AU2020100984A4

AU2020100984A4 - A Method and Apparatus for Ground-tunnel Wide Field Electromagnetic Surveying

Info

Publication number: AU2020100984A4
Application number: AU2020100984A
Authority: AU
Inventors: Wenbo GUO; Yinai LIU; Xishe QIN; Hongyu Wang; Guoqiang Xue
Original assignee: Xi An Northwest Nonferrous Geophysical & Geochemical Exploration Co Ltd
Current assignee: Xi An Northwest Nonferrous Geophysical & Geochemical Exploration Co Ltd
Priority date: 2020-06-11
Filing date: 2020-06-11
Publication date: 2020-08-13
Anticipated expiration: 2028-06-11

Abstract

This invention relates to a method and an apparatus for conducting ground-tunnel wide field electromagnetic geophysical exploration. In a method proposed by the invention, a dipole transmitter coil is placed over the ground, a dipole receiver is arranged in the tunnel along the direction thereof; pseudo-random code is provided to the transmitter, signals are received by the receiver in the tunnel; the apparent resistivity of the geological body below the measured tunnel is obtained by analysing the signals received by the receiver. The invention also provides apparatus for carrying out the method. This apparatus comprises a dipole transmitter coil placed over the ground and a dipole receiver electrode arranged in the tunnel along the direction thereof. 1000 1E 100 C 0 00 7'rud-unlEMSre 0 0 Freauancv/Hz 0 200 400 600 800 l~a()m o0 3. 386 37 346 3.6 3,5 34 3 32 3.2 3A 3 1 3.0 30 EO 30 - 1 2.9 2.8 28 28 8 : 2,7 2.7 26, 2.6 26 2.5 2A4 2.3 22 20 8ft 01P" 1:i0000 100 0 100 0 200 400 600 800 iN0 distance/rnMeem Fig.5

Description

1E 100

C 0

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Fig.5

Editorial Note 2020100984 There is only eleven pages of the description

Description

A Method and Apparatus for Ground-tunnel Wide Field Electromagnetic Surveying

Technical Field The invention relates to the field of electromagnetic geological exploration. More particularly, this invention relates to a method and related apparatus for ground-tunnel wide field electromagnetic geological exploration.

Background At present, the available electromagnetic surveying methods globally for hydrocarbon exploration (or oil and gas exploration) primarily include Magnetotellurics (MT), Controlled Source Audio-frequency Magnetotellurics (CSAMT), and Transient electromagnetic (TEM). MT surveys are typically employed for investigating deep structures, but less accurate and effective. CSAMT has shallow penetration limit despite providing better resolution for shallow geological mapping. The effectiveness of TEM is greatly dependent on terrian condition, which increases the difficulty of geological surveying in some special area. Thus, the current methods can not satisfy the demand for detailed deep target imaging.

In recent years, the exploration of replacement resources for productive mines has been carried out based on the model of making ore search nearby. When making ore search nearby, the geophysical exploration work is generally arranged at the periphery of discovered ore deposits (spots), ore fields (areas), or within the scope of the mineralization concentrated area, with a goal of expanding the scope of known ore deposits (including horizontal expansion and vertical expansion), and looking for new hidden ore deposits (bodies) and new types of ore. The ore search and geophysical prospecting work for productive mines is substantially similar to the geophysical prospecting work in general mineral exploration. Yet, the it requires large invesitigation depth. There are two main types of ore search tasks for productive mines: looking for blind ore bodies in the depths of known mining areas or exploring for extension of known ore bodies; and looking for hidden unknown ore bodies near the periphery thereof. The investigation depth of the former is generally required as from 500 meters to 1000 meters, whereas the latter should be at least greater than 300 meters.

Because of the rapid urbanisation, various types of electromagnetic noise from natural or man-made sources have been generated to inevitably obscure signal for transient electromagnetic survey. This makes it more difficult to obtain transient electromagnetic signals with favorable resolution, especially in the highly explored mining area and the ones adjacent to residential area.

In practice, investigation depth and resolution are greatly dependent of the minimum resolution of surveying instrument and signal - to - noise ratio (SNR). In other words, if the noise level is high,

the SNR will be decreased, resulting in delayed signals, which will obscured by noise. Thus, the

depth of investigation and resolution are negatively affected.

