An efficient high-resolution (HR) three-dimensional (3D) seismic reflection
system for small-scale targets in lacustrine settings was developed.
In Lake Geneva, near the city of Lausanne, Switzerland, the offshore
extension of a complex fault zone well mapped on land was chosen
for testing our system. A preliminary two-dimensional seismic survey
indicated structures that include a thin (<40 m) layer of subhorizontal
Quaternary sediments that unconformably overlie south-east-dipping
Tertiary Molasse beds and a major fault zone (Paudeze Fault Zone)
that separates Plateau and Subalpine Molasse (SM) units.
A 3D survey was conducted over this test site using a newly developed
three-streamer system. It provided high-quality data with a penetration
to depths of 300 m below the water bottom of non-aliased signal for
dips up to 30degrees and with a maximum vertical resolution of 1.1
m. The data were subjected to a conventional 3D processing sequence
that included post-stack time migration. Tests with 3D pre-stack
depth migration showed that such techniques can be applied to HR
seismic surveys.
Delineation of several horizons and fault surfaces reveals the potential
for small-scale geologic and tectonic interpretation in three dimensions.
Five major seismic facies and their detailed 3D geometries can be
distinguished. Three fault surfaces and the top of a molasse surface
were mapped in 3D. Analysis of the geometry of these surfaces and
their relative orientation suggests that pre-existing structures
within the Plateau Molasse (PM) unit influenced later faulting between
the Plateau and SM. In particular, a change in strike of the PM bed
dip may indicate a fold formed by a regional stress regime, the orientation
of which was different from the one responsible for the creation
of the Paudeze Fault Zone. This structure might have later influenced
the local stress regime and caused the curved shape of the Paudeze
Fault in our surveyed area.