AU2020101905A4 - A method for determining middle zone exploration depth of electromagnetic exploration - Google Patents

Abstract

The application discloses a method for determining the detection depth of a transition zone. The method includes: determining the range of the transition zone, determining the detection depth H of the transition zone according to (1-a)H+a-H wherein, a is the proportion of formative wave, H, is the detection depth of far-field induction sounding, and HA is the detection depth of near-field geometric sounding. The device includes: a transition zone determination module configured to determine the transition zone range; and a calculation module configured to determine the detection depth H of the transition zone according to (1-a)H+a-Hh I sky wave observation pomnt source surface wave body wave Fig. 1 near zone transitional far zone Fig. sou t1 t2 Fig. 2

Description

sky wave observation pomnt source surface wave

body wave Fig. 1

near zone transitional far zone sou

t1 t2 Fig.

Fig. 2

A METHOD FOR DETERMINING MIDDLE ZONE EXPLORATION DEPTH OF ELECTROMAGNETIC EXPLORATION

Field of the Invention

The invention relates to the field of electromagnetic geophysical exploration. More particularly, this invention relates to a method for determining middle zone exploration depth of electromagnetic exploration.

Background of the Invention

The electromagnetic wave generated by dipole antenna actually radiate in all

directions for artificial source frequency domain long-grounded wire source. These

waves can be divided into sky wave, surface wave, and formative wave.

For traditional small-scale artificial source frequency domain electromagnetic

methods, for example, controlled source electromagnetic methods (CSEM), have been

developed based on MT method. This method is able to obtain apparent resistivity and

impedance phase by measuring the electromagnetic field of land surface and sea-bed,

respectively, which can then be used to map underground electrical distributions . It

has the advantages of large detection depth, high resolution, and strong

anti-interference abilities. Therefore, it has been widely used in the exploration of oil

and gas resources, mineral resources, geothermal studies, and hydrocarbon

exploration.

Estimation of the depth of detection is an important for electromagnetic methods.

theoretically, the electromagnetic signals at any frequency and at any time exist within

all depths, however, limited by the instrument accuracy and noise level, only a apart

of stronger signal can be observed and utilized, so the actual detection depth is not

enough large. The detection depth can be calculated by the skin depth, which is a very

important concept in frequency domain electromagnetic methods. Generally, it is

defined that the depth of plane electromagnetic wave attenuation to 1/ e (about 37%) of ground amplitude in underground medium propagation is skin depth.

With respect to MT field and far field CSAMT, based on plane wave treatment, the

skin depth calculation formula can be used to estimate the depth of exploration.

However, in CSAMT middle field zone and near zone, electromagnetic wave cannot

be regarded as plane wave, and the formula of plane wave skin depth calculation is no

longer applicable. At this time, if we still use this formula to calculate the depth of

detection, there will be a big error.

In the field exploration, the actual detection depth is also affected by the sensitivity of

the instrument, the noise level and the complexity of geological structure.

When we collecting the field data from far field zone near to middle field zone, the

advantages of large intensity, small static effect, high resolution and high construction

efficiency of the near source detection signal will be earned. When working in the

middle field area, it is necessary to accurately divide each position and the sounding

mode of each frequency point, so that the detection depth can be accurately defined.

In regard to the land CESM, on the basis of the propagation path, the electromagnetic

waves which are generated by the dipole antenna can be divided into sky wave,

surface wave and formative wave (as shown in Fig. 1). The wavelength of the

electromagnetic wave in the sub-surface tend to be much shorter than those in the air.

Therefore, at a certain time (t), the wave which are propagating almost vertically

downward will tend to build based on the wave path difference of the surface and

formative wave. These waves are used for traditional far-source field detection.

However, when detecting in middle field zone, these waves will not propagate almost

vertically downward.

As shown in Fig.2, means far zone; means near zone. where r is the distance between

the transmitting source and the receiving point, represents the wave number of the

electromagnetic wave.

Controlled source audio magnetotelluric sounding (CSAMT) requires collecting data

in far field zone. The length of grounded dipole AB is generally 1-3 km. alternating current is induced down to earth. The data is generally collected in an area of 60 angles on either side of the AB, the survey line direction is parallel to the transmitter line direction. (as shown in Fig. 1).

