CN111965714B - Electromagnetic detection method and device based on transient process and storage device - Google Patents

Electromagnetic detection method and device based on transient process and storage device Download PDF

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CN111965714B
CN111965714B CN202010679071.4A CN202010679071A CN111965714B CN 111965714 B CN111965714 B CN 111965714B CN 202010679071 A CN202010679071 A CN 202010679071A CN 111965714 B CN111965714 B CN 111965714B
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王广君
宋梦兰
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China University of Geosciences
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Abstract

The invention provides an electromagnetic detection method, equipment and storage equipment based on a transient process, wherein the method comprises the following steps: first of all, setLayer model and coil parameters, and fitting parameters alpha and I by using actually measured turn-off current data based on least square method0Then, calculating a frequency domain emission current expression formula I (omega); then, a frequency domain transient electromagnetic response formula in the vertical direction is obtained through calculation by using a frequency domain Maxwell equation, and conversion of the frequency domain to the time domain is realized through G-S conversion; and finally, solving a first-order Bessel integral by utilizing the Hankel transformation, and obtaining time domain induced electromotive force by utilizing an electromagnetic induction formula. The invention has the beneficial effects that: compared with a transient electromagnetic detection method, the method has higher detection precision in a shallow layer, and can effectively reduce a shallow detection blind area; compared with the current element superposition method, the method provided by the invention is also proved to be feasible and has higher calculation speed for the current element superposition method.

Description

Electromagnetic detection method and device based on transient process and storage device
Technical Field
The invention relates to the technical field of electromagnetic detection, in particular to an electromagnetic detection method, electromagnetic detection equipment and storage equipment based on a transient process.
Background
Electromagnetic detection methods are various and include an electrical method, a magnetic method, transient electromagnetism, a ground penetrating radar and the like, and the transient electromagnetic detection technology is a common underground medium detection technology at present. The transient electromagnetic method is one of the main methods for detecting underground structures, a set of measuring device of a transient electromagnetic system consists of a transmitting part and a receiving part, and the working process can be divided into three parts, namely transmitting, electromagnetic induction and receiving. The working process is as follows: step current is introduced into the transmitting coil, and when the stable current in the transmitting coil is suddenly turned off, the current of the transmitting coil suddenly changes to generate a magnetic field which changes with time around, which is called a primary magnetic field, according to the principle of electromagnetic induction. The primary field can be transmitted to the periphery, when meeting with a good geological conductor in the transmission process, the primary field can be excited in the geological body to generate an induced eddy current, and the induced eddy current is attenuated in the underground geological body, so that a new induced electromagnetic field changing along with time, namely a secondary magnetic field, can be generated. The secondary field will contain geological information about the ore body, and the law of change of the secondary magnetic field is received by the receiving coil of the receiver. The observed data is analyzed and processed, and information such as the distribution profile of the underground electric conductors is interpreted according to the data.
In order to achieve the purpose of shallow layer detection, the transient electromagnetic method needs to acquire data as early as possible to obtain more complete secondary field information. However, in practice, due to the limitations of the switching speed and the voltage withstanding capability of the switching device and the distributed capacitance of the transmitting coil, the current turn-off time cannot be instantly turned off, so that early signal distortion is caused, and a shallow detection blind area is caused. Meanwhile, the problem of shallow layer detection is further increased based on the inversion explanation of the ideal step forward evolution. The problem of shallow detection by a transient electromagnetic method is solved by reducing distortion of a secondary field signal or realizing forward and backward modeling based on actual turn-off current.
The document 'shallow transient electromagnetic method based on the principle of equivalent diamagnetic flux' (Sheishun, Longxia, Zhongsheng, etc.. shallow transient electromagnetic method based on the principle of equivalent diamagnetic flux [ J ]. geophysical science, 2016,59(9):3428 and 3435.) proposes a shallow transient electromagnetic method based on the principle of equivalent diamagnetic flux, which adopts two identical coils which are parallel and coaxial up and down and uses reverse current as a transmitting source (a double-coil source) and receives an underground secondary field at the middle plane of the double-coil source. Because the receiving surface is an equivalent diamagnetic flux plane of the upper coil and the lower coil, the magnetic flux of the primary field in the receiving coil is always zero, the secondary field distortion can be reduced from the source theoretically, and the pure secondary field observation is realized. Although the improved coil device based on the equivalent diamagnetic flux principle can approximately measure a pure secondary field signal, the method requires the receiving coil and the transmitting coil to be integrated, so that the coil is smaller, the energy is relatively weakened during transmitting, the detection range is smaller, and the exploration efficiency is much smaller.
