A high-resolution three-dimensional (3D) seismic reflection system
for small-scale targets in lacustrine settings has been developed.
Its main characteristics include navigation and shot-triggering software
that fires the seismic source at regular distance intervals (max.
error of 0.25 m) with real-time control on navigation using differential
GPS (Global Positioning System). Receiver positions are accurately
calculated (error < 0.20 m) with the aid of GPS antennas attached
to the end of each of three 24-channel streamers. Two telescopic
booms hold the streamers at a distance of 7.5 m from each other.
With a receiver spacing of 2.5 m, the bin dimension is 1.25 m in
inline and 3.75 m in crossline direction. To test the system, we
conducted a 3D survey of about 1 km(2) in Lake Geneva, Switzerland,
over a complex fault zone. A 5-m shot spacing resulted in a nominal
fold of 6. A double-chamber bubble-cancelling 15/15 in(3) air gun
(40-650 Hz) operated at 80 bars and 1 m depth gave a signal penetration
of 300 m below water bottom and a best vertical resolution of 1.1
m. Processing followed a conventional scheme, but had to be adapted
to the high sampling rates, and our unconventional navigation data
needed conversion to industry standards. The high-quality data enabled
us to construct maps of seismic horizons and fault surfaces in three
dimensions. The system proves to be well adapted to investigate complex
structures by providing non-aliased images of reflectors with dips
up to 30 degrees.