Controllable source audio magnetotelluric sounding is one kind of electromagnetic sounding method which takes a limited long grounding wire as the field source and simultaneously observes the electrical and magnetic field parameters at a certain distance from the center of the field source. although the CSAMT method should work in the far area, it is also impossible to be too far from the field source to make the signal difficult to observe. Actual observation points cannot be too close to the field source to avoid the near region not satisfying the CSAMT detection principle; the intermediate transition region detection is inevitable. But the existing method is not clear enough to divide the transition zone, and the calculation of the detection depth of the transition zone is not complete, which leads to the low accuracy of the detection results.

Summary The invention resolving the technical issue is to provide an electromagnetic detection method for middle field zone data collection and the calculation of exploration depth, which can be applied to obtain the information of afiner geological target body with middle field zone surveyed data. The methods described include:

Determination of the transition zone;

Determination of the detection depth He of the transition zone according to the

following formula:

H =(1-a)- H + a- H,

where H, is the corrected depth satisfying the transition region condition;

H,5 = 2 is the depth of inductive sounding satisfying the formative wave

AB region conduction; H,, = is the depth of the geometric sounding satisfying 2 surface wave region conduction; and a is the proportion of the formative wave,

which is related to the frequency, resistivity, and offset.

Preferably, the range of identified intermediate areas includes:

a can be used to divide near field zone, middle field zone, and far field zone,

that is, a < A represents far field zone; a > -- A represents near field zone

A: a ! 1- A represents middle field zone.

Preferably, the value of the A is5%.

Preferably, the method for calculating the depth Ha of the far field zone sounding is

as follows:

H = 2

It is assumed that the conductivity 0 and magnetic permeability '" are frequency

independent, and that the displacement currents could be ignored. 6 is circle

frequency.

Preferably, the method for calculating the depth of the near- field zone is as follows:

H,=AB 2

where A and B represent the two end positions of the grounded dipole wire

respectively, AB presents the length of the grounded dipole wire.

Engineering technicians in this field will be more aware of the above advantages and

characteristics of the application in the light of the detailed description of the specific

embodiments of the application in conjunction with the attached drawings below.

Brief Description of the Drawings

The attached drawings are used to provide a further understanding of the technical scheme of the invention and form part of the specification. Together with the embodiment of the present application, the technical scheme used to explain the invention does not constitute a restriction on the technical scheme of the invention. Some specific Auxiliary materials of this application will be described in detail in an illustrative rather than restrictive manner with reference to the accompanying drawings. the same attached drawing marks in the attached drawings indicate the same or similar parts or parts. Technicians in this field should understand that these drawings are not necessarily drawn in proportion.

Fig.1 is the propagation path of sky wave, ground wave and formation wave in background technology.

Fig. 2 is the division of the far and near regions of the background technology. tl and t2 represent the tl time and the t2 time, respectively. S Iand S2 present the body wave and the ground wave, respectively;

Fig. 3 is a configuration diagram of CSAMT field work arrangement in background technology.

Fig. 4 is a schematic flow chart of a method for determining the depth of detection in the transition zone for the embodiment of this application.

Fig. 5 is a plane distribution diagram of formative waves at different frequencies with a half-space resistivity of 100 Ohm-m, in which (a) the frequency is 1000 Hz, (b) the frequency is 500 Hz, (c) the frequency is 100 Hz, (d) the frequency is 10 Hz, (E) the frequency is 1 Hz, and (f) the frequency is 0.1 Hz.

Fig. 6 is a plane distribution diagram of formative waves with different half-space resistivity at 100 Hz in the embodiment of the application, wherein: (a) resistivity is ohm-m, (b) resistivity is 100ohm-m, (c) resistivity is 500ohm-m, (d) resistivity is 1000ohm-m, (E) resistivity is 5000ohm-m, (f) resistivity is 10000ohm-m.

Fig. 7 is a schematic diagram of the variation of formative wave and ground wave occupation ratio with frequency at different measuring points when the half-space resistivity is 100 Ohm-m, in which: (a) coordinate of measuring point is (0,5000), (b) coordinate of measuring point is (0,10000), (c) coordinate of measuring point is

(0,15000), (d) coordinate of measuring point is (0,20000).

