CN110376651B - Time-frequency electromagnetic device based on horizontal bipolar current source and geophysical exploration method - Google Patents
Time-frequency electromagnetic device based on horizontal bipolar current source and geophysical exploration method Download PDFInfo
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- G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
- G01V3/08—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices
- G01V3/082—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices operating with fields produced by spontaneous potentials, e.g. electrochemical or produced by telluric currents
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- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
- G01V3/08—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices
- G01V3/087—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices the earth magnetic field being modified by the objects or geological structures
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Abstract
The invention discloses a time-frequency electromagnetic device based on a horizontal bipolar current source, which comprises bipolar current source emission equipment and measurement equipment, wherein the bipolar current source emission equipment comprises eight groups of sub-bipolar current source emission equipment, each group of sub-bipolar current source emission equipment comprises power supply equipment, a positive electrode end point connected with the positive electrode of the power supply equipment and a negative electrode end point connected with the negative electrode of the power supply equipment, the positive electrode end points and the negative electrode end points form a loop with the power supply equipment through grounding, the eight positive electrode end points are arranged close to each other to form a central point, the eight negative electrode end points radiate outwards in a radial direction by the central point formed by the arrangement of the eight positive electrode end points and are arranged in an annular mode, and the eight negative electrode end points and the eight positive electrode end; the signal input end of the measuring device is connected with the signal output end of the bipolar current source transmitting device. The time-frequency electromagnetic device based on the horizontal bipolar current source can transmit signals on the ground to achieve the effect of a vertical source underground, in a well or in water.
Description
Technical Field
The invention relates to the field of geophysical electromagnetic exploration, in particular to a time-frequency electromagnetic device based on a horizontal bipolar current source and a geophysical exploration method.
Background
Geophysical exploration, abbreviated as geophysical exploration, refers to the detection of geological conditions such as formation lithology, geological structure and the like by researching and observing the changes of various geophysical fields. The electromagnetic exploration method is mainly used for searching ores, water, oil and gas, structural exploration, engineering exploration, environmental geological investigation and the like, and the electromagnetic exploration method is developed in the directions of large exploration depth, high precision, multi-component acquisition, multi-parameter interpretation and the like. The application space is expanded to various aspects such as air (aviation transient electromagnetism, semi-aviation transient electromagnetism), well (well-ground, well-to-well, ground well), ocean and the like, and response data such as multi-component electromagnetic fields, induced electromotive force and the like are collected.
For a complex subsurface geologic structure, these new techniques are increasingly useful in exploring complex formation media. However, currently, mainly applied electromagnetic exploration methods, such as a wire return source (Nabighian,1979) and a ground wire source (Kaufman and Keller,1983), mainly use targets (such as deposited metal ore, groundwater, etc.) with a shallow surface layer or a background with poor conductivity and good conductivity as main materials, so that the application range of the electromagnetic exploration method is limited, and it is imperative to improve the resolution capability of the electromagnetic method for detecting other underground targets with general conductivity.
In the existing literature, a time-frequency electromagnetic (TFEM) method (which develops and soaks, 2013) is that eastern geophysical companies integrate a plurality of methods on the basis of long offset transient electromagnetic (LoTEM), Induced Polarization (IP), frequency domain Controllable Source Electromagnetic (CSEM) and other methods, and use a grounded long lead source as an excitation source to excite continuous positive and negative square waves and measure far-zone electromagnetic response in a large offset measurement zone. The current time-frequency electromagnetic method is mainly used for a new method of oil exploration (the working method is shown in figure 1), adopts a working mode similar to large offset seismic exploration, supplies strong current to the earth to excite an oil-gas exploration target, and measures a secondary electromagnetic field and an electromagnetic field frequency spectrum formed by the discharge of a pore medium of an oil-gas reservoir; the technology simultaneously obtains time domain signals and frequency domain signals, and accurately reconstructs an underground physical property model through the combined processing of the time domain signals and the frequency domain signals to obtain the resistivity and polarizability abnormity of the oil-gas exploration target. The time-frequency electromagnetic technology combines frequency domain sounding and time domain sounding in a system, can select excitation waveforms with different frequencies and different types according to the depth of an exploration target, can provide resistivity information and excitation polarization information, and can detect the oil-gas content of the exploration target while researching the electrical structure. The time-frequency electromagnetic method (TFEM) utilizes a large device to carry out depth measurement on targets with different depths by changing the waveform length and the frequency. The time-frequency electromagnetic method defines possible oil and gas reservoir targets according to the apparent resistivity and the apparent polarizability distribution, is suitable for the exploration of the oil and gas reservoir targets with high polarization and high resistivity, and achieves quite abundant results. Since such time-frequency electromagnetism needs to transmit a large excitation signal, the area for acquiring response data is also very large. There are also transient electromagnetic methods using small offsets, such as the short offset transient electromagnetic method (schumann, 2013), which also allow the exploration of underground electrical structures by observing the response signals in a region closer to the source.
