Tunnel detection at Yuma Proving Ground, Arizona, USA — Part 2: 3D full-waveform inversion experiments

We have applied time-domain 3D elastic full-waveform inversion (FWI) to a known tunnel constructed 10 m below the surface with no distinguishing surface expressions. Multicomponent inversion experiments that use an initial model estimated from surface wave methods suggest that the vertical sources and the combination of vertical and longitudinal receivers result in the clearest image of the tunnel. We obtain an approximate 3D image of the tunnel using 24 vertical sources and 720 vertical and 720 longitudinal receivers. We find that increasing the number of vertical sources to 216 does not significantly improve the details of the tunnel. Further experimentation indicates that we can detect the tunnel using a reduced data set of 6–10 vertical sources and 216 vertical sensors. In addition, calculating the [Formula: see text] ratio helps remove artifacts from the inverted model and highlights the location of the tunnel. We compare the 3D inversion results with the 2D FWI results for the same tunnel that we previously evaluated. The variety of successful inversion experiments suggest that FWI is capable of imaging shallow tunnels in desert geologies.

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