JP3204910B2 - Underground electromagnetic survey method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic exploration method for exploring an underground cavity.

[0002]

2. Description of the Related Art Electromagnetic methods for performing underground exploration using electromagnetic induction have been developed since the beginning of this century, and have been used for exploration of resources such as mines, geothermal, and petroleum. It has been widely used for underground structure surveys. Various methods have been conventionally developed as such an electromagnetic exploration method. In particular, a method of artificially generating an electromagnetic field underground to perform underground exploration has been put to practical use today.

[0003] As such a method, there are known a frequency domain electromagnetic exploration method that treats an electromagnetic response as a function of frequency and a time domain exploration method that treats an electromagnetic response as a function of time. As is well known, the frequency domain and the time domain are a pair of Fourier transforms, and are theoretically equivalent. In particular, the latter performs the measurement in the absence of an electromagnetic field after the transmission current is cut off, and in recent years, a time-domain electromagnetic search method without the influence of a magnetic field directly incident from a transmission loop has attracted attention. .

FIGS. 1 to 3 schematically show the principle of this time domain underground exploration method. At the time of measurement, first, as shown in FIG. 1, a transmission loop 10 is installed on the ground surface, and a reception loop is installed at a measurement point. And
A transmission current is supplied to the transmission loop 10, and this transmission current is
As shown in FIG. As a result, FIG.
As shown in (B), a back electromotive force is generated according to the law of electromagnetic induction to maintain the same magnetic field as before the interruption, and an eddy current is generated on the ground surface. The eddy current on the ground surface attenuates in accordance with the resistivity of the ground, but a new eddy current is generated in the ground to prevent the change of the current. This process is repeated, and a phenomenon occurs as if the eddy current 100 propagates deep underground. This phenomenon is called a smoke ring phenomenon. 2 (A) to 2 (D) show the current density distribution underground at this time in chronological order. It can be seen that the portion showing the maximum value of the current density permeates deep underground with time.

[0005] These eddy currents are attenuated according to the specific resistance of the path formation. For this reason, using a receiving loop installed on the ground surface corresponding to the search position, the eddy current attenuation is detected as a change in the time function of the electromagnetic field as shown in FIG.
Underground resistivity distribution can be known. For example, when the underground has a high specific resistance, the eddy current attenuates rapidly, but when the underground has a low specific resistance, it attenuates slowly.

[0006] Therefore, by moving the reception loop (or the transmission loop and the reception loop) one after another according to the search position, collecting the reception data and analyzing the reception data, for example, as shown in FIG. Underground resistivity distribution can be obtained, and based on this resistivity distribution,
Underground structure can be known.

In recent years, using such an electromagnetic exploration technique,
Attempts have been made to measure underground cavities. For example, underground cavities left after excavation of underground resources and underground cavities left buried after excavation of shafts are detected in advance and safety measures are taken.

[0008] However, the conventional electromagnetic exploration method has a problem that it is difficult to find an underground cavity and to accurately detect the distribution of the cavity.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an underground electromagnetic exploration method capable of accurately exploring the distribution of an underground cavity. is there.

[0010]

To achieve the above object, a first aspect of the present invention is to provide a cavity in an underground exploration area along a survey line on the ground surface side using a current source for generating smoke ring and an electromagnetic wave receiving section. A method for electromagnetically examining the distribution, comprising: a primary model creation step of creating a primary model representing a horizontal multilayer structure model of an underground by a resistivity distribution from measured data of an underground exploration area measured in advance; A secondary model creation step of creating a secondary model for cavity exploration assuming the existence of a virtual cavity layer whose thickness and depth are unknown in an arbitrary layer of the secondary model; A cavity distribution model calculating step of identifying an unknown number of the virtual cavity layer based on the exploration data and calculating a cavity distribution model representing an underground cavity distribution from the secondary model.

In the underground electromagnetic exploration method of the present invention, an underground stratum structure to be an area to be searched is first prepared as a primary model. This primary model considers the underground structure as a horizontal multilayer structure model, and expresses this as a resistivity distribution.