At present, the apparatus of the wide field electromagnetic method can be divided into several

subtypes based on the field source and the observation mode. The E-Ex, a wide field electromagnetic method used for measuring x component of the electric field, is most widely used.

It is characterized by the ground placed the transmitter and the receiver, and the great productive efficiency. Since the wide field electromagnetic method was created, they have been used for large

depth investigation, of which it is greater than 1km. Despite the wide field electromagnetic method have high resolution for geophysical detection in shallow area, very a few studies were

conducted to investigate it. Thus, there are very a few application examples. In addition, the receiver of the wide field electromagnetic method can be placed both on ground and in tunnel,

forming a novel geophysical exploration method ground-tunnel wide field electromagnetic method-can be operated in tunnels. Yet, in practice, the wide field electromagnetic method has

not been studied and applied in tunnels.

Summary A method for conducting ground-tunnel wide field electromagnetic geophysical exploration according to one aspect of the invention is provided. A dipole transmitter coil is placed over the

ground. A dipole receiver is arranged in the tunnel along the direction thereof. A sequence of pseudo-random code is provided to the transmitter. The signals are received by the receiver in the

tunnel. The apparent resistivity of the geological body below the measured tunnel is obtained by

analysing the signals received by the receiver.

An apparatus for carrying out the method is also provided according to one aspect of the invention.

This apparatus comprises a dipole transmitter coil placed over the ground and a dipole receiver

electrode arranged in the tunnel along the direction thereof. Alternatively, the angle between the

receiving electrode in tunnel and the transmitter coil on ground is less than 2°.

Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

The method and the apparatus for ground-tunnel wide field electromagnetic geophysical exploration described have following advantages: (1) The ground-tunnel wide field electromagnetic method not only keeps the technical advantages

of the traditional wide field electromagnetic method, also has greater anti-interference ability. As

the in-tunnel receiver is closer to the surveying target, the investigation depth and the resolution are better than the state of prior art.

(2) The ground-tunnel wide field electromagnetic method holds the potential to (unconventional) hydrocarbon exploration, deeply exploring ore deposits near the discovered mining area,

geophysical exploration in difficult terrain. This novel method provides more opportunities to accurately measuring conductivity of geophysical bodies in deeper and wider area, which is

improved upon the present technology.

Description of Figures FIG. 1 is a schematic diagram of an apparatus of E-Ex wide area electromagnetic method in an

embodiment; FIG. 2 is a schematic diagram of a method and a related apparatus for ground-tunnel wide field

electromagnetic exploration according to an embodiment of the present invention. FIG. 3 shows a relationship between electric field and apparent resistivity measured at six

measuring points and frequency in the embodiment of the present invention. FIG. 4 shows a relationship between adjacent measuring points and electric field amplitude of the ground-tunnel wide field electromagnetic method in an embodiment of the present invention.

FIG. 5 is a cross-sectional view of apparent resistivity sketched according to the results of ground-tunnel electromagnetic method in the embodiment of the present invention.

FIG. 6 is a comprehensive geological interpretation map of the ground-tunnel wide field

electromagnetic measurement for a super-large lead-zinc mine outside China in an embodiment of

the present invention.

Detailed Description of the Invention

Embodiment 1: A method for ground-tunnel wide field electromagnetic geophysical exploration

described in this embodiment comprises: placing a dipole transmitter coil over the ground, pseudo-random codes are provided to the

transmitter; placing a dipole receiver coli in the tunnel along the direction thereof, signals are received by the receiver; wherein the apparent resistivity of the geological body below the measured tunnel is obtained by analyzing the signals received by the receiver.

In order to ensure the defined accuracy of signal strength and apparent resistivity of the receiving end, the angle between the bipole receiver and the dipole transmitter is less than 2°.

Wherein, the formula of the iterative calculation for the apparent resistivity p is:

2rr3E, IdL [1+e-i'(1+ikr)]cosp cos(p -a) -IpdL [2- e-i''(1+ikr)]sinp sin(p -a)

wherein, I represent the emission current, dL represents the length of the current dipole, i represents imaginary argument, k is the wave number, r is the distance between the transmitter

and the receptor; qp is the azimuth angle of the cylindrical coordinate system, Enn is the observed

electric field component; and a is the angle between the dipole transmitter and the dipole receptor.

