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

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.