10:05 AM - 10:25 AM
[HTT17-05] Applicability of Distributed Acoustic Sensing (DAS) technology for surface wave measurement
Keywords:Distributed Acoustic Sensing (DAS), Surface wave,, Multi-channel Analysis of Surface Waves (MASW), Phase velocity dispersion curve
Recently, the distributed acoustic sensing (DAS), which is one of the distributed fiber optic sensing technologies using fiber optics, is receiving increased attention. DAS is promising as an innovative vibration monitoring tool in various fields including geophysics because it utilizes an optical fiber itself as a sensor and it can acquire data over long length at spatially high density. In civil engineering, preliminary investigation of underground conditions with boring survey and/or geophysical survey techniques are often required for the safe and rational construction of underground structures. This study aimed at investigating the applicability of DAS for geophysical exploration using elastic wave and Multi-channel Analysis of Surface Waves (MASW) survey was performed to understand the basic performance of DAS.
A trench with a length of about 100 m was excavated in a research field located in Richmond, California, USA, and fiber optic sensor cables were directly buried in the trench. Surface waves induced by an active source using a sledgehammer were measured at 1 m intervals along with a fiber optic sensor. The result with a tightly coupled sensing cable (i.e. high-sensitivity cable) showed that the clear surface waves propagating to the other end. On the other hand, the wave propagation could not be clearly detected due to background noise in the latter part of the loosely coupled telecommunication-cable (i.e. low-sensitivity cable). These results indicated that the cable structure affects the response sensitivity for the seismic wave. However, stacking of the waveforms in low-sensitivity cables allowed the signal-to-noise ratio to improve and showed almost the same results as high-sensitivity cables. In addition, surface waves were measured using a three-component geophone to compare with DAS measurement results. It could be found that similar waveform recordings and the phase velocity dispersion curve as DAS were obtained from the geophone with the measurement results of the horizontal component in the same direction as the fiber optic cable.
In the frequency band necessary for the analysis of geophysical exploration, surface wave measurement of the active source can be performed with almost the same performance as existing survey method with geophones. DAS, which can simultaneously acquire data at intervals of several meters over several tens of kilometers, will enable an application to a wider range of subjects.
A trench with a length of about 100 m was excavated in a research field located in Richmond, California, USA, and fiber optic sensor cables were directly buried in the trench. Surface waves induced by an active source using a sledgehammer were measured at 1 m intervals along with a fiber optic sensor. The result with a tightly coupled sensing cable (i.e. high-sensitivity cable) showed that the clear surface waves propagating to the other end. On the other hand, the wave propagation could not be clearly detected due to background noise in the latter part of the loosely coupled telecommunication-cable (i.e. low-sensitivity cable). These results indicated that the cable structure affects the response sensitivity for the seismic wave. However, stacking of the waveforms in low-sensitivity cables allowed the signal-to-noise ratio to improve and showed almost the same results as high-sensitivity cables. In addition, surface waves were measured using a three-component geophone to compare with DAS measurement results. It could be found that similar waveform recordings and the phase velocity dispersion curve as DAS were obtained from the geophone with the measurement results of the horizontal component in the same direction as the fiber optic cable.
In the frequency band necessary for the analysis of geophysical exploration, surface wave measurement of the active source can be performed with almost the same performance as existing survey method with geophones. DAS, which can simultaneously acquire data at intervals of several meters over several tens of kilometers, will enable an application to a wider range of subjects.