JP3236455B2 - Underground electromagnetic exploration method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an underground electromagnetic survey method and apparatus, and more particularly, to an underground electromagnetic survey method and apparatus for obtaining a ground structure around a tunnel using a time domain electromagnetic survey technique.

[0002]

2. Description of the Related Art Conventionally, when excavating a tunnel, it is necessary to accurately grasp the conditions in front of the face and around the tunnel, etc. for safe and reliable construction. . In particular, when excavating mountain tunnels, etc., it is necessary to accurately understand the situation in front of and around the face, especially the existence of fault crush zones, and take appropriate measures. Needed.

For this reason, conventionally, a boring survey has been performed at predetermined intervals from the ground surface along a tunnel excavation path,
The underground structure was being measured.

[0004]

However, in such a conventional method, the measurement of the underground structure along the tunnel excavation path not only takes an enormous amount of time and money, but also the ground conditions around the tunnel, especially the face There was a problem that the situation in front could not always be measured accurately.

For example, when excavating a mountain tunnel, a boring survey cannot always be performed, and even when a boring survey is possible, boring equipment and personnel must be transported deep in the mountain. In addition, there is a problem that the work is extremely difficult, and requires much labor and cost.

Moreover, boring is performed at predetermined intervals,
The underground structure between each sampling point can be estimated and estimated from the stratum at the sampling point. Therefore, there is a problem that, for example, when a crush zone or the like partially exists between the sampling points, it cannot be detected.

The present invention has been made in view of such a conventional problem, and an object of the present invention is to provide an underground electromagnetic probe capable of easily and accurately measuring a ground structure around a tunnel, particularly a ground structure in front of a face. It is to provide a method and an apparatus.

[0008]

In order to achieve the above object, the present invention uses a current source for generating smoke ring installed in a tunnel premises and an electromagnetic wave receiving unit installed in a measurement point in the tunnel premises. A method for electromagnetically exploring the resistivity distribution of a ground, wherein a transmission current is supplied to the current source for smoke ring generation, and by interrupting the transmission current, a smoke ring in which an eddy current propagates to the ground is provided. The phenomenon is generated, and the underground electromagnetic exploration process in the time domain in which the attenuation of the eddy current is detected and stored as a change in the time function of the electromagnetic field using the electromagnetic wave receiving unit is repeated for each measurement point, and for each measurement point, It is characterized in that the resistivity distribution of the ground is calculated based on the detection data.

Further, the present invention provides a current source for generating a smoke ring installed in a tunnel premises, an electromagnetic wave receiving unit installed at a measurement point in the tunnel premises, and applying a transmission current to the current source for generating a smoke ring. The energizing means and the attenuation of the underground eddy current caused by the smoke ringing phenomenon that occurs when the energization of the transmission current is interrupted, at each measurement point, based on the change in the time function of the electromagnetic field from the output of the electromagnetic wave receiving unit. And an underground structure calculating means for calculating the specific resistance distribution of the underground structure. By adopting such a configuration, when a transmission current is supplied to the current source for smoke ring generation installed in the tunnel premises and then cut off,
A smoke ring phenomenon occurs in which the eddy current propagates to the ground around the tunnel. The attenuation of the eddy current is detected as a change in a time function of an electromagnetic field using an electromagnetic wave receiving unit provided in the tunnel premises. Such a detection operation,
Repeat for each measurement point in the tunnel.

Then, based on the detection data obtained for each point, a specific resistance distribution of the ground around the tunnel is calculated.

In this manner, the geological structure around the tunnel can be accurately grasped by accurately determining the resistivity structure of the ground around the tunnel.

In particular, by installing the electromagnetic wave receiving unit near the face of the tunnel and obtaining the detection data, the ground structure in front of the face can be accurately grasped.

Here, it is preferable to use a transmission loop and a reception loop installed in the tunnel premise toward the tunnel excavation direction as the smoke ring generation current source and the electromagnetic wave reception unit. Thereby, the smoke ring phenomenon can be generated with a simple structure, and the detection data can be measured at each measurement point.

In particular, with such a configuration,
The transmission loop and the reception loop can be easily moved in the tunnel premises, and data measurement at each measurement point can be easily performed.

[0015] Further, a plurality of the transmission loops may be installed in a tunnel excavation direction.

