Distributed Acoustic Sensing Experiments in Crystalline Rock - A Field Study in Switzerland

Distributed Acoustic Sensing (DAS) has recently indicated its high potential in seismological applications such as ambient noise tomography and the recording of regional earthquakes.

To further investigate this potential, we conduct a series of experiments in a tunnel system in the Grimsel Rock Laboratory in southern Switzerland. Benefits of the site are existing fibers in a range of cemented boreholes with collocated strainmeters, borehole seismometers, a collocated permanent seismometer from the Swiss Seismological Service (SED), relatively low anthropogenic noise, and the availability of multiple active sources. With an array of geophones and seismometers that complement the already mentioned instrumentation, a large variety of comparisons to DAS data can be made.

We assess the quality of DAS data with multiple experiments in a broad range of frequencies, including

- active controlled-source vibroseis experiments

- active source seismics using a sledge hammer as source

- active source sparker-shots in a water-filled borehole

- low-frequency strain induced by pressure variations due to water injection in a borehole

- local and regional earthquake recordings

- passive ambient noise recordings

- recordings of anthropogenic signals from a nearby power station and a dam

Comparisons with other instruments and redundant measurements along the cable allow us to estimate the instrument-noise at any given location. The broad spectrum of the experiments provides detailed insight into the general usefulness of DAS is crystalline environments.

Below is a figure showing a small regional earthquake recorded with our distributed acoustic sensing installation.

Figure:

A small regional earthquake recorded with distributed acoustic sensing in a tunnel-installation in crystalline rock in Switzerland. BH1 consists of 4 separate boreholes and BH2 of 2 separate boreholes next to each other.