CN111694060A - FOOTPRINT technology-based multi-channel transient electromagnetic inversion method and transient electromagnetic surveying device - Google Patents
FOOTPRINT technology-based multi-channel transient electromagnetic inversion method and transient electromagnetic surveying device Download PDFInfo
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Abstract
The invention discloses a transient electromagnetic inversion method and a transient electromagnetic surveying device based on a footprint technology, which can greatly reduce a three-dimensional inversion space corresponding to a single data point and improve the three-dimensional inversion efficiency of transient electromagnetic signals, and comprises the following steps: s1, transmitting a pseudorandom coding current; s2, receiving a pseudo-random response containing geological information; s3, acquiring earth impulse response; s4, loading parameters of an inversion area and a mesh generation model; s5, constructing a minimum parameter functional; s6, constructing a footprint weighting matrix; s7, obtaining the distribution condition of the earth electric structure in the inversion region. The device comprises: transmitting probe, receiver, work platform.
Description
Technical Field
The invention relates to the field of oil and gas resources and geophysical, in particular to a FOOTprint technology-based multi-channel transient electromagnetic inversion method and a transient electromagnetic surveying device.
Background
A Multi-channel transient electromagnetic method (MTEM) is a new technology developed internationally since 2002, mineral products and oil and gas resources in China are in short supply, and iron, copper, aluminum, potassium salts and the like have high dependence on imported products, so that sustainable development of national economy is restricted; aiming at the current situation that the mineral resource reserves in China are seriously insufficient, the 'attack depth blind detection' becomes a main battlefield of a new round of resource exploration.
However, with the increase of the buried depth of the target body, the detection environment is more complex, the detection difficulty is also increased rapidly, the worldwide problem of geophysical exploration is solved by considering both large-depth and high-precision detection, and the conventional exploration equipment cannot meet the requirements of large depth and high precision due to the restriction of factors such as complex landform and high exploration cost; the traditional electromagnetic method is generally relatively sensitive to the detection of low-resistance bodies and relatively poor in the recognition capability of high-resistance bodies; one outstanding problem faced by traditional electromagnetic prospecting is that strong human noise makes the signal-to-noise ratio of the collected data difficult to guarantee, when the interference signal is not very strong, the existing instrument and method can be used to obtain good effect, and it is difficult to obtain electromagnetic signal with high signal-to-noise ratio in the strong interference area; in addition, when the size of a target body is small and the buried depth is large, the traditional electromagnetic method is greatly influenced in detection depth and precision, and resource exploration of deep mineral products is directly limited.
With the remarkable advantages of the multi-channel transient electromagnetic method on the offshore oil and gas storage exploration, the method obtains wide attention of domestic scholars, and compared with the traditional electromagnetic method, the method has the characteristics of high resolution ratio on a high-resistance body, high noise-resistant level and the like. In the aspect of multi-channel transient electromagnetic data processing and interpretation, two-dimensional inversion of one-dimensional inversion and a last-order-wave late-stage approximate direct current method is still focused, a truly effective multi-channel transient electromagnetic three-dimensional inversion method is lacked, the existing inversion method is difficult to meet the requirements of actual three-dimensional space geological exploration, and multi-channel transient electromagnetic response characteristics and inversion interpretation aiming at complex three-dimensional geological bodies are urgently needed to be developed.
The model parameters and data scale of three-dimensional electromagnetic inversion are very large, and the required calculation amount and storage requirement are often difficult to bear by a personal computer; the problem is particularly prominent when a multi-channel transient electromagnetic method is used for three-dimensional inversion; one is that because a multi-channel transient electromagnetic method often has hundreds of emission sources and receivers in a work area, the reciprocity theorem used in the traditional MCSEM can effectively minimize the number of sources to achieve the purpose of fast calculation, but the number of the emission sources and the number of the receivers in the multi-channel transient electromagnetic method are basically equivalent, so the reciprocity theorem cannot be used to improve the calculation efficiency, the sensitivity matrix is too large, and the storage and calculation pressure is very large; secondly, because the transient electromagnetic method uses many more frequency points than the frequency domain electromagnetic method, the calculated amount is multiplied.
Therefore, the method reduces the time domain sensitivity matrix calculation pressure by using the footprint technology, and enables multi-channel transient electromagnetic three-dimensional inversion to be possible.
Disclosure of Invention
The invention aims to solve the problems in three-dimensional inversion of a multi-channel transient electromagnetic method in the prior art, and discloses a multi-channel transient electromagnetic inversion method and a transient electromagnetic surveying device based on a footprint technology.
