CN111323825B - Ground-pit wide-area electromagnetic detection method and device - Google Patents

Ground-pit wide-area electromagnetic detection method and device Download PDF

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CN111323825B
CN111323825B CN202010205929.3A CN202010205929A CN111323825B CN 111323825 B CN111323825 B CN 111323825B CN 202010205929 A CN202010205929 A CN 202010205929A CN 111323825 B CN111323825 B CN 111323825B
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王宏宇
郭文波
薛国强
柳建新
刘银爱
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Xi'an Northwest Coloring Matter Chemical Prospecting Corps Co ltd
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Abstract

A ground-pit wide-area electromagnetic detection method and a device relate to an electromagnetic detection technology for geological detection and aim to make up for the blank of the prior art. The invention relates to a land-pit wide-area electromagnetic detection method, which comprises the following steps: arranging a grounding transmitting lead on the ground, and arranging receiving electrodes in the tunnel point by point along the direction of the tunnel; supplying pseudo-random codes to a grounding transmitting wire, and receiving signals by a receiving electrode in the tunnel; and obtaining the apparent resistivity of the space geologic body below the underground tunnel to be detected according to the signals received by the receiving electrodes. The invention relates to a ground-pit wide-area electromagnetic detection device, which comprises: a grounded transmitting conductor disposed on the ground; and receiving electrodes arranged point by point along the direction of the tunnel.

Description

Land-pit wide-area electromagnetic detection method and device
Technical Field
The invention relates to an electromagnetic detection technology for geological detection, in particular to a wide-area electromagnetic detection method and device in an underground tunnel.
Background
At present, electromagnetic methods which can be used for deep exploration of oil and gas resources at home and abroad mainly comprise a Magnetotelluric (MT) method, a Controlled Source Audio Magnetotelluric (CSAMT) method, a Transient Electromagnetic (TEM) method and the like. The MT detection depth is large, but the measurement precision and the working efficiency are low; CSAMT has high measurement precision in a shallow wave zone, but has small detection depth; the TEM has low working efficiency, is seriously influenced by terrain conditions, and is difficult to increase the detection depth. The original electromagnetic method can not meet the requirements of oil gas resources and mineral resources on large-depth fine exploration.
In recent years, exploration of production mine take-over resources is developed based on the mode of mining and prospecting. In the case of mining, geophysical prospecting work is generally arranged at the periphery of a known deposit (point), a field (zone) or a mineralization concentration region, and aims to enlarge the scale of the known deposit (including horizontal extension and vertical extension), search for new blind deposits (bodies) and new species. The mineral exploration work of the production mine is basically similar to the physical exploration work in the general mineral exploration. A large probing depth is required. The mine exploration task of mine production mainly comprises two types: blind ore bodies are searched in the deep part of the known ore area or the deepening of the known ore bodies is traced; and searching blind unknown ore bodies in the near periphery. The former detection depth generally requires 500-1000 m. The latter should also have a probing depth of at least more than 300 meters.
With the gradual advance of urbanization construction, more and more electromagnetic noise and human interference affect the exploration effect of transient electromagnetic waves. Particularly near rural and urban areas where humans are gathering and in mining areas where development is high, it has become very difficult to obtain high quality transient electromagnetic signals. In actual work, the exploration depth and the exploration precision mainly depend on the minimum resolution level of an instrument and the strong and weak relation between abnormal signals and noise levels, namely the signal-to-noise ratio. If the noise level is strong, the signal-to-noise ratio of the signal is reduced, and the latest delay time of the available signal inevitably moves forward, resulting in a shallow detection depth and a low detection accuracy.
The wide-area electromagnetic method can be divided into various device forms according to field source forms or observation modes at present, a horizontal current source is mostly adopted to transmit signals in field production, and the E-Ex wide-area electromagnetic method for measuring the x component of an electric field is most widely applied and is characterized by ground transmission and ground reception and high production efficiency. Since birth, the wide-area electromagnetic method mainly focuses on large-depth detection, and the detection depth of an application example is generally more than 1 km. The shallow part detection resolution of the wide-area electromagnetic method is higher, but few researches are carried out at present, and the application examples are few. In addition, the wide-area electromagnetic method can be used as an advanced ground geophysical prospecting method, and can also be used for underground receiving and detecting, so that a wide-area electromagnetic method, namely a ground-pit wide-area electromagnetic method, which can be observed and applied in the tunnel is formed, but in the practice of the wide-area electromagnetic method, research and application on a wide-area electromagnetic depth measuring device in the tunnel are not carried out.
