AU2020100497A4 - Transient Electromagnetic Detection Method with Cross-Like Grounded-Wire Source - Google Patents

Abstract

Abstract The present invention discloses a transient electromagnetic detection method with a cross-like grounded-wire source, and belongs to the technical field of transient electromagnetic detection methods. For the sake of better mining of geoelectric information contained in each component of a transient electromagnetic field of a grounded-wire source and further improvements in detection accuracy of the transient electromagnetic method with a grounded-wire source and detection capability thereof on 2D and 3D structures, the present invention provides a transient electromagnetic detection method with a cross or cross-like grounded-wire source. With the addition of a vertical or crossed emission source to a conventional rectilinear wire source, responses in the method of the present invention are overall stronger than responses of the conventional source, and in particular, the field strength amplitudes for an X-direction horizontal magnetic field and a Y-direction horizontal electric field are increased by several orders of magnitude; thus, the problem that such components for the conventional source cannot be effectively observed in the past is solved. Moreover, the resolutions to high-resistivity and low-resistivity anomalies are not weakened. Fig. 1

Description

Description

Transient Electromagnetic Detection Method with Cross-Like Grounded-Wire Source

Technical Field

The present invention particularly relates to a transient electromagnetic detection method with a cross-like grounded-wire source, and belongs to the technical field of transient electromagnetic detection methods.

Background Art

As an important branch of transient electromagnetic methods, transient electromagnetic methods with electric sources have played an important role in the explorations of metal mines, oil and gas resources and geothermal resources in the medium-deep part. A long-offset transient electromagnetic method (LOTEM) and a short-offset transient electromagnetic method (SOTEM) have been mainly developed according to ratios of transmitter-receiver separations to target detection depths.

In the early stage of development, a transient electromagnetic method with an electric source follows the observation mode of a frequency-domain artificial-source electromagnetic method, such as a controlled-source audio magnetotelluric method (CSAMT), for observation with an offset 3-6 times greater than the depth of a detection target, and thus is called long-offset TEM (LOTEM). In LOTEM, a grounded-wire source is taken as an electric dipole source. The formation conditions in an oil and gas resource occurrence area are in line with a ID layer structure, so LOTEM is used in oil and gas resource exploration with a large depth. Similar to CSAMT, theoretically all the measuring points are located in the far zone, ensuring that the main components of the responses are from the ground wave. The responses of an observation point are mainly from below the measuring points. However, in actual observation, measuring points are difficult to meet the observation conditions in the far zone and are mainly located in the medium deep zone or transition zone, and the responses of the measuring points often contain the information of a geological body between a transmitter and a receiver.

SOTEM is used for detection at a smaller separation between transmitter and receiver, where there are large differences between the components of the formation waves and the surface waves contained in the responses of the same measuring point at different times; the responses include

2020100497 31 Mar 2020 both geological information under the measuring points and the information of a geological body between a transmitter and a receiver; and such geological body is often 2D or 3D.

The traditional transient electromagnetic detection method with a single grounded-wire source has three main problems: 1) due to the power supply by a single grounded-wire, the current system formed in the ground is in a single direction and only suitable for detecting a one-dimensional electrical structure; 2) within a certain range, the response field strength is weak or it covers a zero-value zone and an electrically non-sensitive zone, thus affecting the selection of the observation components of the transient electromagnetic method with an electric source and the observation range; and 3) when the direction of the grounded-wire is consistent with the trend of an underground low-resistivity layer, the injected current is absorbed by the low-resistivity layer, which affects the detection effect.

Summary of the Invention

Therefore, to better solve the above problems and fully mine the geoelectric information contained in electric-source transient electromagnetic observation components in different directions, the present invention provides a transient electromagnetic detection method with a cross or cross-like grounded-wire source, wherein an emission system comprises a crossed emission source and a plurality of grounding electrodes; and three magnetic field components and two electric field components are simultaneously observed in an observation system, replacing a conventional observation device that only observes a horizontal electric field component and a vertical magnetic field component.

Specifically, the method comprises:

arranging a cross-like grounded-wire source, namely adding a crossed emission source to a rectilinear wire source, collecting data, and calculating three magnetic field components and two electric field components.

