JP2001337174A - Mapping method for ground impedance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently sample data reflecting dynamic physical properties of the ground by small number of workers and simple work to infer hardness and softness of the ground and the distribution of a cavity and a loosened part in an extremely shallow underground part. SOLUTION: An impedance mapping method for the ground is provided with a base plate 14 placed on a ground surface, a reaction mass 18 vibrating vertically using a piston 16 integrated with the base plate as an axis, and acceleration meters 20, 22 loaded on the base plate and the reaction mass. A vibrator- type hypocenter 10 according to a system in which a reaction force of the reaction mass is transmitted to the ground surface through the base plate is used to obtain an impedance value of the ground at a measurement point from vibration frequency and vibration acceleration, when the vibrator type hypocenter is vibrated, and the operation is done at a plurality of different measurement points to display the impedance distribution of the ground.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hard or soft or very shallow underground surface characteristic obtained by determining the impedance value of the ground at a measurement point from the vibration characteristics of a vibrator-type hypocenter installed on the surface of the ground when vibrating. The present invention relates to an impedance mapping method for a ground, which displays a distribution of cavities and looseness of a part.

[0002]

2. Description of the Related Art In a survey in the field of civil engineering or the environment, it is required to search for a structure, foreign matter, physical properties, etc. of a shallow underground as efficiently and economically as possible. Several meters underground
There are various techniques for exploration techniques for shallow parts up to this point, but surface wave exploration and underground radar are used for changes in stratum composition and physical properties.

The surface wave exploration is a method of observing a surface wave propagating along the ground surface and estimating a ground structure by using the dispersibility thereof. A plurality of geophones are arranged at an appropriate position away from the epicenter at a certain distance, and the ground is excited by the epicenter to detect the vibration with each geophone. As a result, the time difference between the waves propagating between the geophones is known, so that the phase velocity can be obtained. In such surface wave exploration, the phase velocity is assumed to be the average velocity up to half the wavelength, since the high-frequency surface waves have a property of propagating to the shallow depth and the low-frequency surface waves have a property to propagate deeper. By doing so, the distribution of the phase velocity in the depth direction can be obtained. Since the phase velocity is one of the physical properties reflecting the ground characteristics,
It can be used for estimating the stratum composition of the ground.

The underground radar has a transmitting antenna and a receiving antenna installed on the ground surface, radiates electromagnetic waves into the ground, and measures the reflection characteristics of the electromagnetic waves. Thereby, the buried object or the underground structure can be inspected.

[0005]

The surface wave exploration measures the phase velocity, and the underground radar measures the reflection characteristics of electromagnetic waves. Used in combination with survey methods. However, the phase velocity of the surface waves or the reflection characteristics of the electromagnetic waves do not directly represent the mechanical strength of the ground such as softness or looseness. Therefore, there is a demand for the development of an exploration technique capable of directly and accurately obtaining the underground mechanical strength distribution and the like by a simple method.

An object of the present invention is to use a very small number of workers,
To provide a ground impedance mapping method capable of efficiently collecting data reflecting the mechanical properties of the ground by simple operations, and thereby estimating the distribution of hard and soft ground, and cavities and loosening of a very shallow underground. It is.

[0007]

According to the present invention, a base plate placed on the ground surface, a reaction mass vibrating up and down about a piston integrated with the base plate as an axis, and mounted on the base plate and the reaction mass. A vibrator-type hypocenter equipped with an accelerometer and transmitting the reaction force of the reaction mass to the ground surface through the base plate is used. The present invention uses such a vibrator-type hypocenter, obtains the impedance value of the ground at a measurement point from the vibration frequency and the detected vibration acceleration when the vibrator-type hypocenter is vibrated, and performs the operation at a plurality of different measurement points. And a ground impedance mapping method for displaying the impedance distribution of the ground.

Here, the impedance value Z is as follows: Z = σ / V _g where σ is obtained from the stress. Velocity V of the ground at the station
_g is, V _g = a _b / iω However, a _b acceleration record of the base plate, omega can be obtained from the vibration frequency.

When a vibrator-type hypocenter is vibrated, the vibration characteristics of the hypocenter itself vary depending on the hardness of the ground. Therefore, it is possible to estimate the distribution of hard and soft ground, cavities and looseness in the extremely shallow underground by analyzing the difference in vibration characteristics from the records of the accelerometer attached to the hypocenter. The present invention has been completed by conducting research and development on both hardware and software based on such a principle. This impedance mapping method measures the impedance directly on the ground surface, so without any data processing, that is, when the measurement is completed,
It is possible to evaluate the hardness of the ground surface.

