A shear‐wave seismic system using full‐waveform inversion to look ahead of a tunnel‐boring machine

Abstract

In the near surface with unconsolidated soils, shear-wave properties can often be characterised better and with a higher resolution than compressional-wave properties. To enable imaging ahead of a tunnel-boring machine, we developed a seismic prediction system with a few shear-wave vibrators and horizontal receivers. The boring process is interrupted at regular intervals to carry out active surveys. The vibrators are then pushed against the rock or soil in front of the cutting wheel of the machine. The design of the vibrators is based on linear synchronous motor technology that can generate very low frequencies, starting at 5 Hz. These vibrators generate a force in a direction perpendicular to the tunnel axis. Horizontal receivers measure the particle velocity, mainly due to the horizontally polarised shear waves. Because imaging with conventional migration methods suffers from artefacts, caused by the incomplete aperture and inaccuracies in the assumed velocity model, we use two-dimensional horizontally polarised shear full-waveform inversion to resolve the subsurface shear properties. The classic cycle-skipping problem, which can make the application of fullwaveform inversion cumbersome, is avoided by the capacity of the vibrators to generate low frequencies. In this paper, we demonstrate the capabilities of the proposed seismic system through a number of synthetic and field experiments.

Imaging ahead of a tunnel boring machine with DC resistivity: A laboratory and numerical study

“…of the natural mechanical signals being generated by the TBM itself, although there has also been some research into active seismic methods [24], as seen in Figure 1.3. The advantage of seismic is that the method is easy to implement and data can be collected continuously during excavation; however it is not sensitive to everything and may have difficulties with poor coupling [25].…”

Elastic waveform inversion in the frequency domain for an application in mechanized tunneling

“…The potential of full waveform inversion for exploration in a mechanized tunnel domain have already been investigated for several time domain approaches: Bharadwaj et al (2017) use a combination of an adjoint gradient method and a two-dimensional model, which focuses on horizontally polarized shear waves; Lamert & Friederich (2019) use the adjoint gradient method in combination with a nodel discontinuous Galerkin approach; Nguyen & Nestorović (2016) developed a hybrid non-deterministic approach by combining simulated annealing with the unscented Kalman filter, which Trapp & Nestorović (2021) have successfully applied on measured data of a small-scale experimental setup.…”

Web-based visualisation for look-ahead ground imaging in tunnel boring machines

“…Work flow of the web-based information platform for management, visualisation and interpretation of tunnel ground imaging/contextual data. (a) GPR circular scanning options [9](b) Seismic sensors[30].…”

“…Circular scanning configuration of the ground imaging system. (a) GPR sensors; (b) Cutter head of the TBM showing source-receiver acquisition geometries along different diameters[30] initiates the process of data downloading and updating the corresponding database. Communication errors, which may affect the data ready exchange, are handled using the standard Transmission Control Protocol in the POCO C++ library.…”

“…Since both GPR and seismic sensors are reflec-420 tion based techniques, in order to calculate the distance from a 421 probable event to the TBM cutter head, the velocity of signals 422 travelling in the surrounding medium must be known. In this 423 work, the velocity (or ground permittivity) of GPR signals is 424 estimated based on the rough ground characteristics; the veloc-425 ity model of seismic sensors comes out of an inverse modelling 426procedure as detailed in[29].427 Tunnel geology view.…”