In order to realize the purpose of the invention, the technical scheme adopted by the invention is as follows:
a multi-channel transient electromagnetic inversion method based on a footprint technology is disclosed, which comprises the following steps:
s1, transmitting a pseudorandom coding current to the underground;
s2, receiving a pseudo-random response containing geological information through a plurality of receivers;
s3, removing system responses of the transmitter and the receiver from the received signal to obtain an earth impulse response;
s4, loading parameters of an inversion area and a mesh generation model;
s5, constructing a minimum parameter functional;
s6, constructing a footprint weighting matrix;
and S7, solving the minimum functional to obtain the distribution condition of the earth electric structure in the inversion region.
Preferably, the following steps: step S1 specifically includes: a pseudo-randomly coded current is launched into the ground through an electrical source and using a grounded conductor.
Preferably, the following steps: step S2 specifically includes: a pseudo-random response containing geological information is measured and received in an axial direction using a plurality of receivers on an axial device.
Preferably, the following steps: step S3 specifically includes: removing system responses of a transmitter and a receiver from a received signal through deconvolution to obtain an earth impulse response; the method specifically comprises the following steps:
the receiving end measurement signal is convolution of the transmitting current, the earth impulse response and a receiving end instrument:
ak(xC,xD；xA,xB,t)＝ik(xA,xB,t)*g(xC,xD；xA,xB,t)*rCD(t)+nk(xC,xD,t)；
wherein ik(xA,xBT) is the change in current, g (x)C,xD；xA,xBT) is the earth impulse response, rCD(t) is the instrument response.
Extracting earth impulse response g (x) from receiving end signal by deconvolution algorithmC,xD；xA,xB,t)。
Step S4 specifically includes: and loading parameters of an inversion region and a mesh generation model.
Step S5 specifically includes: and constructing an unconstrained target functional and a selected stabilizer.
(1) Constructing an unconstrained target functional to obtain a regularization function:
Pα(m)＝F(m)+αs(m)→min
where α is the regularization factor, s (m) is the stabilizer, F (m) is the error functional:
wherein d is input data, A (m) is model forward response, CdFor the weighting matrix, the weights of different offset data at different times in the inversion can be balanced.
(2) The stabilizer s (m) is chosen as the MVS vertical minimum support function:
where e is the focusing parameter and S is the horizontal cross section of the rectangular field V.
Preferably, the following steps: step S6 specifically includes: according to the three-dimensional spatial distribution characteristics of the sensitivity matrix, constructing a football weighting matrix, which specifically comprises the following steps: each data point is only sensitive to a certain range of space around the data point, the sensitivity of the emitting source and the receiver is larger as the data point is closer to the emitting source, the ratio of a core sensitivity area to the whole inversion area is small, and the core sensitivity area is determined and a footprint weighting matrix is constructed by analyzing the distribution characteristics of sensitivity matrixes with different offset distances and different moments in a three-dimensional space.
Preferably, the following steps: step S7 specifically includes: and solving the minimum functional by using an OCCAM (optical coherence tomography), quick relaxation and conjugate gradient nonlinear inversion algorithm to obtain the distribution of the underground electrical structure.
Preferably, the following steps: before step S1, the pseudo-random encoding parameters of code element width and sequence length are designed according to exploration depth and geological data.
Furthermore, the invention also discloses a transient electromagnetic surveying device, which is applied to a multichannel transient electromagnetic inversion method based on the footprint technology, and comprises the following steps:
the emission source is annular and is communicated with a pseudorandom sequence or a step wave at two ends;
and the receivers are at least two and are arranged along the axial direction of the emission source, and the system response of the transmitter and the receivers is removed from the received signal to obtain the pulse signal.
And the working platform is connected to an upper computer and a power supply on the mobile carrier through cables.
The work platform includes: the lifting module is arranged at the top of the frame body and is connected with the cylindrical connecting part, the steering module is arranged in the middle of the frame body and is connected with the cylindrical connecting part, and the front end of the cylindrical connecting part is provided with an illuminating element and a camera.
The transmitting probe is arranged at the front end of the working platform and connected with the frame body, and the receiver is arranged in the frame body and connected with the bottom of the cylindrical connecting part.
Preferably, the following steps: the transient electromagnetic surveying device extracts earth impulse response through deconvolution of observation signals transmitted by a transmitting probe and receiver response, eliminates noise through multiple superposition and pseudo-random code characteristics, and improves signal-to-noise ratio.
Preferably, the following steps: lifting module comprises the propeller that 4 levels set up, and these 4 propellers divide into two sets ofly and the symmetry sets up in the both sides of tube-shape adapting unit, and every propeller of a set of all distributes around the tube-shape adapting unit axial, turn to the module and constitute by the propeller of 2 vertical settings side by side.
Preferably, the following steps: the propellers are all rotated forwards and backwards by the helical blades at one end of the driving shaft driven by the motor, and the steering module realizes steering and forward and backward rotation in the same direction by the parallel propellers in different directions.