Detailed Description of the Embodiments

Embodiment 1

The embodiment of the application method for determining middle zone exploration

depth of electromagnetic exploration, which generally includes the following steps (as

shown in figure 4):

Step Sl. determining the range of transition zone:

According to the proportion a of formative wave, it can be divided into near area,

transition area and far area, that is, a <A represents far area, a >1-- A

represents near area. A ! a I - A represents transition area, in which the optimal

value of A is 5%, a is related to frequency, resistivity and transmitting-receiver

distance;

Step S2. determining the detection depth He of the transition zone according to the

following formula:

H =(1-a)- H + a- H,

where He is the corrected depth satisfying the transition region condition;

Ha =,= 2 is the depth of inductive sounding satisfying the formative wave

AB region conduction; H,, = is the depth of the geometric sounding satisfying 2 surface wave region conduction; and a is the proportion of the formative wave,

which is related to the frequency, resistivity, and offset.

It is assumed that the conductivity 0 and magnetic permeability 'o are frequency independent, and that the displacement currents could be ignored. CO is circle frequency. A and B represent the two end position of the grounded dipole wire respectively, AB presents the length of the grounded dipole wire.

In land CSEM exploration, the field of electric dipole or magnetic dipole on the ground is often studied. Let the earth be a homogeneous isotropic conductive medium, and there is a harmonic current in the grounded dipole Il located at the origin of the coordinate (=0e(-it), I0 is the amplitude, t is the time, and I is the length and direction of the dipole). According to the image principle, the vector potential A (unit T-m) of the horizontal electric dipole has a component Ax along the electric moment and Az perpendicular to the earth-air interface. Considering the connection condition and ignoring the displacement current, the vector potential Ax] and Az1 can be derived (the subscript indicates that they are the components in x and z directions respectively, and 1 represents the underground area).

dA R

(3) R

N =lo ( +: 4N (R -: (4)

Il a2p a3N E = ( + ) (5) 2ro az2 ay2az)

El = -1 a3N (6 )

2rcuaxayaz

E = (7) 2roaxaz

H = (8) ( 2rck 2 Oxayaz Dxayaz2 )

Il _( (+ + 4)N (9) a p H-- H_=- a3p 2ck27 k2 3 az2 az Ox 2 az

I/ 04 N 202 N 83P H =- ( +k) (10) 2,7k 2 yaz I yaz ayaz2

Where P and N are Sommerfeld integral and Foster integral respectively, where k is the wave number in the earth, R is the distance from the field point to the source point, I0 and KO are the second kind of zero-order Bessel functions.

In step SI, the horizontal component of the electric field is:

Ii 02p a3 N E =--(-+) 2;rca Oz2 O28z

Formative wave:

3ikZ 2 32 82p 1 2 I I kR 2 E-= e' (ik-1-k z - +- )

27raz 2 2ru R R R2

Surface wave:

I 3 8ON Il ik I1IK I K 1+C10IK] 2;rca ay28Oz 2;rca 2 R2

Where, coo,c 11 ,cO 1 ,c 1 is the calculation coefficient, I, K1 are the second kind of

first-order Bessel functions.

When z=0,

Formative wave:

E=- eikR 1 2rca R3

Surface wave:

Il ik 1 3Y2 tjR2j )

2,rca 2R 2

In order to obtain the values of the field components in formulas (5) ~ (10), we need only to find the partial derivatives of Sommerfeld and Foster integrals P and N.

Although the mathematical method of derivation is common, the derivation process is

very complicated. EX, Hyl and Hzl contain partial derivatives of P and N. The

physical significance of underground closed form solution is that Forter integral term

N represents surface wave component, while Sommerfeld integral term P has

exponential attenuation relation, which represents formative wave directly

propagating underground. The closed form of underground gives explicit terms of

formative wave and surface wave, which is the basis of studying field area and

sounding method.

Theoretically speaking, the response at any observational point can be regarded as the

sum of the formative and surface wave. The ratios of these two terms are known to

change with the variation in the observational location, frequency, and resistivity.

E a= = x100% E , + 'E

when a < 5%, the effect of the formative wave could be ignored and the surface

wave is dominant. The area could then be regarded as a surface wave region with

inductive sounding (similar to a far-field zone).

when a > 95%, the effect of the surface wave could be ignored and the formative

wave is dominant. The area could then be regarded as a formative wave region with

geometric sounding (similar to a near-field zone).

Therefore, for the transition region which satisfied 5%<a <95%, the surface and

formative waves coexist. Also, both inductive and geometric sounding exist in

different proportions at the same time.