The primary model is created from actual measurement data of a search target area measured in advance. For example, claim 2
As in the invention of No. 1, an electromagnetic survey technique is used to measure and calculate the underground resistivity distribution without any cavities, and this is used as a primary model. In addition to this, the underground resistivity distribution is measured using, for example, an electrical logging measurement technique, and the
A next model may be created.

Next, a secondary model for cavity exploration is created using the primary model created in this way. This 2
The secondary model is created assuming the existence of a virtual cavity layer whose thickness and depth are unknown in an arbitrary layer of the primary model. For example, at a mining site such as an underground resource, it is presumed that the layer where the underground resource to be mined exists can be easily specified, and that there is a high possibility that a hollow layer exists in this layer. In such a case, it is assumed that the virtual cavity layer exists in the underground resource layer existing in the primary model, and a secondary model for cavity exploration may be created.

Then, the unknown number of the virtual cavity layer is specified based on the electromagnetic survey data from the underground survey area along the survey line, and a cavity distribution model representing the underground cavity distribution is calculated from the secondary model. .

As described above, according to the present invention, a formation existing in advance as a background around a cavity layer is created as a primary model using measured data, and a virtual cavity is formed in an arbitrary layer of the primary model. Assuming the existence of layers, a secondary model for cavity exploration is created.

Therefore, in this secondary model, since the formation existing around the hollow layer has a known value as the background, the secondary model is based on the electromagnetic survey data along the survey line. , It is possible to accurately calculate a cavity distribution model representing the underground cavity distribution.

According to a second aspect of the present invention, in the first aspect, the primary model is created based on electromagnetic survey data along the survey survey line.

According to the present invention, the electromagnetic survey data collected for the operation of the primary model can be used for the operation of the cavity distribution model. Exploration is not required. That is, in this kind of exploration, there is an effect that the cavity distribution model can be accurately calculated only by analyzing the obtained electromagnetic exploration data without increasing the number of actual and most expensive electromagnetic exploration operations. .

According to a third aspect of the present invention, in any one of the first and second aspects, the secondary model has a specific resistance and a thickness below the virtual cavity layer in an arbitrary layer of the primary model. It is created as a model assuming the existence of a virtual hydrous layer as an unknown. In the cavity distribution model calculation step, the unknown number of the virtual hydrous is specified based on the electromagnetic survey data, and the underground cavity distribution is determined from the secondary model. Is characterized by calculating a cavity distribution model representing

As described above, in the present invention, a secondary model is created on the assumption that a virtual cavity layer exists in an arbitrary layer of the primary model. At this time, assuming, for example, a large-scale cavity existing at a mining site or the like, a moat residue of the mining target layer often exists below the cavity. Such moat-remaining areas often contain water in relation to groundwater, and it is preferable to consider the effect of the specific resistance that changes due to the water content.

The invention of claim 3 has been made in consideration of such a point, and the resistivity and the thickness are set as unknowns below the virtual cavity layer in an arbitrary layer of the primary model. A secondary model is created assuming the existence of a virtual hydrous layer.

In the cavity distribution model calculation step, the unknown number of the virtual hydrous layer is specified based on the electromagnetic survey data, and a cavity distribution model representing the underground cavity distribution is calculated from the secondary model.

As a result, the influence of the change in the resistance distribution due to the water-containing layer can be reduced, and the underground cavity distribution can be obtained more accurately.

According to a fourth aspect of the present invention, in any one of the first to third aspects, as the current source for generating smoke ring,
A transmission loop is used, and a reception loop is used as each of the electromagnetic wave receiving units.

With the above configuration, smoke ringing can be effectively generated, and electromagnetic exploration data can be effectively obtained from the search target area.

According to a fifth aspect of the present invention, in the fourth aspect, in the actual measurement step, an electromagnetic survey of the underground survey area is performed using a slinggram measuring method.

Conventionally, it is known to perform electromagnetic exploration in the time domain using a transmission loop and a reception loop.