Preferably, the following steps: the built-in waterproof bearing of propeller, waterproof bearing includes: the sealing side end cover comprises an outer shaft, an inner shaft, balls, a limiting part and a sealing side end cover, wherein a threaded hole matched with a driving shaft to be screwed is formed in the center of the inner shaft, sliding grooves matched with the balls are formed in the inner side face of the outer shaft and the outer side face of the inner shaft, the limiting part is fixedly connected with the outer surface of the inner shaft, the balls are arranged in the limiting part and can freely rotate, an axial reserved groove is formed in the end side face of the outer shaft, a smooth plane is formed at the bottom of the groove, a first clamping groove is formed in the outer surface of the inner shaft, which is close to the edge of the end side, a first clamping block matched with the first clamping groove is arranged.
Preferably, the following steps: and a second clamping groove is formed in the periphery of the bottom of the reserved groove of the outer shaft, a second clamping block matched with the second clamping groove is arranged on the surface of the sealing side end cover, which is in contact with the smooth plane of the bottom of the groove, and the second clamping groove is vertical to the opening direction of the first clamping groove.
Preferably, the following steps: the sealing side end cover is in an annular sheet shape with an internal opening, and the radial width of the sealing side end cover is matched with the distance between the outer shaft reserved groove and the outer surface of the inner shaft.
Preferably, the following steps: the side face of the frame body is provided with a hollowed-out handrail and a mounting hole, and the transmitting probe is connected with the frame body through a telescopic rod.
The invention has the beneficial effects that:
the invention utilizes multi-channel transient electromagnetic surveying, has stronger anti-interference capability and higher detection precision, and has good application prospect; the multi-channel transient electromagnetic method also has extremely high resolution capability on high resistance abnormity, and has advantages in the aspect of expanding the application of the electromagnetic method in the oil gas field.
Different from the traditional surveying method, the method is characterized in that Pseudo-Random current (PRBS) is emitted, measuring devices are arranged in the axial direction of an emission source, the current of an emitting end and the voltage of a receiving end are measured along with time, earth impulse response is obtained through deconvolution operation in a mode that an emitting system and a receiving system move on a section at the same time, the offset distance is 2-4 times of the surveying depth generally, and large-depth and high-precision surveying can be achieved.
The method introduces the footprint technology, establishes the sensitivity range linear relation related to the sensitivity and the offset distance, solves the problem of difficult calculation caused by overlarge sensitivity matrix in three-dimensional inversion, constructs the weighting matrix to weight the sensitivity matrix, and converts the dense matrix into the sparse matrix, thereby effectively reducing the memory pressure of the sensitivity matrix and improving the calculation efficiency.
Drawings
FIG. 1 is a working schematic diagram of a multi-channel transient electromagnetic method;
FIG. 2 is a schematic structural diagram of a detecting device;
FIG. 3 is a schematic view of the positions of the lifting module and the steering module;
FIG. 4 is a sectional view showing the internal structure of the waterproof bearing;
FIG. 5 is an enlarged view of the internal structure at A in FIG. 4;
FIG. 6 is a diagram of the working principle of a multi-channel transient electromagnetic method;
FIG. 7 is a process diagram for obtaining the earth impulse response after adding strong interference;
FIG. 8 is a comparison graph of the detection capability of a multi-channel transient electromagnetic method, a CSAMT method and an MT method on K-type strata (apparent resistivity);
FIG. 9 is a schematic diagram of a transient electromagnetic printing technique;
FIG. 10 is a FOOTPRINT field at different times offset by 1000 m;
FIG. 11 is a FOOTPRINT field at different times offset by 2000 m;
FIG. 12 shows the fotopprint field at different times offset by 3000 m;
FIG. 13 is a distribution diagram of multi-channel transient electromagnetic emission sources and recording points;
FIG. 14 is a three-dimensional spatial subdivision of the salt dome model;
FIG. 15 is a comparison of the salt dome model and the clipped region of the inversion results.
Detailed Description
The invention will be described in more detail below with reference to the following figures and examples:
example 1, a transient electromagnetic survey apparatus, fig. 2-5.
As shown in fig. 2, the method specifically includes:
the transmitting probe 1 is annular and is provided with a pseudo-random sequence or a step wave at two ends.
And the receivers 2 are at least two and are arranged along the axial direction of the emission source, and the system response of the transmitter and the receivers is removed from the received signal to obtain the pulse signal.
And the working platform 3 is connected to an upper computer and a power supply on the mobile carrier through cables 4.