Another approach is to place the source in the well and measure the response signal at the surface so that the source is closer to the target and the depth of investigation and resolution may be improved. The electromagnetic method of the well needs an existing well, for example, an oil well can be used, but in a place without the well, drilling is needed, and the depth of the well needs 3km or more according to the depth of a target layer.
However, the grounded long-wire source used in the current time-frequency electromagnetic method is a transmission source mainly emitting TE mode electromagnetic signals, and is not an excitation source with the best longitudinal resolution, so that an excitation source more sensitive to an electrical anomaly target is needed, so that people can utilize the advantage of being easy to construct, namely emitting on the ground and receiving on the ground, and a detection method with the best detection resolution is available. Therefore, if a longitudinal current excitation is used, so that a relatively strong longitudinal anomaly is generated in the target body, the target body is received by a receiver on the ground in the most convenient path, so that the amplitude of the received response is relatively strong, and the detection resolution is improved. The emission sources that can generate such excitation fields are vertical electric dipole sources or vertical bipolar sources (hellwig, 2013), but unfortunately, the underground mineral exploration is low-frequency band excitation signals, and the situation using the vertical electric dipole sources or the vertical bipolar sources can only be in wells (which and how, 2003; wangxikang, 2007) or in water (Goldman, 2015; Haroon,2016), such as interwell electromagnetism, ground electromagnetism, ocean electromagnetism, etc., while the ground method with convenient construction and low cost cannot use the excitation sources of this type. On the other hand, ground time-frequency electromagnetism needs to be collected in a large area, the signal collection efficiency is low, the working cost is high, and the data collection technology needs to be improved. Finally, the traditional time-frequency electromagnetic method measures data outside a certain offset distance, is not an exploration method of whole-course measurement, and cannot fully utilize time-frequency electromagnetic response information.
Disclosure of Invention
In order to solve the problems, the invention provides a time-frequency electromagnetic device based on a horizontal bipolar current source and an exploration method, which can directly arrange a grounding wire source on the ground, can realize the same effect of longitudinal current excitation without placing a vertical current source in underground drilling in advance, and have high resolution capability of electromagnetic signals.
In order to achieve the purpose, the invention adopts the technical scheme that:
a time-frequency electromagnetic device based on a horizontal bipolar current source comprises bipolar current source emission equipment and measurement equipment, wherein the bipolar current source emission equipment comprises eight groups of sub bipolar current source emission equipment, each group of sub bipolar current source emission equipment comprises power supply equipment, a positive electrode end point connected with the positive electrode of the power supply equipment and a negative electrode end point connected with the negative electrode of the power supply equipment, the positive electrode end points and the negative electrode end points form a loop with the power supply equipment through grounding, the eight positive electrode end points are arranged close to each other to form a central point, the eight negative electrode end points are radially radiated outwards by the central point formed by the arrangement of the eight positive electrode end points and are arranged in a ring shape, and the eight negative electrode end points and the eight positive electrode end points are positioned on the; the signal input end of the measuring device is connected with the signal output end of the bipolar current source transmitting device.
The working process is as follows: a horizontal bipolar current source emitting device is first placed on the ground in a loop. Digging 9 electrode pits, eight groups of sub bipolar current source emission devices, wherein each group of sub bipolar current source emission device comprises a power supply device, a positive electrode end point connected with the positive electrode of the power supply device and a negative electrode end point connected with the negative electrode of the power supply device, the positive electrode end point and the negative electrode end point form a loop with the power supply device through grounding, and the eight groups of positive electrode end points are arranged at the position of 0 electrode pit; electrode pits of the other 8 ground sources are radially radiated outwards by taking O as a center and are annularly arranged, eight groups of negative electrode end points are respectively arranged, the positive electrode end points and the negative electrode end points are grounded to form a grounding loop with power supply equipment, and the current is transmitted to the underground to realize underground detection.
