Aya Kamimura

Evaluation and interpretation of the effects of heterogeneous layers in an OBS/air-gun crustal structure study

Abstract We present a method for interpreting seismic records with arrivals and waveforms having characteristics which could be generated by extremely inhomogeneous velocity structures, such as non-typical oceanic crust, decollement at subduction zones, and seamounts in oceanic regions, by comparing them with synthetic waveforms. Recent extensive refraction and wide-angle reflection surveys in oceanic regions have provided us with a huge number of high-resolution and high-quality seismic records containing characteristic arrivals and waveforms, besides first arrivals and major reflected phases such as PmP. Some characteristic waveforms, with significant later reflected phases or anomalous amplitude decay with offset distance, are difficult to interpret using only a conventional interpretation method such as the traveltime tomographic inversion method. We find the best process for investigating such characteristic phases is to use an interactive interpretation method to compare observed data with synthetic waveforms, and calculate raypaths and traveltimes. This approach enables us to construct a reasonable structural model that includes all of the major characteristics of the observed waveforms. We present results here with some actual observed examples that might be of great help in the interpretation of such problematic phases. Our approach to the analysis of waveform characteristics is endorsed as an innovative method for constructing high-resolution and high-quality crustal structure models, not only in oceanic regions, but also in the continental regions.

Long Duration Time-lapse Experiment in Al Wasse, Saudi Arabia Using an Ultra-stable Seismic Source

We carried out a long-duration seismic time-lapse experiment consisting of two periods in 2012-2013 and 2015 in a water pumping field in the Kingdom of Saudi Arabia using an ultra-stable ACROSS seismic source and a geophone array. The comparison of travel times and amplitudes of P waves show quite small but distinct temporal variation with time. The maximum change is approximately 1.5 ms during two months both in the first and the second periods. Because the total travel time is 0.2 s, the change of 1.5 ms corresponds to 0.75%. The resolution of travel-time change is ~0.1 ms and it is good resolution to detect the change of subsurface caused by CO2 or high-temperature H2O injection to the heavy oil reservoir. We also discuss the NRMS repeatability of the observation system using the first arrivals at the stations within 700 m from the source and found that the NRMS repeatability was better than 5% during 2 month periods. The source itself might have roughly 2% NRMS repeatability. The NRMS repeatability is based on the waveform change. The present result supports that the ACROSS source has excellent repeatability well enough to discuss the time lapse of the subsurface.