Converted wave velocity model using 4C Ocean Bottom Seismometer data

Abstract

Ocean-bottom seismometer (OBS) consists of four components (4C), including one vertical (Z), two horizontal geophones (X and Y) and one hydrophone (H). 4C OBS survey provides us reflected P-wave and converted shear wave (PS) data, which can be used to derive accurate subsurface geological information. In this study 4C data from Hydrate Ridge, offshore Oregon continental margin, USA have been utilized to study the PS mode properties and to investigate the sensitivity of the seismic velocities to the presence of gas hydrate and associated free gas in the sediments. In order to derive a reliable velocity model, the data of 27 OBS from two different profiles have been analysed. Velocity analysis of reflected and PS waves in the tau-p domain has been applied to 4C OBS data. The results reveal a complex distribution of gas along specific sedimentary strata rather than along the base of the hydrate stability zone (HSZ). The stratigraphic control on the gas distribution forces the gas concentration to increase slightly with depth at certain locations. The converted wave velocity does not show velocity anomaly in the hydrate-bearing sediments. The hydrates which form within the pore space of the sediments do not cement sediment grains enough to affect shear properties.