The main idea of the current element superposition method is to divide the emission current during the turn-off period into N small current elements, when the early sampling interval is sufficiently small, each current element is a negative step current element, respectively calculate the transient response generated by each negative step current element, and the total transient response is the sum of transient fields generated by each negative step current element. The method is suitable for any turn-off current waveform, but the current in the turn-off period needs to be accurately sampled, and the method takes the complete turn-off of the current as a starting point and does not utilize signals in the turn-off period; meanwhile, the method needs to solve N times of ideal transient electromagnetic responses every time, so that the calculation time is long and the efficiency is low.
Disclosure of Invention
In order to solve the above problems, the present invention provides an electromagnetic detection method, an apparatus and a storage apparatus based on a transient process, an electromagnetic detection method based on a transient process; the method comprises the following steps:
s101: designing a transient electromagnetic rapid turn-off circuit, and calculating an expression of current i (t) in the transient electromagnetic rapid turn-off circuit; the transient electromagnetic fast turn-off circuit comprises: MOS tube Q1～Q6Resistance R3、R4And RLAnd an inductance L; wherein, MOS tube Q1～Q4Forming a bipolar transmitting bridge circuit, MOS transistor Q5And Q6For controlling the switch, a resistor R3And a resistance R4In order to turn off the damping resistor, the inductor L is equivalent inductor of the transmitting coil, and the resistor RLIs a load resistor;
s102: according to the transient electromagnetic rapid turn-off circuit and the expression of the current i (t) thereof, an exponential turn-off current model is provided: selecting the zero time of sampling to be at the starting point of the turn-off of the emission current, the expression of the exponential decay current is shown as formula (1):
Figure BDA0002585194510000021
I0is the emission current when not turned off; alpha is an attenuation coefficient; t is t1The moment when the emission current starts to be turned off; t is t2The moment when the emission current is completely turned off;
s103: setting a stratum model and coil parameters, and fitting unknown parameters alpha and I in the formula (1) by using the measured turn-off current data of the transient electromagnetic rapid turn-off circuit based on a least square method0(ii) a Then substituting the formula (2) to calculate a frequency domain emission current expression I (omega):
Figure BDA0002585194510000031
in the above formulaI (ω) is the Fourier transform of I (t); t is toff＝t2-t1I.e. current off time;
s104: according to the formula (2), a frequency domain Maxwell equation is utilized to calculate and obtain a frequency domain transient electromagnetic response formula H in the vertical directionz(ω) as shown in formula (3):
Figure BDA0002585194510000032
in the above formula, a is the radius of the circular transmitting loop of the transmitting coil; j. the design is a square1(λ a) is a first order Bessel function, where λ is the Hankel transform integral; z1Is the input impedance of the first layer; z0Being the inherent resistance of the air layer,
Figure BDA0002585194510000033
wherein mu0The value is 4 pi x 10 for vacuum magnetic permeability-7
S105: using G-S transformation to real Hz(omega) converting a frequency domain into a time domain to obtain a time domain transient electromagnetic response containing first-order Bessel integration;
s106: solving the time domain transient electromagnetic response of the first-order Bessel integral by using the Hankel transformation to obtain the time domain transient electromagnetic response; and finally, obtaining time domain induced electromotive force by using an electromagnetic induction formula.
Further, in step S101, the MOS transistor Q1～Q6MOS tubes which are all N-channel; MOS tube Q1Are respectively connected to the resistors R4One end of (1), MOS tube Q5S pole and MOS transistor Q3The D pole of (1); MOS tube Q1D pole respectively connected to positive pole of power supply VCC and MOS tube Q2The D pole of (1); the negative pole of the power VCC is connected to the MOS tube Q3S pole and MOS transistor Q4The S pole of (1); resistance R4The other end of the first resistor is connected to one end of an inductor L and an MOS tube Q5The D pole of (1); the other end of the inductor L is connected to the resistor RLOne terminal of (1), resistance RLAre respectively connected to the resistor R3And MOS transistor Q6The D pole of (1); resistance R3Are respectively connected to the MOS transistor Q2S pole and MOS tube Q6S pole and MOS transistor Q4The D pole of (1); MOS tube Q1～Q6The S pole and the D pole are connected with a diode, the S pole is connected with the anode of the diode, and the D pole is connected with the cathode of the diode.
Further, the expression of the current i (t) in the transient electromagnetic fast turn-off circuit is shown in formula (4):
Figure BDA0002585194510000041
in the above formula, I0For the emission current, the current on the coil is equal to the emission current I when t is 00(ii) a U is the supply voltage and Q2、Q3、Q5The sum of the conduction voltage drops of the corresponding three diodes.
Further, in step S104, if the loop of the transmitting coil is a square loop with a side length of L, the loop is converted into an equivalent radius by area equality:
Figure BDA0002585194510000042
further, in step S105, the time domain induced electromotive force expression represented by equation (9) is as equation (5):
Figure BDA0002585194510000043
in the above formula, m is 10, KjAre G-S transform coefficients.
