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 Cascadia Basin are very thick, over 2 km, and are underconsolidated because of a high rate of sedimentation. Seismic results suggest that the sediment acoustic velocities are low, and increase slowly and monotonically 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 thick sequence of abyssal basin sediments, and for comparison with seismic results. While models with multiple layers can be found which fit the data, the‘best-fitting’models share a common feature—decreasing resistivity with depth, which is suggestive of a gradient in the resistivity. The feature is consistent with the results of other geological and geophysical studies. In particular, the seismic results do not indicate any significant contrast in the physical properties, i.e. layering, in the upper 500 m. We extend the VED theory to the case where resistivity varies continuously with depth, and obtain a gradient model that 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 physical property models, the porosity, for example, are not consistent with the physical properties derived from seismic survey results. The mismatch is diagnostic of the underconsolidation, and is primarily due to the effects of the high rate of sedimentation on the seismic velocity and on the heat flow, which in turn influence the physical properties.