Subproject A2

Development of Effective Concepts for Tunnel Reconnaissance using Acoustic Methods


Aim of project A2 is the development and application of new methods for advance reconnaissance during mechanized tunneling based on elastic wave propagation. We have established stably converging full-waveform-inversion approaches operating in the time and the frequency domain. Synthetic scenarios demonstrate their ability to reliably image disturbances ahead of the tunnel face. We currently validate the methods by means of a small-scale laboratory experiment employing ultrasonic waves excited by piezoelectric transducers and recorded by a laser vibrometer. We plan to apply the methods to in-situ data from current and past tunnel excavations.

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Reconstruction of a fault in front of the tunnel boring machine represented by a discontinuous change in seismic wave velocity. Left: True wave velocity distribution. Center: Reconstruction through full waveform inversion in the time domain. Right: Reconstruction through full waveform inversion in the frequency domain. Both inversions were started from a homogenous wave velocity distribution. Colors indicate changes to initial wave velocity distribution introduced by the inversion. Both inversion techniqus are able to produce a clear image of the fault.

Small scale laboratory experiment for tunnel reconnaissance. A piezo-electric transducer is used to illuminate a rock specimen by elastic waves in the ultrasonic range. The displacement caused by the waves is recorded by a ultra-sensitive laser interferometer. It is planned to detect unknown obstacles within the rock specimen by analyzing the seismic waves scattered from obstacles inside the specimen. Left: Sketch of the experimental setup. Right: Photograph of the experimental setup with the laser measuring elastic waves traveling through the specimen and originating from the piezo transducer at the bottom.