Further, KjThe calculation method of (2) is as in formula (6):
Figure BDA0002585194510000044
in the above formula, M is M/2, and N is (j + 1)/2.
Further, in step S106, the above equation (6) includes a first-order bezier infinite integral, and the first-order bezier function is solved by using the hankel transformation to obtain the transient induced electromotive force, which is expressed by the following equation (7):
Figure BDA0002585194510000045
in the above formula, S is the effective area of the receiving coil, μ0Is the permeability of air, HiAre hankel filter coefficients.
Further, HiThe method is characterized in that the method adopts 140-point filter coefficients as the Hankel filter coefficients, and n is 140; lambda [ alpha ]iIs a sampling point, which takes on the value of
Figure BDA0002585194510000051
A computer-readable storage medium storing instructions and data for implementing a transient process-based electromagnetic detection method.
An electromagnetic detection device based on transient processes, comprising: a processor and the storage device; the processor loads and executes the instructions and data in the storage device for implementing an electromagnetic detection method based on transient processes.
The technical scheme provided by the invention has the beneficial effects that: the transient electromagnetic detection method is completely compatible with the transient electromagnetic detection method, has higher detection precision in a shallow layer than the transient electromagnetic detection method, and can effectively reduce a shallow detection blind area; compared with the current element superposition method, the method provided by the invention is also proved to be feasible and has higher calculation speed for the current element superposition method.
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The invention will be further described with reference to the accompanying drawings and examples, in which:
FIG. 1 is a flow chart of a transient process based electromagnetic detection method in an embodiment of the present invention;
FIG. 2 is a circuit diagram of a transient electromagnetic fast turn-off circuit in an embodiment of the present invention;
FIG. 3 is a schematic diagram of an equivalent circuit of a load inductor discharge circuit according to an embodiment of the present invention;
FIG. 4 is a diagram of the turn-off waveform of the actual emission current in an embodiment of the present invention;
FIG. 5 is a flow chart of forward computing in an embodiment of the present invention;
FIG. 6 is a schematic representation of the transient state of different turn-off times under the single layer model in the embodiment of the present invention;
FIG. 7 is a schematic representation of the transient state of the two-layer D model with different turn-off times according to the embodiment of the present invention;
FIG. 8 is a schematic representation of the transient state of the second layer G model with different turn-off times in the embodiment of the present invention;
FIG. 9 is a schematic representation of the transient state under the three-layer A model with different turn-off times in the embodiment of the present invention;
FIG. 10 is a schematic representation of the transient state of the three-layer Q model with different turn-off times in the embodiment of the present invention;
FIG. 11 is a schematic representation of the transient state of the three-layer H model with different turn-off times in the embodiment of the present invention;
FIG. 12 is a schematic representation of the transient state of the three-layer K model with different turn-off times in the embodiment of the present invention;
FIG. 13 is a schematic diagram illustrating a comparison of the transient process of the three-layer model with the current element superposition forward modeling according to an embodiment of the present invention;
FIG. 14 is a schematic diagram illustrating a comparison of forward results of different N values by a three-layer model current element superposition method according to an embodiment of the present invention;
FIG. 15 is a schematic diagram illustrating a comparison of transient electromagnetic forward, transient forward and measured data in an embodiment of the present invention;
FIG. 16 is a schematic diagram illustrating comparison of transient forward and measured data according to an embodiment of the present invention;
FIG. 17 is a schematic diagram illustrating a comparison of transient electromagnetic forward, transient forward and measured data in an embodiment of the present invention;
FIG. 18 is a schematic diagram illustrating a comparison of transient forward and measured data according to an embodiment of the present invention;
fig. 19 is a schematic diagram of the operation of the hardware device in the embodiment of the present invention.
Detailed Description
For a more clear understanding of the technical features, objects and effects of the present invention, embodiments of the present invention will now be described in detail with reference to the accompanying drawings.
The embodiment of the invention provides an electromagnetic detection method and device based on a transient process and a storage device.
Referring to fig. 1, fig. 1 is a flowchart of an electromagnetic detection method based on a transient process in an embodiment of the present invention, which specifically includes the following steps:
s101: designing a transient electromagnetic fast turn-off circuit, comprising: MOS tube Q1～Q6Resistance R3、R4And RLAnd an inductance L; wherein, MOS tube Q1～Q4Forming a bipolar transmitting bridge circuit, MOS transistor Q5And Q6For controlling the switch, a resistor R3And a resistance R4In order to turn off the damping resistor, the inductor L is equivalent inductor of the transmitting coil, and the resistor RLIs a load resistor;
step signals commonly used by the transient electromagnetic method are used as excitation sources, but when the transmitting current of a transmitter is turned off, instant turning-off cannot be achieved due to reasons such as transmitting coil inductance and the like. The emission current turn-off process is actually the process of inductive discharge.
