Seismic data acquisition using optical ?ber sensing and permanent source ACROSS at Aquistore CCS test site

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  • アクイストアCCS試験場における光ファイバーセンシングおよび常設型震源アクロスを用いた地震探査データ取得<sup> </sup>
  • アクイストアCCS試験場における光ファイバーセンシングおよび常設型震源アクロスを用いた地震探査データ取得
  • アクイストア CCS シケンジョウ ニ オケル ヒカリ ファイバーセンシング オヨビ ジョウセツガタ シンゲン アクロス オ モチイタ ジシン タンサ データ シュトク

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<p>Time-lapse seismic plays an important role in reservoir monitoring. Monitoring of onshore fields is challenging because of insufficient repeatability mainly associated with heterogeneous weathering layers. Permanent monitoring using both stationary sources and sensors can address the problem. However, the number of the sources is very small due to high initial installation cost, resulting in the sparse and spatially limited reflection points. Distributed Acoustic Sensing （DAS） might solve this limitation because it has dense sensor interval along the whole long fiber, although its S/N is low. To investigate the combined use of these two technologies, we analyzed the data acquired in Aquistore CO2 storage field by using DAS and a rotary stationary source called ACROSS. The DAS were installed on the surface and along the vertical well. The ACROSS source had been operated continuously for 8 hours. We estimated S/N by taking the ratio of root-mean-square （RMS） around first break and background noise, and showed that the S/N was improved with the square root of stacking number. The reflection at 3,000 m depth became visible after stacking data for 8 hours. The VSP data were converted from strain rate to strain and particle velocity respectively by integration along time axis and space axis. These dual wavefields successfully separated the upgoing and downgoing waves. The waveform repeatability for 8 hours were estimated by comparing both earlier and later 4 hours data, and the result showed that normalized RMS is around 5 % at 1,500 m depth. The imaging result of the VSP data showed the long areal coverage of DAS extended the imaging area comparing to conventional VSP acquisition geometry. This study confirmed the complementary relationship between the permanent source and DAS acquisition. Combination of these technologies might enable us to monitor small changes of the reservoir in onshore fields.</p>

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