Reconstructing disturbance zones ahead of the tunnel face by elastic waveform inversion supported by a parametric level-set representation

Abstract

This work presents a flexible and effective methodology for locating and characterizing the disturbance zones ahead of the underground tunnel face by elastic full-waveform inversion (FWI) enhanced with the parametric level-set representation. By using the unscented Kalman filter as the inversion machinary, the inversion process is completely free from gradient calculations and able to provide uncertainty bounds of the estimated model. The conceptual methodology is verified through successful reconstructions of single- and multiple-disturbance objects in a simple 2D frequency domain model. In the synthetic tunnel reconnaissance tests, the special characteristics of the tunnel seismic waves in the time domain are described, and the results of SPECFEM2D simulation and a qualitative evaluation of the simulated tunnel seismic waveforms are shown. The computer model and its simulated tunnel seismic waveform data are eventually used to reconstruct the geological scenarios, whose disturbance is present in the form of a single object and multiple discontinuous objects, in a parsimonious and flexible manner. Although further validations using laboratory or in-situ measurements and the use of fully 3D model are needed to prove the practicality of this approach, the current results are encouraging and promising to apply FWI in tunneling practice as an advanced tool for looking ahead of the tunnel face.