The present invention has the following advantages: the transient electromagnetic detection method with a cross-like grounded-wire source provided in the present invention can replace the traditional observation method in which only a horizontal electric field component and a vertical magnetic field component are observed. Responses in the method of the present invention are overall stronger than responses of the conventional source, and in particular, the field strength

2020100497 31 Mar 2020 amplitudes for an X-direction horizontal magnetic field and a Y-direction horizontal electric field are increased by several orders of magnitude; thus, the problem that such components for the conventional source cannot be effectively observed in the past is solved. Moreover, the resolutions to high-resistivity and low-resistivity anomalies are not weakened.

Brief Description of the Drawings

Fig. 1 is an arrangement diagram showing a cross-like grounded-wire source of the present invention.

Fig. 2a and Fig. 2b are schematic diagrams showing an X-direction magnetic field and a Y-direction electric field generated by a conventional emission source, respectively.

Fig. 3 is a schematic diagram showing a current direction of a cross emission source system.

Fig. 4a and Fig. 4b are schematic diagrams showing distribution characteristics (le-3s) of X-direction horizontal electric fields of a conventional source and a cross source, respectively.

Fig. 5a and Fig. 5b are schematic diagrams showing distribution characteristics (le-3s) of Y-direction horizontal electric fields of a conventional source and a cross source, respectively.

Fig. 6a and Fig. 6b are schematic diagrams showing distribution characteristics (le-3s) of X-direction horizontal magnetic fields of a conventional source and a cross source, respectively.

Fig. 7a and Fig. 7b are schematic diagrams showing distribution characteristics (le-3s) of Y-direction horizontal magnetic fields of a conventional source and a cross source, respectively.

Fig. 8 is a schematic diagram showing relative errors of electric field components of a conventional source and a new cross source in the case of a low-resistivity anomaly and no anomaly.

Fig. 9 is a schematic diagram showing relative errors of magnetic field components of a conventional source and a new cross source in the case of a low-resistivity anomaly and no anomaly.

Fig. 10 is a schematic diagram showing relative errors of electric field components of a

2020100497 31 Mar 2020 conventional source and a new cross source in the case of a high-resistivity anomaly and no anomaly.

Fig. 11 is a schematic diagram showing relative errors of magnetic field components of a conventional source and a new cross source in the case of a high-resistivity anomaly and no anomaly.

Fig. 12a and Fig. 12b are schematic diagrams showing attenuation curves of Y-direction horizontal electric fields of a new cross source and a conventional source, respectively.

Fig. 13a and Fig. 13b are schematic diagrams showing attenuation curves of X-direction horizontal electric fields of a new cross source and a conventional source, respectively.

Fig. 14a and Fig. 14b are schematic diagrams showing attenuation curves of X-direction horizontal magnetic fields of a new cross source and a conventional source, respectively.

Fig. 15a and Fig. 15b are schematic diagrams showing attenuation curves of Y-direction horizontal magnetic fields of a new cross source and a conventional source, respectively.

Detailed Description of the Invention

A specific embodiment of the present invention will be described below with reference to the accompanying drawings.

In the embodiment, provided is a transient electromagnetic detection method with a cross or cross-like grounded-wire source. A vertical or crossed emission source is added to a conventional rectilinear wire source; the responses of the cross grounded-wire source are subjected to forward calculation analysis, and compared with the response distribution of the conventional emission source; secondly, the resolution of each component of the new emission source is calculated, and compared with the resolution of a conventional observation component; and finally, field trials are carried out in a typical mining area to demonstrate the feasibility of the new detection method.

Fig. 1 is an arrangement diagram showing a cross-like grounded-wire source of the present invention, in which a transmitter 1, an emission wire 2 and a grounding electrode 3 are shown.

Derivation of response expressions

2020100497 31 Mar 2020

For a conventional single-direction emission wire source, a transient electromagnetic field of an X-direction grounded long wire source transmitted and received on a layered earth surface is expressed as follows:

Ids d x .z/₀ ., Joi,,.