The key advantages of the wide field electromagnetic method include: (1) overcome the

theoretical limitations and technical defects for deep-depth geophysical exploration over the

CSAMT, and solving major problems such as limited detection depth, poor detection efficiency,

and 3D detection capability. This method provides large-area, deep-depth, high-precision, high-efficiency, and multi-parameter geophysical exploration; (2) providing infinite distributed receiver arrays so as to greatly improve the electromagnetic field efficiency, excellent

anti-interference ability, and large amount of obtained information. The method also facilitates quick sweep of a large area with significantly improved measurement accuracy, and laying a solid

foundation for real three-dimensional electromagnetic exploration; (3)with a software system for interpreting the data obtained from wide field electromagnetic method developed based on the

CPU/GPU high-performance computing platform (part of algorithms shared with the integrated system of gravity, magnetoelectricity and three-dimensional inversion imaging interpretation),

using the combination of finite elements and infinite elements to perform the numerical simulation of wide field electromagnetic method, and using the conjugate gradient method to iteratively solve

the linear equations formed by the linearization of nonlinear problems, thereby providing fast and precise inversion imaging of massive data under the condition of any complex terrain with the

wide field electromagnetic method. The wide field electromagnetic method unifies the "near zone", "transition zone", and "far zone",

so as to define the wide field apparent resistivity, and improve the distortion effect in the non-far zone, which enables the deep-depth measurement can be carried out in a large and non-far zone, with a greater exploration depth than CSAMT does at the same transmission and reception distance. The concepts of E-Ex wide field electromagnetic method and wide field resistivity are explained below with the horizontal component Ex of electric field. As shown in FIG. 1, Exl, Ex2 and Ex3 refer to the x component of the electric field measured by three sets of receiver electrodes.

Under the quasi-static limit, the x component of the electric field of the horizontal current supply

on the uniform ground surface is expressed as

IdL2 k Ex = I l [1-3sin2 o+e-i(1+ikr)] 2io-r (1)

and the receptor; c is the conductivity of the ground below the measuring point; qp is the azimuth

angle of the cylindrical coordinate system.

Apparent resistivity is a comprehensive reflection of underground electrical inhomogeneity and topographic relief, mainly reflecting the spatial change of dielectric properties. In other words, the

apparent resistivity is a complex weighted average of the true resistivity of the medium in space.

From the expression (1) of the x component of electric field of the horizontal harmonic electric

dipole on the uniform ground surface, we know that the electric field is related to the underground resistivity parameter. The resistivity parameter can be obtained by iterative calculation of this

formula, such that:

KE-Ex 2u dL -MN (2)

AVMN=E;.MN A VmN x-MN(3)

FE-Ekr)=1-3sin9+ek'(1-ikr) (4)

In formulas (2) to (4): A VMN is the potential difference between point M and point N, and MN is

the pole distance of the measuring electrode. As such, the wide field apparent resistivity can be expressed as:

AVMN a, E-Ex II F -(ikr) (5) The formula (5) is the wide field apparent resistivity defined by the horizontal component Ex of

electric field. As can be seen from its definition, as long as the potential difference, the sending current, and associated polar distance parameters are measured and iteratively calculated, the underground apparent resistivity information can be extracted. It may be seen from the above deduction that there is a strict definition for the wide field apparent resistivity, without any approximation and discard.

The controlled source audio-frequency magnetotelluric method is modeled after MT, with

extracted Cagniard apparent resistivity. It is defined as

1 E,2 Pa cop| H, 2 (6) The formula (6) is an approximate calculation formula obtained by discarding some higher-order

terms while satisfying the condition of "far zone". When the condition of "far zone" is not

satisfied, the formula (6) cannot be established, so CSAMT is only applicable to "far zone"

measurement. However, there is no approximate condition in the definition of wide field apparent resistivity, and it is not necessary to be limited to the "far zone", and can work in a large number

of "non-far zones".

