10:45 〜 11:00
[HDS08-07] Near-surface shear-wave velocity monitoring using seismic ambient noise
★Invited Papers
キーワード：surface waves, microtremor, monitoring, S-wave velocity, ambient noise, spatial autocorrelation
We had continuously measured near-surface shear wave velocity (Vs) using seismic ambient noise for two months in terms of microtremor array measurements. The observed phase velocities of Rayleigh waves clearly changed associated with rainfall. Resultant timelapse Vs profiles implies that the Vs down to 6 m deep changed associated with the rainfall.
The Vs is directly related to small strain shear modulus and one of the most important physical properties in various geotechnical problems, such as site amplification, liquefaction, and slope stability. We used to use invasive methods, such as a velocity logging or a seismic cone penetration test (CSPT), to measure the shallow Vs. Non-invasive methods, such as active surface wave methods or multi-channel analysis of surface waves (MASW) has been getting popular in the 2000s. The active surface wave methods require sources and it is difficult to continuously measure the changes of the Vs in real fields. Passive surface wave methods, such as microtremor array measurements or seismic interferometry, has been getting popular for the last decade. The passive surface wave methods do not require any source and it is easy to deploy in real fields. We installed a small geophone array, continuously recorded ambient noise, processed data in real-time using cloud server for two months, and summarized phase velocity and Vs changes associated with rainfall.
The investigation site was located on a Holocene or Pleistocene alluvium plain in Cupertino, CA, U.S at approximately 70 m elevation. We used a triangular array with sides of 8 m consisting of 24 vertical component geophones (velocity sensor). A 24 channels, 24 bit A/D seismograph continuously recorded the seismic ambient noise with 4 msec sampling intervals. A local PC calculated complex coherencies for all possible pairs and uploaded them to a cloud server in every 10 minutes. The server automatically transformed the coherencies to phase velocity images in frequency domain and picked phase velocities. We had carried out the continuous measurements for two months from the middle of December 2023 to the middle of February 2024.
Figure 1 compares the phase velocity images at Jan. 21st and Feb. 10th respectively. We measured the vertical component of the ambient noise and the phase velocities corresponds to Rayleigh waves. There was rainfall more than 200 mm between the Jan. 21st and Feb. 10th. We can clearly recognize that the phase velocity decreased above 20 Hz.
The server picked phase velocities and averaged them in four frequency ranges every 10 Hz from 10 Hz (10~20, 20~30, 30~40, and 40~50 Hz). Their wavelengths were approximately 15~25, 10~15, 5~10, and 2~5 m, and corresponding depths were approximately 5~8, 3~5, 1.5~3, and 0.5~1.5 m respectively. Figure 2 summarizes timelapse averaged phase velocities compared with cumulative rainfall. Before the middle of January, the highest frequency range (40~50 Hz) changed associated with the rainfall on Dec. 17th to 20th and Dec. 28th to 29th. Other frequency ranges did not show clear change in those periods. There was a considerable amount of rainfalls in the period between Jan. 21st and Feb. 10th. Total rainfall was approximately 225 mm in that period. In that period, phase velocities in the lower frequency ranges, 20~30 Hz and 30~40 Hz significantly decreased from 350 to 200 m/sec and from 300 to 110 m/sec respectively. We applied non-linear inversion to the dispersion curves on Jan. 21st and Feb. 10th and estimated Vs profiles to 10 m deep. Resultant Vs profiles show that Vs from surface to 6 m deep decreased 10~40 %. The Vs particularly decreased about 40 % at 1~2 m deep from 210 to 130 m/sec. It should be noted that the higher frequency range (40~50 Hz) decreased earlier (from Jan. 21st) and lower frequency ranges (20~30 Hz and 30~40 Hz) decreased later (from Jan. 31st). Different change periods might imply water penetration process from shallow to deep.
The Vs is directly related to small strain shear modulus and one of the most important physical properties in various geotechnical problems, such as site amplification, liquefaction, and slope stability. We used to use invasive methods, such as a velocity logging or a seismic cone penetration test (CSPT), to measure the shallow Vs. Non-invasive methods, such as active surface wave methods or multi-channel analysis of surface waves (MASW) has been getting popular in the 2000s. The active surface wave methods require sources and it is difficult to continuously measure the changes of the Vs in real fields. Passive surface wave methods, such as microtremor array measurements or seismic interferometry, has been getting popular for the last decade. The passive surface wave methods do not require any source and it is easy to deploy in real fields. We installed a small geophone array, continuously recorded ambient noise, processed data in real-time using cloud server for two months, and summarized phase velocity and Vs changes associated with rainfall.
The investigation site was located on a Holocene or Pleistocene alluvium plain in Cupertino, CA, U.S at approximately 70 m elevation. We used a triangular array with sides of 8 m consisting of 24 vertical component geophones (velocity sensor). A 24 channels, 24 bit A/D seismograph continuously recorded the seismic ambient noise with 4 msec sampling intervals. A local PC calculated complex coherencies for all possible pairs and uploaded them to a cloud server in every 10 minutes. The server automatically transformed the coherencies to phase velocity images in frequency domain and picked phase velocities. We had carried out the continuous measurements for two months from the middle of December 2023 to the middle of February 2024.
Figure 1 compares the phase velocity images at Jan. 21st and Feb. 10th respectively. We measured the vertical component of the ambient noise and the phase velocities corresponds to Rayleigh waves. There was rainfall more than 200 mm between the Jan. 21st and Feb. 10th. We can clearly recognize that the phase velocity decreased above 20 Hz.
The server picked phase velocities and averaged them in four frequency ranges every 10 Hz from 10 Hz (10~20, 20~30, 30~40, and 40~50 Hz). Their wavelengths were approximately 15~25, 10~15, 5~10, and 2~5 m, and corresponding depths were approximately 5~8, 3~5, 1.5~3, and 0.5~1.5 m respectively. Figure 2 summarizes timelapse averaged phase velocities compared with cumulative rainfall. Before the middle of January, the highest frequency range (40~50 Hz) changed associated with the rainfall on Dec. 17th to 20th and Dec. 28th to 29th. Other frequency ranges did not show clear change in those periods. There was a considerable amount of rainfalls in the period between Jan. 21st and Feb. 10th. Total rainfall was approximately 225 mm in that period. In that period, phase velocities in the lower frequency ranges, 20~30 Hz and 30~40 Hz significantly decreased from 350 to 200 m/sec and from 300 to 110 m/sec respectively. We applied non-linear inversion to the dispersion curves on Jan. 21st and Feb. 10th and estimated Vs profiles to 10 m deep. Resultant Vs profiles show that Vs from surface to 6 m deep decreased 10~40 %. The Vs particularly decreased about 40 % at 1~2 m deep from 210 to 130 m/sec. It should be noted that the higher frequency range (40~50 Hz) decreased earlier (from Jan. 21st) and lower frequency ranges (20~30 Hz and 30~40 Hz) decreased later (from Jan. 31st). Different change periods might imply water penetration process from shallow to deep.