The seismic refraction method is generally utilized for determining depth to and competency of the bedrock surface, overburden classification, geological structure, and earthquake risk assessment. On land and on the ocean bottom, shear wave refraction is used to determine layering and layer elastic moduli.
Electrical resistivity methods have been traditionally used for subsurface layering delineation based on distinct resistivity contrasts between diverse geological materials. Generally utilized to obtain resistivity soundings, the method is also used in the profiling mode to provide more rapid coverage of large survey areas.
Overwater acoustic profiling is carried out to determine sediment layering and depths to bedrock in the marine or fresh water lake and river environments. The tool is usually accompanied by a high frequency fathometer and side-scan sonar. The side-scan sonar is used to scan to the sides of a traverse to determine bottom conditions and locate bottom or shallow sub-bottom hazards. Positioning is generally carried out with standard GPS technology.
Ground penetrating radar is effective in rapidly profiling shallow subsurface layering based on the reflection of electromagnetic waves from subsurface interfaces. It is commonly applied to mapping soil layers, depths to bedrock, buried stream channels, rock fracture zones and cavities, buried waste materials and buried metallic features. The tool can be used on land, overwater in freshwater lakes and rivers and in boreholes, either in the downhole or crosshole modes.
Borehole geophysical methods are used to provide high resolution information on lithology, bulk density, porosity, fracturing, fluid flow, oxidation, and susceptibility. New technology allows multiple logs to be recorded simultaneously. Borehole seismic methods either downhole or crosshole provide information on layering, elastic moduli of layers and bedrock, voids or anomalies in geological layering using tomography mapping, and assist in determining design parameters for structures. Crosshole and tomographic surveys are also carried out with resistivity and electromagnetic methods to provide high resolution images of anomalous conditions having contrasting resistivity or conductivity properties.
As examples of widely used, multi-faceted geophysical investigations we have highlighted dam sites and pipeline crossings. These applications, while specific, emphasize the capability afforded by geophysical surveys to map, assess and evaluate geological conditions in diverse environments and with minimum impact.