LAUSR

Abstract We present an analysis of the wireline sonic logs acquired through Alpine Fault hanging wall rocks during the second phase of the Deep Fault Drilling Project (DFDP-2). Due to inaccuracy in the initial tool-processed slowness logs, a more robust reprocessing of the data using the slowness–time coherence method was required. The reprocessing of the data yielded slowness logs that provide a significantly better estimate of compressional and shear slowness. The average slowness values of compressional and shear waves are 68 μs/ft and 110 μs/ft, respectively, corresponding to velocities of 4.5 km/s and 2.8 km/s, with some excursions related to fractures, borehole conditions, and tool settings, but no systematic downhole trends. The log values are compared to laboratory measurements of P- and S-wave velocity for samples of Alpine schist and mylonitic rocks to examine the scale dependence of the measurement. The average log velocity is 2 to 18% slower than the laboratory measurements due to the presence of mesoscale fractures that are present in situ but are not sampled at the laboratory scale. Using effective medium theory, we estimate the crack density in the hanging wall host rock to be ≤0.4. Comparison to field-scale seismic studies indicates that there is a broad (500–600 m wide) outer damage zone surrounding the intensely deformed inner damage zone of the Alpine Fault. This fractured outer zone facilitates fluid flow through the hanging wall and around the Alpine Fault.