The vibrator-type epicenter is sequentially moved to a plurality of measurement points along one or more measurement lines, and the vibration frequency of the vibrator-type epicenter is changed stepwise at each measurement point, and each vibration frequency and its The ground impedance value at the measurement point is determined for the different vibration frequencies from the vibration acceleration with respect to. Thereby, a two-dimensional or three-dimensional distribution of the impedance of the ground can be displayed. When evaluating the distribution of impedance in the depth direction in this way, an analysis based on a vibration frequency or the like is required.

In the data recording, at the measuring point, an acceleration record is recorded at a certain vibration frequency, and subsequently, an acceleration record is recorded again at the same vibration frequency. Take in the measured value only when it is small.
When the error is large, by repeating the above operation, it is possible to more accurately capture the measured value.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In practice, it is preferable to control the driving state and acceleration detection of a vibrator-type hypocenter by a personal computer. In this case, (1) a drive waveform is created on software of a personal computer, and D
A / A conversion function to generate a control signal, (2) A / D conversion function of a personal computer to capture an electrical detection signal from an accelerometer attached to a vibrator-type seismic source and save the record (3) obtaining the impedance value of the ground from the stored acceleration record; (4) obtaining the impedance value of the ground for each vibration frequency on a survey line or in a grid on the ground surface;
The impedance mapping of the ground can be easily performed by going through the process of displaying it on the contour map.

At this time, the relationship between the vibration frequency and the ground volume reflecting the impedance is obtained, and the distribution of the impedance can be obtained by inverse analysis. In other words, the motion of the reaction mass, base plate, and ground when the vibrator-type epicenter is vibrated is obtained by a combination of the response calculation of the mass-spring system and the model calculation by the difference method in which the ground is replaced with a fine grid. Find the impedance value by the model. Then, the result of the model calculation is compared with the actual measured value, and the impedance distribution that best describes the actual measured value is displayed as the final result.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An impedance mapping measurement system is constructed by incorporating control software for impedance mapping into a portable vibrator type epicenter system. Fig. 1 shows an example of the measurement system.
Shown in The vibrator-type hypocenter 10 is mounted on a base plate 14 placed on the ground surface 12, a reaction mass 18 vibrating up and down around a piston 16 integrated with the base plate 14, and a base plate 14 and a reaction mass 18, respectively. Accelerometer 2
0, 22 in which the reaction force of the reaction mass 18 is transmitted to the ground surface 12 through the base plate 14. The vertical vibration is applied by using an electromagnetic drive mechanism (combination of a coil and a magnetic circuit: not shown) built in the reaction mass 18, and from the power amplifier 26 operated by an AC power supply 24 to the coil Is supplied by supplying a drive current of a predetermined frequency to the device.

The driving and acceleration detection of the vibrator-type hypocenter 10 are controlled by a personal computer 30. control·
The personal computer 30 for data recording and analysis
Sound circuit mounted in the main body, or D / A of sound board 32 or sound card added to the main body
The vibrator-type hypocenter 10 is controlled using a conversion and A / D conversion function. That is, the D / A conversion function of the sound board 32 is controlled by software, and a single-cycle sine wave signal (electric drive signal) specified by a parameter is formed and output. This control signal is transmitted to the power amplifier 2
At 6, it is converted into a drive current, and the drive current causes the vibrator-type hypocenter 10 to vibrate. In addition, both accelerometers 20,
The acceleration recording by the A / D 22
The data is captured by the personal computer 30 using the conversion function and recorded as digital information. Recording of this acceleration record is also controlled by software.

Control by software is performed in the following procedure. (1) a process of creating a drive waveform on software of a personal computer and generating a control signal by a D / A conversion function; (2) an electrical detection signal from an accelerometer attached to a vibrator-type hypocenter. The process of capturing and storing the record by the A / D conversion function of the personal computer, (3) the process of obtaining the impedance value of the ground by the stored acceleration record, and (4) the vibration frequency on the ground surface on a measuring line or grid. For each, there is a step of obtaining the impedance value of the ground and expressing it in a contour map, and (5) a step of obtaining the relationship between the vibration frequency and the ground volume reflecting the impedance and obtaining the distribution of impedance by inverse analysis.