In the step S2, when the formative wave proportion gradually increases, the induction

sounding does not dominate, and the geometric sounding gradually takes the

dominant position, the use of this depth formula will bring large error. For the area dominated by geometric sounding, the detection depth is independent of frequency, that is, direct current sounding. In order to solve the problem of detection depth in the area where the proportion of formative wave and surface wave is equivalent, namely, the transition area, the implementation mode proposes the following functions:

H =(1-a)- H +a- H,

Where, He is the corrected depth, Ha is the depth formula of induction sounding,

2 H , = HF AB '0), Hb is the depth formula of geometric sounding 2 .=

Fig.5 shows the plane distribution of formation waves at different frequencies with

resistivity of 1000hm-m in half space of the application example, where (a) frequency

is 1000 Hz, (b) frequency is 500 Hz, (c) frequency is 100 Hz, (d) frequency is 10 Hz,

(e) frequency is 1 Hz, and (f) frequency is 0.1 Hz. It can be seen from figure (a) that

when the half space resistivity is fixed and the frequency is 1000 Hz, the wave ratio of

the formative wave is basically reduced to 5% when X = Om and Y = 1000m. At this

time, the area with Y greater than 1000m is the far area, which is induction sounding.

The area where the wave ratio of formative wave is more than 95% is near area,

which is geometric sounding. Induction sounding and geometric sounding coexist in

other areas, but their proportions are different. It can be seen from figures (b), (c), (d),

(e) and (f) that with the gradual decrease of the frequency, a further distance is

required to meet the condition that the wave occupation ratio of the formative wave is

less than 5%, that is, the lower the frequency, the further distance is needed to meet

the far-field conditions. When the frequency is 1 Hz, the Y direction needs to be more

than 21 km to meet the strict far-field conditions, that is, induction sounding

conditions, while when the frequency is 0.1 Hz, the Y direction needs to be greater

than 66km to meet the strict far-field conditions. At this time, if we observe and

calculate the detection depth of this frequency in the near area, we need to consider

the induction sounding and geometric sounding comprehensively. At this time, the

traditional depth formula based on induction sounding is very inaccurate.

Fig.6 shows the plane distribution diagram of formation waves with different

half-space resistivity at 100 Hz, where (a) resistivity is 1OOhm-m, (b) resistivity is

OOhm-m, (c) resistivity is 50OOhm-m, (d) resistivity is 100OOhm-m, (e) resistivity

is 500OOhm-m, (f) resistivity is 1000OOhm-m. It can be seen from figure (a) that

when the frequency is 100 Hz and the half-space resistivity is 10 Ohm-m, the wave

ratio of the formative wave is basically reduced to 5% when X = 0 m and Y direction

is more than 1000 m. At this time, the area with Y greater than 1000 m is the far area,

which is induction sounding. For the area where the ratio of formative wave is more

than 95%, it is near area, which is geometric sounding. In other areas, induction

sounding and geometric sounding coexist, but their respective proportions are

different. It can be seen from figures (b), (c), (d), (e) and (f), with the increase of

resistivity, the farther distance is needed to meet the condition that the wave ratio of

formative wave is less than 5%. That is, the greater the resistivity is, the farther the

distance is needed to meet the far-field conditions, so as to achieve the purpose of

induction sounding. When the resistivity is 5000 Ohm-m, the Y direction needs to be

more than 14 km to meet the strict far-field conditions, that is, the induction sounding

conditions. When the resistivity is 10000 Ohm-m, the Y direction needs to be greater

than 21 km to meet the strict far-field conditions. At this time, if we observe and

calculate the detection depth of this resistivity in the near area, we need to

comprehensively consider induction sounding and geometric sounding, while the

depth formula obtained by induction sounding is very inaccurate.

Fig.7 shows the variation of the ratio of formative wave and surface wave with

frequency at different measuring points when the half-space resistivity of the

application example is 100 Ohm-m. It can be seen from figure (a) that for the point

located at (0,5000), when the frequency is 0.1 Hz, the formative wave occupation

ratio is 33%. With the gradual increase of frequency, the formative wave occupation

ratio gradually decreases, and the surface wave occupation ratio gradually increases.

When the frequency reaches about 17 Hz, the formation wave occupation ratio

decreases to 5%. For this point, the frequency greater than 17 Hz is induction sounding, and less than 17 Hz is geometric sounding. It can be seen from figures (b),

(c) and (d) that with the increase of offset, the wave occupation ratio of formative

wave decreases gradually, while that of surface wave increases gradually. More and

more frequency points belong to induction sounding. When the measuring point is

located at (0, 20000) and the frequency is 1 Hz, the formation wave ratio is reduced to

%, that is, for the measuring point, the frequency points with frequency greater than

1 Hz are induction sounding, and the points with frequency less than 1 Hz are

geometric sounding.