Preferably, the included angle between a central point formed by the arrangement of the eight positive electrode end points and a connecting line between any two adjacent negative electrode end points is 45 degrees; when the included angle of a connecting line between the central point of the positive electrode end point and any two adjacent negative electrode end points is 45 degrees, the best effect which can be achieved by setting eight negative electrode end points is obtained.
Preferably, in the eight groups of the sub-bipolar current source emission devices, the distances between the positive electrode terminal point and the negative electrode terminal point are equal; the distances from the positive electrode end point to the eight negative electrode end points are the same, so that signals transmitted by the eight sources can be kept synchronous at any time, finally received signals are better and stable, and the analysis effect and the final exploration effect are better.
Preferably, the measuring device comprises a control device, a data storage device and a display device, wherein a signal input end of the control device is connected with a signal output end of the bipolar current source emission device, and two signal output ends of the control device are respectively connected with a signal input end of the data storage device and a signal input end of the display device; for measuring the signal emitted by the bipolar current source emitting device.
A geophysical exploration method is based on the time-frequency electromagnetic device based on the horizontal bipolar current source and comprises the following arrangement steps:
c1: nine electrode pits which are positioned on the same plane are dug on the ground, and another eight electrode pits are annularly arranged by taking one electrode pit as a center;
c2: setting eight groups of power supply equipment, and eight groups of positive electrode end points connected with the positive electrode of the power supply equipment and negative electrode end points connected with the negative electrode of the power supply equipment respectively;
c3: eight positive electrode end points of the positive electrode of the power supply equipment are installed and connected in the electrode pit in the center, and the negative electrode end points of the negative electrode of the power supply equipment are respectively installed and connected in the other eight electrode pits.
Preferably, the method further comprises a measuring method, wherein the measuring method is a time domain measuring step or a frequency domain measuring step, and the time domain measuring step specifically comprises the following steps:
s1: the host machine controls a transmitting device of the terrestrial space-time frequency electromagnetic device to respectively transmit power-off and non-power-off positive and negative bipolar rectangular pulse current signals;
s2: designing a flight path, a survey line and a measuring point position and an operation flight path of the measuring instrument according to the detection position and the terrain condition;
s3: measuring three-component induced electromotive force Vx, Vy and Vz during the power-off period of the emission pulse at each measuring point position;
s4: performing channel extraction, superposition and filtering processing on the data in the S3 to obtain transient corresponding data;
the frequency domain measuring step includes the steps of:
d1: the host computer controls the transmitting device of the space-time frequency electromagnetic device to contain a plurality of continuous positive and negative square wave signals with different fundamental frequencies;
d2: determining the position of a measuring point and the running route of the measuring instrument according to the detection position and the topographic condition;
d3: measuring three-component magnetic fields Hx, Hy and Hz at the positions of the measuring points;
d4: and (3) after the data in the D3 are subjected to superposition and filtering processing, Fourier transform is carried out, electromagnetic field frequency response under corresponding fundamental frequency is extracted, and frequency domain response data are obtained.
Preferably, in the arranging step, eight positive electrode terminals and eight negative electrode terminals are respectively connected with the positive electrodes and the negative electrodes of eight power supply devices, the distance between a positive electrode terminal connected with the same power supply device and a negative electrode terminal is the same, and the included angle between the positive electrode terminal and any two connecting lines adjacent to the negative electrode terminal is 45 °.
In the above exploration methods, the reference numerals do not represent a sequential order, which the skilled person can vary without departing from the scope of protection of the present invention.