Meanwhile, in the elastic wave exploration, a part of the wave energy propagating from the epicenter is back-propagated and returns to the epicenter. The strength of the back-propagating signal has a large correlation with the hardness of the ground. In a field far enough away from the hypocenter to allow the wavefront to be considered almost planar, its impedance is expressed as the product of density and velocity, which varies with the bulk modulus and shear modulus of the ground. On the other hand, the impedance Z in a broad sense including a field close to the epicenter where the wavefront must be treated as a spherical surface includes a locally acting stress σ and a local velocity (particle velocity) V _g due to the stress σ.
Is defined as the following equation: Z = σ / V _g

Accordingly, it is possible to estimate the ground impedance near the ground surface by generating vibration at a certain point on the ground surface and simultaneously measuring the stress at that point and the particle velocity of the ground. The stress σ transmitted to the ground to the vibrator type hypocenter is expressed by the following equation as a so-called ground force F _g by two accelerometers attached to the hypocenter. _{σ = F g / A = (} M b · a b + M r · a r) / A A: contact area of the base plate M _b: weight of the base plate a _b: a base plate acceleration M _r: the reaction mass weight a _r: Reaction Mass acceleration Further, the velocity can be obtained by the following equation from the integration of the acceleration waveform of the base plate, assuming that the vibration of the base plate is in phase with the vibration of the ground. _{V g = a g / iω =} a b / iω a g: particle acceleration ω: signal frequency

[0019] Therefore, from these _{equations, Z = (M b · a} b + M r · a r) · iω / A · a b , and the impedance from the two acceleration record is required. In practice, the amplitude and phase are evaluated from the impedance waveform obtained from this equation.

FIG. 2 shows a flowchart of a measurement procedure on site in the method of the present invention. First, as a preparation stage, measurement points are determined and marked. The measurement points may be set along one measurement line, or may be set in a grid along a plurality of measurement lines. In addition, search parameters are set. The search parameters include the number of vibration frequencies and frequency steps. Then, a vibrator-type epicenter is installed at the measurement point and automatic measurement is performed. After automatic measurement, move the vibrator-type hypocenter to the next measurement point and perform the next automatic measurement. This operation is repeated by the number of measurement points.

FIG. 3 shows a detailed flowchart of the automatic measurement. In the automatic measurement step, the vibrator-type epicenter is vibrated and an acceleration record is recorded at the same time (first time).
Subsequently, the vibrator-type epicenter is vibrated, and an acceleration record is recorded at the same time (second time). And the first and two
The difference (error) between the measured values at the second time is calculated, and if the error is small (if it is within a preset value), the measured value is stored. If the error is large, the process returns to the first step and the recording of the acceleration record is performed again. If the error is small and the measured value is saved, change to the next frequency. This operation is automatically repeated by the number of frequencies.

The amplitude or phase of the impedance obtained as described above is displayed as follows. A. In the case where a measurement point is provided on one measurement line (in the case of one-dimensional measurement), the measurement point (line distance) is plotted on the horizontal axis, and the frequency is plotted on the vertical axis. The frequency is higher for higher frequencies and lower for lower frequencies. By expressing the impedance of a shallow portion of the ground at a high frequency and expressing the impedance including a deeper portion at a low frequency, the distribution of the impedance of the cross section of the ground can be expressed. B. When measuring points are provided in a grid (in the case of two-dimensional measurement) If a plurality of measuring lines are provided and the impedance is measured at the measuring points set in a grid, the planar impedance distribution for one frequency is contoured. It can be shown in the figure. Since this measurement is performed for a plurality of frequencies, contour plots can be made for the number of frequencies. As described above, the higher frequency indicates the impedance of the shallower portion, and the lower frequency indicates the average impedance including the deeper portion, so the contour diagram for each frequency obtained indicates the depth obtained by cutting the ground in the depth direction. It can be evaluated as a slice cross section.

FIG. 4 shows the results of ground impedance measurement performed by providing measurement points in a grid. The dotted line (2.3 mx 2 m) is used to dig the topsoil once (depth 2
m) This is a place where it is backfilled with a large amount of water while compacting, and it is considered that the impedance is different from that of the surroundings. The measuring points were provided in a grid pattern (13 × 4 points) at 1 m intervals. FIG. 5 shows the rate of change of the impedance value for each frequency as a plane contour as a result of impedance mapping. The change rate is obtained as an average value for each frequency, and is obtained as a ratio (%) between the deviation of each measured value and the average value. It is considered that the measured impedance value reflects a stratum characteristic of a larger volume (up to deeper depth) as the vibration frequency is lower (the wavelength is longer). In this experiment, it was found that the impedance change at a depth of about 2 m from the ground surface (backfilled depth) was most clearly captured by impedance mapping using a frequency of about 230 Hz. On the other hand, the frequency is 134H
As the z and 77 Hz drop, the zone showing a high impedance ratio becomes less significant. This is considered to be because the impedance obtained by the vibration of about 200 Hz or less reflects the influence of the stratum having a larger volume than the backfilled range. Also, when the frequency becomes about 400 Hz or more, the portion showing the high impedance ratio becomes gradually unclear.