Disclosure of Invention
The invention aims to make up for the blank of the prior art and provides a land-pit wide-area electromagnetic detection method and a land-pit wide-area electromagnetic detection device.
The invention relates to a land-pit wide-area electromagnetic detection method, which comprises the following steps:
arranging a grounding transmitting lead on the ground, and arranging receiving electrodes in the tunnel point by point along the direction of the tunnel;
supplying pseudo-random codes to a grounding transmitting wire, and receiving signals by a receiving electrode in the tunnel;
and obtaining the apparent resistivity of the space geologic body below the underground tunnel to be detected according to the signals received by the receiving electrodes.
Optionally, an angle between the receiving electrode and the grounded transmitting wire is less than 2 °.
Optionally, the iterative calculation formula of the apparent resistivity ρ is:
Figure GDA0003794606880000021
wherein, I is the power supply current of the grounding transmitting lead; dL is the length of the electric dipole source; i is a pure imaginary number; k is the wave number of the uniform half-space; r is a transmitting-receiving distance;
Figure GDA0003794606880000022
is the angle between the direction of the electric dipole source and the radial from the midpoint of the source to the receiving point, E mn Alpha is the angle between the receiving electrode and the grounded transmitting wire for the observed electric field component (i.e. the electric field component obtained by the receiving electrode).
The invention relates to a ground-pit wide-area electromagnetic detection device, which comprises:
a grounded transmitting conductor disposed on the ground; and
and receiving electrodes arranged point by point along the tunnel direction.
Optionally, an included angle between the receiving electrode and the grounded transmitting lead is less than 2 °.
The land-pit wide area electromagnetic detection method and the land-pit wide area electromagnetic detection device have the following advantages:
(1) the ground-pit frequency domain electromagnetic method inherits the technical advantages of the traditional ground frequency domain electromagnetic method, has strong anti-interference capability, and has further enhanced deep resolution capability due to the fact that the measurement receiving position is closer to the detection target geologic body, thereby having larger effective detection depth compared with various traditional tunnel geophysical prospecting methods;
(2) the earth-pit frequency domain electromagnetic method has wide application prospect in the detection process of deep blind ores, particularly in the fields of deep prospecting of the deep side of a known mining area, blind ore body detection of the deep side of a metal mine, deep poor geologic body space positioning of a coal field roadway, oil-gas exploration, unconventional oil-gas exploration and the like, is effective supplement of the traditional ground electromagnetic method, and is worthy of further research and technical practicability of the method.
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FIG. 1 is a schematic diagram of an apparatus for E-Ex wide area electromagnetic method in accordance with an embodiment;
fig. 2 is a schematic diagram of a land-pit wide-area electromagnetic detection method and apparatus according to an embodiment of the present invention.
FIG. 3 is a graph of the variation of the measured electric field and apparent resistivity with frequency at 6 stations in accordance with the preferred embodiment of the present invention;
FIG. 4 is a graph of the electric field amplitude of the electromagnetic sounding curve of the ground and pit frequency domains at adjacent points measured in the embodiment of the present invention;
FIG. 5 is a cross-sectional view of apparent resistivity plotted against results of electromagnetic earth-pit sounding in accordance with an embodiment of the present invention;
FIG. 6 is a comprehensive interpretation graph of geology measured by an electromagnetic method in a frequency domain of a mountain region and a pit of a certain ultra-large lead-zinc mine abroad in the specific embodiment of the invention.
Detailed Description
The first embodiment is as follows: the land-pit wide-area electromagnetic detection method according to the embodiment comprises the following steps:
arranging a grounding transmitting wire on the ground, arranging a transmitter at the midpoint of the grounding transmitting wire, and arranging receiving electrodes in the tunnel point by point along the tunnel direction;
supplying pseudo-random codes to a grounding transmission lead, and receiving signals by a receiving electrode in a tunnel;
and obtaining the apparent resistivity of the space geologic body below the underground tunnel to be detected according to the signals received by the receiving electrodes.
In order to ensure the definition precision of the signal strength and the apparent resistivity of the receiving end, the included angle between the receiving electrode and the grounding transmitting wire is less than 2 degrees.