Specifically, the work platform 3 includes: the device comprises a frame body 3-1, a lifting module 3-2, a steering module 3-3 and a cylindrical connecting part 3-4, wherein as shown in figures 2 and 3, the cylindrical connecting part 3-4 is arranged in the center of the frame body 3-1 and is fixedly connected with the frame body, the lifting module 3-2 is arranged at the top of the frame body and is connected with the cylindrical connecting part 3-4, the steering module 3-3 is arranged in the middle of the frame body 3-1 and is connected with the cylindrical connecting part 3-4, and the front end of the cylindrical connecting part 3-4 is provided with an illuminating element 3-5 and a camera.
The transient electromagnetic surveying device extracts earth impulse response through deconvolution of observation signals transmitted by the transmitting probe and the response of the receiver, eliminates noise through multiple superposition and pseudo-random code characteristics, and improves signal-to-noise ratio.
Lifting module 3-2 comprises the propeller of 4 level settings, and these 4 propellers divide into two sets ofly and the symmetry sets up in the both sides of tube-shape adapting unit 3-4, and every propeller of a set of all distributes around the tube-shape adapting unit axial, turn to module 3-3 and constitute by 2 vertical propellers that set up side by side: the propellers are driven by a motor to realize forward and reverse rotation by helical blades at one end of a driving shaft, wherein the steering module realizes steering through the forward and reverse rotation of the propellers side by side in different directions and realizes forward or backward rotation in the same direction, as shown in figure 3.
Further, the propeller is provided with a waterproof bearing 4 inside, as shown in fig. 4, the waterproof bearing 4 includes: the sealing device comprises an outer shaft 4-1, an inner shaft 4-2, balls 4-3, a limiting part 4-4 and a sealing side end cover 4-5, wherein a threaded hole 4-6 matched and screwed with a driving shaft is formed in the center of the inner shaft 4-2, sliding grooves matched with the balls 4-3 are formed in the inner side face of the outer shaft 4-1 and the outer side face of the inner shaft 4-2, the limiting part 4-4 is fixedly connected with the outer surface of the inner shaft 4-2, the balls 4-3 are arranged in the limiting part 4-4 and can freely rotate, an axial reserved groove 4-7 is formed in the end side face of the outer shaft 4-1, a smooth plane is formed in the groove bottom, and a first clamping groove 4-8 is formed in the outer surface of the inner shaft 4-2 close to.
Further, as shown in fig. 4-5, one end of the sealing side end cap 4-5 is provided with a first clamping block 4-9 matched with the first clamping groove 4-8, the inner side surface of the other end is contacted with the smooth plane of the groove bottom, a second clamping groove 4-10 is arranged around the groove bottom of the reserved groove 4-7 of the outer shaft 4-1, a second clamping block 4-11 matched with the second clamping groove 4-10 is arranged on the surface of the sealing side end cap 4-5 contacted with the smooth plane of the groove bottom, the opening direction of the second clamping groove 4-10 and the first clamping groove 4-8 is vertical, the sealing side end cap 4-5 is an annular sheet with an internal opening, the radial width of the sealing side end cap is matched with the distance between the outer shaft reserved groove 4-7 and the outer surface of the inner shaft 4-2, the side surface of the frame body 3-1 is provided with a hollowed handrail and, the emission probe 1 and the frame body 3-1 are connected together through a telescopic rod 5.
In conclusion, the transient electromagnetic surveying device provided by the invention utilizes multi-channel transient electromagnetic surveying, has stronger anti-interference capability and higher detection precision, and has good application prospect; the multi-channel transient electromagnetic method also has extremely high resolution capability on high resistance abnormity, and has advantages in the aspect of expanding the application of the electromagnetic method in the oil gas field.
Structurally surveying the device, survey the environment in order to adapt to the great degree of depth more, the special work platform of structure has been designed, the probe of front end, illumination and camera are used for satisfying the detection of high accuracy, rationally distributed lifting module and turn to the module, utilize lifting module to realize work platform's rising and decline, the utilization turns to the module and has the effect that turns to and go forward and retreat concurrently, make work platform need not the manual work and carry and sneak into and survey, huge work load has been reduced, carry out redesign to the waterproof bearing of propeller, make its water-proof effects outstanding, be applicable to some adverse circumstances.
Example 2, a multichannel transient electromagnetic inversion method based on the footprint technology, see fig. 1, 6-15.
The FOOTprint technology-based multi-channel transient electromagnetic inversion method disclosed by the embodiment of the invention is realized by the transient electromagnetic surveying device disclosed by the embodiment 1.
Specifically, the working principle of the multi-channel transient electromagnetic method and the process of extracting the earth impulse response are explained in detail through fig. 1 and 6 to 8, in order to overcome the disadvantages of the original surveying mode, the invention adopts the multi-channel transient electromagnetic method to survey, and the specific working mode is shown in fig. 1 and 6.