Β_χ — I [0 Gm ) - (l + ^rf) J/JA/jt/A π ox r ^Jo v_n u_n z_nIds e· . λ , _z, _x ,, -4—L (1 + /^-)-Λ(^ΛΓ)^ 4π ^Jo u_n (formula 1) a⁼f«' '™ +% >-μ, w

4π ox r ^Jo y₀ z/₀ (formula 2) ///\' Λ V r Xi

4/r ox r

..... (formula 3)

Ids t) x r® yL v_n,

4π ox r ^Jo

Iflv f*GCi — ί (l-'^W'W

4?r^Jo (formula 4)

Ids v //, =---—

4tt r (formula 5) where ds is a the length of a dipole; (x, y, z) is coordinates of a receiving point; r is a distance between the receiving point and the dipole; ttm and γιε are reflection coefficients in TM and TE modes, respectively; and Ji(/.r) and Jo(7.r) are first-order and zero-order Bessel functions of the first kind, respectively.

Given an X-direction source length of 100 m and an emission current of 10 A, a geoelectric model is calculated as follows:

ρ1 = 100Ωιη, ρ2 = 10Ωιη, ρ3 = 100Ωιη, hl=400m; h2 = 10m

The magnetic field generated in X-direction and the electric field generated in Y-direction are distributed as shown in Fig. 2a and Fig. 2b.

2020100497 31 Mar 2020

The horizontal electric field and magnetic field in the two directions are extremely non-uniformly distributed, and have weak field strengths, which is not conducive to data collection and observation and also is one of the main reasons that the field observation is focused on an X-direction electric field and a vertical magnetic field.

For the electromagnetic field of a cross or cross-like source, responses can be considered as a combination of emission wire sources. In the emission current direction as shown in Fig. 3, taking a measuring point (x, y) between the sources as an example, the coordinates of (x, y) in the coordinate system of 12 source are denoted as (Χ2, y2) after coordinate system conversion. The electromagnetic field generated in each direction at the measuring point is expressed as follows:

£, = Γ[0-frv)^-(l + r_TE)^\J_x(kr_x)dX -^T(l +

4π ox η -ό ,v_u w₀ 4π ^Jo u₀

i.ds . , d v, 1·» . . u_{0 z}, . z₀.,_z, ζ₀/->ί/ό· cos p ·. . x,_z, —5— sin 0—— I [(1 - /'_rw )-Λ- (I + >„.)—y^M^dλ —----1 (1 + r_re)—J₀(2r, )dA

4π ox₂ r₂ ^Jo y0 u0 4π ^Jn i/0

i.ds , 8 x, 1·“ ti₀ ,, . ζ_0Ί,_ζ.

cos<f> I [(1 r_TM) . (1 + /‘_re) ]J,(ai'₂)dA π ox₂ r, ^Jn y₀u (formula 6) „ i.ds d v f®_rz. . u_a ,, χ2_0Ί,., ... fds , d v, c®_rz1 ,u₀

4π οχ io ^[° ^rrw)y0 ⁽ ‘^rc) u0 ^{] ,( 1}} 4^ ^C°^ dx₂ rj ^{[(1 r}™\ /0

-(1 + r_TE )^-P,(zr₂ )άλ - A_sin φ A % P_[(i _ _rTu )^--(1 + _ΓγΕ )£±]J,(A/₂ )άλ «ο fy₂ r₂ J» y₀w

z.,i,ds . , r®,, , λ , ,.

———sin0 (1 + r_TE)—J_a(Xr₂)dA

4π' u (formula 7) ,, i.ds d v t®, . „. , ,. , ,. i.ds . , d v, r ®, /fy =4—--1,, (fr,_w +fo:)^e 7,(Λη)ί/Λ-ή— sin0—— ^-1 (rrM + rTE)e “ Jx(Ar2)dA 4π ox i\ ^Jo 4π dy2 r₂ ^Jo

- —-sin^i (\-r_TE)e^u^aJ0(kr2)dλ + ——cos^---— i (rrw + rTE)e^Uo'Jx(kr₂)dλ

4π ^Jo 4π dx₂ r₂ ^Jo (formula 8) ,, i.ds 0 x r®, , „ ., ,. ,,. i.ds r®,, // — - — J (/_rM + i'_rE)e ° J_l(Ai'_i)dA - J (1 ~i'_TE)e ° Aj_Q(Ai\)dλ

