High resolution P- and S- wave tomography from combined geophone and DAS sensing at the DFDP-2 Borehole, Alpine Fault, New Zealand

Abstract:

A series of high-spatial resolution zero-offset and walk-away VSP (Vertical Seismic Profile) surveys were carried out in the cased DFDP-2 borehole in order to better image the Alpine Fault and to obtain in situ measures of wave speeds. The survey was conducted within the Whataroa Valley, that was originally glacially scoured and now consists of a variety of high-energy fluvial deposits contained within the fault-related metamorphic schists and mylonites. The upper 245 m of the borehole are through the sediments with metamorphic rocks encountered below this depth. The survey incorporated both wall-locking geophones at close vertical spacings as small as 1-m to depths of 394 m simultaneously recorded with a cemented digital acoustic sensing (DAS) cable along the entire 893m of the deviated borehole. The offset records show direct P and S arrivals, the latter interpretation confirmed by eigenanalysis of the 3-component geophone data and enhanced by a polarization filter. These two arrivals are interpreted as direct P-wave and S-wave produced at the lithological boundary between sedimentary rock and the beneath bedrock. All the transit times for P- and S-wave are manually picked and then used in a wavepath eikonal traveltime (WET) type inversion to obtain both high-resolution 2D P- and S- wave velocity tomograms at a number of azimuths extending from borehole. The WET inversion is based on a finite frequency assumption using wavepath with a certain width instead of typical ray tracing. Interpretation of the tomogram panels are complicated by the geometry of the valley. Wave speeds in the fluvial deposits range from 1132.5 m/s to 2057.9 m/s. The wavespeeds in the metamorphic rock mass, under the assumption of isotropy, only reach as high as 4024.4 m/s and 2836.0 m/s for P and S waves, respectively. These are significantly less than that anticipated for intact rock and indicate the high degree of damage of these materials near the fault. Work continues on comparison of the DAS to the high-quality 3-C geophone results.