Comparison of fibre optic sensor and Borehole Seismometer VSP surveys in a Scientific Borehole - DFDP-2B, Alpine Fault, New Zealand

The DFDP-2B scientific borehole was drilled in 2014 to a depth of 893 m into mylonitic fault rocks adjacent to the Alpine Fault, New Zealand, in the second phase of the Deep Fault Drilling Project (DFDP; Sutherland et al, 2015). The borehole was designed to penetrate the fault zone in order to study the physical and chemical properties of a major tectonic boundary in an active continent-continent collision zone. As part of the project, a slim (BQ) casing string containing optical fibre cable was deployed in the borehole from the surface to its base, on the outside of the main casing. This fibre cable was used to measure temperatures at approximately two-month intervals in the year following the completion of drilling using Distributed Temperature Sensing (DTS) techniques. In January 2016, an active source seismic experiment was conducted around the DFDP-2B borehole, as part of which the optical fibre cable was used to collect acoustic data along the length of the borehole. In addition, a conventional four-level, three-component vertical seismic profile (VSP) string was suspended in the accessible top 400 m of the borehole. Zero-offset and multi-azimuth walkaway VSP surveys were undertaken to image around the borehole and determine the velocity structure and reflectivity characteristics in the hanging-wall of the Alpine Fault.