Combination of Least Square and Monte Carlo Methods for OBS Relocation in 3D Seismic Survey Near Bashi Channel

Abstract

The relocation of ocean bottom seismometers (OBSs) is a key step in analyzing the three-dimensional seismic tomographic structure of crust and mantle. In order to get the accurate location of OBSs on the seafloor, we analyze the travel times of direct water waves emitted by air-guns. The Monte Carlo and least square methods have been adopted to calculate the true OBS location. The secondary time correction is necessary if the arrivals of direct water waves show overall time drift during relocation which maybe originates from remnant of linear clock drift correction and average errors of travel time picking, mean water velocity assumption, and experiment geometry. We have improved the original OBS relocation procedure which we used previously for other experiments by deliberateness of a secondary time correction and automatically approaching the really mean water velocity. A series of synthetic tests are carried out firstly to document the feasibility of our procedure and then it is applied on a real experiment. In here, we relocate 28 OBSs in total were relocated in 3D seismic survey near Bashi Channel. Relocation results show that the drifting distances for the 28 OBSs range from 65 to 1136 m between the deployed and relocated locations deduced by relocation results. The Pearson correlation coefficient between OBS drifting direction and sea current direction is 0.79, indicating that the two sets of data are highly linearly related and further manifest the sea current as the most possible driving force for OBS drifting during landing on the seafloor but its detailed influence mechanism is unclear by now. This research is necessary and critical for velocity structure modeling, and the optimal relocation program provides valuable experiences for 3D seismic survey in other area.

Keywords:

• 3D refracted seismic experiment
• Bashi Channel
• least square method
• Monte Carlo method
• OBS relocation

Acknowledgments

We would like to thank the crew of the R/V Shiyan 2 and scientists who took part in the scientific cruise. Thanks to Haibo Huang and Xiqiang Xu for their insightful comments. Most of the figures in this article were prepared using the GMT graphics package (Wessel and Smith 1995).

Additional information

Funding

This work is supported by the Chinese National Natural Science Foundation [contracts 91428204, 41606064, 41674092, 41730532 and 91428205], and Strategic Priority Research Program of the Chinese Academy of Science [XDB06030202].