At this time, the transmission current is applied to all of the plurality of transmission loops in the same direction so that the generated electromagnetic field has directivity, and the ground situation in front of the face can be grasped more accurately. be able to.

When a plurality of the transmission loops are provided, the electromagnetic waves generated in the area of the transmission loop near a noise source such as a large working metal machine become zero. Thus, a current may be supplied in a direction opposite to that of another transmission loop. This makes it possible to accurately detect the ground structure around the tunnel without being affected by the noise source existing in the tunnel premises.

As described above, when a plurality of the transmission loops are provided in the direction of the face, the energizing means may be formed so as to be capable of switching the energizing direction of the transmission current to each transmission loop. preferable.

The above-mentioned transmission loop and reception loop may be used as the current source for generating the smoke ring and the electromagnetic wave reception unit. However, other current sources and reception units, such as a line using a ground electrode or the like, may be used. An antenna or the like may be used as needed.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 shows a preferred embodiment of the time domain underground electromagnetic survey according to the present invention.

The apparatus of the embodiment includes a transmission loop 10 and a reception loop 20 installed on the premises of a mountain tunnel 200.

The transmission loop 10 functions as a current source for generating smoke ring.
At 00, it is installed facing the face. Since the tunnel 200 of the embodiment has a cross-sectional structure with a radius of 4.75 m as shown in FIG.
m, a rectangular shape with a width of 6.75 m.
0 premises. And this transmission loop 10
, The transmission current is supplied from the transmission power supply device 30 as shown in FIG.

The receiving loop 20 functions as an electromagnetic wave receiving unit, and is installed in the tunnel 200 premises toward the face. The receiving loop 20 is formed to have a small diameter of, for example, about 40 to 50 cm so that the receiving loop 20 can be freely moved and installed in the tunnel 200, and its output is input to the receiving device 50 shown in FIG. I have.

When an underground electromagnetic survey around a tunnel is performed using this device, the installation position of the transmission loop 10 and the measurement points P1, P2,. Is set in advance. Then, the measurement used in the transmission loop 10 and the reception loop 20 is repeated for each measurement point, and the measurement data is stored in the memory 58 in the reception device 50.

Then, based on the measurement data obtained for each measurement point, the underground resistivity distribution around the tunnel is calculated, and the underground structure around the tunnel is obtained based on the obtained resistivity distribution.

FIGS. 4 to 6 schematically show the principle of the time domain search method.

At the time of measurement, first, as shown in FIG.
The transmission loop 10 is installed in the premises of the tunnel 200, and the reception loop 20 is installed at a predetermined measurement point P.
Then, a transmission current is supplied to the transmission loop 10, and the transmission current is rapidly cut off as shown in FIG. As a result, as shown in FIG. 6B, an electromotive force for maintaining the same magnetic field as before the interruption is generated by the law of electromagnetic induction, and an eddy current is generated in the ground around the tunnel 200. The eddy current is attenuated in accordance with the resistivity of the ground, but a new eddy current is generated inside the ground to prevent the change of the current. This process is repeated, and a phenomenon occurs as if the eddy current 100 propagates from the periphery of the tunnel 200 to the deep ground, and this phenomenon is called a smoke ring phenomenon.

FIGS. 5A to 5D show the current density distribution of the ground at this time in a time-series manner. It can be seen that the portion showing the maximum value of the current density permeates deep into the mountain with time.

These eddy currents are attenuated according to the specific resistance of the stratum in the propagation path. For this reason, the attenuation of the eddy current is detected as a change in the time function of the electromagnetic field as shown in FIG. 6C using the reception loop 20 installed corresponding to the search position, and the underground resistivity distribution is known. be able to. For example,
When the underground has a high resistivity, the eddy current decays rapidly, but when the resistivity is low, it decays slowly.

Therefore, the reception loop 20 is moved one after another according to the measurement points of the tunnel premises 200, and the reception data obtained thereby is analyzed.
The specific resistance distribution of the ground around the tunnel 200 can be obtained, and the ground structure can be known based on this specific resistance distribution.

FIG. 3 shows a specific configuration of the transmitting energizing device 30 and the receiving device 50.

The transmitting energizing device 30 includes a clock 32
And a transmission current supply unit 34 and a transmission control unit 36.

