Doctoral thesis

Géométrie sédimentaire des bassins glaciaires péri-alpins (Quaternaire – Suisse) : étude par sismique réflexion haute résolution

ContributorsSigner, Claude
Number of pages236
Imprimatur date1995-11-10

The objective of this research is to demonstrate the contribution of high resolution reflection seismic in identifying the nature and spatial distribution of glacial sedimentary bodies.

For the measurement we use a 24 channel engineering seismograph, 50 Hz geophones and impulse, hammer (5 kg) and seisgun (12 gauge). From the sample data set, it appears from spectral analysis that the dominant frequency of the reflection information is ranging between 80 and 350 Hz. Processing was done on a IBM PC running EAVESDROPPER© software.

The two Quaternary basins studied have a sedimentary record characterized by at least two glacial phases separated by warmer climatic intervals:

- In the Rossens area, the aims of the seismic study are twofold: to identify a system of at least two main valleys (lnterglacial canyon and paleo-valley) incised into the Tertiary molasse and to understand the spatial geometry of the Quaternary infilling. Four main units have been distinguished. The top molasse reflection is often weak because of the overlying well compacted lodgement till. ldentifying the Quaternary sedimentary units is important for undersranding groundwater circulation: in particular the porous gravel body can be traced from the subsurface, they are normally sealed by argillaceous till which prevents groundwater contamination. Seismic data show the pinch-out of the till and help identifying the location of "hydraulic windows" through the till, i.e. areas where surface polluants couId reach the groundwater reservoir.

- The "Plaine de l'Aire" area offers the same type of problems as the Rossens area. It is a tectonically induced depression in the molasse, filled by Quaternary sediments. A very high resolution seismic allows the recognition of different seismic facies in the Quaternary sequence. lt comprises of diamict, outwash, braided river and glaciolacustrine deposits. The base of the thick outwash sequence of the "Alluvion Ancienne" is a groundwater reservoir in the Geneva area. With the help of shallow reflection seismics it may be possible to map and estimate the volume of this porous rock body.

ln both case studies the resolution of shallow reflection seismics was sufficient to distinguish most of the sedimentary units known from boreholes and outcrops. Seismic events are identified from depth of 20 to ~170 meters.

Therefore in both studied areas, shallow reflection seismic data contributes to a better understanding of the shape of quaternary valleys and the vertical and lateral developpment of their glacial/periglacial infills. An extensive use of cost effective shallow seismic surveys may provide the basic information required by engineers to solve 3D problerns in glacial basins, with strong implications for hydrogeology rnodelling.

Citation (ISO format)
SIGNER, Claude. Géométrie sédimentaire des bassins glaciaires péri-alpins (Quaternaire – Suisse) : étude par sismique réflexion haute résolution. 1995. doi: 10.13097/archive-ouverte/unige:175758
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