RESISTIVITY STRUCTURE OF THE UNDERCONSOLIDATED SEDIMENTS OF THE CASCADIA BASIN

The Cascadia Basin is located on the abyssal plain of the Juan de Fuca Plate, off the west coast of Vancouver Island. The sediments of the C ascadia Basin are very thick, over 2 km, and are underconsolidated bec ause of a high rate of sedimentation. Seismic results suggest that the sediment acoustic velocities are low, and increase slowly and monoton ically with depth from the sea-floor. A vertical electric dipole (VED) survey was conducted to obtain the resistivity structure of the upper kilometre of sediments, to determine the physical properties of a thi ck sequence of abyssal basin sediments, and for comparison with seismi c results. While models with multiple layers can be found which fit th e data, the 'best-fitting' models share a common feature-decreasing re sistivity with depth, which is suggestive of a gradient in the resisti vity. The feature is consistent with the results of other geological a nd geophysical studies. In particular, the seismic results do not indi cate any significant contrast in the physical properties, i.e. layerin g, in the upper 500 m. We extend the VED theory to the case where resi stivity varies continuously with depth, and obtain a gradient model th at fits the Cascadia Basin data marginally better than simple layered models. The model resistivity is combined with heat-flow measurements for the determination of physical properties versus depth; the physica l property models, the porosity, for example, are not consistent with the physical properties derived from seismic survey results. The misma tch is diagnostic of the underconsolidation, and is primarily due to t he effects of the high rate of sedimentation on the seismic velocity a nd on the heat flow, which in turn influence the physical properties.