Wherein, the iterative calculation formula of the apparent resistivity rho is as follows:
Figure GDA0003794606880000031
wherein, I is the power supply current of the grounding transmitting lead; dL is electric dipole source (i.e. electric dipole formed by grounded transmitting wire)Son) length; i is a pure imaginary number; k is the wave number of the uniform half-space; r is a transmitting-receiving distance;
Figure GDA0003794606880000032
is the angle between the direction of the electric dipole source and the radial from the midpoint of the source to the receiving point, E mn Alpha is the angle between the receiving electrode and the grounded transmitting wire for the observed electric field component.
The core advantages of the wide-area electromagnetic method include: (1) the method breaks through the theoretical limitation and the technical defect that the CSAMT approximate definition of apparent resistivity leads to the incapability of large-depth detection, solves the major problems of small detection depth, low measurement efficiency, poor three-dimensional detection capability and the like of the traditional manual source electromagnetic method, and can carry out large-area, large-depth, high-precision, high-efficiency and multi-parameter detection; (2) the array measurement of unlimited number of distributed receivers is realized, the field efficiency is greatly improved, the anti-interference capability is strong, the information quantity is large, large-area rapid scanning is facilitated, the measurement precision is obviously improved, and a solid foundation is laid for realizing true three-dimensional electromagnetic method exploration; (3) a wide-area electromagnetic method data processing and interpretation software system (sharing part of algorithm with a magnetoelectricity three-dimensional inversion imaging and interpretation integrated system) developed based on a CPU/GPU high-performance computing platform realizes the numerical simulation of the wide-area electromagnetic method by adopting a method of combining a finite element and an infinite element, solves a linear equation set formed by linearization of a nonlinear problem by adopting a conjugate gradient method in an iteration mode, and realizes the rapid and fine inversion imaging of the wide-area electromagnetic method of mass data under any complex terrain condition.
The wide-area electromagnetic method organically unifies a near area, a transition area and a far area from the uniformity of a field, defines the wide-area apparent resistivity, improves the distortion effect of a non-far area, enables depth measurement to be carried out in a wide area which is not limited to the far area, and has larger exploration depth than CSAMT on the same receiving and transmitting distance. The E-Ex wide area electromagnetic method and the concept of wide area resistivity are explained below with the electric field horizontal component Ex. As shown in fig. 1, Ex1, Ex2, Ex3 refer to the x-component of the electric field measured by three sets of receiving electrodes.
The x component of the electric field of the source of the horizontal current at the quasi-static limit on a uniform earth surface is expressed as
Figure GDA0003794606880000041
Wherein: i is the supply current of the grounded transmitting conductor 1; dL is the length of the electric dipole source; i is a pure imaginary number; k is the wavenumber of the uniform half-space; r is a transmitting-receiving distance; sigma is the conductivity of the earth below the measuring point;
Figure GDA0003794606880000046
the included angle between the direction of the electric dipole source and the radial of the source from the midpoint to the receiving point.
Apparent resistivity is a comprehensive reflection of subsurface electrical inhomogeneities and topography, mainly reflecting the spatial variation of the electrical properties of the medium, or is a complex weighted average of the true resistivity of the medium over space. The expression (1) of the x component of the electric field of the horizontal harmonic electric dipole of the uniform large ground surface shows that the electric field is related to the underground resistivity parameter, and the resistivity parameter can be obtained by iterative calculation of the expression. Order:
Figure GDA0003794606880000042
ΔV MN ＝E x ·MN (3)
Figure GDA0003794606880000043
in equations (2) to (4): delta V MN Is the potential difference between M point and N point, and MN is the polar distance of the measuring electrode. The wide area apparent resistivity can then be expressed as:
Figure GDA0003794606880000044
equation (5) is the wide area apparent resistivity defined by the electric field horizontal component Ex. It can be seen from the definition that the apparent resistivity information of the underground can be extracted by measuring the potential difference, the sending current and the related polar distance parameters and adopting an iterative method for calculation. From the above deductions, it is known that the wide area apparent resistivity has a strict definition, in which no approximation or rejection is performed.