The principle of a method for ground-tunnel wide field electromagnetic detection proposed in

this embodiment is, as shown in FIG. 2, placing a dipole transmitter coil AB over the ground; placing a dipole receiver coli MN in the tunnel along the direction thereof, signals are received by

the receiver; where the length pf the transmitter coil on the ground is roughly equal to the buried

depth of a target layer. The transmission power of the transmitter is 200 kilowatts. Pseudo-random

code current is supplied to the transmitter coil AB, and the signal is received by the receiving electrode in the tunnel. The angle between the receiving electrode MN and the transmitter coil AB

is less than 2°, and MN is parallel to AB. The signal is received underground along the tunnel. The

target body is relatively close to the receiving electrode and the received signal is strong, which can improve the resolution and detection accuracy, and enhance the "side-view" capability.

Due to objective conditions, the measurement line may not be straight (the direction of the measurement line is the direction indicated by MN in the figure), which is difficult to be ensured

especially in the actual work of the ground-tunnel wide field electromagnetic method due to the tunnel conditions, which means the observed electric field component is not the Ex component,

but a vector value containing electric fields in other directions, named as Enn. It is of great significance for the ground wide field electromagnetic method or the tunnel wide field

electromagnetic method to deduce the expression of the E. field value component of the wide field electromagnetic method, which is a key technology for the successful application of the

ground-tunnel wide field electromagnetic method. According to the relationship between the components of the electromagnetic field, the E,nn

expression is derived as shown in (7), (8)

Enn = E,.COS((9- a) - E, sin((o - a) (7)

E,= [1+ e-'(1+ikr)]cospcos(qp-a)-IpdI[2-e -r(1+ikr)]singsin(qp-a) 2Tr 2r (8) The iterative calculation of apparent resistivity for the ground-tunnel E-Ex wide area

electromagnetic method is shown in formula (9), and the iterative calculation of apparent resistivity of E-Emn wide area electromagnetic method is shown in formula (10).

2IT r E , IdL [1- 3sin 2 y + e-'T(1+ikr)] (9)

IidL[I+e-'e'(I+ikr)] cos cos(-a)-IpdL[2-e-''(1+ikr)]singsin(p-a) (10)

Through the iterative method in numerical calculation or the inverse spline interpolation technique,

the resistivity values in the calculation formulas (9) and (10) can be solved. According to Maxwell's equations and the basic theory of electromagnetic field electromagnetic

waves, the expression of the electric component of the electromagnetic field emitted by the ground

electric dipole and at the underground z depth is obtained:

IdL r= im18KIdL> Er=-icop cosp - e-")J(mr)dm+ --- icosp e(-"hzmJ,(mr)dm 4;T 0 mU m+M Or 2 0

E,=--icop IdL -sin 4c 2m m+m1 e-"JO(mr)dm+ 1 0 arq - IdL2c cosj )W f-"mJ()d (12)

The basic formulas for the iterative calculation of apparent resistivity of the E-Emn ground-tunnel

wide field electromagnetic method with ground transmission and tunnel reception may be obtained by substituting equations (11) and (12) into equations (7) and (10). This is the theoretical

basis of the ground-tunnel wide field electromagnetic method. In formulas (11) and (12), Er and Ep are the analytic expressions of the electromagnetic field

emitted by ground horizontal electric dipole and at the underground z-depth under the quasi-static limit conditions and within the cylindrical coordinate system of a uniform half space; where JO, J1

are the order 0 and 1 Bessel functions respectively, and m and m, are integral variables,

m= m 2 -k 2

The method for ground-tunnel wide field electromagnetic detection proposed in this embodiment may achieve the purpose of studying the electrical distribution structure around the tunnel by

studying the spatial and temporal changes of the induced electromagnetic field in the tunnel, so

that the spatial distribution and extension direction of the inferred target body may be found. The ground-tunnel wide field electromagnetic method is an application extension of the ground wide

field electromagnetic method in the underground space. This method selects to perform the data observation in the underground space closest to the target body, which provides the characteristics of strong resolution capability, not easily affected by terrain, and the like, and improves the resolution capability to an ore body.

