JP3106183B2 - Ground exploration method and device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for exploring a ground by constantly monitoring microtremors, seismic waves, and the like and exploring the stratum structure of the ground.

[0002]

2. Description of the Related Art Strong ground motions that cause damage during an earthquake are governed by the soil structure at the extremely shallow surface. For this reason, in order to explore the shallow surface structure, methods such as direct boring and investigation, as well as methods of artificially shaking the ground using explosives and exciters and measuring the response are used. ing. However, these methods are expensive for boring surveys, explosives, and exciters, and are not feasible for surveys anywhere, such as in urban areas or near buildings.

[0003] For this reason, recently, a method using the microtremor has attracted attention, and based on this method, the ground structure has been estimated by group observation. The method using the microtremor does not provide the same accuracy as the direct method described above, but is expected to be an inexpensive method with less restrictions on the selection of the survey point.

[0004]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to make it possible to detect a stratum structure of a ground with high accuracy by a method using microtremors.

Therefore, the present invention relates to a ground exploration method for exploring the geological structure of the ground by constantly observing tremors and seismic waves, etc., and observing velocity waves on the surface of the ground and constantly observing horizontal tremors and seismic waves. The main component is regarded as a shear wave, and the depth distribution from the surface in an amount corresponding to the effective value of the shear strain is measured from the velocity wave in which the rising wave and the descending wave of the shear wave overlap with each other, thereby forming a stratum of the ground. It is characterized by performing a structure exploration, as a ground exploration device, a velocity type vibration transducer installed on the ground surface and observing a velocity wave on the ground surface, and reading the velocity wave observed by the vibration transducer, the velocity wave Analysis means for performing a depth distribution analysis of the strain spectrum from, the analysis means, by the depth distribution analysis of the strain spectrum, the depth distribution from the ground surface of the amount corresponding to the effective value of the shear strain in the ground. To measure It is an butterfly.

[0006]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing an embodiment of a ground exploration device according to the present invention, FIG. 2 is a diagram showing an example of the configuration of an analysis device, and FIG. 3 is a diagram for explaining the depth distribution of the effective value of shear strain in the ground. FIG. In the figure, reference numeral 1 denotes an oscillator, 2 denotes an analyzer, 3 denotes an A / D converter, 4 denotes a WD calculation array, 5 denotes a multiplier, 6 denotes an adder, and 7 denotes an integrator.

In FIG. 1, a vibration transducer 1 is a velocity vibration transducer which is installed on the ground surface and observes vibrations on the ground surface. Propagation of seismic waves and microtremors on the ground that is not so deep from the surface of the ground, shear waves (S waves) for horizontal vibration
Is considered to have a main power, and the vibration isolator 1 on the surface of the ground observes and records such an overlapping shear wave and descending wave. The analysis device 2 reads a velocity wave observed on the ground surface by the velocity type vibration isolator 1 and performs a depth distribution analysis of a strain spectrum. As shown in FIG. 2, the analyzing device 2 first converts a velocity wave captured by the vibration transducer 1 into a discrete signal by an A / D converter 3, and converts the signal into a discrete signal by a WD (Wigner distribution) calculation array 4. Is converted into a Wigner distribution (WD) at each sample point. Finally, an instantaneous spectrum of the difference between the rising wave and the falling wave at a specific depth from the addition of the Wigner distribution by the multiplier 5, the adder 6, and the integrator 7. And an amount corresponding to the effective value of the shear strain is obtained from the integral.

[0008] If the depth distribution of the effective value of the shear strain in the ground is obtained, it is possible to obtain the place where one formation comes into contact with the next formation, where the distribution rapidly increases as shown in FIG. The ground surveying device according to the present invention comprises a velocity type vibration transducer that captures vibration on the surface of the ground and an analyzer that processes the signal as described above, and actually performs ascending waves and descending waves that constitute waves propagating in the ground. It has the characteristic that the difference can be evaluated without being separated into a.

Next, the flow of processing by the analyzer will be described. FIG. 4 is a diagram for explaining the depth distribution analysis processing of the strain spectrum by the analyzer.

In the depth distribution analysis of the strain spectrum, as shown in FIG. 4, a velocity wave v (t) observed on the ground is first read (step S11), and an autocorrelation function R _V (t,
τ) is calculated (step S12).

For the sake of simplicity, the ground structure below the vibration isolator 1 is modeled by a two-layer structure having a surface layer and a base as shown in FIG. At this time, the shear strain ε at the depth h of the surface ground
(T; h) is the shear wave (S wave) that is incident vertically.
Using the rising wave f (t) and the falling wave g (t),
Can be expressed by

[0012]

１ (t; h) = {− f (t) + g (t)} / 2V = [− v {t− (h / V)} + vΔt + (h / V)}] / 2Vh : Distance from the ground surface V: S-wave velocity at the surface layer v (t): Velocity response observed at the ground surface Here, the velocity wave v (t) observed at the ground surface was expanded (for unsteady signals. ) The autocorrelation function R _V (t, τ) is represented by the following equation (Equation 2).

[0013]

R _V (t, τ) = v ｛t + (τ / 2)｝ v ｛t
− (Τ / 2)｝ τ: delay time After the autocorrelation function R _V (t, τ) is calculated, next, a Wigner distribution W _v is obtained by Fourier transform (step S13). These are executed by the WD calculation array 4.

