Detecting a known near-surface target through application of frequency-dependent traveltime tomography and full waveform inversion to P- and SH-wave seismic refraction data
Detecting a known near-surface target through application of frequency-dependent traveltime tomography and full waveform inversion to P- and SH-wave seismic refraction data
Jianxiong Chen, Colin A. Zelt and Priyank Jaiswal
Geophysics, 82, R1-R17, 2017.
We have applied a combined workflow of frequency-dependent
traveltime tomography (FDTT) and full-waveform inversion
(FWI) to 2D near-surface P- and SH-wave seismic data
to detect a known target consisting of a buried tunnel with concrete
walls and a void space inside. FDTT inverted the P- and
SH-wave picked traveltimes at 250 Hz to provide long-wavelength
background velocity models as the starting models for
FWI. FWI inverted 18–54 Hz P-wave data and 16–50 Hz
SH-wave data to produce velocity models with subwavelengthand
wavelength-scale features allowing for direct interpretation
of the velocity models as is usually carried out in conventional
imaging using seismic reflection data. The P- and SH-wave
models image the top part of the tunnel at the correct location
at a depth of 1.6 m as a high-velocity anomaly. The P-wave
models also image the air in the void space of the tunnel as
a low-velocity anomaly. The inverted models were assessed
by synthetic tests, the consistency of the inverted sources,
and the fit between the predicted and observed data. As a comparison,
conventional ray-theory infinite-frequency traveltime
tomography (IFTT) was also applied in a combined workflow
with FWI. The comparisons of the inverted models favor the use
of FDTT over IFTT because (1) The FDTT models better recover
the magnitude of the velocity anomalies and (2) the FDTT
model serves as a better starting model for FWI, which results in
a more accurate FWI velocity estimation with better recovery of
the magnitude and location of the key features. FDTT will not
provide significant benefits over IFTT in all studies, particularly
those in which ray theory is valid.
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