Referring to fig. 2, fig. 2 is a circuit diagram of a transient electromagnetic fast turn-off circuit in step S101 according to an embodiment of the present invention; MOS tube Q1～Q6MOS tubes which are all N-channel; MOS tube Q1Are respectively connected to the resistors R4One end of (1), MOS tube Q5S pole and MOS transistor Q3The D pole of (1); MOS tube Q1D pole respectively connected to positive pole of power supply VCC and MOS tube Q2The D pole of (1); the negative pole of the power VCC is connected to the MOS tube Q3S pole and MOS transistor Q4The S pole of (1); resistance R4The other end of the first resistor is connected to one end of an inductor L and an MOS tube Q5The D pole of (1); the other end of the inductor L is connected to the resistor RLOne terminal of (1), resistance RLAre respectively connected to the resistor R3And MOS transistor Q6The D pole of (1); resistance R3Are respectively connected to the MOS transistor Q2S pole and MOS tube Q6S pole and MOS transistor Q4The D pole of (1); mOS tube Q1～Q6The S pole and the D pole are connected with a diode, the S pole is connected with the anode of the diode, and the D pole is connected with the cathode of the diode.
The power supply process of the transmitting coil can be divided into four parts of positive power supply, positive turn-off, reverse power supply and reverse turn-off. When the power is supplied in the positive direction, the positive current flows in the load coil; when the forward current is turned off, Q1、Q4、Q5、Q6While being cut off, the load inductance passes through the resistor R3The parasitic diode of the transistor and the power supply form a discharge loop, and the current flows in the direction of the arrow in fig. 2. During the emission current falling edge, the load inductance discharge equivalent loop is shown in fig. 3.
From the kirchhoff circuit, the linear differential equation for inductance can be listed by fig. 3:
Figure BDA0002585194510000071
u is power supply voltage VCCAnd Q2、Q3、Q5The sum of conduction voltage drops of the corresponding three diodes, i is current, and t represents time;
s102: when t is equal to 0, the current on the coil is equal to the emission current I0Therefore, the expression of the current i (t) in the transient electromagnetic rapid turn-off circuit can be obtained as follows:
Figure BDA0002585194510000072
in the above formula, I0For the emission current, the current on the coil is equal to the emission current I when t is 00。
As can be seen from the above equation (2), the time-off process of the emission current is an exponential decay process. The actual emission current turn-off signal of the transient electromagnetic instrument is shown in fig. 4, and firstly, the emission current is stably output and then is suddenly turned off, the emission current slowly decays to zero after a period of time, and the decay speed of the emission current in the turn-off process is inconsistent.
S103: according to equation (2), an exponential off-current model is proposed: the zero time of sampling is selected to be at the starting point of the turn-off of the emission current, and then the expression of the exponential decay current is:
Figure BDA0002585194510000073
I0is the emission current when not turned off; alpha is an attenuation coefficient; t is t1The moment when the emission current starts to be turned off; t is t2The moment when the emission current is completely turned off;
s104: setting a stratum model and coil parameters, and fitting unknown parameters alpha and I in formulas (2) and (3) by using the actually measured turn-off current data of the transient electromagnetic rapid turn-off circuit based on a least square method0(because of noise error in measuring current, the change curve of current can be obtained by fitting, and I can be accurately obtained0A value); then substituting the formula (4) to calculate a frequency domain emission current expression I (omega):
Figure BDA0002585194510000081
in the above formula, I (ω) is the fourier transform of the off-current I (t); t is toff＝t2-t1I.e. current off time;
at t1Before the moment, the emission current is stable, a stable magnetic field is established in the earth, the first term in the formula can be ignored, and the turn-off starting point is set as the zero moment, t1＝0，toff＝t2-t1＝t2The above formula (4) can be changed to formula (5):
Figure BDA0002585194510000082
equation (5) is the frequency spectrum during the current turn-off process; the formula (2) is an accurate model of the transient electromagnetic rapid turn-off circuit, an exponential turn-off model of the formula (3) is obtained after simplification, and then a frequency domain model required by forward calculation, namely the formula (5), is obtained from the time domain model of the formula (3).