However, when the reception loop is arranged inside the transmission loop (in-loop arrangement) and the underground cavity layer is measured, various kinds of noise existing around the cavity layer and its background are excessively picked up. Exploration accuracy is reduced. In other words, in the electromagnetic survey method of the in-loop arrangement, since the transmission loop must be arranged with a radius of about several hundred meters, when the underground hollow layer has a width of only several tens of meters, In data, data obtained from a region other than the cavity layer and its background due to smoke ring generated in the ground acts as noise, and this is a major factor that lowers the search accuracy.

Further, in the out-loop method in which the transmission loop and the reception loop are arranged apart from each other, it is not necessary to increase the size of the transmission loop as in the in-loop method, so that the influence of noise can be reduced in this respect. However, if the distance between the transmission loop and the reception loop is increased, the received waves from the surrounding strata will be picked up as noise, as in the case of the in-loop.
This is a major factor that lowers the search accuracy.

In consideration of such a problem, the present invention employs a slinggram measuring method in which a receiving loop is arranged outside a transmitting loop and the transmitting loop and the receiving loop are arranged relatively close to each other. . By using this slinggram measurement method and performing electromagnetic surveys in the underground exploration area, data obtained from areas other than the underground cavity layer and its background as noise are mixed into the obtained electromagnetic survey data. It was confirmed that the ratio decreased and the exploration accuracy became extremely high.

[0031]

Next, a preferred embodiment of the present invention will be described in detail with reference to the drawings.

In the present embodiment, an example will be described in which the TDEM method (time domain electromagnetic method) is used as an electromagnetic method and electromagnetic cavities underground are detected.

FIG. 5 schematically shows a cross-sectional view of an underground electromagnetic exploration area to be subjected to electromagnetic exploration. In the electromagnetic exploration area shown in FIG. m
There is a large cavity 200 of thickness.

In the present embodiment, a predetermined search line is set on the ground surface 300 for such a search area, and a total of 35 measurement points No1, No2,... No35 are set along the search line. ing. The interval between each measurement point is 10
m.

FIG. 6 schematically shows the arrangement of the transmission loop 10 and the reception loop 20 used in the electromagnetic survey of this embodiment.

Such an arrangement of the transmission loop 10 and the reception loop 20 is called a slingogram arrangement. The measurement method using this arrangement is performed while keeping the interval between the transmission loop 10 and the reception loop 20 constant, and it is conventionally considered to be effective in capturing a tomogram or the like. Here, one component (Z component, vertical component of the magnetic field) TDEM exploration (measurement interval 10 m, number of measurement points 35) was performed using this measurement method.

The transmission loop 10 functions as a current source for generating smoke ring.
m. The receiving loop 20
Functions as an electromagnetic wave receiving unit (sensor), and is disposed at a position about 10 m behind the transmission loop.

At each measurement point, the transmission loop 10 shown in FIG.
, A DC transmission current of 1.5 amps is intermittently supplied to the receiver, and sampling in the reception loop 20 is performed by u (6.8 to 696 μsec after the interruption of the transmission current) and V (35 to 2792 μsec).
c) and H (88 to 6978 μsec) in each measurement mode. The equipment used is PROTETE manufactured by GEONICS.
M-47.

The analysis was performed by one-dimensional horizontal multilayer inversion, and a measurement point having no cavity and a measurement point having a cavity were compared and examined from the obtained analysis data.

In the measurement of the present embodiment, specifically, a location where a large-scale cavity at the Otani stone mining site in Utsunomiya city exists was set as a search target area, and the measurement was performed.

The measurement site has a stratified structure in which Oya stones are distributed over a depth of 70 to 80 m under a surface soil layer of about 10 m. The quarry was excavated in the horizontal direction after excavation of the shaft, and this exploration was carried out from the ground surface. FIG. 5 is a cross-sectional view of a large-scale cavity (quarry mark) existing below the survey line.

FIG. 7 shows an analysis result of the one-dimensional horizontal multilayer inversion described above. According to this analysis result, this search area is analyzed into a three-layer structure.

In the portion where the cavity 200 does not exist (measurement points No. 1 to 20), the error of the analysis result is small (average error 5.4%), and the specific resistance value and the stratum boundary show almost the same value. . The average of the specific resistance value is 63Ω · m from the upper layer, 18
Ωm, 5 Ωm, and the average value of the layer thickness is the first layer 310
Is 11 m, and the second layer 320 is 67 m.