The following is an application example of the method for ground-tunnel wide field

electromagnetic detection proposed in this embodiment. The example is an example of the

ground-tunnel frequency-domain electromagnetic method for a large-scale lead-zinc mine in a foreign country. In the mining area, Devonian pyroclastic rocks, volcanic metamorphic rocks,

carbonate rocks and early Carboniferous limestones are mainly exposed. The intrusive rocks in the

ore deposit consist of a series of granite-like veins, including granodiorite porphyry, granite porphyry, quartz porphyry, etc., with a distribution substantially controlled by the main fault

structure, and the skarn ore bodies are generally found in the contact zone between the granite-like veins and the limestones. Among them, the granodiorite porphyry veins are of the greatest

significance for mineralization, and the contact zone thereof with the surrounding limestones are the main ore-controlling structures. The metal mineral components mainly include galena,

sphalerite, pyrite, chalcopyrite, and a small amount of magnetite and hematite. A variety of metal minerals are superimposed in the skarn body. The results of physical parameter researches show

that granite and limestone have obvious difference in resistivity. The higher skarnization the limestone has, the more obvious the electrical difference is. The skarnization is the most reliable

ore search indicator for this ore deposit. At present, the mine has an elevation of 1400 m from the earth surface and has been mined to an

elevation of 920 m underground. The mined-out spaces are distributed all over the mine area. At the same time, because the occurrence of the ore body is steeply inclined at 75 to 850, the surface

geophysical prospecting observation technology is inevitably affected by the mined-out spaces and production operations. The geological tasks proposed by the mine require that the geophysical prospecting method can detect to an elevation of 0 m and below. Based on the above

considerations, the ground-tunnel wide field electromagnetic method was selected as the method and technology to solve the deep ore prospecting problem of the mine. This method observes in

the tunnel to weaken the influence by the upper mined-out space, and at the same time overcomes

the difficulty of passage on the ground surface, and meets the requirements for a geophysical

prospecting depth of about 1 km for the tunnel. At a transmission and reception distance of about 8 km, 16 physical points for ground-tunnel

frequency domain electromagnetic method were collected in the tunnel below 500m underground.

It may be seen from FIG. 3 that the curve for individual measurement points is relatively smooth,

without any severe frequency jump and curve distortion. The overall curve is of AK type or AKH type, where at a lower frequency band (below 10 Hz), it shows a downward trend, and then the

electric field value and apparent resistivity rise rapidly, which shows obvious near-field characteristics. The electric field curve and the apparent resistivity in the frequency domain defined by the E-E,, frequency domain electromagnetic method are almost the same in terms of curve type and change trend characteristics, with only slightly difference in the details of individual measurement points. It is noted that, during the measurement process, mine production operations are still being carried out in the tunnel. In the case of obvious interference, the ground-tunnel frequency domain electromagnetic method may still obtain good measured data, as this method has strong anti-interference ability.

FIG. 4 is a graph of the electric field amplitude of the ground-tunnel frequency domain electromagnetic depth sounding curve of adjacent points. It may be seen from comparison

between the electric field curves of the ground electromagnetic method and of the ground-tunnel electromagnetic method that the two curves are of different curve types. However, the

ground-tunnel curve is a typical HK curve, and the fluctuation amplitude of the ground electromagnetic curve at a frequency band below 200hz is not high, with no obvious curve details.

Compared with the ground electric field curve, the ground-tunnel electric field has a higher amplitude at a high value and a lower amplitude in a low value, with more prominent details in the

entire curve. It may be seen from the formula

E I d F,2wxr [ 1 +e-"'(1+ikr)]cospcos(p-a)- Id 2-e '(+ikr)]singsin((p-a) 2wxr2 11k)]iand formula 3 2rr Enn, IdL[1+e'(1+kr) cospcos(Q-a)-IdL[2-e-'1+ikr) sinqsin(Q-a) that in an active electromagnetic method, the

electric field component is more sensitive to the underground resistivity, and the change of the

electric field curve is the response of the underground electrical property. The ground and tunnel measurements at the same point show that the ground-tunnel frequency domain electromagnetic

method has better anomaly resolution than the ground electromagnetic method, and can reflect more anomalous details. The ground-tunnel electromagnetic data contains more abundant

information. FIG. 5 is a cross-sectional view of apparent resistivity sketched according to the results of the

ground-tunnel electromagnetic depth sounding. The ground-tunnel frequency domain depth sounding point in Fig. 4 is located 0-300 m from the left end of the profile. The electrical

distribution on the left side of the profile generally shows a four-layer geoelectric cross section from low to high, but it also has certain response to low resistance of the fifth layer. The depth

sounding curve of a single measuring point can correspond to the geoelectric structure on the

profile. In combination with the known geological conditions of the mine, the electrical structure of this profile basically reflects the local geological structure characteristics. The effective

detection depth in the tunnel reached more than 1 km.

