JPS63269083A - Electromagnetic type underground inspection - Google Patents

Abstract

PURPOSE:To simplify measuring work, by a method wherein a survey side track is divided into a plurality of sections to calculate a section impedance between sections and a spatial filter computation is performed from the resulting number of waves to determine an apparent specific resistance or a true specific resistance from the results. CONSTITUTION:A survey side track S is divided into a plurality of sections S1-Sn to measure position data between the sections. Fluctuating potential differences DELTAVi between the sections are measured simultaneously with remote site stations 31-3m for measuring potential differences as for multiple sections. Magnetic field sensors 4x and 4y are arranged near the line S to measure horizontal two components of fluctuating magnetic field. Then, fluctuating electric fields between sections are determined from the potential differences DELTAVi. A spatial impedance is calculated from these fluctuating magnetic fields and fluctuating electric fields and a spatial filter operator is determined from a wave number thus obtained to perform a spatial filter computation. Then, from the results, an apparent specific resistance or a true specific resistance is determined. This inspection method permits measurement only limited to one direction of electric field and two directions of magnetic field thereby simplifying measuring work.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to an electromagnetic underground exploration method for exploring underground electrical structures using /% electromagnetic waves.

BACKGROUND OF THE INVENTION The basic principles of electromagnetic underground exploration methods were proposed by Cagiard in 1953 (see, eg, US Pat. No. 2,667,801). After assuming the so-called -dimensional structure, in which the underground structure changes only in the depth direction and is uniform and unchanged in the lateral direction, he calculated the apparent resistivity ρ by the following hour wheel. Expressed by the formula.

ρ, = (1/ωμ)・(E/H)”...
・(1) However, ω is the angular frequency, μ is the magnetic permeability, S is the fluctuating electric field, and H is the horizontally fluctuating magnetic field in the direction perpendicular to E.

Here, (E/H) is called wave impedance or simply impedance and is a function of frequency.

The depth of penetration of electromagnetic waves into the underground depends on the frequency of the electromagnetic waves, and when the frequency is high, ρ indicates the influence of shallow underground areas,
The lower the frequency, the deeper the underground shadow is shown. Therefore, by measuring impedance at various frequencies, the apparent resistivity ρ deep underground can be obtained. In other words, if the underground electrical structure is a one-dimensional structure that changes only in the depth direction, by simultaneously measuring the fluctuating electric field E and the horizontally fluctuating magnetic field H perpendicular to it at any one point,
You can learn about the underground electrical structure.

However, in the case of a so-called two-dimensional structure in which the underground structure changes in one horizontal direction as the depth changes, the wave impedance becomes a tensor quantity.

A variety of studies have been conducted regarding this impedance strength, and basically the two components of a fluctuating electric field EX at a certain point.

A total of 5 component signals, ie, E, and the three varying magnetic field components HX, H, ,H, are measured simultaneously. Z XX + Z xy l
When Z FX +Z yy is an impedance tensor, the following equation holds true.

However, A and B are magnetic field conversion functions (Tituber).

Here, if the X-axis is chosen to match the strike direction of the two-dimensional structure, Z□ becomes the wave impedance in the strike direction, which corresponds to the wave impedance obtained with the -dimensional model, and from now on, The apparent resistivity of the underground is determined. Z ITX is the wave impedance in the direction perpendicular to the strike direction, and this function has different properties from Zo, and is used along with A and B when interpreting electrical underground structures (especially when considering the presence of faults). (presence or absence, etc.)

Conventionally, based on the assumption of such a two-dimensional structure, the MT method (MAGNETO-based LU
RIC-METHOD) is generally widely practiced.

For example, see JP-A-60-133387.

"Problems to be Solved by the Invention" This conventional method is effective when the underground structure in the survey area is one-dimensional or two-dimensional. However, actual underground structures are complex, especially in volcanic archipelagoes with active crustal activity like Japan, where -dimensional or two-dimensional structures are rare, and most are three-dimensional structures (electrical underground structures are It has a structure that changes not only in the direction but also in two horizontal directions. For such a three-dimensional structure, the impedance Z1 in the strike direction obtained assuming a two-dimensional structure and the impedance Zv in the direction perpendicular to it are
It is impossible to accurately know the true underground structure from II.

