Active-source seismic imaging of the Alpine Fault at Whataroa

Abstract

The Alpine Fault produces major (magnitude 7.5–8.0) earthquakes approximately every 300 years; the most recent Alpine Fault earthquake occurred in 1717 AD, meaning that the fault is now late in its typical cycle between one earthquake and next. Understanding the structure and earthquake-generating characteristics of the Alpine Fault ahead of a future large earthquake has been the focus since 2011 of a multinational scientific research effort, the Deep Fault Drilling Project (DFDP), centred on the Whataroa district in South Westland.

The second phase of DFDP involved the drilling of the 893 m-deep DFDP-2B borehole in the Whataroa Valley in late 2014. The borehole provided exceptional data regarding the temperature and pressure conditions acting within the fault zone, and provides a means of recording seismic data using instruments within the borehole and deployed on the surface.

In this project, we used several types of controllable sources of seismic waves (including a truck-mounted vibrator and a hydraulic weight drop) and seismic recording devices (including an optical fibre and other sensors installed in the DFDP-2B borehole and moving arrays of seismometers distributed on the valley floor) to generate and record seismic waves. These waves enable a picture of the subsurface structure of the Alpine Fault to be constructed, in a similar manner to medical imaging of a human body. The data contain signal that have bounced off the Alpine Fault, the steep walls of the valley and different stratigraphic layers beneath the valley floor.

Analysis is ongoing to construct a detailed three-dimensional model of the Alpine Fault’s geometry that incorporates knowledge gained from the DFDP-2B borehole and provides a basis for future scientific drilling in this area.

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Technical Abstract

In January 2016, an active source seismic experiment was conducted around the DFDP-2B scientific borehole with the aim of imaging the Alpine Fault and providing velocity information about the overburden and metamorphic basement of the Whataroa Valley. The experiment included a Vertical Seismic Profile (VSP) utilising a slimhole four-stage, three-component geophone string suspended in the accessible top 400 m of the borehole, and the optical fibre cable that had been installed at the time of well completion. These data were acquired with a truck-mounted vibroseis source located at the wellhead (zerooffset), and at a range of locations on profiles radiating away from the well (multi-offset and multi-azimuth).

Co-incident with the VSP experiments, surface geophones were deployed on two profiles to record the signals from the vibroseis source. In addition to the two-dimensional profiles, a set of three-component seismometers were deployed in different arrays around the wellhead to record the vibroseis shots in a narrow-azimuth three-dimensional survey.

During the two weeks of acquisition, all of the receivers were left to record earthquakes and signals from other natural sources during nights and periods when the vibroseis truck was not operating. The experiment is the first use of an optical fibre cable for distributed acoustic sensing in a research borehole in New Zealand. The optical fibre cable had already been used for repeated temperature sensing so was known to be in satisfactory condition to a depth of 893 m. The acoustic sensing performed using Schlumberger’s heterodyne Distributed Vibration Sensing (hDVS) equipment, demonstrated that the cable was coupled sufficiently well with the casing to allow detection of the active shots and earthquakes.