysis method, full-waveform characteristics of an induction-polarization symbiotic effect are analyzed, and a targeted excitation relation is constructed; a time domain electromagnetic transmitter based on full-waveform current control realizes bipolar trapezoidal wave current transmission with linear ascending and descending and controllable slope, and respectively excites an induction effect and a polarization effect of an underground medium; and after the received full-waveform data is preprocessed, parameters such as polarizability, resistivity and the like are extracted, and imaging is carried out. The full-waveform target detection of the time domain electromagnetic induction-polarization effect is realized, the detection blind area is eliminated, and the purpose of improving the detection precision and efficiency of the time domain electromagnetic method is achieved.

Description

Technical Field

[0001] This application belongs to the field of geophysical exploration, relates to time-domain electromagnetic method exploration technology, and specifically relates to a full-waveform targeted detection method for time-domain electromagnetic induction-polarization effect. Background Art

[0002] Time-domain electromagnetic detection methods often obtain single information by measuring electricity or magnetism. However, as the Earth is a non-uniform and strongly dissipative medium, the lithology and physical properties underground show strong non-uniformity and non-linearity. Therefore, it is very important to measure multi-scale characteristics and parameters for complex media.

[0003] In common underground multi-phase conductive media, the induction and polarization effects coexist and accompany each other. It is difficult to accurately interpret the detection results by only considering a single electromagnetic response or polarization response. Therefore, it is very necessary to conduct targeted detection on the induction-polarization effect of underground media. The existing detection methods only perform targeted excitation on the falling edge of a bipolar trapezoidal wave and observe in the tail section, with insufficient efficiency. Summary of the Invention

[0004] To solve the above technical problems, the embodiments of this application provide a full-waveform targeted detection method for time-domain electromagnetic induction-polarization effect, which realizes the full-waveform targeted detection of time-domain electromagnetic induction-polarization effect, eliminates detection blind spots, and achieves the purpose of improving the detection accuracy and efficiency of time-domain electromagnetic methods.

[0005] This application is realized through the following technical solutions:

[0006] A full-waveform targeted detection method for time-domain electromagnetic induction-polarization effect, comprising:

[0007] Construct a multi-phase conductive medium model according to the polarization parameters of the underground medium, and the multi-phase conductive medium model adopts the representation form of conductivity and the representation form of equivalent polarization rate;

[0008] Obtain diffusion equations representing electromagnetic intensity and electromagnetic intensity respectively according to the multi-phase conductive medium model and Maxwell's equations;

[0009] According to the diffusion equations, use the fractional-order finite-difference time-domain method to solve the fractional-order terms in the multi-phase conductive medium model, and discretely recursively the component form containing fractional-order differential terms for calculating the magnetic field response of the induction-polarization coexistence effect under different excitation current parameters;

[0010] According to the magnetic field response of the induction-polarization coexistence effect under different excitation current parameters, simulate the electromagnetic response characteristics at different excitation times, calculate the electromagnetic response at different turn-on times for the polarization field and calculate the electromagnetic response at different turn-off times for the induction field, and determine the optimal turn-on time and the optimal turn-off time according to the electromagnetic response;

[0011] The time-domain electromagnetic transmitter based on full-waveform current control realizes full-waveform targeted excitation by using the optimal turn-on time and the optimal turn-off time.

[0012] Furthermore, the time-domain electromagnetic transmitter with full-waveform current control sets the DC voltage regulation and clamping module and the energy feedback voltage regulation and clamping module in the transmitter. For the polarization field, based on the DC voltage regulation and clamping module, the linear slow rise of the emission current is realized, and the rise time is controlled as the optimal turn-on excitation time to target and excite the polarization effect.

[0013] For the induction field, based on the energy feedback voltage regulation and clamping module, the linear fast turn-off of the emission current is realized, and the turn-off time is controlled as the optimal turn-off excitation time to target and excite the induction effect.