The AB end is communicated with a pseudorandom sequence or step wave, observation is carried out on an axial device (the prior art), a receiving end is provided with a plurality of receivers during measurement, the change of current or voltage of a transmitting end along with time and the change of voltage of the receiving end along with time are measured simultaneously, a mode that a transmitting system and a receiving system move on a section simultaneously is adopted, the working mode is similar to a 2D seismic reflection method, and one measuring point has a plurality of pieces of data information under transmitting and receiving distances; removing system responses of a transmitter and a receiver from a received signal through deconvolution operation to obtain an earth pulse signal; the receiving and transmitting distance r and the target body depth d should satisfy the following relation:
r is more than or equal to 2d and less than or equal to 4d formula (1)
The specific value of the transmitting-receiving distance r is related to the resistivity above a target layer, and some values even exceed 4 d; the signals collected by the receiving end need to take into account the response of the transmitting end instrument, the response of the receiving end instrument, the earth impulse response and the noise interference, and the signals collected by the receiving end can be expressed as follows:
y(t)＝i(t)*hs(t)*hr(t) g (t) + n (t) formula (2)
Wherein, denotes convolution, i (t) is known as pseudo-random current, hs(t) is the response of the transmitting end system, which is the impulse response generated by the equivalent circuit of the circuit in the transmitter, the power supply electrode of the transmitting end A, B, the long wire grounded and the like, and h is the unknown impedance (including resistance, inductance and capacitance) of the equivalent circuits(t) is unknown, hr(t) is the impulse response generated by the equivalent circuit formed by the receiving end instrument circuit and the receiving electrode MN, and is unknown, g (t) is the unknown earth impulse response, and n (t) is noise.
In order to obtain a transmitting end system response hs(t), need measure voltage variation (the distance unit is centimetre level, also can measure current variation) in the place very close to the transmitting terminal, the instrument response of transmitting terminal measured voltage must be unanimous with the instrument response of receiving terminal measured voltage, owing to be very close to the transmitting terminal, so only the impulse response that the receiving terminal return circuit produced:
vs(t)＝i(t)*hs(t)*hr(t) formula (3)
In the formula, hs(t) is the transmitting end instrument response.
The multi-channel transient electromagnetic method extracts earth impulse response through deconvolution of emission observation signals and receiving end system response, eliminates noise through multiple superposition and pseudo-random code characteristics, and improves signal-to-noise ratio.
The difference between the multi-channel transient electromagnetic method and other electromagnetic methods is summarized, and the multi-channel transient electromagnetic method mainly has the following outstanding characteristics:
because the pseudo-random sequence is adopted to transmit signals to the underground and the earth pulse signals are obtained by deconvoluting data of the transmitting end and the receiving end, the problem that the traditional transient electromagnetic method cannot extract the early field can be solved, and the resolution capability of the shallow layer is greatly improved.
The autocorrelation function of the pseudo-random sequence resembles white noise, which is statistically uncorrelated with any signal, which greatly improves the noise immunity of the method.
Observed with axial means and measuring only ExCompared with the traditional electromagnetic method, the method greatly improves the detection of the high-resistance body capability, so that the method is more suitable for the detection of oil and gas resources.
The method adopts pseudo-random current coding emission, array type multichannel receiving and earthquake-like three-dimensional (3D) imaging processing technologies, has the characteristics of high detection precision, deep depth and strong anti-interference capability, and is successful in detecting the high-resistance form of oil gas and distinguishing an oil gas-water interface.
The invention has made comparison research on the resolving power of a multi-channel transient electromagnetic method and the visual resistivity contrast curve of a CSAMT and MT method, and the resolving power of the multi-channel transient electromagnetic method and the CSAMT and MT method on a middle high-resistance body (K-type stratum) is shown in figure 8, for example, the E excited by pulse wave in the multi-channel transient electromagnetic method in the model is shown in figure 8xE, with strongest detection capability of field to middle high-resistance layer and excited by step wavexField-by-field, pulse-wave excited HzField-second, step-wave excited HzThe field and CSAMT, MT methods have the worst detection capability to the middle high-resistance layer, and the multichannel transient electromagnetic method can find boundary and dynamic change easily through analysisThe reaction is better.
(III), specifically, the multichannel transient electromagnetic inversion method based on the footprint technology is explained with reference to FIGS. 9 to 12, and specifically includes:
the loading of model parameters such as inversion areas, mesh generation and the like comprises the following steps:
determining the core area of three-dimensional inversion, the subdivision size and the subdivision quantity of the grid in the directions of x, y and z according to the data acquisition range, the detection depth and the like, initializing the grid, and setting the maximum iteration times and the minimum error.
