Evaluation of time lapse of the near-surface layer due to precipitation

Several factors affect time-lapse studies on oil, gas and CO2 storage. One important factor is temporal change in near-surface layers due to precipitation and/or seasonal variations caused by wet and dry seasons. We applied the time-lapse method to evaluate the temporal change in the near-surface layer. We used 100 m of optical fiber, 100 4.5-Hz vertical geophones on the ground at every meter along the 100 m of fiber and a Mini-Vib seismic vibrator as the horizontal vibration source. We ran the Mini-Vib every 30 min during daytime. We also tested the use of the optical fibers with a distributed acoustic sensor (DAS). There was 23 mm of precipitation during the 4-day test period. We compared two sets of waveforms, phases and travel times for (a) both before the rain, (b) both after the rain and (c) before and after the rain. The direct comparison of waveforms in each of (a) and (b) showed extremely small changes. The largest change was due to precipitation; the comparison in case (c) revealed clear temporal change in waveforms, phases and travel times. The precipitation effects on body waves and Rayleigh waves resulted in <1 ms faster and 15 ms (∼1%) slower travel times, respectively. This discrepancy between body waves and surface waves can be explained by their different paths.