[0014] Furthermore, the optimal turn-on excitation time is the turn-off time corresponding to the earliest sign reversal when the maximum negative response amplitude in the polarization characteristics of all response curves decays by 10%. The polarization characteristics include: the sign reversal time and the maximum negative response amplitude.

[0015] Furthermore, the optimal turn-off excitation time is the shortest turn-off time of the emission circuit, which is calculated by the following formula:

[0016]

[0017] In the formula, V off is the falling-edge clamping voltage value, the power supply voltage U s , and the impedance R and inductive reactance L of the emission bridge circuit and the emission load coil.

[0018] Furthermore, it also includes:

[0019] Performing continuous full-waveform differential synchronous acquisition of response data;

[0020] Performing data baseline correction, low-frequency noise filtering, and full-waveform primary field rejection on the response data to obtain the pure secondary field response;

[0021] Characterizing the polarization effect of the underground medium using the multi-phase conductive medium model for the pure secondary field response, constructing an inversion objective function of the data error and the regularization term of the polarization model parameters, and using the Gauss-Newton optimization algorithm for multi-parameter iterative inversion and fast imaging of the one-dimensional full-waveform time-domain polarization response.

[0022] Compared with the existing methods, the full-waveform targeted detection method provided by this application has the following beneficial effects: Through full-waveform numerical simulation, this application obtains the targeted excitation characteristics with different rise and fall times; through a full-waveform current transmitter, full-waveform targeted excitation is achieved. After effectively processing the data, the conductive and polarization information of the underground medium can be obtained simultaneously, realizing the full-waveform targeted detection of time-domain electromagnetic induction-polarization effect, eliminating the detection blind area, and achieving the purpose of improving the detection accuracy and efficiency of the time-domain electromagnetic method. Description of the Drawings

[0023] Figure 1 is the flow chart of the time-domain electromagnetic full-waveform detection method provided by this application;

[0024] Figure 2 is the schematic structural diagram of the full-waveform current transmitter system provided by this application;

[0025] Figure 3 is the response data after baseline filtering of the full-waveform receiver data provided by this application;

[0026] Figure 4 is the pure secondary field response result diagram after the primary field is removed throughout the process provided by this application;

[0027] Figure 5 is the single-point one-dimensional resistivity extraction imaging result diagram provided by this application; Detailed Embodiments

[0028] In order to make the objectives, technical solutions and advantages of this application clearer, the following further elaborates on this application in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not used to limit this application.

[0029] See Figure 1 As shown, the flow chart of a full-waveform targeted detection method for time-domain electromagnetic induction-polarization effect in an embodiment of this application, a full-waveform targeted detection method for time-domain electromagnetic induction-polarization effect: includes:

[0030] S1 Construct a multi-phase conductive medium model according to the polarization parameters of the underground medium, and the multi-phase conductive medium model adopts the representation form of conductivity and the representation form of equivalent polarization rate;

[0031] The polarization parameters of the underground medium are obtained by referring to geological data, and a multi-phase conductive medium model is established based on the polarization parameters of the underground medium; here, multi-phase refers to a composite medium composed of multiple components with different physical properties such as conductivity and dielectric constant, which can be uniformly or non-uniformly distributed in space.

[0032] The conductivity expression of the multi-phase conductive medium model is written as:

[0033]

[0034] In the formula There are a total of 11 parameters, which are divided into the conductivity σ0 of the surrounding rock, 5 strong polarization medium parameters and 5 weak polarization medium parameters. Among them, the strong polarization medium parameters include the conductivity σ1 of the strong polarization medium, the strong polarization dispersion coefficient C1, the strong polarization volume fraction f1, the strong polarization particle radius a1 and the strong surface polarization coefficient α1; among them, the weak polarization medium parameters include the conductivity σ2 of the weak polarization medium, the weak polarization dispersion coefficient C2, the weak polarization volume fraction f2, the weak polarization particle radius a2 and the weak surface polarization coefficient α2, where ω is the angular frequency.

[0035] The multi-phase conductive medium model is established for simulation calculations on a computer. It is used to simulate real media.