The controllable source audio frequency magnetotelluric method imitates the method of MT, and the extracted electrical resistivity of Carniian is defined as
Figure GDA0003794606880000045
The formula (6) is an approximate calculation formula obtained by discarding some high-order terms under the condition that the 'far zone' is satisfied. Equation (6) cannot be satisfied when the "far zone" condition is not satisfied, and thus CSAMT is only applicable to the "far zone" measurement. And the wide-area apparent resistivity definition has no approximate condition, is not limited to a far zone and can work in a vast non-far zone.
The principle of the ground-pit wide-area electromagnetic detection method provided by the embodiment is shown in fig. 2, and a grounding transmitting lead AB is arranged on the ground; arranging a receiving electrode MN point by point along the tunnel direction in the tunnel; the length of the grounded transmitting wire is approximately equal to the buried depth of the target layer. The transmitting power of the grounding transmitting wire is 200 kilowatts. Supplying pseudo-random code current to a grounded transmission wire AB, and receiving signals by a receiving electrode in the tunnel; the included angle between the receiving electrode MN and the transmitting electrode AB is less than 2 degrees, and the MN is parallel to the AB. The underground receiving is carried out along the roadway, the receiving electrode target body is close, the receiving signal is strong, the resolution ratio and the detection precision can be improved, and the side-looking capability is enhanced.
Under the influence of objective conditions, the survey line may not be straight (the survey line direction is the direction indicated by MN in the figure), especially in the actual work of the earth-pit wide area electromagnetic method, the survey line is difficult to be ensured under the influence of tunnel conditions, namely, the observed electric field is not an Ex component, but contains vector values of electric fields in other directions, which is named as E mn . Deducing a wide-area electromagnetic method E mn The expression of the field value component has great significance for the ground wide-area electromagnetic method or the tunnel wide-area electromagnetic method, and is a key technology for successful application of the ground-pit wide-area electromagnetic method.
Deriving E from the relationship between the components of the electromagnetic field mn The expressions are shown as (7) and (8)
Figure GDA0003794606880000051
Figure GDA0003794606880000052
An iterative calculation formula of apparent resistivity of the E-Ex wide-area electromagnetic method of the pit is shown in a formula (9), and an iterative calculation formula of apparent resistivity of the E-Emn wide-area electromagnetic method is shown in a formula (10).
Figure GDA0003794606880000053
Figure GDA0003794606880000054
The resistivity values in the calculation formulas (9) and (10) can be obtained by an iteration method or an inverse spline interpolation technology in the numerical calculation.
Obtaining the electric component expressions of the electromagnetic field of the ground electric dipole emission and the underground z depth according to the Maxwell equation set and the basic theory of the electromagnetic field and the electromagnetic wave:
Figure GDA0003794606880000055
Figure GDA0003794606880000061
and (3) carrying the formulas (11) and (12) into the formulas (7) and (10) to obtain a basic formula for iterative calculation of the apparent resistivity of the E-Emn earth-pit wide area electromagnetic method for earth surface transmission and pit reception. This is the theoretical basis of the method of the earth-pit wide area electromagnetic method.
Formula (II)(11) In (12), Er,
Figure GDA0003794606880000063
Respectively are analytical expressions of an electromagnetic field of a horizontal electric dipole emission on the ground and an underground z depth under a cylindrical coordinate system of a uniform half space under a quasi-static limit condition; j0 and J1 are 0 th order and 1 st order Bessel functions, m and m 1 In order to be able to integrate the variables,
Figure GDA0003794606880000062
according to the ground-pit wide-area electromagnetic detection method provided by the embodiment, the change characteristics of the induction electromagnetic field in the tunnel in space and time are researched, so that the purpose of researching the electric distribution structure around the tunnel can be achieved, and the space distribution and the extension direction of an inferred target body can be found. The ground-pit wide-area electromagnetic method is an application expansion of the ground wide-area electromagnetic method in underground space, selects the underground space closest to a target body to carry out data observation, has the characteristics of strong resolving power, difficult influence by terrain and the like, and can improve the resolving power of ore bodies.