4π ox η ·ό 4π ^Jo i₂ds . , 5 x, r® _z , ,, ,. .,. i₂ds . 0 x, r®, . , , 1 .,+ — Sin0— I (/7-5/ + )<? )ί/Λ 4-— COS^-^ I (/_rv 4/_ΓΑ-)^ ^|(λ/₂)ί/Α

4π oy₂ r₂ ^Jo 4π ox₂ r₂ ^Jo + ——cos^£ (I - r_TE )e“^a' aJ₀(λ/₂ )dA

2020100497 31 Mar 2020 (formula 9) ,, i.ds V. r® ., , λ² , , . ,,. i.ds v, r ®,, , ,. . λ² , ,. ,,.

H.=-——I (I + r_Tt:) e ⁰ --J i(A>\)d Λ—-—— (1 + r_r£. )e “ --J|(A/₂)i/A

4π η ^Ju tt0 4π r, ^Jo uQ (formula 10) where φ is an included angle between two source currents ii and Ϊ2; / 2 2” / 2 7

/] — ajJC + _y i ^f2 ⁼ ++2 Ji(kr) and Jo(kr) are first-order and zero-order Bessel functions of the first kind, respectively; z represents a burial depth of a receiving point, which is zero in case of surface observation;

y _y „ _ ^Z0__^J1 v _ ^U0 7 _ ^WU 'TE ~ _v Λ ^Z0~“

F_o + ξ , z₀ , ₅ z₀ = ιωμ_ϋ, y₀ = ιωε₉ *

In practical use, different emission combinations can be chosen according to the response characteristics of different components.

Analysis of response distribution characteristics

Given an X-direction source length of 100 m and an emission current of 10 A, a geoelectric model is calculated as follows:

pl = ΙΟΟΩγπ, p2= ΙΟΩιτι, p3 = ΙΟΟΩηι, hl =400m; h2 = 10m

Electromagnetic field responses of conventional and cross sources are calculated according to formulas (1)-(5) and (6)-(10), respectively, and the calculation results are as shown in figures 4a, 4b, 5a, 5b, 6a, 6b, 7a and 7b.

By comparison, responses in the method of the present invention are overall stronger than responses of the conventional source, and in particular, the field strength amplitudes for an X-direction horizontal magnetic field and a Y-direction horizontal electric field are increased by several orders of magnitude; thus, the problem that such components for the conventional source cannot be effectively observed in the past is solved.

The resolution of each component of the new source is analyzed by calculating a relative error between different geoelectric model responses. The relative error is calculated according to a formula as follows:

2020100497 31 Mar 2020 η = 2x x 100% (formula 11) where F_a represents a response with an anomaly body, and F represents a response with no anomaly body; and

K and H represent models with high resistivity and low resistivity, respectively, with model parameters as shown below:

Η: pl = 100Qm, ρ2=10Ωιη, ρ3 = 50Ωιη, hl = 100m; h2=10m

K: pl = Ι00Ωιη, ρ2=1000Ωιη, ρ3 = 50Ωιη, hl = 100m; h2=10m

D: pl — ΙΟΟΩιη, ρ2 = 50Ωιη, hl = 100m.

Point (80, 200) is taken for example to analyze resolutions.

Fig. 8 shows relative errors of electric field components of a conventional source and a new cross source in the case of a low-resistivity anomaly and no anomaly. As shown in Fig. 8, the relative error of the Y-direction component of the conventional source is larger. However, even when no interface is encountered in the early stage, a large relative error may exist, affecting the determination of a low-resistivity abnormal burial depth. Moreover, the Y-direction electrical field component and the X-direction electrical field component of the new source are similar, and the relative error appearing at the moment corresponding to the low-resistivity abnormal burial depth is obviously greater than those at other moments, providing a good resolution to a low-resistivity body.

Fig. 9 shows relative errors of magnetic field components of a conventional source and a new cross source in the case of a low-resistivity anomaly and no anomaly. As shown in Fig. 9, the magnetic field component of the new source is similar to the magnetic field component of the conventional source, and the relative error appearing at the moment corresponding to the low-resistivity abnormal burial depth is obviously greater than those at other moments, providing a good resolution to a low-resistivity body.

