Surface wave exploration technology for the tunnel overlying strata with vehicle vibration source

Abstract

Subway tunnels are often built under the arterial roads in cities and the safety of overlying strata is important. Due to the limitation of urban space, many geophysical exploration methods maybe not be applicable. Most of the near surface exploration are carried out along the roads. This makes the continuously moving sources, including the vehicle and metro, to become important passive sources in the seismic detection. This type of signal has the characteristics of wide distribution, strong amplitudes, easy collection, and strong correlation. It carries abundant near-surface information and has high potential in non-destructive detection and monitoring. We obtain both the S-wave and P-wave profile with vehicle signals. Firstly, we propose an efficient method for equidistant linear array to extract the dispersion curves. The multiple vehicles signals were simulated with path integration method and the extracted dispersion curve has a good agreement with the theoretical data. In engineering and petroleum exploration, the P-wave velocity is more important than S-wave velocity sometimes. Previous study has shown that Poisson’s ratio is sensitive to the wavelength (W)-depth (D) relationship. Based on this observation, we proposed an inversion method to estimate the P-wave velocity structures directly. The traditional method is improved by adding the W/D-D information to the objective function, which is more sensitive to superficial layers. We select a high unevenness road above the operating subway. The vehicle signals are recorded along the roadside and the dispersion data is extracted. The inverted S-wave velocity profile can be used to identify warning geological positions such as water-rich weak interlayers. The resulting P-wave profile is also consistent with the drilling data. Our work successfully proves the effectiveness of vehicle signals and maps the overlying layers profile of subway tunnel.