Earth Resistivity Structures and their Effects on Geomagnetic Induction in Pipelines.

摘要

Geomagnetically induced (telluric) currents flowing in pipelines can interfere with their cathodic corrosion protection systems. Intensity of geomagnetic activity and pipeline structure influences the magnitude of telluric currents, a problem particularly acute in northern regions where geomagnetic disturbances are stronger and more frequent. Furthermore, lateral variations of subsurface resistivity are suspected to be a contributing cause of large pipe-to-soil potentials (PSP) observed locally on some pipelines worldwide. However, the extent to which the geological conditions beneath a pipeline influences telluric currents and PSP has not been fully investigated. Therefore, this study tested the hypothesis that large PSP amplitudes are, in part, a consequence of differing Earth resistivity structure, both vertically and laterally, along a pipeline route. As well, statistics regarding exceedance of telluric activity and PSP over the operational lifetime of a northern pipeline are provided.;This study demonstrated that literature-derived 1D Earth resistivity models can be used to predict PSP variations over a broad scale (Alaska, Mackenzie Valley study areas), and are reasonably accurate in comparison to a model obtained from 1D inversion of MT data (Mackenzie Valley study area). In addition, it was shown that PSP can be modelled (Ottawa Valley study area) using a spatially-variant surface geoelectric field. This new approach effectively accommodates a geological complex area where rapid lateral changes of subsurface resistivity occur along a pipeline route. Future work would include spectral analysis to better isolate and identify the Earth response causing large amplitude geoelectric field and PSP observed near Pakenham along the Ottawa Valley pipeline, and forward modelling to identify the kind of geological feature responsible.;A combination of geological literature compilation, magnetotelluric (MT) and PSP surveys, and numerical analysis was applied to test the hypothesis. A progressive approach was undertaken, examining three separate pipeline routes, two of which are proposed and one existing. The work resulted in the first regional MT soundings along the length of the Mackenzie River valley, and detailed combined MT and PSP surveys along part of the Ottawa Valley.