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| Titel |
Characterization of the Neuhauserwald Quaternary valley, northern Switzerland, using high-resolution seismic-reflection and seismic-refraction imaging |
| VerfasserIn |
Fabienne Reiser, Cedric Schmelzbach, Heinrich Horstmeyer, David Sollberger, Lasse Rabenstein, Hansruedi Maurer, Johan Robertsson |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250089844
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| Publikation (Nr.) |
 EGU/EGU2014-4057.pdf |
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| Zusammenfassung |
| The Swiss Molasse basin is largely covered by Quaternary sediments which have a thickness
ranging from a few meters to several hundred meters. These glacial, glaciofluvial, and
glaciolacustrine sedimentary deposits are of high interest for a number of reasons; for
example, they contain a large part of Switzerland’s underground freshwater supplies, and
resolving their structure and deposition processes is important to reconstruct the climate
history. Furthermore, this usually thin, but highly heterogeneous near-surface cover can have
a significant deleterious effect on subsurface imaging by regional-scale seismic-reflection
surveying. The study presented here was motivated by the observation of a hithertofore
unknown Quaternary valley observed on recently acquired regional-scale seismic-reflection
data.
To characterize the depth and internal structure of the Neuhauserwald Quaternary valley,
two high-resolution seismic-reflection/refraction datasets were acquired. The approximately
900m long line 1 runs parallel to the valley axis, whereas the ~ 700m long line 3 is
oriented perpendicular to it. A borehole on line 1 provides lithological information and
seismic velocities for the upper 150 m, which were determined by means of a check-shot
experiment. The lithological sequence consists of alternating sand and gravel units over
lacustrine silty sands. Mesozoic limestones are found at 128 m depth below surface. The final
processed seismic reflection images show reflections down to around 200 ms traveltime (~
130 m). The first-arrival traveltime tomography models show a distinct velocity
increase from around 500 m/s at the surface to around 4000 m/s at about 150 m
depth.
For line 1, velocity variations between 500 m/s and 2000 m/s indicate vertical and lateral
changes within the valley infill. The depth to the high-velocity basement, however, is only
poorly constrained by a few rays in the refraction tomogram resulting from the paucity of
long-offset traveltime picks due to the low signal-to-noise ratio. In contrast, the
seismic-reflection images are of relatively high quality, showing a number of sub-horizontal
reflections. Correlation with the borehole data reveals that the interface between the morainal
sand-gravel unit and the underlying lacustrine sediment at around 160 ms (90m depth) is
highly reflective, while the reflection from the basement at around 200 ms (130 m depth) is
relatively weak. Without borehole control, the strong reflection from the top of the lacustrine
sediments would probably have been mis-interpreted as the reflection from the basement.
The refraction tomographic image of line 3 yields a clear image of the dipping
basement. The quality of the line 3 reflection images is inferior compared to that
of line 1, which is due to a not yet resolved static correction issue. The complex
structure of the heterogeneous Quaternary infill and significant topographic variations
complicate the static correction estimation. As a consequence, only a few short
reflection segments are observed on line 3, which may indicate a more complex
sedimentary structure close to the valley flank compared to the central part of the valley. |
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