Nankai ACORK Tidel Response: Phase Lead/Delay as a Potential Indicator for Fracture Zones?

For more than 12 years we have been conducting a continuous monitoring of downhole pore pressures at multiple sub-bottom intervals in ODP Holes 808I and 1173B situated landward and seaward of the deformation front in the Nankai Trough off Cape Muroto. We found that the pressure response to the semi-diurnal ocean tide (M2), both amplitude and phase, gradually changed during the observed period. The M2 amplitudes at most depths in Hole 808I decay as their phase delay (up to 45 degrees), only if the amplitude is larger than ~0.2 of that for the seafloor. On the other hand, we observe an anomalous phase lead (up to -40 degrees) if the relative amplitude is smaller than ~0.2. We hypothesize that the recorded pore pressure is a combination of two components; one with larger amplitudes and phase-delay and the other with small amplitude and phase-lead. The former is interpreted to be caused by a direct communication between sensors and the seafloor through the space of the annulus. It gradually attenuated by the decrease in hydraulic diffusivity around the sensors, as the annulus space is packed off by a gradual collapsing of the formation. After that, we hypothesize that the sensor is in good mechanical/hydrological contact with the formation, and that the latter variation is identical or close to that of formation pressure variation. The mechanism of the latter variation remains enigmatic, but a feasible explanation is the existence of a layer, next to the sensor, with the contrast in fluid bulk modulus, frame bulk modulus, or permeability (Wang and Davis, 1996). At Site 808, numerous small fractures were detected in cores/logs, which can generate such contrast in bulk modulus or permeability. Other causes, such as solid earth tide or thermal expansion/contraction caused by a tidally-induced flow within the casing cannot explain both the amplitude and phase simultaneously. We propose that a tidal response signal can be used to detect local fracture zones or gas-rich layers.