Constructing a minimum parameter functional comprises:
constructing a minimum parameter functional comprises: constructing an unconstrained target functional to obtain a regularization function:
Pα(m)＝F(m)+αs(m)→min
where α is the regularization factor, s (m) is the stabilizer, F (m) is the error functional:
wherein d is input data, A (m) is model forward response, CdThe weight of different offset data at different moments in inversion can be balanced as a weighting matrix; the stabilizer s (m) is chosen as the MVS vertical minimum support function:
where e is the focusing parameter and S is the horizontal cross section of the rectangular field V.
According to the three-dimensional spatial distribution characteristics of the sensitivity matrix, constructing a football weighting matrix comprises the following steps:
the fixed working device is sensitive to only a limited area, which is called a footprint, i.e. the sensitivity domain, which contains most of the overall sensitivity of the fixed working device; as shown in fig. 9, the influence range of the towed electromagnetic system is significantly smaller than that of the whole measurement region, so that the sensitivity of the whole inversion region is formed by overlapping the small-range sensitivity matrices formed by each transceiver, and the storage and calculation requirements of the sensitivity matrices can be reduced by several orders of magnitude, so that the large-scale three-dimensional electromagnetic inversion becomes an easily-handled problem.
The multichannel transient electromagnetism is transmitted by using an electrical source, and the sensitivity distribution of the array receiving Ex field response is very complex; in order to optimally apply the footprint technology, the calculation efficiency and the inversion accuracy are considered, and the sensitivity space distribution of different offset distances needs to be analyzed. The method for calculating the overall sensitivity is shown as follows:
sE(rj|r′)＝GE(rj|r′)·E(r′)
GE(rjr ') represents the electric field generated by a unit electric dipole source at the conductivity perturbation point r'.
FIGS. 10-12 are the fotopprint fields at offsets 1000m, 2000m, 3000m, respectively, each offset in turn calculating the fotopprint fields at times 0.001s, 0.01s, and 0.1s, respectively; the graph analysis shows that the variation range of the sensitive areas with the same offset distance and different moments is not large, so that the variation range of the sensitive areas with the same offset distance and different moments is only discussed, the variation of the footprint domains with different moments is not discussed, and the core sensitive areas with the same offset distance and the later period are selected as the footprint domains; the linear relation between the side length of the footprint domain and the offset distance approximation is determined, a linear function is synthesized by using a least square method, and the length of the footprint domain in the inline direction is 1.75 xL +566.67m, the length of the crossline direction is L +533.33m, and the length of the depth direction is 0.5 xL +266.67 m; and constructing a sensitivity weight matrix, wherein the weight coefficient is 1 within the set detection range, and the weight coefficient is 0 outside the set detection range, so that the dense sensitivity matrix becomes a sparse matrix, and the memory consumption and the calculation amount are reduced by more than one order of magnitude.
Solving the minimum functional, and acquiring the distribution condition of the geoelectric structure in the inversion region comprises the following steps: and solving the minimum functional by utilizing non-linear inversion algorithms such as OCCAM, NLCG, Quasi-Newton, RRI and the like, so as to obtain the distribution of the underground electrical structure.
Examples of specific applications
The specific application example of the multichannel transient electromagnetic inversion method based on the footprint technology is as follows:
in this embodiment, the multi-channel transient electromagnetic data is three-dimensionally inverted by the described multi-channel transient electromagnetic inversion algorithm based on the footprint technology.
Setting a three-dimensional model as shown in FIG. 13, a salt dome exists in the uniform half space, the size of the salt dome area is 1850m < x <6350m, 2350m < y <7250m, 250m < z <2350m, the resistivity of the salt dome is set to be 1000 Ω -m, and the background model is 100 Ω -m; adopting 14 exploration lines extending along the X direction, simulating a land multi-channel transient electromagnetic method measuring mode, wherein the distance between measuring lines is 500m, and the measuring lines are uniformly distributed on Y [1500:500:8000] m; each measurement line is measured by using a dipole-dipole device, the point distance of the receiving points is 500m, the receiving points are uniformly distributed on an X-axis direction X [1000:500:8000] m, the point distance of the emission source is 1000m, the emitting sources and the recording points are uniformly distributed on the X [ -3000:1000:12000] m, and the distribution of the emission sources and the recording points is shown in FIG. 14.
The inversion region ranges from 1000m to 7000m in the X direction, 1900m to 7900m in the Y direction, the sizes of the unit cells are 300m in the X, Y direction, the thickness of the front 5 layers is 50m from 100m to 2557.235m in the z direction, the increment coefficient is 1.1 after the thickness is increased to 229.75m, and a bottom layer with the thickness of 230m is added for 22 layers.
