Unlike conventional sensors that measure the passage of seismic waves at a single position, distributed vibration sensing systems, also known as distributed acoustic sensing systems, detect the passage of seismic waves by averaging a measurement of strain over a section of fibre‐optic cable. Distributed vibration sensing systems work by transmitting pulses of light down the fibre and measuring the phase of the Rayleigh backscatter. At random positions along the fibre, however, fading occurs; this is where the amplitude of the backscattered signal is very small due to cancellation of the scattered electric fields, resulting in anomalously noisy traces in a common source gather.This paper addresses the problem of fading in a particular form of distributed vibration sensors: a new optical arrangement of the instrumentation is described that allows the measurement to be carried out quasi‐simultaneously at multiple optical interrogation frequencies. The interrogation frequencies are chosen to be sufficiently different that their fading properties are distinct and the diversity thus obtained is used to aggregate the data obtained to substantially reduce the noise caused by fading. As well as reducing the effects of fading, the aggregation of the independent results can also help to reduce the overall noise of the measurement and improve the linearity of the distributed vibration sensing system.

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The New Zealand Alpine Fault is a major plate boundary that is expected to be close to rupture, allowing a unique study of fault properties prior to a future earthquake. Here we present 3-D seismic data from the DFDP-2 drill site in Whataroa to constrain valley structures that were obscured in previous 2-D seismic data. The new data consist of a 3-D extended vertical seismic profiling (VSP) survey using three-component and fiber optic receivers in the DFDP-2B borehole and a variety of receivers deployed at the surface. The data set enables us to derive a detailed 3-D P wave velocity model by first-arrival traveltime tomography. We identify a 100-460 m thick sediment layer (mean velocity 2,200 ± 400 m/s) above the basement (mean velocity 4,200 ± 500 m/s). Particularly on the western valley side, a region of high velocities rises steeply to the surface and mimics the topography. We interpret this to be the infilled flank of the glacial valley that has been eroded into the basement. In general, the 3-D structures revealed by the velocity model on the hanging wall of the Alpine Fault correlate well with the surface topography and borehole findings. As a reliable velocity model is not only valuable in itself but also crucial for static corrections and migration algorithms, the Whataroa Valley P wave velocity model we have derived will be of great importance for ongoing seismic imaging. Our results highlight the importance of 3-D seismic data for investigating glacial valley structures in general and the Alpine Fault and adjacent structures in particular.Viewed on a regional scale, the central Alpine Fault appears as a straight boundary (e.g., Norris & Cooper, 2001;Sutherland et al., 2006). Crustal-scale seismic reflection data show a single oblique fault striking northeastward and dipping 40−60 • to the southeast at depths of 15-30 km (e.g., Davey et al.