  Distributed acoustic sensing (DAS) is a relatively new technology for
recording stress wave propagation, with promising applications in both
engineering and geophysics. DAS's ability to simultaneously collect high
spatial resolution data over long arrays suggests that it is especially well
suited for near-surface imaging applications such as 2D MASW (multi-channel
analysis of surface waves). 2D MASW aims to produce a pseudo-2D cross-section
of shear-wave velocity (Vs) for the purpose of identifying and characterizing
subsurface layering and anomalies. These cross-sections are produced by
interpolating numerous 1D Vs profiles extracted from overlapping sub-arrays
along the testing alignment. When using traditional seismic equipment, these
sub-arrays are typically collected in a roll-along configuration, where the
equipment is moved along the alignment at some predetermined sub-array
interval. DAS does not have the same limitations, as data from all shot
locations are simultaneously recorded along the entire length of the DAS array.
This alleviates the requirements to pre-determine sub-array length and interval
during the acquisition stage and allows for multiple geometries to be
investigated during the processing stage. The present study utilizes DAS data
to evaluate the effects of sub-array length on 2D MASW results at a
well-characterized field test site. We organize the DAS waveforms into multiple
sets of overlapping MASW sub-arrays of differing lengths, allowing for direct
comparison of the derived results at the same site. We show that the length of
the individual MASW sub-arrays has a significant effect on the resolution of
the resulting cross-sections as well as the resolved location of large
impedance contrasts at our study site and evaluate those locations compared to
invasive testing.