The following is an application example of a land-pit wide-area electromagnetic detection method proposed by the present embodiment, which is an example of a foreign ultra-large lead zinc mine mountain-pit frequency domain electromagnetic method. Pyroclastic rock, volcanic metamorphic rock, carbonate rock and limestone of early stone-carbon period mainly grow in the mine area. The invasion rock in the deposit is composed of a series of granite veins, such as granite spangle, granite porphyry and quartz porphyry, the distribution is obviously controlled by a main fracture structure, and the silicalite ore body is generally present in a contact zone of the granite veins and limestone. Wherein the granite spangle rock veins have the maximum mineralization significance, and the contact zones of the granite spangle rock veins and surrounding limestone are main mineral control structures. The metal mineral components mainly comprise galena, sphalerite, pyrite, chalcopyrite and a small amount of magnetite and hematite, and a plurality of metal minerals are distributed in the skarn body in an overlapping mode. The research result of physical property parameters shows that granite and limestone have obvious resistivity difference, the higher the skarning degree of the limestone is, the more obvious the electrical property difference is, and the skarning is the most reliable ore finding mark of the ore deposit at the place.
At present, 1400m elevation of a mine from the earth surface is mined to 920m elevation position underground, and goafs in a mining area are distributed; meanwhile, as the ore body is steeply inclined at 75-85 degrees, the ground geophysical prospecting observation technology is inevitably influenced by a goaf and production operation; geological tasks proposed by mines require that a geophysical prospecting method can detect the elevation of 0m to be shallow. Based on the above consideration, the method technology is selectively carried out by a pit-wide area electromagnetic method so as to solve the deep prospecting problem of the mine. The method is used for observing in the tunnel to weaken the influence of the upper goaf, simultaneously overcomes the difficulty of adverse passing of the earth surface, and meets the requirement of the tunnel geophysical prospecting for the detection depth of nearly 1 km.
At a transmitting and receiving distance of nearly 8km, 16 earth-pit frequency domain electromagnetic method physical points are collected in a tunnel below 500m of depth underground. It can be seen from fig. 3 that the curve of each measuring point is relatively smooth, no serious frequency point hopping and curve distortion exist, the overall curve is AK type or AKH type, the curve has a downward probing trend in a low frequency range (below 10 Hz), and then the electric field value and the apparent resistivity rapidly rise, thereby showing an obvious near field characteristic. Electric field profile and use of E-E mn The frequency domain defined by the frequency domain electromagnetic method is almost consistent in curve type and change trend characteristics according to the resistivity, and only slightly different in details of individual measuring points. It is worth mentioning that in the measuring process, the production operation of the mine is still carried out in the tunnel, and under the condition of obvious interference, the ground-pit frequency domain electromagnetic method can still obtain better measured data, and the method has strong anti-interference capability.
FIG. 4 is a graph of the electric field amplitude of the electromagnetic sounding curve of the ground and the pit frequency domain of adjacent points. Comparing the electric field curves of the ground electromagnetic method and the earth-pit electromagnetic method, it can be seen that the curve types of the two curves are different, but the earth-pit curve is a typical HK curve, the fluctuation range of the ground electromagnetic curve in the frequency band below 200hz is not large, and the details of the curve are not obvious. Compared with the ground electric field curve, the ground-pit electric field amplitude is higher in high value amplitude and lower in low value, and the details in the whole curve are more prominent. By the formula
Figure GDA0003794606880000071
And formula
Figure GDA0003794606880000072
It can be known that in the active electromagnetic method, the electric field component is more sensitive to the subsurface resistivity, and the change of the electric field curve is the reaction of the subsurface electrical property. The results of ground and tunnel measurements at the same point indicate that the ground-pit frequency domain electromagnetic method has better abnormality resolution capability and can reflect more abnormal details compared with the ground electromagnetic method, and the ground-pit electromagnetic data contains more abundant information.
FIG. 5 is a plot of apparent resistivity profile plotted from the results of electromagnetic earth-pit sounding. The earth-pit frequency domain sounding point in fig. 4 is located at 0-300m from the left end of the section, the electrical distribution on the left side of the section is generally shown by a four-layer earth section from low to high, but has a certain response to the low resistance of the fifth layer. The sounding curve of the single-point can correspond to the geoelectrical configuration on the section. In conjunction with the known geology of the mine, the electrical configuration of the profile substantially reflects the local geologic formation characteristics. The effective detection depth in the tunnel reaches more than 1 km.
