Seismic Full Waveform Inversion for Sinkhole Assessment and Remediation Monitoring: A Case Study on a Roadway in Montgomery County, Pennsylvania

Karst-induced sinkholes present a significant challenge to infrastructure, particularly in regions underlain by carbonate bedrock. This case study examined the use of two-dimensional (2D) seismic full waveform tomography using surface waves to monitor the effects of grouting on a sinkhole located along a roadway in Montgomery County, Pennsylvania. FWI is an advanced geophysical technique that iteratively adjusts an initial model of subsurface properties to minimize the misfit between observed (field data) and synthetic (numerically generated data) seismic waveforms, providing higher resolution images of subsurface variations in seismic velocity compared to conventional surface wave methods like Multichannel Analysis of Surface Waves (MASW). After extensive field data collection and analysis, the pre-grouting FWI results identified significant low-velocity zones (VS < 300 m/s) in the upper 7–8 m of the test site, correlating with mapped sinkholes under the roadway. The post-grouting FWI efforts demonstrated a notable reduction in both the lateral extent and the intensity of the low-VS zone in the same region. In the previously identified zone, shear-wave velocities now exceed 400–500 m/s, indicating increased stiffness and density attributable to successful grout injection. The observed rise in VS values, together with the confinement of the low-velocity zone to a depth of 4–5 m (reduced from 7–8 m prior to grouting), provides clear evidence of effective ground improvement, particularly where the anomalies were most prominent between 100 and 140 m of distance along the FWI survey line. These findings suggest that grouting effectively stabilized the subsurface, and they support the efficacy of an FWI approach for monitoring grouting-based ground improvement efforts, particularly when such efforts are used to mitigate sinkhole hazards in karst-prone environments.