In order to compensate for this with the conventional MT method, a total of five components, two electric field components and three magnetic field components, must be observed along the survey line in sufficient quantities and at as many points as possible, usually at intervals of about 1 km. , for example, at intervals of 200 m), perform two-dimensional analysis at each point, arrange the results in the direction of the survey line as shown in Figure 7, and correlate them with similar properties.
The purpose is to interpret the electrical underground structure of the survey line as closely as possible using human judgment.

However, this method has the following problems.

■ Observation of five components at one measurement point usually requires about 10 hours of measurement time, so carrying out multiple measurements increases time and cost and is not practical from an economic standpoint.

■ There are individual differences in correlations and interpretations, and the results will vary from person to person, and objective reliability is weak.Also, if the underground structure is three-dimensional, even if you make great efforts, you will only get results with low reliability. I can't get it.

■ If there is a three-dimensional anomaly in a particularly shallow part (high frequency part) (normally, the shallower the underground structure is, the more the wind is flowing).

The deep part (low frequency part)
This results in distorting the signal and impeding correct analysis of the deep structure (the so-called masking phenomenon of the deep structure by the shallow abnormal structure).

An object of the present invention is to solve the above-mentioned problems (1), (2), and (3).

"Means for Solving the Problems" In the method of the present invention, a survey line is divided into a plurality of sections, and for each section, the fluctuating potential difference between the survey points at both ends and the horizontal two-component fluctuating magnetic field near the survey line are calculated. Measure at the same time. Then, after determining the fluctuating electric field of each section from the position data of each survey point and its fluctuating potential difference data, the section impedance is calculated from the fluctuating electric field and fluctuating magnetic field, and the spatial filter operator is found from the wave number to filter the spatial filter. do calculations,
From the results, determine the apparent resistivity or true resistivity. Then, if necessary, a cross-sectional view along the survey line is plotted based on the apparent resistivity or true resistivity.

"Function" In the present invention, a total of three components are measured: an electric field in one direction and a magnetic field in two directions. The distance of each section obtained from the position data is Δ1.
N=1.2.3-. n), (the oblique angle is Qi (the angle between the section direction and the horizontal plane), and the fluctuating potential difference in each section is Δ■1
Then, the fluctuating electric field E in each section can be found by the following equation.

E8=(ΔVi) /(Δj! r ・cos Q +
) -=13) Furthermore, if the simultaneously measured horizontal two-component fluctuating magnetic field is H,,H, then the impedance in each section (section impedance) Z,...Ziy can be obtained by the following equation.

El = Z □ H Obtain wave number characteristics, that is, wave number conversion (K conversion)
administer.

However, K and I are the wave numbers of the horizontal impedance Z ix, and K is the wave number of Z iy.

In the impedance expression in the frequency-wavenumber domain obtained by this conversion, components due to three-dimensional effects have the property that the distribution of high wavenumbers at high frequencies extends to the low frequency domain around the same wavenumber.

Therefore, in order to prevent this, various basic types of transfer functions W (f, K) are used to create an optimal filter W,
Set in wave number domain. This filter W.

The optimal filter operator F1 is given by inverse Fourier transform of F1, and its formula is, for example, as follows.

However, C is a coefficient and j is an imaginary unit.

F obtained in this manner usually has stronger low-pass filter characteristics as the frequency becomes lower (deeper).

A spatial filter operation is performed using this spatial filter operator F. For example, the effective azimuth angle φ of the spatial filter is determined by the following equation.

ΣFls1nθ.

φ−tan"' ΣFICO5θ, ・・・・・・
(8) However, θ8 is the angle between the section direction and the X axis.

Next, the section impedance Zi obtained as above
A spatial filter operator F is applied to y+ZiV according to the following equation. This value at a distance of 2 along the survey line is 2. ．． 2. shall be.

ΣF, Z, ......(9) Z, =
ΣFtZi, ...0ωHere, K is 2
．． ．． 2. To standardize! It is the coefficient at F8.
It is obtained from φ and the length of the section.

Then the impedance 2. for the effective orientation at point 2. ．． 2. is calculated using the following formula.

Z, = −Z, sinφ+Z, cosφ −−
--・-00Zt-Z, 1cosφ+Z, sinφ
-····−cm Here, Zl corresponds to the impedance obtained from the electric field in the tangential direction to the measuring line at point 2 and the magnetic field in the direction perpendicular to this, and corresponds to Zo in the above equation (2).

Z2 is the impedance obtained using a magnetic field parallel to the tangent.

