Comparison of DAS (distributed acoustic sensor) and seismometer measurements to evaluate physical quantities in the field

Seismic exploration and time-lapse study require knowledge of the physical quantity of measurement. Although a DAS (distributed acoustic sensor) is very useful because it can provide very dense seismic records telemetered to a data center 20 km away, the quality of data and comparison of waveforms to those for conventional geophones or seismometers are not well understood. According to Schlumberger Co., their DAS measures the strain rate parallel to the fiber length in contrast to a geophone measuring the particle motion of displacement. To confirm the physical quantity of DAS measurement in a field study, we carried out experiments using DAS and three-component (3C) seismometers at the same location. We used three 100-m-long optical fibers buried at 20 cm depth and 18 units of low-frequency 3C seismometers. The 4-day experiment was carried out in September 2017. The three fibers located at approximately the same positions showed nearly the same waveforms with some subtle differences. This suggests that differences among the three fibers in casings, qualities of optical fibers and coupling to the ground made no significant difference to waveforms. For two natural earthquakes, we compared the DAS records to the calculated strain rate using horizontal seismometer records. The comparison of S-wave phases showed surprisingly good matching with each other. The phase-to-phase fitting of DAS and seismometers suggests that theoretical and observed strain records can be compared. In addition, although the incident angle to the fibers was nearly vertical, the horizontal DAS records showed clear P-wave arrivals. Presentation Date: Tuesday, October 16, 2018 Start Time: 1:50:00 PM Location: Poster Station 10 Presentation Type: Poster