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.