[0036] S2 Further, according to the multi-phase conductive medium model and Maxwell's equations, diffusion equations representing electromagnetic intensity and electromagnetic intensity are obtained respectively.

[0037] In one embodiment, a three-dimensional induced-polarization electromagnetic response numerical simulation of the multi-phase conductive medium model is carried out according to the finite-difference time-domain method, and the accuracy of the three-dimensional numerical simulation algorithm is verified through a homogeneous half-space model and a layered polarization model.

[0038] Substituting the conductivity expression of the multi-phase conductive medium model into Maxwell's equations, it can be obtained that

[0039]

[0040] In the formula Among them is the electromagnetic intensity, is the magnetic field intensity, k is the wave number, ε is the dielectric constant, μ is the magnetic permeability, σ e ′ = σ0 + σ0f1M1 + σ0f2M2, σ e ″1 = σ0f1M1, σ e ″2 = σ0f2M2.

[0041] S3 According to the diffusion equation, the fractional-order terms in the multi-phase conductive medium model are solved by the fractional-order finite-difference time-domain method, and the component form containing fractional-order differential terms is discretely recursed to calculate the induced-polarization coexistence effect magnetic field response under different excitation current parameters. Specifically, according to the established diffusion equations (2) and (3), the fractional-order finite-difference time-domain method is used to directly solve the fractional-order terms in the multi-phase conductive medium model, and the component form containing fractional-order differential terms is discretely recursed to realize the three-dimensional electromagnetic numerical simulation of the double-time-scale induced-polarization effect, and the induced-polarization coexistence effect magnetic field response is obtained through simulation.

[0042] S4 analyzes the electromagnetic response characteristics at different excitation times based on the induced-polarization response with different excitation current parameters, and constructs a targeted excitation relationship. That is, according to the above numerical simulation, calculate the response at different turn-on times for the polarization field; calculate the response at different turn-off times for the induction field; finally, determine the optimal turn-on time and the optimal turn-off time based on the electromagnetic response.

[0043] The optimal turn-on excitation time is the turn-off time corresponding to the earliest sign reversal moment when the maximum negative response amplitude decays by 10% in the polarization characteristics of all response curves according to the calculation results of the induced-polarization electromagnetic response at different turn-off times. The polarization characteristics include: the sign reversal moment and the maximum negative response amplitude;

[0044] The optimal turn-off excitation time is the shortest turn-off time of the circuit, which is calculated by the following formula:

[0045]

[0046] where V off is the falling-edge clamping voltage value, the power supply voltage U s , the impedance R and inductive reactance L of the transmitting bridge circuit and the transmitting load coil.

[0047] S5 is a time-domain electromagnetic transmitter based on full-waveform current control. The time-domain electromagnetic transmitter based on full-waveform current control uses the optimal turn-on time and the optimal turn-off time to achieve full-waveform targeted excitation.

[0048] For the structure of the time-domain electromagnetic transmitter, see Figure 2 :

[0049] See Figure 2 , the transmitter includes a power supply, a combined clamping unit, a matching unit, a transmitting bridge circuit and a main controller. The combined clamping unit includes a DC voltage-regulating clamping module and a feed energy voltage-regulating clamping module, both of which are powered by the power supply.

[0050] The transmitting bridge circuit includes power IGBT devices Q1, Q2, Q3, and Q4, diodes D1, D2, and D3, and a transmitting load coil. The series-connected power IGBT devices Q1 and Q2 are in parallel with the series-connected power IGBT devices Q3 and Q4. The two ends of the parallel connection form the first end and the second end of the transmitting bridge circuit. One end of the power supply is connected to the first end through the series-connected diodes D1 and D2, and the first end is output through the diode D3. The second end of the transmitting bridge circuit is connected to the other end of the power supply. According to the switching signal of the main controller, the transmitting bridge circuit forms a bipolar current. One end of the transmitting coil is connected between the power IGBT devices Q1 and Q2, and the other end is connected between the power IGBT devices Q3 and Q4.