The transmission current supply section 34 is formed so as to supply the transmission current shown in FIG. 6A in accordance with a command from the transmission control section 36.

The receiving device 50 includes a clock 52
, An amplifier 62, an A / D converter 54, and a CPU 56
, A memory 58, and an input / output unit 60. Then, the output of the receiving loop 20 is amplified by the amplifier 62, and is then converted from analog to digital by the A / D converter 54 and input to the CPU 56. The CPU 56
Based on a command from the input / output unit 60, data from the A / D converter 54 is stored in the memory 58.

The transmitting energizing device 30 and the receiving device 50 are configured to operate while being synchronized with each other by clocks 32 and 52. Specifically, since the energizing device 30 and the receiving device 50 are formed separately, the clocks 32 and 52 are adjusted so as to be completely synchronized before the start of measurement. As a result, even if the two devices 32 and 52 are completely separated from each other, the two devices will operate in perfect synchronization with each other.

FIG. 7 shows a flowchart of a procedure for performing measurement while moving the receiving loop 20 to each of the measurement points P1, P2.

First, the receiving loop 20 is set at the first measurement point P1 (step S1).

Next, a transmission current shown in FIG. 6A is supplied to the transmission loop 10, and the transmission current is rapidly cut off (step S2).

When the transmission current is cut off, as shown in FIGS. 4 and 5, a smoke ring phenomenon occurs in which the eddy current propagates to the ground around the tunnel. At this time, the attenuation of the eddy current is detected as a change in the time function of the electromagnetic wave using the reception loop 20 installed at the measurement point P1, as shown in FIG. 6C. The detection data is output from the amplifier 54, converted into a digital signal by the A / D converter 54, and written and stored in the memory 58 using the CPU 56.

A series of search processes including steps S1, S2, and S3 are repeatedly performed on all measurement points P1, P2,... Shown in FIG.

When the measurement at all the measurement points is completed (step S4), the CPU 56 functions as an underground structure calculation means, and based on the measurement data at each measurement point stored in the memory 58, the ground level is calculated. Is calculated to create, for example, an underground structure model shown in FIG. 1 (step S5).

In particular, in the time domain underground electromagnetic survey method as in the embodiment, the underground structure model is estimated and calculated from the measurement data at each measurement point by using the least squares inversion or another method. Such calculation prescriptions are well known and do not differ from technology. By doing so, the structural model of the ground around the tunnel 200 can be accurately obtained, and in particular, the crush zone 2 existing in front of the face of the tunnel 200, that is, on the planned tunnel excavation route 210.
20 can be detected accurately.

FIG. 8 shows another embodiment of the present invention.

The system according to the embodiment includes a plurality of transmission loops 10-1 and 10- as current sources for generating smoke ring.
, 10-5, these transmission loops 10-1,
.., 10-5 are installed at predetermined intervals in the face direction.

By adopting such a configuration, for example, all the transmission loops 10-1, 10-2,.
By transmitting a transmission current in the same direction, a magnetic field having directivity is generated in the direction of the face, whereby the ground structure in front of the face can be accurately measured.

When there is any noise source 40 near the transmission source in the tunnel premises (for example, when there is an object made of metal or the like having a small resistance), this causes a measurement error. May cause In this case, the direction in which the transmission current is applied to the transmission loops 10-3 and 10-4 located before and after the noise source 40 is set to be opposite so that the magnetic field near the noise source 40 becomes zero. Thereby, the influence of the noise source 40 can be reduced, and the ground structure can be measured more accurately.

In this case, the transmission current supply unit 34 controls the transmission loops 10-1 and 10
It is preferable that the direction of power supply to −2,..., 10-5 be individually switched and controlled.

Experimental Data Using the measuring apparatus of the embodiment, the inventor conducted an experiment for exploring the front face of a face and grasping the state of the mountain with the already-excavated mountain tunnel as a measurement object. In the experiment,
Since the ground structure around the tunnel 200 has already been elucidated, the ground structure is compared with the measured ground structure obtained using the device of the present embodiment, and the device of the present embodiment is evaluated. Was.

FIG. 9 schematically shows the ground structure around the tunnel where the experiment was performed. Here, a region A having a specific resistance of 400 Ωm is provided around the tunnel 200.
A crush zone B with a specific resistance of 50 Ωm and 200 Ωm in front of it,
C exists.

