The Geophysical Toolbox: A Practical Approach to Pipeline Design and Construction

Geophysical surveys map variations in physical properties of subsurface materials, many of which can have a direct impact on pipeline design and construction. This paper provides an overview of complementary geophysical methods available in the geophysical toolbox and shows, through the use of case histories, examples of the applicability of the methods for specific pipeline design and construction scenarios. In the context of pipeline design and construction, the objectives of a geophysical survey typically include one or more of the following applications: • muskeg mapping (thickness, lateral extent); • permafrost delineation (variations in ice content, frozen/unfrozen boundaries); • depth to bedrock; • rippability of bedrock; • soil type delineation (corrosion protection, granular inventories); • subsurface conditions at water crossings for horizontal directional drill planning using detailed investigations (boulder horizons, abandoned workings, depth to bedrock). To successfully address these objectives, it is often necessary to utilize more than one geophysical technique. Geophysical methods commonly employed in pipeline investigations include the following: • seismic refraction (marine and land based); • seismic reflection (marine and land based); • electromagnetics; • electrical imaging; • ground penetrating radar (marine and land based); • sonar. The fullest utility of geophysical information is achieved when combined with complementary approaches to provide the end-user with a value-added, cost effective approach. These other method include: airphoto interpretation, satellite imagery, and drilling. The incorporation of auxiliary data sets results in geophysical sections that provide a means of interpolating subsurface conditions between drill holes and reducing the risk associated with encountering surprises. These sections can also be used to provide for more accurate cost estimates by their inclusion in bid documents while at the same time ensuring a better data base for pipeline design. In addition to the advantages of using a geophysical toolbox, the ramifications of the pitfalls of geophysical approaches will also be discussed through the use of case histories illustrating situations in which an inappropriate geophysical technique was applied.