FIG. 6 is the result of comprehensive geological interpretation based on the results of ground-tunnel frequency domain electromagnetic depth sounding and known geological information, where the X-axis represents the horizontal distance between the measuring point and the beginning of the measuring line. According to the geological mineralization law of the mining area, the skarnization of limestone is an important ore search indicator, and the pyritization and sericitization of silicate are also meaningful signs of mineralization and alteration. The range of skarnization and pyrite sericitization inferred from results of the ground-tunnel frequency domain electromagnetic depth sounding may provide important indicators for deep prospecting. Below the tunnel, a total of two favorable potential mineralization zones are speculated, which are located in the depth range of -650m to -750m, and the depth range of -650m to -850m. The two favorable zones correspond to the medium and high resistance anomalies in the resistivity profile respectively, which may be geologically interpreted as the contact parts of limestone and intrusive rock. In addition, a strong hydrothermal alteration occurred, showing a phenomenon of reduced resistivity. The two favorable zones have yet to be verified by drilling work.

The method for ground-tunnel wide field electromagnetic detection described in this embodiment has the following advantages:

(1) The ground-tunnel wide field electromagnetic method not only keeps the technical advantages of the traditional wide field electromagnetic method, also has greater anti-interference ability. As

geophysical exploration in difficult terrain. This novel method provides more opportunities to accurately measuring conductivity of geophysical bodies in deeper and wider area, which is improved upon the present technology.

Embodiment 2: An apparatus for ground-tunnel wide field electromagnetic detection described in this embodiment comprises:

a dipole transmitter coil placed over the ground;

a dipole receiver electrode placed in the tunnel along the direction thereof.

alternatively, angle between the dipole receiver and the dipole transmitter is less than 2°.

By processing the signal received by the receiving electrode, the apparent resistivity of the

measured point can be obtained. The formula of the iterative calculation for the apparent

resistivity p is:

2[rr3 In2 IdL [1+e-'*'(1+ikr)]cosqucos( -a) -IpdL [2-5''(1+ikr)]sinqu'sin(Q -a) wherein, I represent the emission current, dL represents the length of the current dipole, i represents imaginary argument, k is the wave number, r is the distance between the transmitter and the receptor; qp is the azimuth angle of the cylindrical coordinate system, Enn is the observed electric field component; and a is the angle between the dipole transmitter and the dipole receptor.

The method for ground-tunnel wide field electromagnetic detection using the abovementioned apparatus for ground-tunnel wide field electromagnetic detection is as described in the first

embodiment, and will not be repeated here.

Claims

Editorial Note 2020100984 There is only one page of the claim Claims

1. A method for conducting ground-tunnel wide field electromagnetic geophysical exploration, the method comprising: placing a dipole transmitter coil over the ground, pseudo-random codes are provided to the transmitter; placing a dipole receiver coli in the tunnel along the direction thereof, signals are received by the receiver; wherein the apparent resistivity of the geological body below the measured tunnel is obtained by analyzing the signals received by the receiver.

2. A method according to claim 1 wherein the angle between the bipole receiver and the dipole transmitter is less than 2°.

3. A method according to claim 1 wherein the formula of the iterative calculation for the apparent resistivity p is:

2 rcr3Enin cosP cos(p -a)-IpdL[2-e-'k'(1+ikr)]sin psin(p -a) IdL[l+e-'r'(1+ikr)]

and the receptor; (p is the azimuth angle of the cylindrical coordinate system, Emn is the observed electric field component; and a is the angle between the dipole transmitter and the dipole receptor.

4. An apparatus for conducting ground-tunnel wide-field electromagnetic geophysical exploration, the apparatus comprising a dipole transmitter coil placed over the ground, and a dipole receiver electrode placed in the tunnel along the direction thereof

5. An apparatus according to claim 12 wherein the angle between the dipole receiver electrode and the depole transmitter is less than 2°.