On the other hand, the measured values at the portion where the large-scale cavity 200 exists (measurement points Nos. 21 to 35) have only a slightly high specific resistance, and a high specific resistance layer considered to be a cavity is not recognized. The error is also large (average error 12.5%) as compared with the portion without a cavity, and the specific resistance value and the layer thickness also have large variations.

As described above, in the ordinary time domain electromagnetic survey, the area where the cavity 200 does not exist (measurement points No. 1 to No. 2)
Although the underground exploration of 0) is performed accurately, the underground exploration of the area where the cavity 200 exists (measurement points Nos. 21 to 35) cannot be performed accurately.

The present invention, as described in more detail below,
From the viewpoint that the existence and distribution of the cavity layer existing underground can be accurately measured by effectively utilizing the results of the underground electromagnetic survey in the area where no 00 exists (measurement points No. 1 to 20). It was conducted.

FIG. 10 schematically shows the procedure of the underground electromagnetic survey method according to the present embodiment.

First, in step S10, the underground exploration area is electromagnetically surveyed along the exploration survey line by using the measurement method of the slinggram arrangement as described above.

Next, in step S12, a primary model of the underground exploration target area is created.

In the primary model, an underground structure serving as an underground exploration area is preliminarily regarded as a horizontal multilayer structure model, and is represented by a resistivity distribution.

Such a primary model can be obtained as shown in FIG. 8, for example, from the data of the measurement points No. 1 to No. 20 in the measurement data shown in FIG. Specifically, the underground exploration area includes the first layer 310, the second layer 320, and the third layer.
It can be considered as a three-layer structure of the layer 330. And the first
The layer is set at a depth of 11 m, the specific resistance is 63 Ωm, the second layer 320 is set at a depth of 67 m, the specific resistance is 18 Ωm, and the third layer is set at a specific resistance of 5 Ωm.

Next, in step S14, a secondary model for cavity exploration is created from the primary model.

As shown in FIG. 8, the secondary model for cavity search is a model on the assumption that a virtual cavity layer exists in the primary model.

Usually, it can be relatively easily determined which of the first to third layers of the primary model has the cavity layer 200. For example, at the valley stone mining site of the embodiment, it can be estimated that the cavity layer exists in the second layer 320 where the valley stone exists.

Therefore, in this embodiment, the second layer 320
It is assumed that a virtual cavity layer 400 having a thickness of H3 exists at a depth position of H2 in the inside, and a virtual water-containing layer 410 having a specific resistance ρ4 and a thickness of H4 exists below the virtual cavity layer 400. Create a secondary model for cavity exploration. It is assumed that the virtual cavity layer 400 is a layer having a high specific resistance, and here, the specific resistance is set to 10000Ω · m.

Next, in step S16, a cavity distribution model shown in FIG. 9 is calculated from the secondary model and the electromagnetic survey data.

That is, the unknowns H 2, H 3, which specify the depth and thickness of the virtual cavity layer 400 and the virtual water-containing layer 410 based on the survey data of the measurement points No 21 to No 35 obtained by the electromagnetic survey of the step S 10. H4 is calculated, and the specific resistance ρ of the virtual water-containing layer 410 is calculated.

Here, the specific resistance of the first layer 310, the second layer 320, and the third layer 330 and the layer thickness H of the first layer 310 and the second layer 320 are fixed, and the virtual cavity layer 400 and the virtual water-containing layer 4 are fixed.
Ten unknowns H2, H3, H4 and ρ4 were determined. FIG. 9 shows data of each measurement point obtained in this way, that is, a cavity distribution model representing the underground cavity distribution.

The distribution of the cavity layer shown in FIG. 9 almost exactly matches the distribution of the actual large-scale cavity 200 shown in FIG.

Note that a portion having no cavity (measurement points No. 1 to 2)
In 0), similar calculations were performed on the assumption that the virtual cavity layer existed. As a result, in these regions, the result was obtained that the cavity layer was distributed with a layer thickness of several m, but these values were found to have an allowable error. There is no problem because it can be processed as a range.