Constructing an unconstrained target functional to obtain a regularization function:
Pα(m)＝F(m)+αs(m)→min
with the scheme in the specific implementation, for each offset, a football field is constructed, whose length in the inline direction is: 1.75 XL +566.67m, crossline directional length: l +533.33m, depth direction length: 0.5 xl +266.67m in length; the weight coefficient is 1 in the set detection range, and the weight coefficient is 0 outside the set detection range, so that the dense sensitivity matrix is changed into a sparse matrix.
In order to solve the minimum parameter functional, a conjugate gradient method is used for solving, which is consistent with the idea of the steepest descent method, and only the step search direction is changed:
wherein the model update amount
Fig. 15 is a comparison graph of an inversion result and a real model by using the method of the present invention, and it can be seen from a three-dimensional cutting graph that a multi-channel transient electromagnetic method has a good high-resistance model boundary delineation, has a high degree of identification of an interface on the model in the vertical direction, and cannot be obviously inverted for a high-resistance part with a large depth, mainly because the electromagnetic method is not sensitive to a high-resistance body, and the field intensity of a secondary field excited by a deep high-resistance body is small, so that the secondary field is difficult to be identified.
In the aspect of inversion time consumption, 60 hours are required for one time of global inversion iteration, and 12 hours are required for one time of Footprint inversion iteration, which proves that the Footprint domain technology provided by the invention can greatly increase inversion efficiency and reduce memory occupation.
In conclusion, the multichannel transient electromagnetic inversion method based on the footprint technology disclosed by the invention utilizes multichannel transient electromagnetic surveying, so that the multichannel transient electromagnetic inversion method has stronger anti-interference capability, higher detection precision and good application prospect; the multi-channel transient electromagnetic method also has extremely high resolution capability on high resistance abnormity, and has advantages in the aspect of expanding the application of the electromagnetic method in the oil gas field.
Different from the traditional surveying method, the method is characterized in that Pseudo-Random current (PRBS) is emitted, measuring devices are arranged in the axial direction of an emission source, the current of an emitting end and the voltage of a receiving end are measured along with time, earth impulse response is obtained through deconvolution operation in a mode that an emitting system and a receiving system move on a section simultaneously, the offset distance generally takes 2-4 times of exploration depth, and large-depth and high-precision surveying can be achieved.
In the multichannel transient electromagnetic method three-dimensional inversion, a footprint technology is introduced, a sensitivity range linear relation related to sensitivity and offset is established, a dense matrix is converted into a sparse matrix, the memory pressure of the sensitivity matrix is effectively reduced, the calculation efficiency is improved, and the problem of difficult calculation caused by overlarge sensitivity matrix in the three-dimensional inversion is solved.
The embodiments of the present invention are disclosed as the preferred embodiments, but not limited thereto, and those skilled in the art can easily understand the spirit of the present invention and make various extensions and changes without departing from the spirit of the present invention.
Claims (10)
1. A multi-channel transient electromagnetic inversion method based on a footprint technology is characterized by comprising the following steps: the method comprises the following steps:
s1, transmitting a pseudorandom coding current to the underground;
s2, receiving a pseudo-random response containing geological information through a plurality of receivers;
s3, removing system responses of the transmitter and the receiver from the received signal to obtain an earth impulse response;
s4, loading parameters of an inversion area and a mesh generation model;
s5, constructing a minimum parameter functional;
s6, constructing a footprint weighting matrix;
and S7, solving the minimum functional to obtain the distribution condition of the earth electric structure in the inversion region.
2. The method for multichannel transient electromagnetic inversion based on the footprint technology of claim 1, wherein: step S1 specifically includes: a pseudo-randomly coded current is launched into the ground using a grounded conductor.
3. The method for multichannel transient electromagnetic inversion based on the footprint technology of claim 1, wherein: step S2 specifically includes: a pseudo-random response containing geological information is measured and received in an axial direction using a plurality of receivers on an axial device.
4. The method for multichannel transient electromagnetic inversion based on the footprint technology of claim 1, wherein: step S3 specifically includes: removing system responses of a transmitter and a receiver from a received signal through deconvolution to obtain an earth impulse response; the method specifically comprises the following steps:
the receiving end measurement signal is convolution of the transmitting current, the earth impulse response and a receiving end instrument:
ak(xC,xD；xA,xB,t)＝ik(xA,xB,t)*g(xC,xD；xA,xB,t)*rCD(t)+nk(xC,xD,t)；
wherein ik(xA,xBT) is the change in current, g (x)C,xD；xA,xBT) is the earth impulse response, rCD(t) is the instrument response;
extracting earth impulse response g (x) from receiving end signal by deconvolution algorithmC,xD；xA,xB,t)。
5. The method for multichannel transient electromagnetic inversion based on the footprint technology of claim 1, wherein: step S4 specifically includes: and loading parameters of an inversion region and a mesh generation model.
