Electrical Properties of the Hanging Wall of the Alpine Fault, New Zealand, from DFDP-2 Wireline Data

Abstract:

The DFDP-2B borehole drilled at Whataroa, New Zealand, provides a first-hand rare opportunity to investigate the damage pattern next to a major active fault. It was drilled along more than 893 m (820 m TVD) within hanging-wall protomylonites and mylonites. The interval between 264 m and 886 m (measured depth) was intensively investigated by wireline logging. Notably, electrical laterolog data were recorded over almost 3 km of cumulative logs, providing a homogeneous, uniformly sampled recording of the electrical properties of the borehole wall.

The laterolog tool measures resistivity with two different electrode configurations, and hence achieves two different depths of penetration. Numerical simulations of the tool’s response show that the true resistivity of the rock is close to the deep resistivity measurement, which in DFDP-2 varies between 300 Ω.m and 700 Ω.m. The shallow resistivity is about 75% of this value, as it is more sensitive to the presence of conductive borehole fluid. However, the large borehole diameter (averaging 8.5 inch = or 21.59 cm) only partially explains this value. The strong anisotropy suggested by laboratory measurements on outcrop samples also contributes to the separation between deep and shallow resistivity.

The shallow and deep resistivities exhibit many significant drops that are coincident with the presence of fractures detected in borehole televiewer data. More than 700 electrical anomalies have been manually picked. The major ones are correlated with attenuation of the sonic data and sometimes with anomalies in fluid conductivity (temperature and conductivity). Their frequency gradually increases with depth, reaching a plateau below 700 m. This increase with depth may be related to closer proximity to the Alpine Fault.