The problem of geophysical exploration is solved by forward modeling and inversion, the inversion result directly reflects the quality of geological exploration, and the forward quality determines the inversion effect. The forward calculation is to assume that the conductive structure (such as resistivity, layer thickness and the like) of the underground medium (geologic body) is known, and then calculate the space and time distribution rule of the electromagnetic field according to the partial differential equation and the boundary condition thereof satisfied by the electromagnetic field, and the forward calculation flow chart is as shown in fig. 5:
s105: according to the formula (5), a frequency domain Maxwell equation is utilized to calculate and obtain a frequency domain transient electromagnetic response formula H in the vertical directionz(ω) as shown in formula (6):
Figure BDA0002585194510000083
in the above formula, a is the radius of the circular transmitting loop, and the unit is m, if the transmitting loop is a square loop with a side length of L, the equivalent radius can be converted through the equal area:
Figure BDA0002585194510000084
J1(λ a) is a first order Bessel function, where λ is the Hankel transform integral; z1Is the input impedance of the first layer; z0Being the inherent resistance of the air layer,
Figure BDA0002585194510000091
wherein mu0The value is 4 pi x 10 for vacuum magnetic permeability-7。
According to the law of electromagnetic induction, the rate of change of the magnetic field with time, i.e., the induced electromotive force, is solved as in equation (7):
Figure BDA0002585194510000092
in the above formula, S is the effective surface of the receiving coilProduct in m2，μ0Is the magnetic permeability of air, and has a value of mu0＝4π×10-7The unit is H/m.
Fourier transform conversion into the frequency domain by the above equation (7) yields equation (8):
V(ω)＝-iωSμ0Hz(ω) (8)
substituting the formula (6) into the formula (8) can obtain the induced electromotive force of the frequency domain under the slope step current turn-off, as shown in the formula (9):
Figure BDA0002585194510000093
as can be seen from the above equation (9), solving the transient electromagnetic forward modeling requires solving two problems: first, the frequency-time domain conversion problem; the second is the solution of infinite integral of a first-order Bessel function.
S106: using G-S transformation to real Hz(omega) frequency domain to time domain conversion (refer to: Lifengping, Possian, Denghui, etc.. several frequency-time domain conversion methods of TEM forward response calculation compare [ J]Geophysical prospecting and chemical prospecting, 2016,40(4): 743-;
the G-S transformation algorithm is an inverse Laplace transformation algorithm which is commonly used in the transient electromagnetic field response calculation research in the geophysical field. Compared with other methods, the G-S transformation method is simple and convenient to calculate by pure real numbers, only calculates a small number of S values, is suitable for calculating a complex site model, and has high calculation speed, high calculation precision and strong practical terrain applicability.
According to the time domain characteristic of G-S transformation, replacing i omega in the formula with a Laplace transformation variable S, and substituting into
Figure BDA0002585194510000094
The expression of time domain induced electromotive force by the formula (9) is as the formula (10):
Figure BDA0002585194510000095
in the above formula, the value of m is not unified at present, and m is 10, K is selected in the embodiment of the inventionjThe G-S transformation coefficient is calculated according to the formula (11):
Figure BDA0002585194510000101
in the above formula, M ═ M/2, N ═ j + 1)/2; here k is just a number counted during the accumulation process and can be replaced by any non-conflicting letter without any practical meaning.
S107: solving the time domain transient electromagnetic response of the first-order Bessel (Bessel) integral (refer to Zhangwei, Wangwei and Stazechtis) by using a Hankel transformation, and obtaining the time domain transient electromagnetic response by comparing the numerical calculation and the precision of the Hankel transformation [ J ]. geophysical prospecting and chemical prospecting, 2010,34(006): 753-755.); and finally, obtaining time domain induced electromotive force by using an electromagnetic induction formula.
The above equation (11) contains a first-order bezier infinite integral, and the first-order bezier function is solved by using the hankerr transformation, so as to obtain the induced electromotive force in the transient process, as shown in the following equation (12):
Figure BDA0002585194510000102
in the above formula, HiThe filter coefficient is a hankel filter coefficient, n is the number of filter coefficient points, and the embodiment of the invention adopts a 140-point filter coefficient, so that n is 140; n corresponds to a hankel filter coefficient of only one count, 140 points; lambda [ alpha ]iIs the ith sampling point, and the value of the sampling point is
Figure BDA0002585194510000103
The following description will be made of simulation analysis for an electromagnetic detection method based on a transient process proposed in the present application:
(1) simulation analysis of transient process forward modeling and transient process forward modeling
Transient electromagnetic detection is obtained with zero off-timeThe equation was conceived to be forward and successfully applied. The embodiment of the invention utilizes different turn-off times and different geological models of the transient process to compare with the transient electromagnetic detection process. The simulation conditions are as follows: the transmitting current is 10A, the transmitting coil is 80m multiplied by 80m, and the receiving coil area is 100m2Under the condition that the number of turns is 1, the simulation results under different stratum models (model parameters are shown in the following table 1) are shown in fig. 6 to 12, assuming that the turn-off time is respectively 0.1 μ s, 10 μ s, 50 μ s and 100 μ s.