As described above, according to the present embodiment, not only the existence of a large-scale cavity but also its size and distribution can be clarified by effectively using the results of the conventional electromagnetic survey for analyzing the underground cavity. It was confirmed that analysis was possible.

The present invention is not limited to the above embodiment, and various modifications can be made within the scope of the present invention.

For example, the creation of the primary model in step 12 may be performed not by using the results of the electromagnetic survey in step S10 but by using other measurement data. For example, the primary model may be created by measuring a specific resistance distribution in a region where no cavity exists by using an electric logging method. Specifically, a measurement hole was dug from the surface of the earth to a depth of several tens of meters below the ground, and the resistivity measurement device was sequentially moved along the measurement hole to measure the resistivity distribution under the ground. The primary model may be created based on

[0064]

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view of the principle of smoke ring generation.

FIGS. 2A to 2D are explanatory diagrams showing a state in which an eddy current spreads underground over time.

FIG. 3 is a timing chart in the case of performing electromagnetic exploration in the time domain.

FIG. 4 is an explanatory diagram of an underground structure obtained using a time-domain underground electromagnetic survey method, in which the horizontal axis represents the length in the horizontal direction and the vertical axis represents the sea level.

FIG. 5 is a schematic explanatory view of an underground electromagnetic survey area.

FIG. 6 is a schematic explanatory diagram of a measurement method using a slingogram arrangement.

FIG. 7 is an explanatory diagram of an underground analysis structure model obtained by using a general underground electromagnetic survey analysis method.

FIG. 8 is a schematic explanatory diagram of a primary model and a secondary model.

FIG. 9 is an explanatory diagram of a measured underground cavity distribution model.

FIG. 10 is a flowchart illustrating a measurement procedure according to the present embodiment.

[Explanation of symbols]

 Reference Signs List 10 transmission loop 20 reception loop 200 large-scale cavity 310 first layer 320 second layer 330 third layer 400 virtual cavity layer 410 virtual water layer

────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Ichiro Sekine 1-7-1, Kyobashi, Chuo-ku, Tokyo Toda Construction Co., Ltd. (72) Inventor Kazunari Wada 1-2-1, Oi, Shinagawa-ku, Tokyo Mitsui Inside Metal Resources Development Co., Ltd. (72) Inventor Akira Saito 1-23-1 Oi, Shinagawa-ku, Tokyo Mitsui Metal Resources Development Co., Ltd. (56) References JP-A-7-110382 (JP, A) 31st Presentation on Ground Engineering Research Group, July 17-19, 1996, Published by Japan Geotechnical Society, pp. 459-460 (58) Fields investigated (Int. Cl. ⁷ , DB name) G01V 3/12 G01S 13 / 88

Claims (4)

(57) [Claims] 1. A method of electromagnetically exploring a cavity distribution in an underground exploration area along an exploration survey line on the ground surface using a current source for generating smoke ring and an electromagnetic wave receiving unit, the method comprising: A primary model creation step of creating a primary model representing a horizontal multi-layer structure model of the underground by a resistivity distribution from actual measurement data; and, in an arbitrary layer of the primary model, thickness and depth are set to unknowns. A secondary model creation step of creating a secondary model for cavity exploration assuming the existence of a virtual cavity layer, and treating the virtual cavity layer as a layer with a high resistivity based on electromagnetic survey data along the survey survey line A cavity distribution model computing step of identifying the unknown by computation and obtaining a cavity distribution model representing the underground cavity distribution from the secondary model. 2. The underground electromagnetic survey method according to claim 1, wherein the primary model is created based on electromagnetic survey data along the survey survey line. 3. In any of claims 1, 2, wherein a current source for smoke ring generator, using a transmission loop, wherein the respective electromagnetic wave receiving unit, underground electromagnetic exploration method characterized by using a receiving loop . 4. The underground electromagnetic exploration method according to claim 3 , wherein in the actual measurement step, an electromagnetic exploration of the underground exploration area is performed using a slingogram measurement method.