FIG. 6 is a comprehensive interpretation result of the proposed geology according to the electromagnetic sounding result of the earth-pit frequency domain and the known geological information, and the horizontal coordinate represents the horizontal distance between the measuring point and the head end of the measuring line. According to the geological mineralization rule of a mining area, silicalite of limestone is an important mineral finding mark, and yellow ferration and sericite of silicate are also meaningful mineralization and alteration marks. By using the ranges of silicalite and yellow iron sericite presumed by the electromagnetic sounding result of the earth-pit frequency domain, important indication information can be provided for deep exploration. Two beneficial regions of potential mineralizers are presumed below the tunnel and are respectively positioned in the depth range of-650 m to-750 m and the depth range of-650 m to-850 m, the two beneficial regions respectively correspond to medium-high resistance abnormity of the resistivity profile, the medium-high resistance abnormity is geologically interpreted as the contact part of limestone and invaded rock, stronger hydrothermal alteration effect is generated, and the phenomenon of resistivity reduction is presented. Two favorable areas are yet to be verified by drilling engineering.
The land-pit wide-area electromagnetic detection method has the following advantages:
(1) the ground-pit frequency domain electromagnetic method inherits the technical advantages of the traditional ground frequency domain electromagnetic method, has strong anti-interference capability, and has further enhanced deep resolution capability due to the fact that the measurement receiving position is closer to the detection target geologic body, thereby having larger effective detection depth compared with various traditional tunnel geophysical prospecting methods;
(2) the earth-pit frequency domain electromagnetic method has wide application prospect in the deep blind mine detection process, particularly in the deep mine exploration of the deep side part of the known mine area, is effective supplement of the traditional ground electromagnetic method, and is worthy of further research and method technology practicability.
The second embodiment is as follows: the earth-pit wide area electromagnetic detection device according to the present embodiment includes:
a grounded transmitting conductor disposed on the ground; and
and receiving electrodes arranged point by point along the tunnel direction.
The transmitter is arranged at the midpoint of the grounded transmission conductor.
Optionally, an angle between the receiving electrode and the grounded transmitting wire is less than 2 °.
The signal received by the receiving electrode is processed to obtain the apparent resistivity of the measured point, and the iterative calculation formula of the apparent resistivity rho is as follows:
Figure GDA0003794606880000081
wherein, I is the power supply current of the grounding transmitting lead; dL is the length of the electric dipole source; i is a pure imaginary number; k is the wave number of the uniform half-space; r is a transmitting-receiving distance;
Figure GDA0003794606880000082
is the angle between the direction of the electric dipole source and the radial from the midpoint of the source to the receiving point, E mn Alpha is the angle between the receiving electrode and the grounded transmitting wire for the observed electric field vector.
The method for performing the earth-pit wide-area electromagnetic detection by using the earth-pit wide-area electromagnetic detection device is as described in the first embodiment, and will not be described herein again.

Claims (3)

1. A land-pit wide-area electromagnetic surveying method, comprising:
arranging a grounding transmitting wire on the ground, arranging a transmitter at the midpoint of the grounding transmitting wire, and arranging receiving electrodes in the tunnel point by point along the tunnel direction; the length of the grounding transmitting lead is equal to the buried depth of the target layer;
supplying pseudo-random codes to a grounding transmitting wire, and receiving signals by a receiving electrode in the tunnel;
obtaining apparent resistivity of a space geologic body below the underground tunnel to be detected according to the signals received by the receiving electrode;
the method of combining finite elements and infinite elements is adopted to realize numerical simulation of the wide-area electromagnetic method, a conjugate gradient method is adopted to iteratively solve a linear equation set formed by linearization of a nonlinear problem, and wide-area electromagnetic inversion imaging of mass data under any terrain condition is realized.
2. A method for wide-area electromagnetic survey of a ground-pit according to claim 1, wherein the angle between the receiving electrode and the grounded transmitting wire is less than 2 °.
3. A land-pit wide-area electromagnetic survey method according to claim 1 or 2, characterized in that said iterative calculation formula of apparent resistivity p is:
Figure FDA0003799425480000011
wherein, I is the power supply current of the grounding transmitting lead; dL is the length of the electric dipole source; i is a pure imaginary number; k is the wave number of the uniform half-space; r is a transmitting-receiving distance;
Figure FDA0003799425480000012
from the direction of the electric dipole source and the midpoint of the source toReceiving the angle between the radial edges of the points, E mn Alpha is the angle between the receiving electrode and the grounded transmitting wire for the observed electric field component.
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