JP2012108072A - Q-value measuring method utilizing vertical array seismograph - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable a Q-value to be easily measured in-situ by utilizing a vertical array seismograph buried underground and using a frequency-variable vibrator type vibration source installed on the ground surface.SOLUTION: A plurality of vibration receivers 10 are buried underground at different depths respectively as a vertical array seismograph, while a frequency-variable vibrator type vibration source 12 is installed on the ground. Vibrations are generated at different oscillation frequencies of 10 Hz to 50 Hz by the vibration source and received by the vibration receivers. Then, each of vibration reception waveforms is recorded at time intervals of 4 ms or less; the recorded waveform record is Fourier-transformed to obtain an amplitude spectrum; an amplitude spectrum ratio between arbitrary two vibration receivers is calculated; and an attenuation coefficient is obtained by dividing the natural logarithm of the amplitude spectrum by the distance between the vibration receivers. Thus, a Q-value of the ground between the two vibration receivers is calculated from the attenuation coefficient and an elastic wave velocity.

Description

  The present invention relates to a method for obtaining a Q value of a ground (reciprocal of an attenuation constant) using a vertical array seismometer buried in the ground, and more specifically, a variable frequency vibrator type vibration source installed on the ground surface. It is related with the method of measuring Q value in-situ using.

  In examining the propagation of seismic waves, the attenuation characteristics of the ground are an important parameter. Regarding the Q value, which is a parameter indicating the attenuation of the ground, frequency dependence, splay dependence, pressure dependence, the influence of non-homogeneous ground, etc. are discussed, and the Q value varies greatly even with similar ground (elastic wave velocity). The phenomenon has been confirmed to occur. From this, it is considered that the measurement of the Q value at the in-situ is important in evaluating the attenuation of the ground, and the Q value is measured at the in-situ when constructing an important structure. There is a tendency.

  In general, there are two methods for measuring the Q value at the original position: a method using PS logging (for example, see Non-Patent Document 1) and a method using a natural earthquake. In the former, a geophone is inserted into the borehole, and the vibration waveform generated by the artificial vibration source is recorded and analyzed. The latter records and analyzes seismic waveforms from natural earthquakes using geophones buried in the ground.

  The PS logging method can arbitrarily set the measurement interval in the depth direction in the borehole, so it is possible to grasp the Q value for each geological division in detail. Becomes narrower. The range of frequencies oscillated and received by the PS logging varies depending on the size of the source used. If the size of the source is increased, vibration at a low frequency can be generated. However, the size of the vibration source is naturally limited, and the range of the oscillation frequency is generally about 30 Hz to 100 Hz. Therefore, there is a large divergence from the natural seismic wave frequency (about 10 Hz or less), and there arises a debate as to whether evaluation in such a frequency band is appropriate for seismic design.

  Although the method using a natural earthquake can be evaluated in the frequency band of the natural earthquake, it must wait for the occurrence of the natural earthquake. Further, the obtained Q value is a Q value between seismometers buried in the ground, and is also affected by the propagation of vibration such as the arrival direction of the ground motion and the incident angle. Furthermore, the data sampling interval of the recorder is rough (the data sampling interval in natural earthquake observation is generally 20 ms to 5 ms). This is because if the frequency of the seismic wave is taken into consideration, it is not necessary to sample more finely, and if it is made finer than necessary, the number of data that can be recorded in the recorder is reduced, the frequency of data collection is increased, and the cost is increased.

  Also, there may be a difference between the Q value obtained by the PS logging method and the Q value obtained by the natural earthquake method, and it is difficult to judge whether this is due to frequency dependence or due to the difference in method. It is often taken up as a problem. Therefore, there is a demand for the development of a method that can easily investigate the frequency dependence of the Q value at the original position.

"Trial of Q measurement during PS logging" Keiji Tonouchi et al., Proceedings of the 60th Scientific Lecture Meeting (1979) p26-27

  The problem to be solved by the present invention is to use a vertical array seismometer buried in the ground, and simply measure the Q value at the in-situ position using a variable frequency vibrator type vibration source installed on the ground surface. Is to be able to.

  In the present invention, a plurality of geophones are embedded in the ground at different depths to form a vertical array seismometer, and a vibrator type vibration source having a variable frequency is installed on the ground surface, and the frequency varies from 10 Hz to 50 Hz depending on the vibration source. Vibrates at the oscillation frequency and is received by each of the receivers. The received waveform is recorded at a sampling interval of 4 ms or less. The recorded waveform record is Fourier transformed to obtain an amplitude spectrum, and the amplitude between any two receivers. Calculating the spectral ratio, dividing the natural logarithm of the amplitude spectrum by the distance between the geophones, obtaining an attenuation coefficient, and calculating the Q value of the ground between the two geophones from the attenuation coefficient and the elastic wave velocity Q value measurement method using a vertical array seismometer characterized by

  Here, the frequency of the vibrator-type vibration source is set to 10 Hz to 50 Hz because it is difficult to generate vibration below 10 Hz, and such low frequency vibration inevitably causes a decrease in resolution, and exceeds 50 Hz. This is because the commercial power supply noise becomes prominent as it approaches 60 Hz, making it difficult to accurately evaluate the Q value. The reason why the sampling interval is 4 ms or less is to enable investigation using an artificial vibration source. More preferably, the sampling interval is about 0.5 ms (1 ms to 0.25 ms). If it is 1 ms to 0.25 ms, the resolution is sufficient and the amount of data is not too large.