The invention has the beneficial effects that:
(1) the time-frequency electromagnetic device based on the horizontal bipolar current source can transmit signals on the ground to achieve the effect of a vertical source underground, in a well or in water;
(2) the time-frequency electromagnetic device based on the horizontal bipolar current source does not need to be used for large-scale construction operations such as drilling in an exploration field, and the cost is saved;
(3) according to the time-frequency electromagnetic device based on the horizontal bipolar current source, when the included angle of a connecting line between the central point where the positive electrode end point is located and any two adjacent negative electrode end points is 45 degrees, the best effect which can be achieved by setting eight negative electrode end points is obtained;
(4) the distances from the positive electrode end point to the eight negative electrode end points of the time-frequency electromagnetic device based on the horizontal bipolar current source are the same, so that signals transmitted by the eight sources can be kept synchronous at any time, finally received signals are better and stable, and the analysis effect and the final exploration effect are better;
(5) according to the time-frequency electromagnetic device based on the horizontal bipolar current source, when the included angle of a connecting line between the central point where the positive electrode end point is located and any two adjacent negative electrode end points is 45 degrees, the best effect which can be achieved by setting eight negative electrode end points is obtained;
(6) the distances from the positive electrode end point to the eight negative electrode end points of the time-frequency electromagnetic device based on the horizontal bipolar current source are the same, so that signals transmitted by the eight sources can be kept synchronous at any time, finally received signals are better and stable, and the analysis effect and the final exploration effect are better;
(7) the time-frequency electromagnetic device based on the horizontal bipolar current source is applicable to various terrains and has good practicability;
(8) the time-frequency electromagnetic device based on the horizontal bipolar current source has the advantages of remarkable measuring effect, high intelligent degree, and good economical efficiency and practicability;
(9) the time-frequency electromagnetic device detection method based on the horizontal bipolar current source uses the unmanned aerial vehicle receiving land space-time-frequency electromagnetic exploration technology, so that a signal source with high power transmitted on the ground can be ensured, and the efficiency of performing large-area regional exploration work in the air can be improved.
Drawings
FIG. 1 is a diagram of the working method of the present invention for oil exploration;
FIG. 2 is a graph comparing the effect of a time-frequency electromagnetic device based on horizontal bipolar current sources and a vertical bipolar source exploration device in a subterranean well according to an embodiment of the present invention;
fig. 3 is a schematic structural diagram of a time-frequency electromagnetic device ground signal source based on a horizontal bipolar current source according to an embodiment of the present invention;
FIG. 4 is a diagram of transmit pulse waveforms for a time domain method and a frequency domain method for a horizontal bipolar current source based time-frequency electromagnetic device survey method according to an embodiment of the present invention;
FIG. 5 is a graph comparing the response of the surface electric field of a 1ms vertical bipolar source in a well based on a circularly arranged eight-way horizontal bipolar current source according to an embodiment of the present invention;
in FIG. 5, FIG. 5-1-1 is the ground Ex component of the circularly positioned eight-way horizontal dipole source; FIG. 5-1-2 is a surface Ex component of a bipolar source in a well;
FIG. 5-2-1 is a surface Ey component of a circularly-positioned eight-way horizontal bipolar source, and FIG. 5-2-2 is a surface Ey component of a bipolar source in a well;
FIG. 5-3-1 is the Ez component of a subsurface 1cm cross section of a circularly positioned eight-way horizontal bipolar source, and FIG. 5-3-2 is the Ez component of a subsurface 1cm cross section of a bipolar source in a well;
FIG. 6 is a graph comparing the response of the surface electric field based on a ring-mounted eight-way horizontal bipolar current source with a vertical bipolar source in a well for 10ms according to an embodiment of the present invention;
in FIG. 6, FIG. 6-1-1 is the ground Ex component of the circularly-arranged eight-way horizontal bipolar source; FIG. 6-1-2 is a surface Ex component of a bipolar source in a well;
FIG. 6-2-1 is a ground Ey component of a circularly positioned eight-way horizontal bipolar source; FIG. 6-2-2 is a surface Ey component of a dipole source in a well;
FIG. 6-3-1 is an Ez component of a subsurface 1cm cross section of a circularly positioned eight-way horizontal dipole source; 6-3-2 is the Ez component of a subsurface 1cm cross section of a bipolar source in a well;
FIG. 7 is a real (a) and imaginary (b) Ez components at 0.1Hz for a ring-mounted eight-way horizontal dipole source according to an embodiment of the present invention;
FIG. 8 is the real (a) and imaginary (b) parts of the Ez component at 0.1Hz for a vertical bipolar source in a well;
FIG. 9 is a real (a) and imaginary (b) Ez components at 100Hz for a ring-mounted eight-way horizontal dipole source according to an embodiment of the present invention;
FIG. 10 is a graph of real (a) and imaginary (b) components of the Ez component at 100Hz for a vertical dipole source in a well;
FIG. 11 is a plot of the relative residual of the response component versus the background model response for a time delay of 1ms for a flight profile at 10 meters altitude, y 1500 m;
in FIG. 11, FIG. 11-1 is the absolute value/%, and FIG. 11-2 is the absolute value/%, of the relative residual of the Vy component and the Vz component;
FIG. 12 is a plot of the relative residual of the response component versus the background model response for a 10ms delay time for a 10m altitude flight line, y 1500m flight profile;
in FIG. 12, FIG. 12-1 is the absolute value/%, and FIG. 12-2 is the absolute value/%, of the relative residual of the Vy component and the Vz component;
FIG. 13 is a plot of the relative residual response of the magnetic field component at 1Hz, with a flight height of 150m and a course y of 1500 m;
in FIG. 13, FIG. 13-1 is the absolute value/%, of the relative residual of the Hx component, and FIG. 13-2 is the absolute value/%, of the relative residual of the Hz component;
FIG. 14 is a plot of the relative residual of the three-component magnetic field response amplitude at 1000Hz with a flight height of 150m and a course y of 1500 m;
in FIG. 14, FIG. 14-1 is the Hx component, FIG. 14-2 is the Hy component, and FIG. 14-3 is the Hz component;
Detailed Description
Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.