Fig. 10 shows relative errors of electric field components of a conventional source and a new cross source in the case of a high-resistivity anomaly and no anomaly. As shown in Fig. 10, the relative

2020100497 31 Mar 2020 error of the Y-direction component of the conventional source is larger. However, even when no interface is encountered in the early stage, a large relative error may exist, affecting the determination of a high-resistivity abnormal burial depth. Moreover, the Y-direction electrical field component and the X-direction electrical field component of the new source are similar, and the relative error appearing at the moment corresponding to the high-resistivity abnormal burial depth is obviously greater than those at other moments, providing a good resolution to a high-resistivity body.

Fig. 11 shows relative errors of magnetic field components of a conventional source and a new cross source in the case of a high-resistivity anomaly and no anomaly. As shown in Fig. 11, the magnetic field component of the new source is similar to the magnetic field component of the conventional source, and the relative error appearing at the moment corresponding to the high-resistivity abnormal burial depth is obviously greater than those at other moments, indicating that the resolution of the magnetic field component of the new source to a high-resistivity body is not weakened.

In general, the response amplitude of the new source is greater, and especially for an X-direction horizontal magnetic field of (X-direction emission source) and a Y-direction horizontal electric field, such field strengths may be increased by several orders of magnitude; moreover, the resolutions to high-resistivity and low-resistivity anomalies are not weakened.

Example of Use

Exploration area is located in a mining area of Datong City, Shanxi Province. The mining area has such main coal-bearing strata as Taiyuan Formation of Upper Carboniferous, Shanxi Formation of Lower Permian and Datong Formation of Middle Jurassic, and is a typical representative of double-system coalfields in North China. The basic structural form of Datong coalfield is a syncline structure, which is located between the Ordos stable deformation region and the eastern extension deformation region, and the deformation of the coal series is characterized by compression-extension transition. The surface layer of the coal mine is covered by Quaternary loess with vertical and horizontal gullies and complex topography. Coal mining has brought a series of goaf problems.

In this work of SOTEM, V8 integrated electrical prospecting apparatus from Canada Phoenix corporation was employed to collect data. With an emission source having a length of N300+E300

2020100497 31 Mar 2020 m, an emission current of 10 A, fundamental emission frequencies of 25 Hz and 8.33 HZ, and an emission power of 30 Kw and taking the center of two crossed sources as the origin of coordinates, resources of a conventional single emission source and the crossed sources were observed at measuring point (300, 200), respectively.

Fig. 12a and Fig. 12b, and Fig. 13a and Fig. 13b show attenuation curves of horizontal electric fields of a new cross source and a conventional source, respectively. By comparison, it could be found that the Y-direction horizontal electric fields of the two sources were significantly different, and the new source had stronger response field strength with no reverse sign. The X-direction horizontal electric fields were less different.

Fig. 14a and Fig. 14b, and Fig. 15a and Fig. 15b show attenuation curves of horizontal magnetic fields of a new cross source and a conventional source, respectively. By comparison, it could be found that the Y-direction horizontal magnetic fields of the two sources were significantly different, and the new source had stronger response field strength. The Y-direction horizontal magnetic fields were less different.

While the foregoing are descriptions of the preferred embodiment of the present invention, it will be appreciated by those of ordinary skill in the art that numerous modifications and adaptations may be made without departing from the principles of the invention, and such modifications and adaptations are to be considered within the scope of protection of the invention.

Claims (3)

1. Claims

   1. A transient electromagnetic detection method with a cross-like grounded-wire source, characterized in that the method comprises:

   arranging a cross-like grounded-wire source, namely adding a crossed emission source to a rectilinear wire source, collecting data, and calculating three magnetic field components and two electric field components;

   wherein specific calculations are as follows:

   through coordinate system conversion, coordinates of a point (x, y) of the rectilinear wire source in the coordinate system of the crossed emission source are denoted as (x2, y2); and an electromagnetic field in each direction generated at a measuring point is expressed as follows:

   = [0 -)^- (1 + )dAJ-(1 ₊ r_rE)-J₀(Ar_{)dA

   4π ox /*y_Q u₀ 4π ^J0 u₀ i-fds . ,d v₂ f®., ., zJ^dscosφ r* , Λ _ ..

