HR: 1330h
AN: S52D-0165    [PDF]
TI: A beam-forming experiment of airgun array in seismic experiments
AU: * Miura, S
EM: miuras@jamstec.go.jp
AF: IFREE, JAMSTEC, 3173-25 Showa-machi, Kanazawaku, Yokohama, 236-0001 Japan
AU: Tsuru, T
EM: tsurut@jamstec.go.jp
AF: IFREE, JAMSTEC, 3173-25 Showa-machi, Kanazawaku, Yokohama, 236-0001 Japan
AU: Kasahara, J
EM: kasa2@eri.u-tokyo.ac.jp
AF: ERI, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: Seismic experiments with airguns, which are useful to understand the crustal structure, have been_@widely used. Multi-channel seismic reflection (MCS) method using multi-channel hydrophone streamer is an effective method to image highly resolved geological structures. Tuned-array composed by several airguns is often used for the signal source of the MCS. It has a significant vertical directivity of energy, which effectively enables to observe airgun signals reflected just beneath the airgun array. However in two-ship survery, which uses two ships for shooting and recording independently, the airgun signals recorded ineffectively since the large incident angle in large offset distance. In OBS reflection and refraction survey, the oblique incidence waves may generate high amplitude P-S converted waves which travel through the basement by shear waves.. In order to record PS converted phases, airgun signals with vertical directivity are less effective. Therefore, two seismic experiments have been conducted using beam forming airgun array to develop the signal source with oblique directivity (beam forming). The first experiment was conducted during a MCS survey off Boso in 2001. The signal source directivity was successfully changed from vertical to oblique in the seafloor reflection amplitude records with shooting delay between airgun arrays. The second experiment was carried out in a two-ship and ocean bottom seismometer (OBS) experiment off Kii in 2002. In the OBS data, the amplitudes of airgun signals with beam forming are improved not only vertical component but also horizontal components. From the experiment, the effectiveness of beam forming airgun signal was confirmed in OBS records. Seismic survey using the beam forming source, two-ships and/or OBS is probably an effective way in secondary experiments as S-wave velocity structure or amplitude variation with offset (AVO), which are associated with the physical property.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3025 Marine seismics (0935)
DE: 3094 Instruments and techniques
DE: 7200 SEISMOLOGY
DE: 7294 Instruments and techniques
SC: Seismology [S]
MN: 2003 Fall Meeting