TABLE 1 stratum model parameter Table
Figure BDA0002585194510000111
As can be seen from the results shown in fig. 6 to 12, the forward curves of the transient processes at different turn-off times are similar in characteristic, and as the turn-off time decreases, the exponential turn-off forward curve gradually approaches the forward curve at the ideal step, and under the experimental conditions of this section, when the turn-off time reaches 0.1 μ s, the exponential turn-off forward curve almost coincides with the forward curve at the ideal step; compared with the ideal step transient electromagnetic forward modeling, the forward modeling of the transient process has an obvious ascending process in the early stage and finally reaches the maximum value, which accords with the actual detection condition, because the release of the underground secondary field is a gradual increasing process along with the reduction of the current; the late data almost coincides with the forward curve of the ideal transient electromagnetism, and the influence of the turn-off time on the late signal is small, so the transient electromagnetism has a good effect on deep detection. Therefore, transient electromagnetic detection is completely compatible with transient electromagnetic detection, has higher detection precision than the transient electromagnetic detection, and can eliminate detection blind zones.
(2) Forward modeling simulation analysis of transient process forward modeling and current element superposition
The current element superposition method is an approximation algorithm of transient electromagnetism. Let the transmitting current be 10A, the transmitting coil be 80m, and the receiving coil area be 100m2Under the condition that the number of turns is 1 and under the condition that the turn-off time is respectively 10 mus, 50 mus and 100 mus, selecting the Q-type three-layer model in the table 1 to carry out transient process forward modeling and transient current elementThe simulation experiment is overlaid, wherein N is 100, and the simulation results are shown in fig. 13-14.
As can be seen from fig. 13, at different turn-off times, the result obtained by the current element superposition method is close to the result obtained by forward modeling of the transient process. Theoretically, the larger the value of the number N of current elements in the current element superposition method, the more accurate the result but the longer the time consumption. In order to further verify the superiority of the transient electromagnetic detection method relative to the current element superposition method, a comparison experiment is carried out when N values are different in the current element superposition method. When the transmitting current is 10A, the transmitting coil is 80m multiplied by 80m, and the receiving coil area is 100m2The number of turns is 1, the turn-off time is 50 mus, the stratum model is a three-layer Q-type, and the simulation result is shown in FIG. 14.
TABLE 2 calculated time comparison
Figure BDA0002585194510000121
As can be seen from fig. 12, as the value of N increases, the forward results of the current element superposition method and the transient electromagnetic detection method gradually approach each other. However, the current element superposition method needs to calculate and accumulate transient electromagnetic responses of N small current elements, that is, forward calculation under N ideal steps is needed, which results in low calculation efficiency. Table 2 above shows that the calculation time of each method is much longer than that of the transient process forward, where only when N is 10, the time consumption of the current element superposition method is shorter than that of the direct forward method, but the simulation result error is large when N is 10. With the increase of the N value, the time consumption of the current element superposition method is greatly increased, and the forward calculation efficiency is lower and lower. The forward modeling provides a basis for inversion, and a currently common electromagnetic detection inversion method is a process of continuously fitting and correcting a forward modeling result and measured data, wherein multiple forward modeling processes are included, and if one forward modeling process takes too long, the inversion efficiency is low. The transient electromagnetic detection method provided by the invention can improve the shallow layer detection precision and can obviously improve the calculation efficiency compared with a current element superposition method.
(3) Transient forward modeling, comparison of transient forward modeling and measured data
Fig. 15-18 compare measured data with simulated data of transient forward versus transient forward. The measured data test sites are as follows: a second imperial concubine mountain area in Wuhan City of Hubei province; and (3) testing conditions are as follows: the receiving coils are all 10m multiplied by 10m, the transmitting coils are respectively 45m multiplied by 45m and 67.5m multiplied by 67.5m, the transmitting current is 5A, and the turn-off time is respectively 50 mus and 70 mus. In the figure, red is a transient process forward curve, a red band curve is a transient electromagnetic forward curve, and blue is an actually measured induced electromotive force curve. Because of the shallow layer detection experiment, a simple single-layer model is adopted in the simulation, and the earth resistivity obtained by inversion is approximately set to be 5 omega m, which is very consistent with the actual situation.
As can be seen from fig. 15-18, the transient forward curve and the measured value are substantially the same, and the transient forward curve and the measured value are far from being inverted. Because the transient electromagnetic forward modeling and the actual test result have larger errors, the transient electromagnetic detection method has a detection blind zone, and the electromagnetic detection method based on the transient process provided by the embodiment of the invention can effectively solve the problem of the shallow detection blind zone of the transient electromagnetic method.
Referring to fig. 19, fig. 19 is a schematic diagram of a hardware device according to an embodiment of the present invention, where the hardware device specifically includes: an electromagnetic detection device 1901, a processor 1902 and a storage device 1903 based on transient processes.
