Case Studies - Engineering

Mapping Conductivity with Helicopter Electromagnetic Surveys as an Aid to Planning and Monitoring Pipeline Construction

Abstract

Helicopter EM surveys have been used to map apparent conductivity as an aid to characterising ground conditions in advance of pipeline construction. The cost of pipeline construction is strongly dependent on the ground conditions encountered, and accurate prediction of these conditions can reduce the planning risk considerably.

A DIGHEM^v conductivity survey was used to map ground conditions along approximately 130km of prospective pipeline corridor, 400m in width. The survey took about two days to complete, providing a map of apparent conductivity with a resolution of approximately 10m. The results are interpreted to determine the extent of shallow bedrock (which would require blasting) and deeper overburden which could be trenched to the depth necessary for the pipeline. Over much of the survey area it is possible to define a single apparent conductivity value as the borderline between soils, which could be trenched, and rock which would have to be blasted. The survey maps the depth to bedrock, and gives some indication of the soil types based on ground conductivity measurements.

The airborne EM survey reduced the time and cost associated with gaining land access and permission for drilling. The survey also served as a check for buried, unknown power lines and pipelines. Airborne EM surveys have also been used to map ground conductivity after the pipelines have been constructed to detect areas of high ground conductivity due to clays or saline soils. These soils can create conditions in which pipeline corrosion is accelerated.

Airborne EM surveys are an effective means of reducing cost and risk in the planning, construction and monitoring of pipelines. Integrated with ground-based measurements and a drilling program, airborne EM apparent conductivity results can serve to gain a more complete understanding of ground characteristics across the areas of interest.

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Greg Hodges, Chief Geophysicist, Geoterrex-Dighem
Jonathan Rudd, P.Eng., Geophysicist, Geoterrex-Dighem
Dominique Boitier, Compagnie General de Geophysique