High-Resolution Surface Geophysics to Guide Pipeline Construction Projects

Three case studies are presented for the application of electrical resistivity and shear wave velocity geophysical techniques to pipeline projects in marine, terrestrial, riverine, salt pond, and estuarine environments. The first case study used resistivity mapping to delineate the geologic stratigraphy and define the boring profiles for the construction of four 20-in. diameter, 165 and 220 m long horizontal flushing channels at a resort property in the Caribbean. The profiles traversed terrestrial, marine, and hypersaline pond environments, and the completed flushing channels resulted in the conversion of a stagnant, 13-acre salt pond into a functional marine habitat. The second case study utilized resistivity and shear wave velocity mapping in a riverine environment in Florida to direct the location of a 20-in. diameter, horizontally-drilled gas line traversing three river crossings. The geophysics provided a detailed geologic stratigraphy of the sub-surface sediment and rock units and further allowed for identification of karst features through which the boring profile would encounter loose, flowing sediments and drilling fluid circulation losses. The results allowed for guidance of the horizontal boring profile through dense shell and limestone strata. The third study utilized resistivity and shear wave velocity mapping on a section of the Hudson River to identify the top of rock for aid in designing a directional boring profile. Past marine seismic reflection surveys were unsuccessful in this environment due to the presence of gas pockets within the overburden sediments. However, the resistivity data beneath the seabed differentiated the sand, gravel, clay, and rock units, which were confirmed by correlation to borings collected on the landward side of the boring route. Shear wave velocity mapping was utilized for the landward completion of the route. The seismic data identified the top of dense rock to within 1 m of what the boring data showed. Results of these studies have provided the geologic framework for engineers to implement successful construction strategies in designing subsurface boring profiles and show the invaluable nature of utilizing geophysics as a first-order technique in guiding rapid, accurate, and cost-effective geotechnical investigations.