  As the vertical array seismometer, existing natural earthquake observation equipment can be used as it is, for example, it is composed of a plurality of identical accelerometers, and the received waveform at the geophone is an elastic wave exploration data recording device. Record.

  In the Q value measuring method according to the present invention, a vibrator type vibration source having a variable frequency is used as a vibration source. Therefore, the oscillation frequency is discretely changed or swept or excited and received by a vertical array seismometer. By recording the waveform, the frequency dependence of the Q value can be accurately evaluated. In addition, since the vertical array seismometer, which is an existing natural earthquake observation facility, can be used in the present invention, a signal cable is connected to an elastic wave exploration data recording apparatus capable of high-speed sampling, and a vibrator type vibration source is installed. It can be a cost-effective survey. Furthermore, by measuring with a vertical array seismometer that conducts natural earthquake observations, it can be compared with the results obtained by natural earthquake motion analysis.

1 is a schematic diagram of a test system for implementing a Q value measurement method according to the present invention. The analysis flow figure of Q value measurement. The graph which shows an example of a Q value analysis result.

  As shown in FIG. 1, a plurality of geophones 10 are buried in the ground at different depths to form a vertical array seismometer. Here, as the vertical array seismometer, existing natural earthquake observation equipment is used as it is. Each geophone 10 is a three-component (horizontal × 2, vertical × 1) servo accelerometer with a natural frequency of 5 Hz. A vibrator-type vibration source 12 with variable frequency is installed on the ground surface. This vibrator-type vibration source 12 can be switched between P wave / S wave, can be swept in an arbitrary range of 10 Hz to 100 Hz, and can change a single frequency at a predetermined time interval and a predetermined frequency interval. It is. The P wave (dense wave) is generated by vibrating the vibrating body up and down, and the S wave (shear wave) is generated by vibrating the vibrating body from side to side. The oscillation frequency is controlled by controlling the frequency of the drive signal created by the personal computer. A drive signal created by a personal computer is transmitted to a valve in a hydraulic circuit to generate vibration at a predetermined frequency.

  The signal cable 14 from each geophone 10 is disconnected from the normal natural earthquake observation system in the observation building 16 and connected to an elastic wave exploration data recording device 18 capable of high-speed sampling for measurement.

  A vibrator-type vibration source 12 installed on the ground surface vibrates at different oscillation frequencies within a predetermined frequency range, and the generated vibration is received by each geophone 10 in the ground. The received waveform is A / D converted by the elastic wave exploration data recording device 18 at a sampling interval of 4 ms to 0.03 ms (preferably about 0.5 ms) to record the waveform record.

Using the waveform recording by the geophones at two different depths, the ground Q value between the two geophones is measured according to the analysis flow shown in FIG.
(1) Recording of waveform record: Record waveform record with elastic wave exploration data recording device.
(2) Waveform recording filter processing: If the recorded waveform contains noise signals (electrical noise, commercial noise, natural noise, etc.) other than those caused by vibration, they are removed by a frequency filter or the like.
(3) Amplitude spectrum calculation: The waveform spectrum is converted from the time domain to the frequency domain by Fourier transform to calculate the amplitude spectrum.
(4) Spectrum flattening: A smoothing process is performed on the calculated amplitude spectrum for the purpose of stabilizing the Q value calculation.
(5) Correction processing: The obtained amplitude spectrum includes attenuation due to the distance from the source and attenuation due to the formation boundary (reflection boundary). Therefore, in order to remove these effects, geometric correction and transmission correction are performed. Apply.
(6) Calculation of spectral ratio: The amplitude spectral ratio between the geophones after correction is calculated.
(7) Calculation of attenuation coefficient: The attenuation coefficient α is calculated by dividing the natural logarithm of the amplitude spectrum ratio by the distance between the geophones. That is, the attenuation coefficient α is
α = {ln (U1 / U2)} / (r1-r2)
It becomes. U1 and U2 are amplitude spectra, and r1 and r2 are distances from the vibration source to the geophone.
(8) Calculation of Q value: The Q value of the ground between the two geophones is calculated from the attenuation coefficient α and the elastic wave velocity V. That is, the Q value is
Q = 2π / {1-exp (−2α · V / f)}
It becomes. Where f is the frequency.

