Insight on the physical properties of sediments within the Nankai inner accretionary prism, from repeated borehole geophysics of NanTroSEIZE IODP expeditions 348 and 358.

We report a comprehensive set of geophysical data acquired in the heart of the Nankai accretionary prism during IODP expedition 358. The full LWD suite included two types of resistivity and measurements, gamma-ray and state-of-the-art borehole imaging tools.

Expedition 358 is the final expedition of the NanTroSEIZE project, targeting the plate boundary fault zone below the Nankai accretionary prism. The primary objective was to extend preexisting hole C0002 from 2900 mbsf to 5200 mbsf to reach the fault at seismogenic depths. However, departing from previous hole C0002P proved challenging and the four sidetracking holes did not exceed 3262.5 mbsf, which is still the greatest depth ever drilled during an IODP expedition.

The repeated LWD logs, either in reduced or full configuration, explain part of the difficulties encountered during drilling. Indeed, the gamma-ray, sonic and resistivity data depart significantly from the logging data first acquired during expedition 348, when the borehole was first extended. The large depth of investigation of the propagation resistivity tool helps identifying 3 situations: (1) large cavings in the proximity of hole C0002P, (2) pristine formation with properties similar to those encountered in hole C0002P and (3) homogeneously disturbed zone with distinctively lower resistivities and sonic velocities. Resistivity is the best marker for damage, rather than gamma-ray.

The latter suggests that previous drilling affected the rock matrix itself rather than localizing damage along discrete fractures. Another surprising feature is the abrupt transition from the disturbed zone to the intact formation. Possible processes explaining these observations are invasion of the formation by reactive drilling fluids or dynamic damage induced by drilling or borehole reentry. This shows how the sediments of the inner accretionary prism are sensitive either to mechanical loading or change in geochemistry.

The repeated logs within the intact formation also show that the resistivities measured with high frequency propagation methods are systematically higher from the low frequency galvanic methods. Such differences are theoretically predicted for clays. This suggests a new method to determine in situ the evolution of illite-smectite ratio with depth.