An electromagnetic detection device 1901 based on transient processes: the one transient process based electromagnetic detection device 1901 implements the one transient process based electromagnetic detection method.
The processor 1902: the processor 1902 loads and executes the instructions and data in the storage device 1903 to implement the one transient process-based electromagnetic detection method.
Computer-readable storage medium 1903: the computer-readable storage media 1903 stores instructions and data; the storage device 1903 is used to implement the electromagnetic detection method based on transient process.
The invention has the beneficial effects that: the invention provides an electromagnetic detection method based on a transient process based on a transient physical equation of current turn-off, and deduces a solving method of transient electromagnetic detection. Simulation experiment results show that the transient electromagnetic detection method is completely compatible with the transient electromagnetic detection method, has higher detection precision in a shallow layer than the transient electromagnetic detection method, and can effectively reduce a shallow detection blind area; compared with the current element superposition method, the method provided by the invention is also proved to be feasible and has higher calculation speed for the current element superposition method.
The detection data of the field actual measurement experiment is completely consistent with the forward curve of the transient electromagnetic detection method provided by the invention, and the correctness of the transient electromagnetic detection method is verified.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.

Claims (7)

1. An electromagnetic detection method based on a transient process is characterized in that: the method comprises the following steps:
s101: designing a transient electromagnetic rapid turn-off circuit, and calculating an expression of current i (t) in the transient electromagnetic rapid turn-off circuit; the transient electromagnetic fast turn-off circuit comprises: MOS tube Q1～Q6Resistance R3、R4And RLAnd an inductance L; wherein, MOS tube Q1～Q4Forming a bipolar transmitting bridge circuit, MOS transistor Q5And Q6For controlling the switch, a resistor R3And a resistance R4In order to turn off the damping resistor, the inductor L is equivalent inductor of the transmitting coil, and the resistor RLIs a load resistor;
s102: according to the transient electromagnetic rapid turn-off circuit and the expression of the current i (t) thereof, an exponential turn-off current model is provided: selecting the zero time of sampling to be at the starting point of the turn-off of the emission current, the expression of the exponential decay current is shown as formula (1):
Figure FDA0003030201830000011
I0is the emission current when not turned off; alpha is an attenuation coefficient; t is t1The moment when the emission current starts to be turned off; t is t2The moment when the emission current is completely turned off;
s103: setting a stratum model and coil parameters, and fitting unknown parameters alpha and I in the formula (1) by using the measured turn-off current data of the transient electromagnetic rapid turn-off circuit based on a least square method0(ii) a Then substituting the formula (2) to calculate a frequency domain emission current expression I (omega):
Figure FDA0003030201830000012
in the above formula, I (ω) is the Fourier transform of I (t); t is toff＝t2-t1I.e. current off time;
s104: according to the formula (2), a frequency domain Maxwell equation is utilized to calculate and obtain a frequency domain transient electromagnetic response formula H in the vertical directionz(ω) as shown in formula (3):
Figure FDA0003030201830000013
in the above formula, a is the radius of the circular transmitting loop of the transmitting coil; j. the design is a square1(λ a) is a first order Bessel function, where λ is the Hankel transform integral; z1Is the input impedance of the first layer; z0Being the inherent resistance of the air layer,
Figure FDA0003030201830000021
wherein mu0The value is 4 pi x 10 for vacuum magnetic permeability-7；
S105: using G-S transformation to real Hz(omega) converting a frequency domain into a time domain to obtain a time domain transient electromagnetic response containing first-order Bessel integration;
in step S105, the time domain induced electromotive force expression represented by equation (9) is as shown in equation (5):
Figure FDA0003030201830000022
in the above formula, m is 10, KjIs G-S transform coefficient;
according to the formula (5), a frequency domain Maxwell equation is utilized to calculate and obtain a frequency domain transient electromagnetic response formula H in the vertical directionz(ω) as shown in formula (6):
Figure FDA0003030201830000023
in the above formula, a is the radius of the circular transmitting loop, and the unit is m, if the transmitting loop is a square loop with a side length of L, the equivalent radius can be converted through the equal area:
Figure FDA0003030201830000024
J1(λ a) is a first order Bessel function, where λ is the Hankel transform integral; z1Is the input impedance of the first layer; z0Being the inherent resistance of the air layer,
Figure FDA0003030201830000025
wherein mu0The value is 4 pi x 10 for vacuum magnetic permeability-7；
According to the law of electromagnetic induction, the rate of change of the magnetic field with time, i.e., the induced electromotive force, is solved as in equation (7):
Figure FDA0003030201830000026
