An ultra-high-resolution 3D marine seismic system for detailed site investigation
Currently, marine ultra-high-resolution 3D surveys tend to be characterized by signal frequency ranges of 0–600 Hz and bin sizes of the order of 3 to 6 metres. This may be acceptable in industry geohazard studies for top-hole well drilling, but it does not have sufficient resolution for the very detailed information required by geotechnical engineers for the design and positioning of the turbine foundations in offshore wind farms. We present a newly developed ultra-high-resolution 3D system, which takes the 3D-resolution and 3D-imaging one step further: to frequency ranges up to 2.5 kHz and 1-m bin sizes. This system is based on an ultra-high-resolution sparker source utilising negative discharge technology, thus guaranteeing a stable and repeatable source signature. The high-fidelity multi-channel recording system is composed of four 24-trace streamers with GPS positioning on both sources and receivers. Sophisticated navigation processing guarantees decimetre precision and the resultant 1-m bin sizes. In addition, slant streamer technology is applied to ensure a broadband seismic response. Processing techniques, including critical corrections for wave motion and tides, have been developed. Trial surveys were conducted in Thailand to validate the method and later in Croatia to study the shallow geology for the foundations of a 2.5-km bridge across a sea inlet. The resultant 3D cubes gave very detailed views of the subsurface allowing the imaging of stratigraphic structures and small-scale features, such as individual boulders in a boulder bed, that had hitherto not been identified.