Soil/rock interface profiling using a new passive seismic survey: Autocorrelation seismic interferometry

Soil/rock interface detection is an essential and critical task in geological site investigation for underground and tunnel projects. The required accuracy is within a few meters both vertically and horizontally. The required resolution poses big challenges to geophysical surveys to be applied in near-surface engineering works. This paper presents a new approach of non-invasive geophysical survey, namely autocorrelation. It is originally derived from seismic interferometry cross-correlation, which can convert a physical geophone to a virtual source. To the extreme is autocorrelation. It turns a single geophone into a self-source receiver. With this concept, the proposed approach includes two methods aiming to reduce the reliance on reference boreholes. A field testing carried out in Singapore is presented to demonstrate the concept and results step by step. The case study also demonstrates that the proposed method can fine-tune the autocorrelograms to achieve the required resolution both laterally and vertically. A by-product of realistic average Vp of the soil layers above soil/rock interface can be estimated reasonably by the proposed method. Two more case studies are followed to illustrate the proposed methods can produce reasonable 2D and 3D soil/rock interface profiles. Due to the self-source feature, this approach is convenient to implement on site. The proposed approach can be an attractive option for geotechnical and geological site investigation especially for delineating the soil/rock interface topography.

Automated summary

Soil/rock interface detection is a critical task in geological site investigation for underground and tunnel projects, requiring high accuracy both vertically and horizontally. This study introduces a new non-invasive geophysical survey method, autocorrelation, to reduce reliance on reference boreholes and achieve the required resolution. The proposed approach can reasonably estimate soil layers' average Vp above the soil/rock interface and produce reasonable 2D and 3D interface profiles. Due to its self-source feature, this approach is convenient for on-site implementation and can be a promising option for geotechnical and geological site investigation.