Deformation-rate distributed acoustic sensing: filtering and coupling considerations

Deformation-rate distributed acoustic sensing (DAS), made available by the unique designs of some interrogator units, can acquire seismic data that are equivalent to the along-fiber particle velocity motion recorded by geophones for scenarios involving (near-)elastic ground-fiber coupling. While near-elastic coupling is straightforward to achieve in cemented downhole installations, it is less clear how to do so in lower-cost horizontal fiber deployments. This investigation illustrates that near-elastic coupling can be achieved by installing and freezing in fiber in shallow backfilled trenches. Any existing residual unwanted signals can then be removed through 1D or our preferred 2D dip-velocity filtering approaches. The processed deformation-rate field records acquired at the surface and two different trenching depths show clear surface- and refracted-wave arrivals; however, the surface-deployed fiber records exhibit significant time-lapse amplitude variations corresponding variable ground-fiber coupling due to air-temperature fluctuations about 0ºC. These results indicate that ground freezing provides improved but more complex fiber coupling.

Methods
This dataset is a subset of a Deformation-rate Distributed Acoustic Sensing, recorded by using Terra15 Treble DAS interrogator unit. We deployed three parallel fiber segments of 120 m total length in a shallow trench in the frozen earth that was watered down and left to freeze in the ground overnight. Over the following day, we acquired repeat sledgehammer shots. The portion of the processed dataset used for the figures is contained in a separate zip file.