Ocean Downhole Seismic Project

Abstract

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 part of the Yamato Basin about 160km west from the northern Japan west coast. The instrument was clamped within a basalt section beneath 543m of sediments at 715m below seafloor and 3, 523m below sea surface. We obtained a continuous real-time seismic recording of 60h length and an off-line and intermittent sea-floor recording of 30 days operation. The dataset includes airgun signals for studying the local crustal structure, several local earthquakes, and a teleseismic event. An array of OBS's was also deployed during the airgun shooting to study detailed local crustal structure including anisotropy. The results of our experiment may be summarized as follows. We obtained for the first time a digital long-period seismic record from beneath the seafloor. The structural environment of OBDS could be constrained from results of drilling and airgun profiling; the crust is nearly identical to that of the southern Yamato Basin, which is about 14km thick, and the upper crust possesses anisotropy in agreement with the present E-W compressive state of stress. Noise is generally lower than island or seafloor stations, and is as low as about 104 (nm/s)²/Hz at about 0.2Hz, where normally a noise peak is found. Signals from local events and airguns exhibit less reverberations as compared with OBS records; i.e. they are less affected by signal-generated noise, thus allowing better phase identifications. Surface wave dispersion from an event (_Mb=5.4) at 39° distance was clearly observed. Thus, our results indicate that downhole emplacement of a seismometer beneath an ocean provides higher quality data than alternative seafloor or island observations. At frequencies lower than 0.1Hz, however, our OBDS did not have sufficient sensitivity to detect the background noise level.