JP2002055172A - Method of investigating cavity in ground - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of investigating a cavity in the ground by which the presence of a cavity under the underground water surface is made easily and accurately graspable. SOLUTION: A boring hole is drilled in the ground being an investigation object, the elastic wave velocity is measured in the periphery of a hole wall for every prescribed depth, and a density distribution is measured in the periphery of the hole wall. The measurement of the elastic wave velocity is carried out by a method analyzing a propagation phenomenon based on differences in a density and rigidity of a layer, a sensor is set on the ground level or underground, and an arrival time of an elastic wave oscillated artificially and the like are measured in a site of some distance therefrom. A concrete measuring method for the density distribution is also known, for example, a formation density logging method using a γ-ray or the like, or the like is adopted therefor. An electric specific resistance in the periphery of the hole wall of the boring hole is measured in every prescribed depth, and the presence of the cavity or a ground characteristic resulting from the cavity is verified based on a correlation between a measured value herein and the density distribution.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for investigating a cavity in a ground.

[0002]

2. Description of the Related Art Conventionally, there are often known business projects in which the existence of an existing underground cavity is a problem when designing and constructing. As such an existing cavity,
This includes small-scale cavities and mining sites in former coal mines, small-scale mining sites and tunnels in metal deposits, and small-scale limestone caves found in Okinawa Ryukyu Limestone. Large-scale cavities can be dealt with in advance because there is a record,
Smaller ones have no records and are often a problem when planning the construction of structures.

[0003] Such a cavity has problems of safety as a support base, problems of environmental measures such as construction sludge, problems of salt water intrusion such as backflow of seawater, groundwater obstruction due to a cavity as a waterway, and There is also a risk of various troubles during construction, so it is essential to know in advance.

Conventionally, various types of physical exploration techniques from the surface of the earth have been mainly used, such as shallow reflection surveys, physical surveys such as ground surface radar, observations in the borehole by a TV such as a borehole TV, and a borehole using specific resistance including geotomography. Measurement methods have been implemented.

[0005]

However, these conventional geophysical exploration techniques are very limited by the size of the cavity, the presence or absence of groundwater, the depth of the survey, the ground conditions, and the like. And it was not easy to actually detect accurately. In particular, when the location of the cavity inside the ground or rock is deep, for example, several tens of meters, and exists below the groundwater level, there is currently no survey technology that can practically cope with it. It can be said that there was no effective survey method.

[0006] The above-mentioned physical exploration method mainly determines the layer structure section of the stratum. Among them, in the shallow layer reflection survey, the diameter of the cavity is several meters or more, and a large-scale cavity in the shallow layer portion is used. Other exploration is difficult. In addition, the ground surface radar is capable of exploring a cavity with a diameter of several tens of cm or more, but the exploration within about 4 m below the ground surface is the limit and cannot be used for the exploration below the groundwater level.

On the other hand, it is conceivable to improve the accuracy by combining various exploration techniques, but there is a problem that the cost is large.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide a method for investigating a cavity in a ground, which makes it possible to easily and accurately grasp the presence or absence of a cavity below groundwater. is there.

[0009]

According to the present invention, a boring hole is drilled in a ground to be surveyed, and an elastic wave velocity around a hole wall is measured at predetermined depths. The method is characterized in that the density distribution around the wall is measured, and the presence or absence of a cavity or the ground characteristics caused by the cavity is verified from the correlation between the two measured values obtained for each depth (claim 1). In the above, the measured values of the elastic wave and the density have a certain correlation. Therefore, according to the method of the present invention, when there is a measured value deviated from the correlation due to the data reduction, there is a cavity at that position or other It can be determined that the ground characteristics are significantly different from each other.

Further, the present invention is characterized in that the electrical resistivity around the hole wall of the borehole is measured at every predetermined depth, and the ground characteristics are verified from the correlation between the measured value and the density distribution. (Claim 2). Therefore, in the method of the present invention, the measurement accuracy can be further improved.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described. First, a boring hole is drilled in the ground to be surveyed, the elastic wave velocity around the hole wall is measured at each predetermined depth, and the density distribution around the hole wall is measured.

The measurement of the elastic wave velocity is performed by a method of analyzing a propagation phenomenon of an elastic wave due to a difference in density, stiffness, etc. of a stratum. The arrival time and the like of elastically oscillated elastic waves are measured. Elastic waves include P waves and S waves. The P wave is a compression wave having a vibration component in the traveling direction of the wave, and is also called a longitudinal wave. On the other hand, the S wave is a shear wave whose vibration direction is perpendicular to the direction in which the wave propagates, and is also called a transverse wave or a torsional wave. Specific measurement methods are well known and include a suspension type PS logging method, a VSP measurement method, an elastic wave tomography method, and the like.

A specific method of measuring the density distribution is also well known, and for example, a density logging method using γ-rays or the like can be adopted.

Further, in addition to the above, the method of the present invention measures the electrical resistivity around the hole wall of the boring hole at every predetermined depth, and determines whether there is a cavity or not based on the correlation between the measured value and the density distribution. The ground characteristics caused by the cavity can be verified. Specific methods for measuring the electrical resistivity are also well known, and an electrical logging method, a resistivity tomography method, a resistivity two-dimensional exploration method, etc. are recommended.

Next, by arranging the data obtained by the above-described measurements, it is possible to determine a portion having a cavity or having a significantly different ground characteristic from other portions.