The Wigner distribution W _v of the velocity wave v (t) observed on the ground surface is

[0015]

(Equation 3)

Is defined as Hereinafter, the integral is from -∞ to + ∞
And The Wigner distribution represents the instantaneous power spectral density function extended to a non-stationary signal. Also, by the inverse Fourier transform,

[0017]

(Equation 4)

## EQU1 ## Assuming that τ = 0 in the equation (Equation 4),

[0019]

(Equation 5)

Which is equal to the instantaneous power of the velocity wave, and with respect to the time shift of the velocity wave v (t),

[0021]

(Equation 6)

The following holds. Also, according to equation [Equation 3]

[0023]

(Equation 7)

The following holds. here,

[0025]

(Equation 8)

Subsequently, the multiplier 5 and the adder 6 calculate a shear strain instantaneous power spectrum density function Wε (step S14). In this operation, Wε is repeatedly calculated to a required depth while changing the depth h, for example, starting from h = 0 and calculating up to h = 40 m or h = 100 m. The Wigner distribution of shear strain ε (t; h) (instantaneous power spectrum density function of shear strain) can be expressed as follows by applying equation [1] to equations [6] and [7]. it can.

[0027]

(Equation 9)

The instantaneous power is

[0029]

(Equation 10)

The instantaneous shear strain power (effective value) is calculated by the integrator 7 in this manner (steps S15 and S16).

Expression (1) using h (depth) as a parameter
0], the effective value of the shear strain is plotted, and a change point of the ground component is detected. FIG. 5 is a diagram illustrating an example of an S-wave travel time curve obtained by performing a geophysical survey, and FIG. 6 is a diagram illustrating an example of an average shear strain instantaneous power. These are examples of data at the observation point in Hadano City, Kanagawa Prefecture. Two points a and b are different at the point where the stratum changes dynamically from, for example, embankment to sand, and from sand to rock, that is, at this observation point It is the connection point of the stratum having the velocity structure. Fig. 5 shows this change point as the arrival time (S-wave travel time) counted from the surface of the ground, and shows how the distortion of the ground under the ground is estimated as a function of the depth. FIG.

At the place where the stratum changes (boundary of the stratum), the distortion is as shown in FIG. 3, and the vertical axis in FIG. 3 and the horizontal axis in FIG. 6 are the same. Therefore, the formation change point in FIG.
In the example of FIG. 6, the vertical axis is multiplied by 10 and corresponds to the horizontal axis of FIG. This is because V =
This is because the calculation was performed at 100 m / s, and the change point appearing during the traveling of the S wave can be obtained from the local peak of the distortion. A comparison between the S-wave travel time curve shown in FIG. 5 and the instantaneous shear strain power of the equation [Equation 10] shown in FIG. 6 obtained by applying the ground survey device according to the present invention shows that at points a and b, It turns out that they match.

It should be noted that the present invention is not limited to the above embodiment, and various modifications are possible. For example, in the above-described embodiment, a description has been given of a case where microtremors or seismic waves are always observed, but it goes without saying that conventional explosives and exciters may be used in combination. In addition, the term cos in the equation (Equation 9) is practically sufficient even if it is omitted.

[0034]

As is apparent from the above description, according to the present invention, a microtremor is always used as an input for ground exploration, and the depth at which the stratum structure of the ground changes (time from the ground surface).
, The information on the ground structure can be obtained. Needless to say, actual application requires reference boring data and the like. However, by extending or interpolating the results regionally, it is possible to obtain the detailed detailed structure of the ground in a specific region. Determining the distribution of shaking intensity during an earthquake is indispensable for earthquake disaster prevention, and the present invention can be a basis for obtaining such a vibration distribution. Further, the present invention can be applied to the detection of foreign substances in the ground.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of a ground exploration device according to the present invention.

FIG. 2 is a diagram illustrating a configuration example of an analysis device.

FIG. 3 is a diagram for explaining a depth distribution of an effective value of shear strain in the ground.

FIG. 4 is a diagram for explaining a depth distribution analysis process of a distortion spectrum by the analysis device.

FIG. 5 is a diagram illustrating an example of an S-wave travel time curve obtained by performing a geophysical survey.

FIG. 6 is a diagram illustrating an example of average shear strain instantaneous power.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Transducer, 2 ... Analyzer, 3 ... A / D converter, 4 ... W
D calculation array, 5: multiplier, 6: adder, 7: integrator

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G01V 1/00 G01H 1/00 G01V 1/30

Claims (2)

(57) [Claims] 1. A ground exploration method for exploring the geological structure of the ground by observing microtremors, seismic waves, etc., and observing velocity waves on the surface of the ground, and determining main components in the horizontal direction such as microtremors, seismic waves, etc. Exploring the geological structure of the ground by measuring the depth distribution from the surface in an amount corresponding to the effective value of the shear strain from the velocity wave where the rising wave and the descending wave of the shear wave overlap each other, assuming that the shear wave is a shear wave. A ground exploration method characterized by performing. 2. A ground exploration apparatus for constantly detecting tremors, seismic waves, and the like, and for exploring the geological structure of the ground, comprising: a speed-type vibrator installed on the ground surface for observing a velocity wave on the ground surface; Analyzing means for reading a velocity wave observed by the shaker and performing a depth distribution analysis of a strain spectrum from the velocity wave, wherein the analyzing means includes an analysis of the depth distribution of the strain spectrum by the depth distribution analysis of the strain spectrum. A ground surveying device for measuring a depth distribution from the surface in an amount corresponding to an effective value.