Abstract
A significant increase in using distributed acoustic sensing (DAS) as a
main sensor of choice in borehole seismic applications prompts more
fundamental research in understanding what DAS measures, the limitations
and performance of the technology, and how it compares to the
conventional sensors. Here we use borehole seismic data collected with
conventional three-component geophone and distributed acoustic sensing
to quantify and compare seismic measurements at the GeoLab facility at
Curtin University campus (Perth, Western Australia). This facility
allows repetition of the experiment using different sensors in
controlled, stable conditions; deployment fibre cables as borehole and
surface arrays; and utilisation of different types of seismic sources.
In this presentation we compare the two datasets acquired with the
conventional Sercel SlimWave VSP tool and Silixa iDAS v2. Data collected
with geophones correspond to particle velocity measurements and can be
calibrated to this velocity from native system units (mV) to m/s. By
differentiating the measurements over the 10-meter interval, we get
“converted geophone” data, which now has a property of the strain rate
with unit [1/s]. In addition to this, DAS data which natively
measure phase variation over time, can be calibrated to the absolute
strain rate with units [1/s]. Although calibrated to the same
property, these two datasets are impacted by different factors that
could affect their amplitudes’ absolute values. For example, the
geophone amplitudes are affected by the type of geophone and its
performance, probe’s housing, the quality of the probe’s coupling to the
formation or the casing, etc. The DAS amplitudes are affected by cable
design, the cable’s coupling to the formation, optical parameters,
interrogator design, etc. We use both the peak-time amplitudes and the
entire wavefield to compare the absolute values of the strain rate of
both types of sensors. For the given study area and survey design (local
geology, type of geophone and fibre-optic cable), it appears that
amplitudes of the strain rate have similar absolute values.