[0051] The matching unit is connected to the transmitting bridge circuit and includes a series connection of a power IGBT device Q5, a power IGBT device Q6, and a resistive load. The emitters of the power IGBT devices Q5 and Q6 are both connected to the resistive load, and the two collectors are respectively connected between the power IGBT devices Q1 and Q2 and between the power IGBT devices Q3 and Q4. When the four power IGBT devices of the transmitting bridge circuit are turned off, the matching unit is connected to the circuit to absorb the tail overshoot of the transmitting current.

[0052] The main controller is a microprocessor that controls the switching devices of the transmitting bridge circuit through the main controller to realize the power supply to the transmitting bridge circuit by the power supply and the combined clamping unit alternately. The gates of the power IGBT devices are all controlled by the drive signals issued by the main controller to realize the conduction and turn-off of the power devices.

[0053] The DC voltage regulating and clamping module is connected in parallel across the input terminals of the power supply. One end of the energy feedback voltage regulating and clamping module is connected to the output terminal of the diode D3, and the other end is connected to the second end of the transmitting bridge circuit.

[0054] Among them, the DC voltage regulating and clamping module is designed based on the boost chopper technology. The module has four ports: input positive, input negative, output positive, and output negative. The input positive is connected to the positive pole of the power supply, the input and output negatives are both connected to the negative pole of the power supply, and the output positive is connected to the collector of the power device Q7. Q7 is turned on at the rising edge to realize high-voltage clamping, so that the bipolar transmitting current rises linearly according to the preset rising time. The output of the module is in a constant voltage mode, and the output voltage is continuously adjustable.

[0055] The energy feedback voltage clamping module includes an energy storage capacitor and an energy feedback unit. The energy storage capacitor is a large-capacity capacitor bank, which is connected in parallel to the emitter of diode D3 and power IGBT device Q4. The energy feedback unit includes three resistors connected in series, one of which is a voltage-dividing resistor. The adjustment terminal of the voltage-dividing resistor is connected to the non-inverting input terminal of a comparator. The inverting input terminal of the comparator is grounded. The output terminal of the comparator is connected to the base of power device Q8. The emitter of power device Q8 is connected to the positive pole of the power supply through an energy feedback resistor. The collector of power device Q8 is connected to the positive pole of the energy storage capacitor.

[0056] During the falling edge, the current flows into the energy storage capacitor through diode D3, and the energy storage capacitor continuously absorbs energy to form a voltage to clamp the current. When the voltage of the energy storage capacitor exceeds the preset value, power device Q8 conducts, and the energy of the energy storage capacitor is fed back to the power supply through the energy feedback resistor. The function of the energy feedback resistor is to control the energy feedback current.

[0057] The embodiments of the present invention can generate a full time-domain electromagnetic waveform. Under a full-wave bipolar trapezoidal wave with one positive and negative cycle:

[0058] During the rising edge stage, power IGBT device Q1, power IGBT device Q4, and power device Q7 are turned on in the positive half-cycle, and power IGBT device Q2, power IGBT device Q3, and power device Q7 are turned on in the negative half-cycle. The DC voltage regulation and clamping module is connected to the transmitting bridge circuit, and the transmitting current is linearly clamped and increased.

[0059] During the flat top section, the value of the transmitting current reaches the optimal current value I best , power device Q7 is turned off, and the DC voltage regulation and clamping module loses its function. Instead, the power supply provides energy to the transmitting bridge circuit through diode D1 and diode D2, and the current enters the flat top constant value stage.

[0060] During the falling edge stage, power IGBT device Q1 and power IGBT device Q4 are turned off in the positive half-cycle, and power IGBT device Q2 and power IGBT device Q3 are turned off in the negative half-cycle. The energy feedback voltage clamping module is connected to the transmitting bridge circuit through diode D3 to linearly clamp the transmitting current.

[0061] At the end of the transmission, power IGBT device Q1, power IGBT device Q2, power IGBT device Q3, and power IGBT device Q4 are all turned off. Power IGBT device Q6 is turned on in the positive half-cycle, and power IGBT device Q5 is turned on in the negative half-cycle, and the matching unit is switched in to absorb the overshoot at the end of the current.

