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

Kayoko Tsuruga; Junzo Kasahara; Ryuji Kubota; Eiichiro Nishiyama; Aya Kamimura; Yoshihiro Naito; Fuminori Honda; Nobutaka Oikawa; Yasuo Tamura; Azusa Nishizawa; Kentaro Kaneda

doi:10.1071/EG08004

RESEARCH ARTICLE

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

Kayoko Tsuruga ¹ ⁵ ⁷ Junzo Kasahara ¹ ⁶ Ryuji Kubota ² Eiichiro Nishiyama ² Aya Kamimura ² Yoshihiro Naito ³ Fuminori Honda ³ Nobutaka Oikawa ³ Yasuo Tamura ¹ Azusa Nishizawa ⁴ Kentaro Kaneda ⁴

+ Author Affiliations

- Author Affiliations

¹ Japan Continental Shelf Survey Co. Ltd., NTC bldg. 3F, Kyobashi 1-11-2, Chuo-ku, Tokyo 104-0031, Japan.

² Kawasaki Geological Engineering Co. Ltd., Mita-Kawasaki bldg. 6F, Mita 2-11-15, Minato-ku, Tokyo 108-8337, Japan.

³ JGI, Inc., Meikei Building, Otsuka 1-5-21, Bunkyo-Ku, Tokyo 112-0012, Japan.

⁴ Hydrographic and Oceanographic Department, Japan Coast Guard, Tsukiji 5-3-1, Chuo-ku, Tokyo 104-0045, Japan.

⁵ Ocean Research Institute, University of Tokyo, Minamidai 1-15-1, Nakano-ku, Tokyo 164-8639, Japan.

⁶ Department of Geosciences, Shizuoka University, Oya 836, Shizuoka 422-8529, Japan.

⁷ Corresponding author. Email: tsuruga.k@tairikudana.com

Exploration Geophysics 39(1) 1-14 https://doi.org/10.1071/EG08004
Submitted: 18 September 2007 Accepted: 27 December 2007 Published: 5 March 2008

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.

Key words: crustal structure, OBS, air gun, refraction, wide-angle reflection, synthetic seismogram, FDM, inserted layer, low-velocity layer, thin high-velocity layer.

Acknowledgments

We appreciate the cooperation of all the staff of the Japan Continental Shelf Survey Co. Ltd, JGI, Inc., and Kawasaki Geological Engineering Co. Ltd, in developing this technique. All datasets described in this paper were obtained by the Japan Coast Guard as part of the continental shelf survey project in Japan. We also thank Dr Eiichi Asakawa of JGI, Inc., for his very useful comments on our manuscript, and Prof. Jun Korenaga of Yale University for his valuable suggestions. We express our thanks to Mr Hiroyuki Kato of the Japan Coast Guard for a careful reading of our manuscript, and we also thank two reviewers, Dr Toshi Yokota of the National Institute of Advanced Industrial Science and Technology, and Dr Jun Matsushima of the University of Tokyo, for their valuable comments for revising our manuscript. The English of this article was greatly improved by an English editor.

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