FIG. 5 shows an example of a procedure for performing an inverse analysis of the impedance distribution. Vibrator external force is divided into the force of the reaction mass and the force of the base plate. The mass-spring system response is calculated based on the acceleration of the reaction mass, and the mass-spring system force is obtained, and this is reflected in the vibrator internal force. The internal force of the vibrator is divided into the force of the reaction mass and the force of the base plate. The acceleration of the base plate reflects the distortion of the stratum model. Based on this, the model calculation of the ground model is performed, and the stress and the ground force of the stratum model are obtained. That is reflected in the power of the base plate. By circulating such operations, the vibration waveform falls in the same sine curve shape, and the impedance values (amplitude and phase) are obtained from the sine curve. By repeating such a procedure, the distribution of impedance can be inversely analyzed.

[0025]

According to the present invention, as described above, only the vibrator-type hypocenter is moved within the exploration range, so that it is not necessary to install a geophone as compared with the reflection method exploration, etc. With the number of workers and simple work,
Data reflecting the mechanical properties of the ground can be collected efficiently, which makes it possible to estimate the distribution of hard and soft ground, and cavities and loosening in the extremely shallow underground.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an example of an impedance mapping measurement system according to the present invention.

FIG. 2 is a flowchart showing an example of the measurement procedure.

FIG. 3 is a flowchart showing an example of the automatic measurement.

FIG. 4 is an explanatory diagram showing an example of impedance mapping measurement results on a plurality of measurement lines.

FIG. 5 is a flowchart illustrating an example of a procedure of an inverse analysis.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Vibrator-type epicenter 12 Ground surface 14 Base plate 16 Piston 18 Reaction mass 20, 22 Accelerometer 24 AC power supply 26 Power amplifier 30 Personal computer 32 Sound board

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Vincent Niekhoff New Bergen Archimedes van 16 in the Netherlands 16 Within the Oryo Center of Applied Geosciences (72) Inventor Jan Blauer New Bergen Archimedes van 16 in the Netherlands Within the Oryo Center of Applied Geoscience

Claims (6)

[Claims] A base plate mounted on the ground surface;
A vibrator comprising a reaction mass vibrating up and down around a piston integrated with the base plate, and an accelerometer mounted on the base plate and the reaction mass, wherein a reaction force of the reaction mass is transmitted to the ground surface through the base plate. Using the seismic source, obtain the impedance value of the ground at the measurement point from the vibration frequency when the vibrator type seismic source is vibrated and the detected vibration acceleration, perform the operation at multiple different measurement points, and display the impedance distribution of the ground An impedance mapping method for a ground, characterized in that: 2. The particle velocity V _g of the ground at the measuring point is given by: V _g = _ab / iω where a _b is the acceleration recording of the base plate, ω is obtained from the vibration frequency, and the impedance value Z is given by Z = σ / V _g However, sigma impedance mapping method of the ground according to claim 1, wherein determining from the stress. 3. The vibrator-type epicenter is sequentially moved to a plurality of measurement points along one or more measurement lines, and the vibration frequency of the vibrator-type epicenter is changed in a step-like manner at each of the measurement points. 3. The impedance mapping method for a ground according to claim 1, wherein a ground impedance value at the measurement point is obtained for the different vibration frequencies from the vibration acceleration corresponding thereto and a two-dimensional or three-dimensional distribution of the ground impedance is displayed. 4. At a measurement point, an acceleration record at a certain vibration frequency is recorded, and subsequently, an acceleration record is recorded again at the same vibration frequency, and when both acceleration records are compared, an error is smaller than a set value. 4. The method according to claim 3, wherein only the measured values are fetched, and when the error is large, the above operation is repeated to fetch the measured values. 5. A method for controlling the drive current and acceleration detection of a vibrator-type seismic source by a personal computer. (1) A drive waveform is created on software of the personal computer, and a control signal is generated by a D / A conversion function. The process of generating, (2) The process of taking in the electrical detection signal from the accelerometer attached to the vibrator type epicenter by the A / D conversion function of the personal computer, and storing the record, (3) The stored acceleration 5. The method according to claim 4, further comprising the steps of: obtaining a ground impedance value by recording; and (4) obtaining a ground impedance value for each vibration frequency on a measurement line or in a grid on the ground surface and displaying the obtained impedance value as a contour map. Ground impedance mapping method. 6. The method according to claim 5, wherein the relationship between the vibration frequency and the ground volume reflecting the impedance is obtained, and the distribution of the impedance is obtained by inverse analysis and displayed.