Analysis of nonlinear elastic spatiotemporal characteristics of the shallow subsurface

During and after the destruction of underground structures, the elastic properties of subsurface rocks may undergo significant changes. Monitoring and detecting these subtle changes in elastic properties require techniques with high precision and high spatiotemporal resolution, which are challenging to achieve with conventional geophysical methods. Although nonlinear elastic characteristics have been used in seismic monitoring of faults and volcanoes, monitoring on fine spatiotemporal scales remains difficult. In this study, we developed a new technique to simultaneously monitor the nonlinear elastic properties of near-surface structures from both temporal and spatial perspectives by recording continuous seismic noise. We conducted a four-day continuous environmental noise monitoring campaign along a dense seismic array in Singapore. During the monitoring period, rock mass damage occurred along the measurement line. Using an approach analogous to “pump-probe” techniques, we analyzed the changes in nonlinear elastic properties during the local damage process at high spatiotemporal resolution. The results indicate that local damage can influence the nonlinear elastic behavior across the entire array. However, there are significant differences between the undamaged and damaged areas, as well as across different stages of damage. These observations and insights can provide timely warnings of minor changes in urban underground spaces, helping to prevent more significant damage and allowing for the implementation of effective, targeted measures. Moreover, this technique can accurately detect existing underground collapses and anomalous geological structures, which has important engineering implications for the detection of urban subsidence and the exploration of underground cavities in mining areas. It can play a crucial role in monitoring urban underground structural changes and providing early warnings of natural disasters areas.