[0062] Connect the DC voltage regulating clamping module and the energy feedback voltage regulating clamping module respectively at the rising edge and falling edge of the emission current, so as to realize the emission of bipolar trapezoidal wave current with linear rising edge and falling edge currents and controllable slopes, and constant currents in the flat top section and the tail section; Connect the current Hall sensor to the emission loop, and synchronously collect the emission current with a 24-bit high sampling rate differential receiver;

[0063] S6 performs continuous full-waveform differential synchronous acquisition of response data;

[0064] S7 performs data baseline correction, low-frequency noise filtering and full-waveform primary field rejection on the response data to obtain the pure secondary field response;

[0065] S8 uses the multi-phase conductive medium model to characterize the polarization effect of the underground medium on the pure secondary field response, constructs the inversion objective function of the data error and the regularization term of the polarization model parameters, and uses the Gauss-Newton optimization algorithm for multi-parameter iterative inversion and fast imaging of the one-dimensional full-waveform time-domain polarization response.

[0066] The receiving sensor can adopt a superconducting quantum sensor with extremely high sensitivity, or a receiving coil with large bandwidth and high sensitivity to measure the induced-polarization symbiotic effect for observation; Use a 24-bit high sampling rate differential receiver to perform continuous full-waveform differential synchronous acquisition of the sensor data;

[0067] The collected data is saved according to the geographical location of the measuring point, time and emission parameters for the next step of processing.

[0068] Preprocess the measured data received by the receiving sensor, and its steps include:

[0069] (I) Data baseline correction: Calibrate the system under different conditions, establish a baseline drift model, and then perform compensation according to the actual conditions; The method of establishing the baseline drift model is to adopt a dual-receiving system, where one set collects response data, and the other set collects the baseline data after filtering the response data, and saves them respectively for the next step of processing.

[0070] As Figure 3 shown, the emission parameters of an emission example are: a 24V power supply, a current assignment of 10A, an emission loop of 100m x 100m, and an emission frequency of 6.25Hz; Then, the data of the high-temperature SQUID induction signal continuously collected at a sampling rate of 47KHz with a 24-bit acquisition card after baseline compensation has a peak-to-peak value of about 2.4V, and the frequency is the same as the emission.

[0071] (II) Low-frequency noise filtering: Establish a noise model for the background field, and use a specific frequency notch algorithm to filter the background noise; First, collect the ambient magnetic field noise data in the long period, analyze the characteristics of the ambient noise, perform notch filtering on the specific frequency noise, and remove the random noise by superposition.

[0072] (III) Full waveform primary field rejection: Based on the transmitted current collected during the operation of the transmitter, calculate the induced value of the primary field in the receiving sensor through a formula and reject it from the data;

[0073] The data is shown in Figure 4 , based on the full waveform primary field rejection method, the acquisition of pure secondary field response is realized, which is more conducive to improving the data interpretation accuracy.

[0074] After the measured data is preprocessed through the above three steps, it can be used for the next processing.

[0075] Use the multi-phase conductive medium model (GEMTIP) to characterize the polarization effect of the underground medium, construct the inversion objective function of the data error and the regularization term of the polarization model parameters, and use the Gauss-Newton optimization algorithm for multi-parameter (resistivity, polarizability, time constant, frequency correlation coefficient) iterative inversion and fast imaging of the one-dimensional full waveform time-domain polarization response;

[0076] The processed data is output as measured data with induced-polarization effect, and the optimization particle swarm algorithm is used for intelligent extraction of multi-parameters such as resistivity, polarizability, and dispersion coefficient.

[0077] Then substitute the extracted polarization multi-parameters into the generalized skin depth formula for calculation, and the result is used with the Gauss-Newton optimization algorithm to achieve conductivity and polarizability-depth imaging.