Example 1:
a time-frequency electromagnetic device based on a horizontal bipolar current source comprises bipolar current source emission equipment and measurement equipment, wherein the bipolar current source emission equipment comprises eight groups of sub bipolar current source emission equipment, each group of sub bipolar current source emission equipment comprises power supply equipment, a positive electrode end point connected with the positive electrode of the power supply equipment and a negative electrode end point connected with the negative electrode of the power supply equipment, the positive electrode end points and the negative electrode end points form a loop with the power supply equipment through grounding, the eight positive electrode end points are arranged close to each other to form a central point, the eight negative electrode end points are radially radiated outwards by the central point formed by the arrangement of the eight positive electrode end points and are arranged in a ring shape, and the eight negative electrode end points and the eight positive electrode end points are positioned on the; the signal input end of the measuring device is connected with the signal output end of the bipolar current source transmitting device.
In embodiment 1, with reference to fig. 2, it can be obtained from the right-hand rule analysis that the distribution of the power lines and the magnetic lines of the vertical bipolar source in the underground well is consistent with the distribution characteristics of the power lines and the magnetic lines of the eight radial earth current sources which are annularly arranged; each grounding source synchronously emits continuous positive and negative square wave pulses with power failure and without power failure, transient response signals (or called time domain response signals) and frequency domain response signals are respectively measured on measuring points of a flight plane, and the underground electrical structure can be inverted according to the collected response signals. Thus, the construction can be carried out on land-in-the-air, but the performance similar to the marine electromagnetic method can be obtained, such as exploration of high-resistance oil and gas or resources containing CO 2; meanwhile, in signal measurement, the land space-time-frequency electromagnetic method utilizes the advantages of strong and stable ground emission source signals, adopts a semi-aviation electromagnetic acquisition technology based on the unmanned aerial vehicle, and utilizes the unmanned aerial vehicle to acquire three-component induced electromotive force response in the air flight plane. Therefore, the data acquisition efficiency can be greatly improved, and the area measurement of time-frequency electromagnetism can be rapidly completed. Meanwhile, the unmanned aerial vehicle is used for large-area flight measurement in the air, the offset distance of a measuring point is not required, so that signals of long and short offset distances can be acquired, the unmanned aerial vehicle or other flight devices carry measuring instruments for measurement, and the route of the unmanned aerial vehicle is designed by geophysical prospecting technicians according to the detection position and the terrain condition.
Example 2:
in example 2, the line angle between the center point formed by the arrangement of the eight positive electrode terminals and any two adjacent negative electrode terminals is 45 °. In the eight groups of the sub-bipolar current source emitting devices, the distances between the positive electrode terminal and the negative electrode terminal are equal. The measuring equipment comprises control equipment, data storage equipment and display equipment, wherein a signal input end of the control equipment is connected with a signal output end of the bipolar current source transmitting equipment, and two signal output ends of the control equipment are respectively connected with a signal input end of the data storage equipment and a signal input end of the display equipment.
In embodiment 2, eight sets of sub-bipolar current source emission devices are respectively provided with eight power supply devices, eight sets of positive electrode end points respectively connected with the positive electrodes of the power supply devices and eight sets of negative electrode end points connected with the negative electrodes of the power supply devices, the positive electrode end points and the negative electrode end points respectively form a loop with the power supply devices through grounding, excitation sources formed by the positive electrode end points and connecting lines of the negative electrode end points are respectively equivalent to a grounded direct-current wire source, when an included angle between a central point where the positive electrode end points are located and any two adjacent negative electrode end points is 45 degrees, the optimal effect which can be achieved by the eight negative electrode end point settings is obtained, when the included angle is not 45 degrees, signal surveying can be achieved, but because the angles are different, the emitting effect of signals is not uniform, and the final surveying effect is reduced to a certain extent; the current size that eight sources launched is the same, and the direction is unanimous, and the time synchronization of current power supply and outage, the distance that eight negative electrode end points are all the same to positive electrode end point distance, can guarantee that the signal that eight sources launched keeps the synchronization at any time for the signal of final receipt is better stable, and the analysis effect and the effect of final exploration are better.
