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Seismic experiment from the surface to the mine in the Mont Terri rock laboratory

In the Mont Terri rock laboratory, another seismic survey was carried out by the GFZ in collaboration with Swisstopo. 32 anchors with 3-component geophones were installed along the safety gallery and the gallery 08. In addition, 16 surface geophones were installed along the gallery walls to record data.

A vibro-truck (Fig. 1) was used on the surface approximately 300 m above the underground laboratory to generate seismic waves along two lines with 50 shot points each. A linear sweep with frequencies from 10 to 150 Hz was excited as the source signal. The geophones in the subsurface recorded the waves and provided valuable data for structural analysis of the rock formation.

The comparison of the surface geophones with the 3-component geophones (Fig. 2) located 2 meters deep in the rock helps to select suitable sensors for the planned main experiment. Both the different transmission properties of the rock formations and possible interference signals caused by the operation of the underground laboratory must be taken into account.

The test also served to verify the technical realization of a GPS time-accurate seismic registration over a long measurement exposure of approx. 1.2 km during the continuous data recording in the underground area.

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Fig. 1: GFZ vibrator truck type Innova Univib-325
Fig. 2: View into the safety gallery of the Mont Terri rock laboratory with DAQLink 4 recording unit and data storage on a laptop. The seismic signals were recorded using three-component geophones (installed 2 m deep in the rock) and surface geophones installed on the side walls of the galleries.