OCEAN DOWNHOLE SEISMIC PROJECT

In the fall of 1989, a broadband digital seismic sensor system (OBDS: Ocean Broadband Downhole Seismometer) was successfully emplaced in an ODP drill hole in the Japan Sea. The site is located in the northern p art of the Yamato Basin about 160 km west from the northern Japan west coast. The instrument was clamped within a basalt section beneath 543 m of sediments at 715 m below seafloor and 3,523 m below sea surface. We obtained a continuous real-time seismic recording of 60 h length a nd an off-line and intermittent sea-floor recording of 30 days operati on. The dataset includes airgun signals for studying the local crustal structure, several local earthquakes, and a teleseismic event. An arr ay of OBS's was also deployed during the airgun shooting to study deta iled local crustal structure including anisotropy. The results of our experiment may be summarized as follows. We obtained for the first tim e a digital long-period seismic record from beneath the seafloor. The structural environment of OBDS could be constrained from results of dr illing and airgun profiling; the crust is nearly identical to that of the southern Yamato Basin, which is about 14 km thick, and the upper c rust possesses anisotropy in agreement with the present E-W compressiv e state of stress. Noise is generally lower than island or seafloor st ations, and is as low as about 10(4) (nm/s)(2)/Hz at about 0.2 Hz, whe re normally a noise peak is found. Signals from local events and airgu ns exhibit less reverberations as compared with OBS records; i.e. they are less affected by signal-generated noise, thus allowing better pha se identifications. Surface wave dispersion from an event (M(b) = 5.4) at 39 degrees distance was clearly observed. Thus, our results indica te that downhole emplacement of a seismometer beneath an ocean provide s higher quality data than alternative seafloor or island observations . At frequencies lower than 0.1 Hz, however, our OBDS did not have suf ficient sensitivity to detect the background noise level.