Then, a transmission loop 10 and a reception loop 20 were set in the tunnel 200, and an experiment was performed.
In the figure, reference numeral 300 denotes the ground surface.

FIG. 10 shows an arrangement of the transmission loop 10 and the reception loop 20 employed in the experiment.

FIGS. 7A, 7B and 7C show the measurement in the background portion.

In FIG. 10A, one transmission loop 10 is set at a position 600 m from the entrance of the tunnel 200, and
Measurement points P at 25m, 650m, and 675m
The receiving loop 20 was set as 1, P2, and P3, and measurement was performed.

In FIG. 9B, two transmission loops 10-1 and 10-2 are set at 575 m and 600 m, respectively.
The transmission electromagnetic waves in the same direction were applied to both, and the measurement was performed.

In FIG. 6C, the measurement is performed by applying a transmission current in the opposite direction to each of the transmission loops 10-1 and 10-2, contrary to FIG.

(D) and (E) show the measurements near the crush zone.

In FIG. 4D, one transmission loop 10 was installed at a position of 740 m, and measurement was performed.

Also, in FIG.
Two transmission loops 10-1 and 10-2 were set at the position of m, and measurement was performed.

While changing the measurement conditions in this way, the transient phenomenon of the secondary magnetic field after the DC current interruption of the transmission loop 10 was measured in the background portion and near the crush zone. The distance between the transmission loop 10 and the reception loop 20 is 25 m, 50 m, 7
Each is set to 5 m.

FIG. 11 shows a transient curve of the measured data in the background portion. Each of the receiving loops 20 was set at a position of 625 m. In the figure, A625, B625, and C625INV correspond to FIG.
0 (A), (B), and (C) are experimental data.
Until 0.01 ms after the current interruption, there is no difference between the data, and thereafter, the influence of the transmission loop 10 far from the reception loop 20 is shown. This is for each transmission loop 1
It can be explained by the eddy current propagation phenomenon from zero. FIG.
In the background part (A625, A650, A67
5) and the vicinity of the crush zone (C765, C790, C8)
15C is a comparison of a transient curve of the normalized voltage value of the reception loop 20 when an experiment is performed using the single-loop transmission antenna 10 shown in FIGS. 10A and 10D in the region 15). . That is, in FIG.
5, A650 and A675 are measurement data when the receiving loop 20 is set at the positions P1, P2 and P3 under the measurement conditions of FIG. In FIG. 12, C765, C790, and C815 are measurement data when the receiving loop 20 is set at the positions of P4, P5, and P6 in FIG. With these data,
Compared with the background part, the transient curve near the crush zone indicates that the propagation speed of the overcurrent decreases near the crush zone and the transient phenomenon is delayed. It is considered that the polarity reversal part seen in the early time of A675 and C815 is due to a two-dimensional specific resistance structure.

From the above experimental results, when the time domain measurement method of the present invention is used, a decrease in the eddy current propagation speed near the low resistivity band around the tunnel is measured. The effectiveness of applying the present invention to exploration in front of the ground and the face was confirmed.

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, in the above-mentioned embodiment, the case where the ground structure near the tunnel face is measured has been described as an example. It is also very suitable for measuring the ground around the tunnel.

In the above-described embodiment, the case where the transmission loop is used as the current source for generating the smoke ring and the reception loop is used as the electromagnetic wave receiving section has been described as an example. However, the present invention is not limited to this. For example, a linear transmission source may be used as a current source for generating smoke ring. In this case, both ends of the linear transmission source are grounded to the ground,
A transmission current supply source may be provided on the way. Even if a transmission source having such a configuration is used, if the transmission current is interrupted, a smoke ring phenomenon can be generated.

Similarly, as the electric wave receiving unit, for example, a line-shaped receiving unit whose both ends are grounded on the ground surface may be used to detect eddy current attenuation as a current change.

[0067]

As described above, according to the present invention,
By installing a current source for generating smoke ring in the tunnel premises and an electric wave receiving unit, it is possible to easily and reliably measure the ground around the tunnel, especially the ground structure in front of the face, and an underground electromagnetic exploration method and There is an effect that a device can be obtained.

[0068]

[Brief description of the drawings]

FIG. 1 is a schematic explanatory diagram in the case of measuring the ground structure around a tunnel using the underground electromagnetic survey technique of the present invention.

