HR: 1340h
AN: S23A-1112    [Abstracts]
TI: Depth Imaging of OBS Reflection Data With Wave Field Separation
AU: * Asakawa, E
EM: asakawa@jgi.co.jp
AF: JGI, Inc., 1-5-21, Otsuka, Bunkyo-ku, Tokyo, 112-0012, Japan
AU: Mizohata, S
EM: mizo@jgi.co.jp
AF: JGI, Inc., 1-5-21, Otsuka, Bunkyo-ku, Tokyo, 112-0012, Japan
AU: Tanaka, H
EM: hiroaki.tanaka@mpdc.co.jp
AF: MC Exploration Co., Ltd., 2-3-1, Marunouchi, Chiyoda-ku, Tokyo, 100-8086, Japan
AU: Mikada, H
EM: mikada@gakushikai.jp
AF: Kyoto University, Kyodai-Katsura, Nishikyo-ku, Kyoto, 615-8246, Japan
AU: Nishizawa, A
EM: azusa@jodc.go.jp
AF: Japan Coast Guard, 5-3-1, Tsukiji, Chuo-ku, Tokyo, 104-0045,
AB: We propose a newly-developed depth imaging approach for OBS (Ocean Bottom Seismometer) reflection data in active-source structural survey using wavefield separation and PSDM (Prestack Depth Migration). OBS data includes a lot of valuable signals not only reflection but refraction. However its wavefield is contaminated with a various kind of waves that degrade the quality of the depth imaging of OBS reflection. Water reverberations, for example, have been thought as a strong source of noise in the imaging. Surprisingly, we found that the multiples reflection waves have the wide spreads of reflection points and we take advantage of this feature to improve the depth imaging after careful processing acquired OBS data. We would like to demonstrate that multiples could be utilized to enhance signal-to-noise ratio. The processing of OBS data in this study is summarized as follows. First, we categorize the OBS wavefield into two parts, i.e., near and far offset data. The near and far offset data inhere are waves that arrive after and before the direct water arrival, respectively. Then we separate the both near and far wavefields into two parts at the arrival of the first-order multiple. The reflection signals before the multiple are primary and up-going waves, whereas the reflection after the multiple events are mainly multiple and down-going in a common receiver gather. After these time-based separations, we apply up/downgoing wave field separation using geophone vertical-component and hydrophone data. Hydrophone records water pressure and, hence, are omni-directional while the vertical component of geophone measures a component of the vector response. These characteristic difference leads us to the separation of upgoing primary reflections and downgoing multiples using the polarity differences due to propagation direction of incoming waves. Finally, we obtain the OBS reflections to 4 domains, near offset primary, near offset multiple, far offset primary and far offset multiple after all the above decompositions. Aside from applying PSDM straightforward to the primary reflections, we could image the same reflectors using decomposed multiples. In this processing, we use a method of mirror-image in PSDM to migrate the multiple reflections under the assumption that the data were acquired at a virtual receiver position located at the top of the virtual water layer above the sea surface whose thickness is exactly same as the sea depth. For OBS geometry in which both primary and multiples are acquired, we found that the multiples could give the significant improvement in the PSDM imaging especially for near offset data, because the spatial redundancy of each reflection point which is very narrow for primary reflection becomes improved after the inclusion of multiples.
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: 2007 Fall Meeting