The calculations from (4) to θ as described above are performed for all sections along the survey line and for all frequencies. Next, the apparent resistivity or true resistivity is calculated from the obtained impedance in the same manner as in the conventional method.

When the results are plotted, for example, as a cross-sectional view along the survey line, an electrical underground structure diagram as shown in FIG. 6 is obtained.

"Example" Examples of the present invention will be described in detail below with reference to the same page.

FIG. 1 is a conceptual diagram of implementation B of the method of the present invention, FIG. 2 is a block diagram of the entire measuring device, and FIG. 4 is a flowchart outlining the work procedure.

First, survey points are determined along the survey line S at required intervals (this interval depends on the required resolution, but for example, about 100 m to 200 m).
1 to S7, and the position data (coordinates and altitude) of measurement points at both ends of each section are measured by the positioning device 1 installed at the site. Then, electrodes 2 are placed at measurement points at both ends of each section, and the fluctuating potential difference Δv8 for each section is simultaneously measured for multiple sections using a plurality of potential difference measurement remote site stations 31 to 31 installed at each site. do. Now, the measured section S
, ~S7 are assumed to be Δ■1 to Δ■7, respectively.

At the same time, a conventionally known magnetic field sensor 4. ,4. The magnetic field measurement remote site station 5 measures the horizontal two-component fluctuating magnetic field. The measured fluctuating magnetic field is H,,H.

shall be. 1 if there are too many survey points to measure at once.
．． Divide into several groups and perform similar measurements.

Remote site stationary run for each potential difference measurement 3°~3.
The control circuit 8 includes a pair of preamplifiers 6, a pair of postamplifiers and a bandpass filter 7, an AGC amplifier, an A/D converter, a multiplexer, a programmer, etc., and the output of the preamplifier 6 is calibrated by a signal from the control circuit 8. A calibrator 9 and a modem 10 are provided.

Also, in the remote site station 5 for magnetic field measurement,
A pair of preamplifiers 11, a pair of postamplifiers and bandpass filters 12, a control circuit 13 including an AGC amplifier, an A/D converter, a multiplexer, a programmer, etc., a calibrator 14, and a modem 15 are provided.

Therefore, each potentiometric remote site station 3
1-3. , the varying potential difference Δve for each section is converted into a digital signal and transmitted from the modem 10. In addition, magnetic field sensor 4. ．． 4. The horizontal two-component fluctuating magnetic field H, , H, detected by the modem 1 is also converted into a digital signal, and
Sent from 5. And these digital signals are
The signal is transmitted via the digital signal line 16 to a remote base site station 18 via a relay point 17 as required, and is received by the modem 19 of the base site station 18, respectively.

The received data (fluctuating potential difference Δ in sections S, ~S9
■, ~Δ■7 and the fluctuating magnetic field H,,H,) are calculated by the CPU 20
The data is stored as raw data in the data buffer memory 22 via the interface unit 21 together with the position data from the positioning device l. In this case, the position data can also be input into the data buffer memory 22 via a storage medium such as a magnetic tape or by the keyboard 36. This raw data is read out under the control of the CPU 20 and computed by the high speed computing device 23.

This high-speed arithmetic unit 23 is one microprocessor,
It is controlled by the CPU 20 to perform a plurality of functions, and when broken down by function, it can be seen that it is composed of a circuit as shown in FIG. 3. That is,
Viewed according to the order of functions, there is a variable electric field calculation circuit 24, an interval impedance calculation circuit 25, a K conversion function 3γ circuit 26,
It is composed of a spatial filter operator calculation circuit 27, an effective azimuth calculation circuit 28, a spatial filter calculation circuit 29, an effective azimuth impedance calculation circuit 30, a calculation completion determination circuit 31, and a specific resistance calculation circuit 32. Therefore, the arithmetic processing performed by this will be explained in detail in accordance with the procedure (see the flowchart in FIG. 5), although the explanation partially overlaps with the above-mentioned "effect" section.

■ Calculation of fluctuating electric field The position data from the positioning device 1, that is, the distance of a certain section S, is Δ1. (i=1.2.3...
・, n), the inclination angle is Qi (the angle between the section direction and the horizontal plane), and the remote site station for potential difference measurement 3. The fluctuating potential difference in the section S obtained by ~3° is Δ
■, then from these, the fluctuating electric field El in that section
is calculated using the following formula.