Example 3:
the method for geophysical exploration by the time-frequency electromagnetic device comprises the following steps of:
c1: nine electrode pits which are positioned on the same plane are dug on the ground, and another eight electrode pits are annularly arranged by taking one electrode pit as a center;
c2: setting eight groups of power supply equipment, and eight groups of positive electrode end points connected with the positive electrode of the power supply equipment and negative electrode end points connected with the negative electrode of the power supply equipment respectively;
c3: eight positive electrode end points of the positive electrode of the power supply equipment are installed and connected in the electrode pit in the center, and the negative electrode end points of the negative electrode of the power supply equipment are respectively installed and connected in the other eight electrode pits.
Preferably, the method further comprises a measuring method, wherein the measuring method is a time domain measuring step or a frequency domain measuring step, and the time domain measuring step specifically comprises the following steps:
s1: the host machine controls a transmitting device of the terrestrial space-time frequency electromagnetic device to respectively transmit power-off and non-power-off positive and negative bipolar rectangular pulse current signals;
s2: designing a flight path, a survey line and a measuring point position and an operation flight path of the measuring instrument according to the detection position and the terrain condition;
s3: measuring three-component induced electromotive force Vx, Vy and Vz during the power-off period of the emission pulse at each measuring point position;
s4: performing channel extraction, superposition and filtering processing on the data in the S3 to obtain transient corresponding data;
the frequency domain measurement steps are as follows:
d1: the host computer controls the transmitting device of the space-time frequency electromagnetic device to contain a plurality of continuous positive and negative square wave signals with different fundamental frequencies;
d2: determining the position of a measuring point and the running route of the measuring instrument according to the detection position and the topographic condition;
d3: measuring three-component magnetic fields Hx, Hy and Hz at the positions of the measuring points;
d4: and (3) after the data in the D3 are subjected to superposition and filtering processing, Fourier transform is carried out, electromagnetic field frequency response under corresponding fundamental frequency is extracted, and frequency domain response data are obtained.
Preferably, in the arranging step, eight positive electrode terminals and eight negative electrode terminals are respectively connected with the positive electrodes and the negative electrodes of eight power supply devices, the distance between a positive electrode terminal connected with the same power supply device and a negative electrode terminal is the same, and the included angle between the positive electrode terminal and any two connecting lines adjacent to the negative electrode terminal is 45 °.
In embodiment 3, with reference to fig. 3 and 4, eight grounded long-conductor bipolar sources with the same length are arranged on the ground, one electrode end point of each grounded long-conductor bipolar source is arranged at the same point O on the ground, the conductors are arranged in a radial direction, the other electrode end point is respectively located at 1-8 points, the grounded long conductors are uniformly arranged, the mutual included angle is 45 degrees, and a combined source of eight grounded long-conductor bipolar sources arranged in a ring shape is formed. The positive electrode end point of eight groups of sub bipolar current source transmitting equipment is arranged on the position O, the negative electrode end point is sequentially arranged on the electrode end points 1-8, continuous positive and negative square wave current pulses with the duty ratio of 1 shown in figure 4 are transmitted, the unmanned aerial vehicle flies along a designed air route on an air flight plane, and three-component induced electromotive force transient response data (time domain) or magnetic field response (frequency domain) are collected by using a carried receiving instrument. In the transmission waveform diagram of fig. 4 in which the pulse for power-off is the pulse used for acquiring the time domain response signal (the upper diagram of fig. 4), and the lower diagram of fig. 4 is the pulse used for acquiring the frequency domain response signal, the magnetic field signal is continuously sampled at equal time intervals at the measurement points, and then the spectral response data at the corresponding fundamental frequency signal is calculated from fourier transform.
