Deformation-rate distributed acoustic sensing: filtering and coupling considerations

<p class="MsoBodyText">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.</p>