Test and Evaluation of Monitoring Systems (TEM) - Non-Intrusive Monitoring
Non-intrusive monitoring techniques have the advantage that the technical barrier and surrounding excavation damaged zone are not disturbed by the monitoring system.
Seismic tomography was selected as the non-intrusive monitoring technique to be evaluated at the GTS. Numerical simulations demonstrated that temporal changes in the shotcrete plug setup should produce significant changes in the seismic waveforms.
Numerical simulations of pressure wavefields for (a) dry bentonite and (b) fully water-saturated bentonite with the source position being indicated by the red dot. (d) and (e) are the corresponding seismic sections as they would be recorded in the receiver borehole. (c) and (f) are the differences between (a) and (b) and between (d) and (e), respectively, with their amplitudes magnified by a factor of two.
For the tomographic field experiments, six gently dipping boreholes of 25 m length were drilled at regular intervals around the circumference of the gallery, shotcrete plug, and bentonite mass.
During each seismic campaign, seismic energy is released at 0.25 m intervals along boreholes 3, 4, and 5. The source employed is a P-wave sparker characterised by a nominally repeatable broadband spectrum up to several kHz.
The seismic waves are primarily recorded by an acquisition system that includes three multi-element hydrophone chains placed in boreholes 1, 2, and 6, and a composite Geometrics Geode recording unit. The three hydrophone chains are each equipped with 24 hydrophones spaced at 1 m intervals. During the surveys, a 0.25 m hydrophone spacing is synthesized by shifting the hydrophone chains by 0.25 m along the boreholes and repeating the experiments. In addition to the hydrophone data, information from 24 100-Hz vertical-component geophones rigidly mounted to the front wall of the shotcrete plug is also recorded.
Two vertical cross sections that show the experimental configuration for the seismic tomography. The upper drawing is perpendicular to the tunnel axis, the lower drawing shows the borehole inclinations and the relative positions of the tunnel, shotcrete plug, and bentonite layers.
Seismic measurements at the ESDRED / TEM project site
Test and Evaluation of Monitoring Systems (ESDRED / TEM)