| 講演要旨(和文) | | サウジアラビアの揚水場において、超安定的な震源を用いたACROSSによるタイムラプス実験を行い、これまでに大きな波形の時間変化を観測している。しかし地震波の性質と経路および再現性の理解が必要である。ACROSS震源を用いた屈折法地震探査を行った結果、オフセット距離700 m以遠でシャドーゾーンのために初動が見えなくなり、1500 m以遠で深度900 mからの強い屈折波が確認された。ACROSS震源のNRMS再現性を議論するために距離700 m以内の初動を用いたところ、2か月間でのNRMS再現性は5%を下回った。2か月間の走時変化は最も良い2観測点で0.2 msであった。震源自体のNRMS再現性はおよそ2%程度と推定される。この結果から、ACROSS震源は地下のタイムラプスを議論するのに十分な再現性を持っていると言える。 |
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| | 講演要旨(英文) | | We carried out the seismic time-lapse experiment in water pumping field in the Kingdom of Saudi Arabia using ACROSS with an ultra-stable seismic source. By the present study, we identified huge temporal changes of waveforms. It is, however, necessary to understand the nature of seismic arrivals and their travel paths, and magnitude of repeatability. We carried out refraction study using the ACROSS seismic source and found that the first arrivals disappear around 700 m offset distance due to the presence of the shadow zone. A strong arrival appears at the distance further than 1500 m refracted at 900 m depth. In this paper, we discuss the NRMS repeatability of the ACROSS seismic source. Because the first arrivals disappear around 700 m distance, we used those shorter than 700 m and found that the NRMS repeatability was better than 5% during 2 month periods. The travel-time drift was 0.2 ms during 2 months at the best two stations. The source itself might have roughly 2% NRMS repeatability. The present result supports that the ACROSS source has excellent repeatability well enough to discuss the time lapse of the subsurface. |
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