FIG. 2 is a schematic explanatory diagram of a transmission loop and a reception loop installed in a tunnel premises.

FIG. 3 is a block diagram showing a preferred example of the underground electromagnetic survey device of the present embodiment.

FIG. 4 is a schematic explanatory view of the principle of smoke ring generation.

FIGS. 5A to 5D are explanatory diagrams showing a state in which an eddy current spreads over the ground with the passage of time.

FIG. 6 is a timing chart in the case of performing a time domain electromagnetic search.

FIG. 7 is a flowchart of an underground electromagnetic survey method according to the embodiment.

FIG. 8 is an explanatory diagram showing another example of the present invention.

FIG. 9 is an explanatory diagram of an experiment performed using the apparatus of the example.

FIG. 10 is a schematic explanatory view showing various experimental patterns in the tunnel premises.

FIG. 11 is an explanatory diagram of data obtained by the experiment described above.

FIG. 12 is an explanatory diagram of data obtained by the experiment described above.

[Description of Signs] 10 transmission loop 20 reception loop 30 transmission device 50 reception device 56 CPU 58 memory 200 tunnel

────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Hitoshi Nishimaki 1-7-1, Kyobashi, Chuo-ku, Tokyo Toda Construction Co., Ltd. (72) Inventor Kazunari Wada 1-23-1 Oi, Shinagawa-ku, Tokyo Mitsui Inside Metal Resources Development Co., Ltd. (72) Inventor Akira Saito 1-23-1 Oi, Shinagawa-ku, Tokyo Mitsui Inside Metal Resources Development Co., Ltd. (72) Inventor Hidehiro Ishikawa 1-23-1 Oi, Shinagawa-ku, Tokyo Mitsui (56) References JP-A-7-110382 (JP, A) JP-A-8-226977 (JP, A) "Illustrated physics exploration" Japan Society of Geophysical Exploration published on June 1, 1992 Pages 63-76 (58) Fields investigated (Int.Cl. ⁷ , DB name) G01V 3/00 G01N 22/00-22/04 G01N 27/72-27/90

Claims (5)

(57) [Claims] 1. A method for electromagnetically exploring a resistivity distribution of a ground using a current source for generating smoke ring installed in a tunnel premises and an electromagnetic wave receiving unit installed at a measurement point in the tunnel premises, As the smoke ring generation current source, a plurality of transmission loops are installed facing the face of the tunnel premises, the electromagnetic wave receiving unit is installed at a measurement point in the tunnel premises, and a transmission current is supplied to each of the transmission loops. By interrupting the transmission current, a smoke ringing phenomenon in which the eddy current propagates to the ground occurs, and the attenuation of the eddy current is detected and stored as a change in the time function of the electromagnetic field using the electromagnetic wave receiving unit. The feature is that the underground electromagnetic survey process in the time domain is repeated for each measurement point, and the resistivity distribution of the ground is calculated based on the detection data at each measurement point. Underground electromagnetic exploration method that. 2. The underground electromagnetic survey method according to claim 1, wherein a transmission current is supplied to the plurality of transmission loops in the same direction. 3. The method of claim 2, wherein a transmission current is supplied to at least one of the plurality of transmission loops in a direction different from that of the other transmission loops. 4. A current source for generating a smoke ring installed in a tunnel premises, an electromagnetic wave receiving unit installed at a measurement point in the tunnel premises, and a current supply for selectively applying a transmission current to the current source for generating a smoke ring. Means, the attenuation of the underground eddy current caused by the smoke ringing phenomenon that occurs when the transmission current is cut off, for each measurement point, detected as a change in the time function of the electromagnetic field from the output of the electromagnetic wave receiving unit, and obtained. Underground structure calculating means for calculating the resistivity distribution of the ground, based on a change in the time function of the electromagnetic field, wherein the plurality of current sources for generating smoke ring are installed in the tunnel yard in the face direction. An underground electromagnetic exploration device formed by using the transmission loop of (1), wherein an underground structure is obtained based on the underground resistivity distribution. 5. The underground electromagnetic exploration apparatus according to claim 4, wherein the energizing means is formed so as to be capable of switching a direction of energizing a transmission current to at least one of the transmission loops. apparatus.