E1 = (ΔVi) / (Δ1 = ・cosQt) −
・=(3)■ Calculation of section impedance At the same time as measuring the fluctuation potential difference, the horizontal two-component fluctuating magnetic field measured by the remote site station 5 for magnetic field measurement is
,,H, then the impedance (section impedance) ZI-, Zt in the section S+ is determined by the following equation.

E,=Z1. H1+ z 1yHy --------(4
)■ K conversion calculation Interval impedance obtained by equation (4) 2. ,．． 2. y is subjected to one-dimensional Fourier transform in the horizontal direction (distance axis) as follows to obtain wave number characteristics, that is, wave number transform (K variable tA) is performed.

k knee, where K is the wave number of the horizontal impedance Z i X, and K is the wave number of Z iy. Also, in this case,
The Nyquist frequency is k, = 1/(2Δ1), and the actual calculation is performed for k, ≧lk, 1 and ≧1kyl.

■ In the impedance expression in the frequency-wavenumber domain obtained by the spatial filter operator calculation ■, the component due to three-dimensional effects is that the distribution of high wavenumbers at high frequencies spreads to the low frequency domain around the same wavenumber. It has a nature.

Therefore, in order to prevent this, various basic types of transfer functions W (f, K) are used to create an optimal filter W8 between the frequency and
Set in wave number domain. This filter W.

By performing inverse Fourier transform on F, the optimal filter operator F is determined by the following equation.

Leh However, C is a coefficient and j is an imaginary unit.

F obtained in this way usually has stronger low-pass filter characteristics as the frequency becomes lower (the deeper it goes), and it is affected by the impedance as a dimensionless convolution operator. .

■ Execution azimuth angle calculation Using this spatial filter operator F, the effective azimuth angle φ of the spatial filter is determined by the following equation.

Σ Fl srnθ.

φ= tan-' ΣFICO5θ, ・・・・・・
・(8) However, θ is the angle between the section direction and the X axis.

■ Execution of spatial filter calculation Sectional impedance determined in the above 2. ,.

A spatial filter operator F is applied to Z and y using the following equation. This angle at a distance l along the survey line is 2X, 2. shall be.

Zx= to ΣFiZi, -------・(9) Zy=
ΣFiZ1y...00) Here, K is 2X, 2. It is a coefficient at two points to standardize F,
, φ and the length of the interval.

■ Execution azimuth impedance calculation Next, impedance 2. for the effective azimuth at point 2. ．． 2. is calculated using the following formula.

Z l ”” ZxSInφ+Z , cosφ ・・
...-CI+1Z, = ZXcosφ+Zy5tn
φ...02) Here, Zl corresponds to the impedance obtained from the electric field in the tangential direction to the measuring line at point p, and the magnetic field in the direction perpendicular to this, and Zl in the above equation (2)
Corresponds to o.

Zt is the impedance obtained using a magnetic field parallel to the tangent.

■ Calculation completion determination The calculations from ■ to ■ as described above (the calculation data are stored in the memory 33) have been completed for the entire section S along the survey line S.
, -S, and whether it has been done for all of the handled frequencies.

■ Specific resistance value calculation Once all impedance calculations have been performed, the apparent resistivity or true resistivity is calculated from the obtained impedance in the same way as before, and the results are read from the memory 34 at any time and used along the measurement line. When plotted using the plotter 34 as a cross-sectional diagram, for example, an electrical underground structure cross-sectional diagram corresponding to the actual strata, including three-dimensional elements, is automatically generated without human interpretation, as shown in Figure 6. can get.

Further, a cross-sectional view of apparent resistivity versus frequency is also plotted as necessary.

In the conventional MT method, after obtaining an impedance curve diagram along a survey line as shown in Figure 7, it is difficult for people to correlate these impedance curves in order to create a cross-sectional diagram that takes three-dimensional elements into account. However, according to the present invention, a cross-sectional view as shown in FIG. 6 can be automatically obtained, and the influence of three-dimensional distortion is much smaller than in the past.

The processed data stored in the memory 33 and the raw data stored in the data buffer memory 22 are magnetically recorded on a magnetic tape by a magnetic tape recorder 35, and are used for subsequent analysis as required. Base site station 1
8 operations, management commands, process designations, parameter designations, etc., are performed manually using keys from the keyboard 36.