[0078] According to the multi-phase conductive medium model, the complex wave number expression is derived as:

[0079]

[0080] Let k 2 =(α + iβ) 2 , and we can get:

[0081]

[0082] Among them,

[0083] According to the definition formula of skin depth, the skin depth d is equal to the distance that the field quantity amplitude decays to its surface value of e -1 , and thus the generalized skin depth formula of the porous polarization medium is obtained.

[0084] See Figure 5 , the resistivity extraction imaging results of the single-point one-dimensional full waveform time-domain polarization response are carried out using the Gauss-Newton optimization algorithm, and the comparison between the target parameters and the actual parameter extraction results is made, and the effect is good.

[0085] Finally, valuable economic mineral resources and valueless mineralized zones are identified and delineated based on the imaging results.

[0086] The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included within the protection scope of the present application.

Claims

1. A full-waveform targeted detection method for time-domain electromagnetic induction-polarization effect, characterized in that, Including: Construct a multi-phase conductive medium model according to the polarization parameters of the underground medium. The multi-phase conductive medium model adopts the representation forms of conductivity and equivalent polarizability; Obtain the diffusion equations representing electromagnetic intensity and electromagnetic intensity respectively according to the multi-phase conductive medium model and Maxwell's equations; According to the diffusion equations, use the finite-difference time-domain method for fractional-order terms to solve the fractional-order terms in the multi-phase conductive medium model, and discretely recursively the component form containing fractional-order differential terms to calculate the induced-polarization co-existing effect magnetic field response under different excitation current parameters; According to the induced-polarization co-existing effect magnetic field response under different excitation current parameters, simulate the electromagnetic response characteristics at different excitation times, calculate the electromagnetic response at different turn-on times for the polarization field and the electromagnetic response at different turn-off times for the induction field, and determine the optimal turn-on time and the optimal turn-off time according to the electromagnetic response; Based on a time-domain electromagnetic transmitter with full-waveform current control, use the optimal turn-on time and the optimal turn-off time to achieve full-waveform targeted excitation.

2. The full-waveform targeted detection method for time-domain electromagnetic induction-polarization effect according to claim 1, characterized in that The time-domain electromagnetic transmitter with full-waveform current control sets a DC voltage regulation clamping module and an energy feedback voltage regulation clamping module in the transmitter. For the polarization field, based on the DC voltage regulation clamping module, the transmitted current is linearly slowly increased, and the rise time is controlled to be the optimal turn-on excitation time to target and excite the polarization effect; For the induction field, based on the energy feedback voltage regulation clamping module, the transmitted current is linearly quickly turned off, and the turn-off time is controlled to be the optimal turn-off excitation time to target and excite the induction effect.

3. A full-waveform targeted detection method for time-domain electromagnetic induction-polarization effect according to claim 1, characterized in that, The optimal turn-on excitation time is the turn-off time corresponding to the earliest sign reversal moment when the maximum negative response amplitude decays by 10% in the polarization characteristics among all response curves. The polarization characteristics include: the sign reversal moment and the maximum negative response amplitude.

4. A full-waveform targeted detection method for time-domain electromagnetic induction-polarization effect according to claim 1, characterized in that The optimal turn-off excitation time is the shortest turn-off time of the transmitting circuit, and is calculated by the following formula: where V off is the falling-edge clamping voltage value, the power supply voltage U s , the impedance R and inductive reactance L of the transmitting bridge circuit and the transmitting load coil.

5. A full-waveform targeted detection method for time-domain electromagnetic induction-polarization effect according to claim 1, characterized in that Also including: Perform continuous full-waveform differential synchronous acquisition of response data; Perform data baseline correction, low-frequency noise filtering, and full-waveform primary field removal on the response data to obtain a pure secondary field response; Use the multi-phase conductive medium model to characterize the polarization effect of the underground medium on the pure secondary field response, construct an inversion objective function of data error and polarization model parameter regularization term, and use the Gauss-Newton optimization algorithm for multi-parameter iterative inversion and fast imaging of one-dimensional full-waveform time-domain polarization response.

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