6. The method for multichannel transient electromagnetic inversion based on the footprint technology of claim 1, wherein: step S5 specifically includes: constructing an unconstrained target functional and a selected stabilizer;
(1) constructing an unconstrained target functional to obtain a regularization function:
Pα(m)＝F(m)+αs(m)→min
where α is the regularization factor, s (m) is the stabilizer, F (m) is the error functional:
wherein d is input data, A (m) is model forward response, CdThe weight of different offset data at different moments in inversion can be balanced as a weighting matrix;
(2) the stabilizer s (m) is chosen as the MVS vertical minimum support function:
where e is the focusing parameter and S is the horizontal cross section of the rectangular field V.
7. The method for multichannel transient electromagnetic inversion based on the footprint technology of claim 1, wherein: step S6 specifically includes:
according to the three-dimensional spatial distribution characteristics of the sensitivity matrix, constructing a football weighting matrix, which specifically comprises the following steps: each data point is only sensitive to a certain range of space around the data point, the sensitivity of the emitting source and the receiver is larger as the data point is closer to the emitting source, the ratio of a core sensitivity area to the whole inversion area is small, and the core sensitivity area is determined and a footprint weighting matrix is constructed by analyzing the distribution characteristics of sensitivity matrixes with different offset distances and different moments in a three-dimensional space.
8. The method for multichannel transient electromagnetic inversion based on the footprint technology of claim 1, wherein: step S7 specifically includes: and solving the minimum functional by using an OCCAM (optical coherence tomography), quick relaxation and conjugate gradient nonlinear inversion algorithm to obtain the distribution of the underground electrical structure.
9. A transient electromagnetic survey apparatus for use in a method for multichannel transient electromagnetic inversion based on a footprint technique as claimed in any one of claims 1 to 8, comprising:
the emission source is annular and is communicated with a pseudorandom sequence or a step wave at two ends;
the number of the receivers is at least two, the receivers are arranged along the axial direction of the emission source, and the system responses of the transmitter and the receivers are removed from the received signals to obtain pulse signals;
the working platform is connected to an upper computer and a power supply on the mobile carrier through cables;
the work platform includes: the lifting module is arranged at the top of the frame body and connected with the cylindrical connecting part, the steering module is arranged in the middle of the frame body and connected with the cylindrical connecting part, and the front end of the cylindrical connecting part is provided with an illuminating element and a camera;
the transmitting probe is arranged at the front end of the working platform and connected with the frame body, and the receiver is arranged in the frame body and connected with the bottom of the cylindrical connecting part.
10. The transient electromagnetic survey device of claim 9, wherein: the transient electromagnetic surveying device extracts earth impulse response through deconvolution of an observation signal transmitted by a transmitting probe and the response of a receiver, eliminates noise through multiple superposition and pseudo-random code characteristics, and improves signal-to-noise ratio;
the lifting module consists of 4 horizontally arranged propellers, the 4 propellers are divided into two groups and symmetrically arranged on two sides of the cylindrical connecting part, the propellers of each group are axially distributed forwards and backwards along the cylindrical connecting part, and the steering module consists of 2 vertically arranged propellers in parallel;
the propellers are driven by a motor to realize forward and reverse rotation by a helical blade at one end of a driving shaft, wherein the steering module realizes steering through the forward and reverse rotation of the propellers side by side in different directions and realizes forward or backward rotation in the same direction;
the built-in waterproof bearing of propeller, waterproof bearing includes: the sealing side end cover comprises an outer shaft, an inner shaft, balls, a limiting part and a sealing side end cover, wherein a threaded hole matched with a driving shaft to be screwed is formed in the center of the inner shaft, sliding grooves matched with the balls are formed in the inner side surface of the outer shaft and the outer side surface of the inner shaft, the limiting part is fixedly connected with the outer surface of the inner shaft, the balls are arranged in the limiting part and can freely rotate, an axial reserved groove is formed in the end side surface of the outer shaft, a smooth plane is formed at the bottom of the groove, a first clamping groove is formed in the outer surface of the inner shaft close to the end side edge, a first clamping block matched with the first clamping groove is arranged at one end of the;
a second clamping groove is formed in the periphery of the bottom of the reserved groove of the outer shaft, a second clamping block matched with the second clamping groove is arranged on the surface, in contact with the smooth plane of the bottom, of the sealing side end cover, and the second clamping groove is perpendicular to the opening direction of the first clamping groove;
the sealing side end cover is in an annular sheet shape with an internal opening, and the radial width of the sealing side end cover is matched with the distance between the outer shaft reserved groove and the outer surface of the inner shaft.
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