As mentioned above, this underground exploration device is separated into remote site stations 1 and 5 and base site station 18, and remote site station 1.5.
Since it has a relatively simple structure and can be miniaturized, it can be carried to the site and measurements can be made relatively easily even in places with poor conditions that cannot be accessed by means of transportation such as automobiles. In addition, the remote site station 1.5 performs signal processing in advance before sending data to the base site station 18, and shares part of the signal processing with the remote site station 1.5, so the base site station 18 Even if the distance is 10 km, a signal with a high S/N ratio can be sent, and a highly accurate cross-sectional view can therefore be obtained.

In the above embodiment, data recording and calculation processing were performed by one base site station 18, but in order to improve work efficiency at the measurement site, it may be divided into two or more stations. When simultaneously measuring multiple fluctuating electric fields along a survey line and horizontal two-component fluctuating magnetic fields in the vicinity, if the fluctuating electric fields and fluctuating magnetic fields are separated and recorded in separate devices, it is possible to detect obstacles or distances between the two. Measurements can be made even if the devices are far apart, without the need to connect them with cables. However, in this case, each device needs to be equipped with a clock and synchronized.

Alternatively, the recording device and the calculation device may be separated to perform recording and calculation independently. Normally, naturally occurring electromagnetic waves are used as an energy source when measuring fluctuating magnetic fields, so measurements are generally carried out at night to reduce noise caused by artificial electromagnetic waves.
By separating the recording device and the computing device, they can be used during day and night. In this case, the measurement data is recorded on a recording medium such as a magnetic tape so that it can be carried.

Although the embodiments of the present invention have been described above, the present invention is not limited to those described above, and various modifications and additions of functions can be made without departing from the basic technical idea. .

"Effects of the Invention" As detailed above, the method of the present invention has the following effects.

1. Conventionally, it was necessary to measure a total of 5 components: 3 directions of electric field and 2 directions of magnetic field, but according to the present invention, it is only necessary to measure 3 components of 1 direction of electric field and 2 directions of magnetic field, simplifying the measurement work. Become.

2. Conventionally, impedance was determined by two-dimensional calculations, but the present invention uses calculations that deal with three dimensions by setting a spatial filter operator using wave number conversion and performing spatial filter calculations. can be removed, and data with a high S/N ratio can be obtained.

3. Conventionally, creating a cross-sectional view along a survey line required human interpretation and lacked objectivity due to individual differences, but according to the present invention, a cross-sectional view along a survey line can be automatically created. Since the cross-sectional view is displayed in front of the screen, an objective and highly accurate cross-sectional view can be obtained.

4. Conventionally, creating a cross-sectional view in a region with a three-dimensional structure required a huge amount of cost and it was not possible to remove three-dimensional distortions, but according to the present invention, this can be achieved at a relatively low cost. ,
Additionally, three-dimensional distortion can be removed.

[Brief explanation of drawings]

Fig. 1 is a conceptual diagram explaining the present invention in detail, Fig. 2 is a block diagram of a device used to implement the invention, Fig. 3 is a block diagram of a high-speed arithmetic unit broken down into its functions, and Fig. 4 is a work procedure. FIG. 5 is a flowchart of the calculation procedure of the high-speed calculation device, and FIG. 6 is an example of a sectional view of an electrical underground structure obtained by the present invention. 7th
The figure is an impedance curve diagram obtained by the conventional MT method. S... Survey survey line, S, -S,... Section, 2... Electric bridge, 4. ．． 4y... Magnetic field sensor, 24... Fluctuation electric field calculation circuit, 25...
...Sectional impedance calculation circuit, 26...
K conversion calculation circuit, 27... Spatial filter operator calculation circuit, 28... Effective azimuth calculation circuit,
29... Spatial filter calculation circuit, 30...
・・Effective azimuth impedance calculation circuit, 31・・・・
- Calculation completion determination circuit, 32... Specific resistance value calculation circuit. Figure 5

Claims (1)

[Claims] 1. Divide the survey line into a plurality of sections, simultaneously measure the fluctuating potential difference between the survey points at both ends of each section and the horizontal two-component fluctuating magnetic field near the survey line, and After determining the fluctuating electric field of each section from the position data and its fluctuating potential difference data, calculate the interval impedance from the fluctuating electric field and fluctuating magnetic field, find the spatial filter operator from the wave number, perform spatial filter calculation, and calculate the result. An electromagnetic underground exploration method characterized by determining apparent resistivity or true resistivity from . 2. The electromagnetic underground exploration method according to claim 1, wherein a cross-sectional view along the survey line is plotted based on the apparent resistivity or the true resistivity.