Table 1 shows the minimum frequency of induction sounding at a certain offset under

different geoelectric conditions. For the same offset, with the increase of resistivity in

half space, the minimum frequency point gradually increases, but the corresponding

detection depth is basically unchanged. This shows that, when the offset is fixed,

although the resistivity in half space will change and the minimum frequency point

satisfying induction sounding will change, the corresponding detection depth will

remain basically unchanged. Moreover, the detection depth increases with the increase

of offset.

Table 1 Minimum frequency and corresponding depth under different geoelectric

conditions

Depth Resistivity (Ohm-rn) Offset(in) Frequency (Hz) (n (M) 5000 17.25 1212 10000 4.3 2428 5000 86.3 1212 10000 21.5 2428 5000 172.5 1212 10000 43 2428 5000 863 1212 10000 215 2428 5000 1725 1212 10000 430 2428

When the formative wave ratio increases gradually, the induction sounding does not play a dominant role, and the geometric sounding gradually occupies the dominant

H= 2 position, the use of depth formula '0 will bring large errors. For the area dominated by geometric sounding, the detection depth is independent of frequency, that is, direct current sounding. In order to solve the problem of detection depth in the area where the proportion of formative wave and surface wave is equivalent, namely, the transition area, the following function is constructed in the embodiment:

H = (1-a)- H +a- H,,

According to the above analysis, when the offset is 5000 m, the lowest frequency satisfying induction sounding under different resistivity can be obtained. At this time, for lower frequency, the transition zone conditions are met, including induction sounding and geometric sounding. The detection depth can be calculated by using the above formula (as shown in table 2). When the resistivity is 100 Ohm-m, the minimum frequency of induction sounding is 17.25 Hz. For the lower frequency points of 10, 5, 1 and 0.1 Hz, the detection depth is calculated by the above formula.

Table2 Detection depth of the transition region under different geoelectric conditions

Resistivity Frequency Depth (Ohm-m) Offset(m) (Hz) (M) 10 1435 5 1861 100 5000 1 380 1 3580 0.1 10675 70 1255 40 1560 500 50001025 10 2658 1 7607 150 1223 50 1861 1000 5000 10 380 10 3580 1 10675

500 1435 100 2657 5000 5000 10 7607 10 7607 1 23862

Fig.8 is a structural diagram of a transition zone detection depth determination device according to the embodiment of the application. The device may generally include:

A transition zone determination module 1 configured to determine the transition zone

range, and a calculation module 2 configured to determine the detection depth H, of

the transition zone according to the following formula:

HC = (1-a)- Ha+ a- H1,

Among them, a is the proportion of formative wave, H, is the detection depth of

far-field induction sounding, H. is the detection depth of near-field geometric sounding. According to the proportion a of formative wave, it can be divided into near area, transition area and far area, that is, a < A represents far area and a > 1- A

represents near area. A a 1- A represents transition area, in which the optimal value of A is 5%, a is related to frequency, resistivity and transmitting-receiver distance.

In the calculation module 2, the H, calculation formula is H =, is the

earth conductivity, po is the earth permeability, 0) is the electromagnetic wave AB circular frequency, H. calculation formula is H, = , A and B are respectively 2 the two ends of the grounded wire, AB is the length of the grounded wire.

The transition area detection depth determination device of the embodiment can perform the steps of the transition area detection depth determination method.

A method and device for determining the detection depth of transition area in the embodiment of the application effectively utilizes the advantages of high signal intensity and small static effect of near-source detection to advance the observation area of artificial source frequency domain electromagnetic method to the near-source area. Starting from the basic theoretical formula of grounding long wire source, the embodiment studies the field characteristics according to the expressions of formative wave and surface wave. On the one hand, the plane distribution characteristics of formative wave and surface wave are calculated when resistivity and frequency change in half space, and according to the proportion of formative wave and surface wave, the conditions satisfying induction sounding and geometric sounding are discussed, and the judgment basis is given. On the other hand, compared with the traditional method of using the skin depth formula applicable to the far area to calculate the detection depth of the transition area, this embodiment redefines the detection depth of the transition area according to different sounding methods, which can more accurately calculate the detection depth of the transition area. The research results can be used as a reference for the selection of field measurement parameters and data processing and interpretation.