   sm φ—— I [(1 - r_nt (1 + >'_TF)— J/Ufi~„ (' + r_TE)—J_a(Ar₂)dA

   4π ox₂ r₂ y₀ w„ 4πu

   -—cos φ4~— L [(1 - i'tm - (1 + Γ_τε))dλ

   4π ox₂ r₂ y₀u formula 1 i_xds d

   4π dx v ptx> li

   4₀ [(l-frv)^-(l + 'fr) >\ To (Ar_t )dA + —cos φ o

   d cx₂

   

   _r ¹ TM f *

   Vo

   -(1 + r^^J^dA -^-sin φΓ[(1 -r_m (1 + r_TE)^-\J^Ar₂)dA »o 4zr oy, r₂ ^Jo y₀ u₀ z_ni,ds . , r⁰⁰ _z. , A , ,, ,., —²T—sin¢( (1 + r_TE)—J₀(2i₂)d2

   4π -¹⁰ formula 2 „ i,ds d v . Lds · , 5 V, r®. . , ,, , ,, //_x =4———L (frv (^λ'\ —^sin^—— L ('™ J_{(2r₂)dk

   4π ox i\ ^Jo 4π oy, r, ^Jo

   - sin(1 - r_TE)e“°'AJ₀(Ar,)d2 + cosφ (r_TM + o_re)e⁰'J_t(Ai\)dλ

   4π * 4π dx₂ r, ^Jo formula 3

   2020100497 31 Mar 2020 ///v r ι»® /z/vi·® //,. =-J_-U_f (,-_ni + r_TE)e“°^J_}(7r\)dλ -[ (1 -r_TE)e^λ^λ,^Ιλ

   4π ox i] ^Jo 4π ^Jo i-,ds . . 8 x, (*>, . „. . .. ... i,ds . δ x, r®. . „ . , ., ...

   + =—sin0--=- (r_rw + r_TE)e^t~J_t(4r₂)d7 + -—cos^---I (r_TM + r_TE)e ’/(Χ/^ί/Ζ

   4π oy₂ r₂ ^Jo ‘ 4ττ δχ₂ r₂ ^Jo + h^L_C0S<z)f (1 - r_TE)e“⁰'7J₀(7i₂)d7

   4π ^Jo formula 4 // = J (i ₊ ,._{te )e}^ *L_{J{ (Α/}- )dA - ^£(1 _{+ r}-_TE )_e“- — J, (λγ₂ )dz 4π η ^Ju w0 4π r2 ^J(l u0 formula 5

   I 2 , Γ

   Π “ V^x + V where φ is an included angle between two source currents 11 and 12; ^v ' and

   I 2 Γ / * — [χ Ί^- V
2. 2 v 2 -- represent the values of separation between receiver and transmitter for the measuring point in the two coordinate systems; ds represents a source length; Ji (/+) and Jo(kr) are first-order and zero-order Bessel functions of the first kind, respectively, with λ representing a variable related to a wave number and r representing the separation between receiver and transmitter; z represents a burial depth of a receiving point, which is zero in case of surface y _ γ _ ⁷o 'i observation; Ge ^—~ represents a reflection coefficient of electromagnetic wave when an ^yo + ^}'ί electric field component is incident on plane;

   represents a reflection coefficient

   Y of electromagnetic wave when a vertical magnetic field is incident on plane; ^zo represents an intrinsic admittance of free space and ' represents a surface admittance; _ u„ — v represents intrinsic impedance of free space and Zj represents surface impedance; the +o surface impedance and the surface admittance are obtained by bottom-up recursion, ; ω represents an angular frequency; i represents a plural number; ® represents an dielectric coefficient of underground homogeneous half-space; and [0 represents a magnetic conductivity of underground homogeneous half-space.

   2. The transient electromagnetic detection method with a cross-like grounded-wire source of claim

   1, characterized in that a V8 integrated electrical prospecting apparatus is employed to collect

   2020100497 31 Mar 2020 data.
3. 3. The transient electromagnetic detection method with a cross-like grounded-wire source of claim

   1, characterized in that the emission source has a length of N300+E300 m, an emission current of

   10 A, fundamental emission frequencies of 25 Hz and 8.33 HZ, and an emission power of 30 Kw.