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| | Abstract | | In order to monitor the temporal changes in subsurface, we have carried out the time-lapse experiment in Al Wasse field, Saudi Arabia using the ACROSS seismic source since 2012. The waveforms showed arrival-time changes about 1 ms for the first arrivals, but large temporal changes in the later phases. Because the sparse geophone spacing and no existing seismic surveys in this region make us difficult to interpret the phase with large temporal changes, we carried out reflection and refraction surveys. In this report, we describe the results of refraction survey. An ACROSS seismic source was used and 1-Hz geophones were placed along a 3 km long line with 50 m spacing. In the result, the first-arrival phase has the apparent velocity 3.5 km/s, which disappears at >0.7 km distance. Very strong arrivals appear at the distances further than 1.5 km with the apparent velocity 4.5 km/s, suggesting the presence of low velocity layer just below the top layer with 3.5 km/s. We compared the observed data and the synthetic seismograms and obtained the 1D VP and VS structure. The basement with 4.5 km/s exists at 900 m depth. The 3.5-km/s layer might be inter-bedding of sandstone and limestone with aquifers. |
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