Geophysical Methods for Site Characterization for Marine Foundations-PDF

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Offshore geophysical surveys provide a rapid, cost-effective, non-intrusive way to characterize marine foundation sites. Compared with boreholes, test pits, and diver inspections, they cover larger areas and produce continuous information about seabed conditions and subsurface geology. Their findings can guide borehole placement, reduce uncertainty in design, and identify hazards such as scour holes, submarine slides, debris, and shallow bedrock.<br /><br />The principal acoustic methods are precision echosounding for water depth and seabed relief; multibeam bathymetry for dense, detailed depth coverage; sidescan sonar for imaging seabed materials, debris, and surface features; and subbottom or seismic reflection profiling for mapping sediment layers and depth to bedrock. GPS, commonly differential GPS, positions the survey data along planned transects. The methods can be used together and their results presented as maps, images, and interpreted geological profiles.<br /><br />Three case studies illustrate their application. At Ketchikan, Alaska, surveys assessed a proposed cruise terminal’s water depth, seabed features, sediment thickness, and bedrock. They found no need for dredging but identified a nearby bedrock shoal and highly variable, thin sediment that could affect pile installation. In Alaska’s Knik Arm, seismic surveys complemented limited borehole data, mapped sediment layers, and found no evidence of bedrock above roughly 900 feet. At the Antioch and Dumbarton Bridges in California, multibeam, sidescan, and seismic methods mapped seabed relief, scour-related features, debris, and subsurface stratigraphy.<br /><br />Successful investigations depend on defining project objectives, data needs, survey conditions, and intended deliverables in advance. Used alongside geotechnical sampling, offshore geophysics supports better-informed and potentially less costly foundation design.

Keywords

offshore geophysical surveys

marine foundation sites

multibeam bathymetry

sidescan sonar

subbottom profiling

seabed conditions

subsurface geology

bedrock mapping

scour hazards

foundation design