The embodiment of the application also provided a computing device, as shown in

Fig.9, which included a memory 1120, a processor 1110 and a computer program

storedin 1120 andcapable ofbeingrunby 1110. The computer program was storedin

space 1130, which was in 1120 for storing program code, and any step 1131 of the

invention could be performed by the processor 1110.

The embodiment also provided a computer-readable storage medium. As shown in

Fig.10, the medium included a storage unit for storing program code, which was

provided with a program 1131' for executing steps, which was executed by a

processor.

The embodiment of the application also provides a computer program product

containing instructions. When the computer program product is run on a computer, the

computer will perform the steps according to the invention.

In the above-mentioned embodiment, it can be realized in whole or in part by software, hardware, firmware or any combination thereof. When implemented by software, it can be realized in the form of computer program products in whole or in part. The computer program product includes one or more computer instructions. When the computer loads and executes the computer program instructions, the process or function described in the embodiment of the present application is generated in whole or in part. The computer can be a general-purpose computer, a special-purpose computer, a computer network and other programmable devices. The computer instructions may be stored in a computer-readable storage medium or transmitted from one computer-readable storage medium to another. For example, the computer instructions may be transmitted from a website site, computer, server or data center through wired (e.g., coaxial cable, optical fiber, digital subscriber line (DSL)) or wireless (e.g., infrared, wireless, microwave, etc.) to another website site, computer, server or data center. The computer-readable storage medium can be any available medium that can be accessed by a computer or a data storage device such as a server, a data center and the like containing one or more available media integration. The available media may be magnetic media (e.g., floppy disk, hard disk, magnetic tape), optical medium (e.g., DVD), or semiconductor medium (e.g., solid state disk (SSD)) and the like.

Professionals should be further aware that the units and algorithm steps of each

example described in connection with the embodiments disclosed herein can be

implemented in electronic hardware, computer software or a combination of the two.

In order to clearly illustrate the interchangeability of hardware and software, the

composition and steps of each example have been described in general terms of

function in the above description. Whether these functions are implemented in

hardware or software depends on the specific application and design constraints of the

technical solution. Professionals may use different methods to implement the

described functions for each specific application, but such implementation shall not be

considered beyond the scope of this application.

A person of ordinary skill in the art can understand that all or part of the steps in the method for realizing the above embodiment can be completed by instructing the processor to complete the above-mentioned embodiment method. The program can be stored in a computer-readable storage medium, which is a non-transitory medium, such as random access memory, read-only memory, flash memory and hard disk, solid state disk, magnetic tape, floppy disk, optical disc and any combination.

The above is only a better specific implementation method of the application, but the protection scope of the application is not limited to this. Any change or replacement that can be easily thought of by any technical personnel familiar with the technical field within the technical scope disclosed by the application shall be included in the protection scope of the application. Therefore, the protection scope of the application shall be subject to the protection scope of the claim.

Claims (5)

Claims

1. A method for determining middle zone exploration depth of electromagnetic exploration,

include:

Determining the transition zone;

Determining the detection depth H of the transition zone according to the following

formula:

HC=(1-a)-Ha+a-H,,

where H. is the corrected depth satisfying the transition region condition;

H, =5 3= 2 is the depth of inductive sounding satisfying the formative wave region

AB conduction; H = - is the depth of the geometric sounding satisfying surface wave 2 region conduction; and a is the proportion of the formative wave, which is related to the

frequency, resistivity, and offset.

2. The method according to claim 1, wherein the scope for determining the transition zone

includes:

acan be used to divide near field zone, middle field zone, and far field zone, that is,

a<A represents far field zone; a >1-A represents near field zone;

A a A r-epresents middle field zone.

3. The method according to claim 2, wherein the value of A is 5%.

H

4. The method according to claims 3, wherein method for calculating the depth a of the

far-field exploration is:

Ha = 2 a UO

It is assumed that the conductivity U~ and magnetic permeability '"O are frequency

independent, and that the displacement currents could be ignored. (9 is circle frequency.

5. The method according to claims 4, wherein the method for calculating the near-field detection

depth HA is:

H,=AB ' 2 where A and B represent the two end positions of the grounded dipole wire respectively, AB

presents the length of the grounded dipole wire.

Fig. 2 Fig. 1

Fig.4 Fig. 3

Fig. 5

Fig. 6

Fig. 8 Fig. 7