Specifically, the following method is used. First, a primary correlation between the P-wave velocity (Vp) and the density (unit volume weight) is determined. Since the P-wave velocity is affected by water,
This correlation line differs between above and below the groundwater table, and the correlation line is a broken line. At this time, the velocity of the P-wave does not change much due to the presence of the cavity, but the density decreases, so that this portion is shown as an abnormal distribution that largely deviates from the regression equation. First-order correlation between S-wave velocity (Vs) and density is obtained. Since the S-wave velocity is not affected by water, it is considered that a portion largely deviating from this correlation corresponds to a weak layer of the rock. A first-order correlation between the electrical resistivity and the density is obtained. For example, porous Ryukyu limestone has a low density (large gap,
That is, the more water is included), the greater the specific resistance value is, and the influence of groundwater flow as a water path is large.

By specifying the above-mentioned correlation, it is possible to know the presence or absence of a cavity at a portion deemed to deviate from the correlation or at the measurement azimuth position, and the change in ground characteristics due to this.

[0018]

<Experimental Example 1> FIGS. 1 and 2 show the first order correlation between Vp and density and Vs and density based on specific measurement results. FIG. 1 shows a first-order correlation based on a measurement result in an orientation without a cavity or the like. First, the relationship between the P-wave velocity (Vp) and the density is as follows: a naturally dried sample, that is, a position on the groundwater table, a water-saturated sample, In other words, the slope of the correlation line is different from below the water table, and rises sharply below the water table. In the figure, the sample collection numbers S80 and C56
Indicates a boring hole number, and dry and we
t indicates a measurement result on the groundwater table and a measurement result on the groundwater table. The first-order correlation between the S-wave velocity (Vs) and the density has a constant gradient without a difference due to drying and saturation.

On the other hand, FIG. 2 shows a first-order correlation based on a measurement result in a stratum with a high groundwater level. First, the relationship between the P-wave velocity (Vp) and the density is shown in FIG. There is no bending due to the correlation as shown, and it is constant with a large gradient. In addition, the X portion surrounded by a broken line is a portion deviating from the correlation, and all are recognized as small-scale hollow portions.

The primary correlation between the S-wave velocity (Vs) and the density is as shown in the figure, but the Y portion surrounded by a broken line frame is a position deviating from the correlation, and indicates the existence of the weak layer portion. Suggests.

<Experimental Example 2> FIG. 3 shows the correlation between the elastic wave velocity / density and the depth based on FIG. P in FIG.
From the wave velocity / density distribution, the actual measurement at part A is from depth -7 to
Small cavities in the bedrock are concentrated at about -10 m, part B at a depth of about -10 m to -16 m, and depth at part C is
About 25-30 m, depth D-31 -36 m in D part
It can be seen that there are small cavities scattered to the extent. Also,
From the S-wave velocity / density distribution shown in the figure, it can be seen that the portion E is a weak layer portion of the rock. In this case, as described above,
It is possible to estimate the depth of the cavity without actually measuring it.

<Experimental Example 3> FIG. 4 shows a first-order correlation between specific resistance and density based on specific measurement results. It is apparent from the figure that the specific resistance decreases as the ground density increases. From this figure, the rocky part of the stratum is small when the density is low,
The higher the density, the higher the porosity. The lower the density, the higher the porosity. The higher the density, the lower the porosity. Further, calcareous (silt content) is small when the density is low, and increases as the density increases. The water content increases when the density is low and decreases when the density increases.

Therefore, the ground properties can be accurately grasped by obtaining the above-mentioned measurements and the primary correlation based on the above-mentioned measurements in addition to the sampling and observation of the sample core. In addition, the experimental example 1
By collating with the primary correlation shown in (1), it can be used as an accurate reference value.

In each of the above experimental examples, when measuring and correlating values are obtained, each measuring device is connected to a personal computer, each measured value at each depth is recorded, and a software is prepared in advance based on the measured result. Calculation of primary correlation by regression equation etc. built in, identification of parts deviating from correlation,
Also, if various numerical values or graphic results of the results can be displayed, printed, stored, etc., data processing at the time of measurement can be easily performed.

[0025]

As is apparent from the above description, the method for investigating a cavity in the ground according to the present invention has the following effects. (1) It is possible to identify small-scale cavities that are actually problematic compared to the geophysical exploration method from the ground surface. (2) It is possible to detect cavities below the groundwater surface, which was almost impossible in the past. (3) Physical properties that cannot be determined by observation of the core sample can be evaluated. (4) Since a deformation test, a water permeability test, a tracer test, a grout test, and the like can be performed on the section in question using the borehole, the investigation borehole is used as a reference borehole, and each survey is conducted as necessary. Items can be added.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between elastic wave velocity and ground density.

FIG. 2 is a graph showing a correlation between elastic wave velocity and ground density.

FIG. 3 is a graph showing a correlation between elastic wave velocity / density and depth.

FIG. 4 is a graph showing a correlation between a specific resistance value and a ground density.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (Reference) G01N 29/18 G01N 29/18 G01V 3/20 G01V 3/20 5/12 5/12 F term (reference) 2G001 AA02 BA11 CA02 GA01 KA01 KA04 LA03 2G047 AA10 BC00 BC02 2G060 AA14 AE01 AF07 HC10

Claims (2)

[Claims] 1. A boring hole is drilled in the ground to be surveyed, an elastic wave velocity around the hole wall is measured at each predetermined depth, and a density distribution around the hole wall is measured. A method for investigating a cavity in a ground, characterized by verifying the presence or absence of a cavity or a ground characteristic caused by the cavity from a correlation between the two measured values. 2. The method according to claim 1, wherein an electrical resistivity value around the hole wall of the boring hole is measured at predetermined depths, and a ground characteristic is verified from a correlation between the measured value and the density distribution. Survey method for cavities in the ground described in.