Abstract
The Nankai Trough is a convergent plate boundary which lies under the Pacific Ocean, off southwest Japan. Borehole drilling surveys by IODP (Integrated Ocean Drilling Program) and a 3-D multi-channel seismic reflection survey by JAMSTEC (Japan Agency for Marine-Earth Science and Technology) were examined around Kashinosaki Knoll which located on the Nankai Trough. We implement seismic inversion analysis for the seabed sediment layers with these data: computing acoustic impedance depth profiles for the drilling sites from P-wave velocity and density data, constructing seismic convolution models with the acoustic impedance profiles and a source wavelet which is extracted from the seismic data, and adjusting the seismic models to observed seismic traces with inversion method. A 3-D acoustic impedance data is obtained from the 3-D seismic data with seismic inversion analysis, and the data is also converted to a 3-D porosity data. This porosity data shows that porosity values are generally decreasing together with TWT in the 3-D porosity data, though in some areas porosity does not subject this trend; we focus such porosity anomaly areas and describe them in relation with the horizons. We confirm such layers as the porosity anomaly area: having high and low porosity alterations, or continuous and clear low porosity stratum. All these porosity anomaly areas become obvious toward the landward part of the region. We conclude that the continuous low porosity layer represents ancient channel sediments, and show the prospects of planar and confined distribution of channel sediments around Kashinosaki Knoll with comparison on the rock stratigraphy between preceding boreholes.
