Exploration Geophysics Exploration Geophysics Society
Journal of the Australian Society of Exploration Geophysicists
RESEARCH ARTICLE

A new method for determining OBS positions for crustal structure studies, using airgun shots and precise bathymetric data

Atsushi Oshida 1 4 Ryuji Kubota 1 Eiichiro Nishiyama 1 Jun Ando 1 Junzo Kasahara 2 Azusa Nishizawa 3 Kentaro Kaneda 3
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- Author Affiliations

1 Kawasaki Geological Engineering Co., Ltd., 2-11-15 Mita, Minato-ku, Tokyo 108-8337, Japan.

2 Japan Continental Shelf Survey Co., Ltd., 1-11-2 Kyobashi, Chuo-ku, Tokyo 104-0031, Japan.

3 Hydrographic and Oceanographic Department, Japan Coast Guard, 5-3-1 Tsukiji, Chuo-ku, Tokyo 104-0045, Japan.

4 Corresponding author. Email: oshidaa@kge.co.jp

Exploration Geophysics 39(1) 15-25 https://doi.org/10.1071/EG08005
Submitted: 11 September 2007  Accepted: 28 November 2007   Published: 5 March 2008

Abstract

Ocean-bottom seismometer (OBS) positions are one of the key parameters in an OBS-airgun seismic survey for crustal structure study. To improve the quality of these parameters, we have developed a new method of determining OBS positions, using airgun shot data and bathymetric data in addition to available distance measurements by acoustic transponders. The traveltimes of direct water waves emitted by airgun shots and recorded by OBSs are used as important information for determining OBS locations, in cases where there are few acoustic transponder data (<3 sites).

The new method consists of two steps. A global search is performed as the first step, to find nodes of the bathymetric grid that are the closest to explaining the observed direct water-wave traveltimes from airgun shots, and acoustic ranging using a transponder system. The use of precise 2D bathymetric data is most important if the bottom topography near the OBS is extremely rough. The locations of the nodes obtained by the first step are used as initial values for the second step, to avoid falling into local convergence minima. In the second step, a non-linear inverse method is executed.

If the OBS internal clock shows large drift, a secondary correction for the OBS internal clock is obtained, as well as the OBS location, as final results by this method. We discuss the error and the influence of each measurement used in the determination of OBS location.

Key words: OBS, crustal structure, OBS position, airgun, bathymetric grid data, acoustic transponder.


Acknowledgments

We give our thanks to the staff of Japan Continental Survey Co. Ltd., Kawasaki Geological Engineering Co. Ltd., and JGI Inc. We particularly thank Dr. Kayoko Tsuruga, Mr Shigeharu Mizohata and Mr Yasuo Tamura for their valuable comments during this study. The authors are also grateful to two anonymous reviewers for their valuable comments. Most of the figures in this paper were prepared using the GMT graphics package of Wessel and Smith (1998).


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