Using a relatively new semi-automated acquisition system, we have
collected a 3-D georadar data set across a section of the San Andreas
Fault that is buried beneath fine-grain sediments. By recording the
23.2×72 m data set in a continuous fashion and eliminating the need
for an independent topographic survey, the acquisition speed was
approximately five times that of standard step-mode methods. Flex-binning
was used to sort the data into 0.4×0.8 m bins regularly spaced at
0.2×0.4 m intervals. Because the bin size was set to twice the bin
spacing, all traces contributed to multiple bins and adjacent bins
overlapped. Early parts of the initial georadar volume were dominated
by direct air and ground waves, whereas later parts were contaminated
with system ringing typical of data recorded across moderately to
highly conducting fine-grain sediments. To obtain useful subsurface
information, the data were subjected to an extensive processing scheme
that included: (i) scaling of each trace based on its Hilbert transform
to increase the relative amplitudes of later arrivals, (ii) Karhunen?Loéve
filtering to reduce significantly the direct waves and system ringing,
(iii) 3-D phase-shift migration to position correctly dipping reflections
and collapse diffractions, and (iv) F?XY deconvolution to reduce
incoherent noise. The fully processed georadar volume contained vivid
images of subhorizontal and trough-shaped reflections from a number
of fluvial paleochannels. These reflections were truncated and offset
by two distinct branches of the San Andreas Fault.