The following effects of the survey, illustrated with reference to figures 5 to 14, demonstrate the uniformity of the induced current distribution in the excited medium by the similarity of the surface electric field responses of the two sources, through the response of a subsurface layered model. The resistivity of the upper layer and the lower layer of the three-layer model is 100 ohm meters, the resistivity of the middle layer is 10 ohm meters, the buried depth is 200 meters, and the thickness is 300 meters. The length of each grounding wire of the annular horizontal bipolar current source is 500 meters, and the excitation current intensity is 1A. For comparison, the response of a vertical bipolar source in a well of the same model was calculated, with the depth of the two endpoints for the powered device being 100 meters and 600 meters, respectively. FIG. 5 is a graph of the response of the Ex and Ey components of the horizontal field at 1ms after the two excitation sources were powered down, and the Ez component on a horizontal section 1cm below the surface. Fig. 6 is a response profile 10ms after power-off. It can be seen that the distribution forms of the two sources are approximate, and the distribution characteristics of the response field under the excitation of the time-frequency electromagnetic emission source and the underground vertical bipolar source are consistent. Fig. 7-10 show the frequency responses of the Ez component at 0.1Hz and 100Hz excitation by two emission sources, with uniform distribution characteristics for both the real and imaginary parts. Therefore, the excitation source provided by the invention can reach the surface response characteristic of the underground vertical bipolar source.
In order to illustrate the effect of the invention, the sensitivity of the invention to the detection target layer is calculated by using the calculation results of the three-layer model, and compared with the results of a conventional grounded long-wire source (such as a traditional ground time-frequency electromagnetic emission source, a frequency-domain CSAMT emission source and a time-domain LoTEM emission source). Sensitivity is described in terms of the relative residual percentage value between the amplitude of the three-layer model response and the amplitude of its background half-space model:
fig. 11 and fig. 12 show the relative residual error curves corresponding to the induced electromotive force transient response of the measuring line with flying height 10m and y being 1500m respectively at 1ms and 10ms, and it can be seen that the relative residual error amplitude corresponding to each component response of the present invention is substantially larger than that of the conventional grounded long-wire source. Fig. 13 and 14 are response component comparison results of the frequency domain of 1Hz and 1000Hz, respectively, which are generally consistent with the situation of the time domain, and the excitation source of the present invention can be more sensitive to the intermediate target layer, which fully proves the effect of the present invention.
The working principle of the invention is as follows: a horizontal bipolar current source emitting device is first placed on the ground in a loop. Digging 9 electrode pits, eight groups of sub bipolar current source emission devices, wherein each group of sub bipolar current source emission device comprises a power supply device, a positive electrode end point connected with the positive electrode of the power supply device and a negative electrode end point connected with the negative electrode of the power supply device, the positive electrode end point and the negative electrode end point form a loop with the power supply device through grounding, and the eight groups of positive electrode end points are arranged at the position of 0 electrode pit; electrode pits of the other 8 ground sources are radially distributed on a circle which takes O as the center and has the radius equal to the length of the ground lead source, eight groups of negative electrode end points are respectively arranged, the included angle between the center point where the positive electrode end points are located and the connecting line between any two adjacent negative electrode end points is 45 degrees, the grounding of the positive and negative electrode end points and power supply equipment form a grounding loop, and the underground detection is realized in the process of transmitting current to the underground. The control system of the transmitting device is controlled by a host computer, respectively transmits power-off and non-power-off positive and negative bipolar rectangular pulse current signals, controls the same power supply current and synchronizes the power-off and power-on time, and is used for exciting source signals of a time domain and frequency domain detection method. Secondly, arranging measuring instruments on the designed measuring point positions, measuring three-component induced electromotive force Vx, Vy and Vz during the outage of the transmitted pulse on each flight measuring point position by a time domain method, transmitting a plurality of sections of uninterrupted continuous positive and negative square wave signals containing different fundamental frequencies by a frequency domain method, and measuring three-component magnetic fields Hx, Hy and Hz on each flight measuring point position. Finally, performing channel extraction, superposition and filtering processing on the observation data of the time domain method according to a transient electromagnetic data processing method to obtain transient response data; and (3) after the observation data of the frequency domain method is subjected to superposition and filtering treatment, Fourier transform is carried out, and electromagnetic field frequency response under corresponding fundamental frequency is extracted to obtain frequency domain response data. By using the technical scheme, the convenient construction mode that the current time-frequency electromagnetic method is used for transmitting a high-power excitation current source on the ground is utilized, meanwhile, the longitudinal current detection effect of a TM mode excitation source which cannot be achieved by a land method can be obtained, eight radial horizontal bipolar sources are annularly arranged on the ground and used as a time-frequency electromagnetic emission source, an excitation field signal can be generated to be equivalent to an excitation field generated by a vertical bipolar source in the underground (in a well or in seawater), and therefore in land electromagnetic exploration, expensive and time-consuming drilling work is not needed, and a transmission system can be arranged at any place on the ground. The method can improve the working efficiency, save a large amount of cost and improve the resolution ratio of the underground target body.