in the above formula, S is the effective area of the receiving coil and is expressed in m2，μ0Is the magnetic permeability of air, and has a value of mu0＝4π×10-7The unit is H/m;
fourier transform conversion into the frequency domain by the above equation (7) yields equation (8):
V(ω)＝-iωSμ0Hz(ω) (8)
substituting the formula (6) into the formula (8) can obtain the induced electromotive force of the frequency domain under the slope step current turn-off, as shown in the formula (9):
Figure FDA0003030201830000031
s106: solving the time domain transient electromagnetic response of the first-order Bessel integral by using the Hankel transformation to obtain the time domain transient electromagnetic response; finally, obtaining time domain induced electromotive force by utilizing an electromagnetic induction formula;
the specific solving steps of the time domain induced electromotive force are as follows:
according to the time domain characteristic of G-S transformation, replacing i omega in the formula with a Laplace transformation variable S, and substituting into
Figure FDA0003030201830000032
The expression of time domain induced electromotive force by the formula (9) is as the formula (10):
Figure FDA0003030201830000033
in the above formula, m is 10, KjThe G-S transformation coefficient is calculated according to the formula (11):
Figure FDA0003030201830000034
in the above formula, M ═ M/2, N ═ j + 1)/2; k is only one letter which is counted in the accumulation process and can be replaced by any non-conflicting letter without actual meaning;
s107: solving the time domain transient electromagnetic response of the first-order Bessel (Bessel) integral by using a Hankel transformation to obtain the time domain transient electromagnetic response; finally, obtaining time domain induced electromotive force by utilizing an electromagnetic induction formula;
the above equation (11) contains a first-order bezier infinite integral, and the first-order bezier function is solved by using the hankerr transformation, so as to obtain the induced electromotive force in the transient process, as shown in the following equation (12):
Figure FDA0003030201830000035
in the above formula, HiThe filter coefficient is a Hankel filter coefficient, n is the number of filter coefficient points, and if a filter coefficient of 140 points is adopted, n is 140; n corresponds to a hankel filter coefficient of only one count, 140 points; lambda [ alpha ]iIs the ith sampling point, and the value of the sampling point is
Figure FDA0003030201830000036
2. The electromagnetic detection method based on transient process as claimed in claim 1, characterized in that: in step S101, MOS transistor Q1～Q6MOS tubes which are all N-channel; MOS tube Q1Are respectively connected to the resistors R4One end of (1), MOS tube Q5S pole and MOS transistor Q3The D pole of (1); MOS tube Q1D pole respectively connected to positive pole of power supply VCC and MOS tube Q2The D pole of (1); the negative pole of the power VCC is connected to the MOS tube Q3S pole and MOS transistor Q4The S pole of (1); resistance R4The other end of the first resistor is connected to one end of an inductor L and an MOS tube Q5The D pole of (1); the other end of the inductor L is connected to the resistor RLOne terminal of (1), resistance RLAre respectively connected to the resistor R3And MOS transistor Q6The D pole of (1); resistance R3Are respectively connected to the MOS transistor Q2S pole and MOS tube Q6S pole and MOS transistor Q4The D pole of (1); MOS tube Q1～Q6The S pole and the D pole are connected with a diode, the S pole is connected with the anode of the diode, and the D pole is connected with the cathode of the diode.
3. A transient process based electromagnetic surveying method as claimed in claim 2, characterized by: the expression of the current i (t) in the transient electromagnetic quick turn-off circuit is shown as the formula (4):
Figure FDA0003030201830000041
in the above formula, I0For the emission current, the current on the coil is equal to the emission current I when t is 00(ii) a U is the supply voltage and Q2、Q3、Q5The sum of the conduction voltage drops of the corresponding three diodes.
4. The electromagnetic detection method based on transient process as claimed in claim 1, characterized in that: in step S104, if the loop of the transmitting coil is a square loop with a side length of L, the loop is converted into an equivalent radius by area equality:
Figure FDA0003030201830000042
5. the electromagnetic detection method based on transient process as claimed in claim 1, characterized in that: in step S106, the above formula (6) includes a first-order bezier infinite integral, and a first-order bezier function is solved by using a hankerr transformation to obtain an induced electromotive force in the transient process, as shown in the following formula (12):
Figure FDA0003030201830000043
in the above formula, S is the effective area of the receiving coil, μ0Is the permeability of air, HiAre hankel filter coefficients.
6. A computer-readable storage medium characterized by: the computer readable storage medium stores instructions and data for implementing any of the transient process based electromagnetic detection methods of claims 1-5.
7. An electromagnetic detection device based on transient processes, characterized by: the method comprises the following steps: a processor and a storage device;
the processor loads and executes instructions and data in the storage device to realize the electromagnetic detection method based on the transient process as claimed in any one of claims 1 to 5.
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