  As described above, the vibrator type vibration source can be generated by switching between the P wave and the S wave. However, since the S wave is important for the seismic design, the Q value by the S wave is regarded as important. By making the frequency variable as described above, it is possible to evaluate the presence or absence of the frequency dependence of the Q value. Therefore, making the frequency variable as in the present invention has great significance. At that time, there is no significant difference in the result between the case where the single frequency is changed stepwise and the case where the vibration is made by sweeping the frequency. However, it is preferable to sweep from the viewpoint of work efficiency. In the present invention, since the measurement is performed with the same seismometer arrangement as that in which the natural earthquake observation is performed, it is possible to compare with the result obtained by the natural earthquake motion analysis.

  The Q value was measured in-situ, and the attenuation characteristics of the survey site were examined. As for the geology of the survey site, the depth of 7m from the surface is composed of fan-shaped sediments composed of sand and gravel and made of silt layer, and the depth of 7m and below is granite. Granite is divided into three categories according to the velocity layer classification obtained by PS logging.

  The vertical array seismometer installed at the survey site consists of four geophones embedded at four depths of 1 m, 50 m, 140 m, and 340 m. Each geophone has the same model and structure, and has a natural frequency of 5 Hz. Servo type accelerometer. These are already used for natural earthquake observation. The oscillation frequency was controlled by using a vibrator type vibration source with variable frequency as the artificial vibration source. The existing vertical array seismometer system at the survey site is intended for natural seismic observations, so the data sampling interval of the recorder is rough, and it is difficult to apply it to surveys using artificial sources. Therefore, the signal cable of each geophone was removed from the existing system and connected to an elastic wave exploration data recording device to record the data.

  When carrying out the measurement, first the background noise of the survey site was measured. As a result, it was confirmed that the commercial power supply noise of 60 Hz was excellent and that the absolute value of the noise level was different depending on the high frequency component. For this reason, it was estimated that it was difficult to evaluate the Q value in a frequency band of 50 Hz or higher.

Measurements at the survey site were performed using the following oscillation and measurement patterns.
・ When the oscillation type is sweep, 10 Hz to 40 Hz, 10 Hz to 50 Hz, 10 Hz to 60 Hz, 10 Hz to 80 Hz, 10 Hz to 100 Hz, all have a vibration time of 5 seconds. ・ When the oscillation type is a single frequency, 5 Hz to 4 patterns of 20 Hz section, 1 Hz interval, 20 Hz-50 Hz section, 2 Hz interval, 50 Hz-80 Hz section, 5 Hz interval, 80 Hz-100 Hz section, 10 Hz interval, vibration time is 5 seconds, and waveform recording sampling interval is any In the case of the case, it was performed at intervals of 0.5 ms.

  The analysis of Q value was implemented along the flow of FIG. In consideration of the fact that the vertical array seismometer is not embedded in the same layer, the amplitude correction of the recorded records was applied in consideration of the effect of transmission attenuation in addition to the geometric correction. The Q value obtained by the analysis is shown in FIG.

  At the survey site, the Q value for each geological division has already been obtained by the Q value measurement by PS logging. is there. On the other hand, the results shown in FIG. 3 indicate that the Q value between the geophones having a depth of 140 m and a depth of 340 m is Qs≈17 in the low frequency range, and the others are about Qs = 8 to 10 in general. Further, it was confirmed that the Q value was substantially constant with respect to the frequency between the frequencies of 15 Hz to 45 Hz in the survey site. The Q value obtained by such an analysis almost corresponded to the past survey results.

  As described above, the Q value for each frequency is obtained by the method of calculating the Q value from the amplitude spectrum ratio between the geophones, and the effectiveness of the Q value measurement using the existing vertical array seismometer for natural earthquake observation is shown. It was. This indicates that the Q value can be easily measured if a vertical array seismometer is already installed. By investigating with variable frequency in this way, cost-effective and highly accurate measurement can be performed.

DESCRIPTION OF SYMBOLS 10 Geophone 12 Vibrator type vibration source 14 Signal cable 18 Elastic wave exploration data recording device

Claims (2)

  A plurality of geophones are buried in the ground at different depths to form a vertical array seismometer, and a vibrator type vibration source with a variable frequency is installed on the surface of the earth, and the vibration source generates different oscillation frequencies from 10 Hz to 50 Hz. The received waveform is recorded at a sampling interval of 4 ms or less, and the recorded waveform record is Fourier transformed to obtain an amplitude spectrum, and the amplitude spectrum ratio between any two receivers is calculated. The attenuation coefficient is obtained by dividing the natural logarithm of the amplitude spectrum by the distance between the geophones, and the Q value of the ground between the two geophones is calculated from the damping coefficient and the elastic wave velocity. Q value measurement method using vertical array seismometer.   The vertical array seismometer is an existing facility for observing natural earthquakes and is composed of a plurality of identical accelerometers, and the received waveform at the geophone is recorded by an elastic wave exploration data recording device. Q value measurement method using vertical array seismometer.

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