Full acoustic waveform inversion in a pml-truncated tunnel environment

Abstract

Prediction of geological changes, such as caverns, fault zones, weakened zones, etc., ahead of a tunnel is crucial for the safety of the tunnel, the structures above it, and the tunnel boring machines in the case of the mechanized tunnelling. Enough number of boreholes can give an insight of the geological structure below the surface. However, too many boreholes, which are very costly in terms of both time and money, are usually needed to scan the geological structure of the ground. To avoid the high cost, looking ahead of the tunnel is usually done by carrying out a seismic survey. In this work, full waveform inversion (FWI) of the acoustic waves is used in a tunnel environment to predict geological changes in the domain. The aim of FWI is to find such an optimal computer model that matches with the field waveforms with all arrivals and amplitudes of the waves. To have a proper inverse model, the first step is to have a precise forward model. One of the crucial issues in forward wave modelling is the existence of the artificial boundaries which are treated by absorbing boundary techniques. For this study, perfectly matched layer (PML) technique is applied to have sharp absorbing boundaries at the spurious edges.