The above-mentioned embodiments only express the specific embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the present invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention.
Claims (7)
1. A time-frequency electromagnetic device based on a horizontal bipolar current source is characterized by comprising bipolar current source emission equipment and measurement equipment, wherein the bipolar current source emission equipment comprises eight groups of sub-bipolar current source emission equipment, each group of sub-bipolar current source emission equipment comprises power supply equipment, a positive electrode end point connected with the positive electrode of the power supply equipment and a negative electrode end point connected with the negative electrode of the power supply equipment, the positive electrode end points and the negative electrode end points form a loop with the power supply equipment through grounding, the eight positive electrode end points are arranged close to each other to form a central point, the eight negative electrode end points radiate outwards in a radial direction by the central point formed by the arrangement of the eight positive electrode end points and are arranged in an annular mode, and the eight negative electrode end points and the eight positive electrode end points; the signal input end of the measuring equipment is connected with the signal output end of the bipolar current source transmitting equipment;
in the eight groups of the sub bipolar current source transmitting devices, the distances between a positive electrode terminal and a negative electrode terminal are equal;
the measuring equipment comprises control equipment, data storage equipment and display equipment, wherein a signal input end of the control equipment is connected with a signal output end of the bipolar current source emission equipment, and two signal output ends of the control equipment are respectively connected with a signal input end of the data storage equipment and a signal input end of the display equipment.
2. The time-frequency electromagnetic device based on the horizontal bipolar current source of claim 1, wherein an included angle between a central point formed by the arrangement of the eight positive electrode terminals and a connecting line between any two adjacent negative electrode terminals is 45 °.
3. A method for geophysical prospecting using the time-frequency electromagnetic device according to any one of claims 1 to 2, characterized in that it comprises an arrangement step, in particular as follows:
c1: nine electrode pits which are positioned on the same plane are dug on the ground, and another eight electrode pits are annularly arranged by taking one electrode pit as a center;
c2: setting eight groups of power supply equipment, and eight groups of positive electrode end points connected with the positive electrode of the power supply equipment and negative electrode end points connected with the negative electrode of the power supply equipment respectively;
c3: eight positive electrode end points of the positive electrode of the power supply equipment are installed and connected in the electrode pit in the center, and the negative electrode end points of the negative electrode of the power supply equipment are respectively installed and connected in the other eight electrode pits.
4. The method of claim 3, further comprising a measuring step, wherein the measuring step is a time domain measuring step or a frequency domain measuring step.
5. The method according to claim 4, characterized in that the time domain measuring step is as follows:
s1: the host machine controls a transmitting device of the terrestrial space-time frequency electromagnetic device to respectively transmit power-off and non-power-off positive and negative bipolar rectangular pulse current signals;
s2: designing a flight path, a survey line and a measuring point position and an operation flight path of the measuring instrument according to the detection position and the terrain condition;
s3: measuring three-component induced electromotive force Vx, Vy and Vz during the power-off period of the emission pulse at each measuring point position;
s4: and (5) performing channel extraction, superposition and filtering processing on the data in the step (S3) to obtain transient corresponding data.
6. The method according to claim 4, characterized in that the frequency domain measuring step is as follows:
d1: the host computer controls the transmitting device of the space-time frequency electromagnetic device to contain a plurality of continuous positive and negative square wave signals with different fundamental frequencies;
d2: determining the position of a measuring point and the running route of the measuring instrument according to the detection position and the topographic condition;
d3: measuring three-component magnetic fields Hx, Hy and Hz at the positions of the measuring points;
d4: and D3, after data are subjected to superposition and filtering processing, Fourier transform is carried out, electromagnetic field frequency response under corresponding fundamental frequency is extracted, and frequency domain response data are obtained.
7. The method according to claim 3, wherein in the arranging step, the positive electrode terminal and the negative electrode terminal of the sub-bipolar current source emission device are respectively connected with the positive electrode and the negative electrode of the power supply device, the distance between the positive electrode terminal and the negative electrode terminal connected with the same power supply device is the same, and the included angle between the positive electrode terminal and any two connecting lines adjacent to the negative electrode terminal is 45 degrees.
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