An ocean broadband downhole seismometer (OBDS) was emplaced in Hole 794D in the northern Yamato Basin, the Japan Sea during ODP Leg 128 by D/V JOIDES Resolution. The seismometer capsule was clamped within basaltic rock section at 714.5 m below sea floor at a water depth of 2,807 m. The OBDS has a three-component feedback type accelerometer and the digital signals were telemetered via a logging cable to shipboard for real-time recording during the first phase of the experiment. Real-time recording allowed full recovery of six-channel data (two gains per component, 16-bit each) at 80Hz/ch sampling rate. A high resolution seismic reflection/refraction experiment was conducted during the real-time recording. The OBDS, nine ocean bottom seismometers (OBS's) which were deployed by R/ V Tansei-maru, and a single channel hydrophone streamer were used to obtain detailed local crustal structure from the controlled-source experiment. Airguns were used as a controlled source and were shot with two 9-liter or a single 17-liter chamber. Profiles consisted of two circles and two linear lines.
We determined the velocity structure near the linear profiles using records of the OBDS, four OBS's which were deployed every 25 km and a hydrophone streamer. First, one-dimensional structures under each OBS were derived by the tau-sum inversion. Second, two-dimensional structures were obtained by a forward 2-D ray tracing method. The sedimentary layer has a P-wave velocity of 1.7 km/s. An acoustic basement with a velocity of 4.5 km/s underlies the sedimentary layer. A layer with a velocity of 6.2 km/s exists under the 4.5 km/s layer. Though we could not determine the thickness of the crust in the study area, the depth of Moho must be more than 14 km from sea surface. The upper crustal structure which is neither typically oceanic nor continental is similar to that in the southern Yamato Basin.