Detection of subbottom ice‐bonded permafrost on the Canadian Beaufort Shelf by ground electromagnetic measurements

Abstract

Accurate knowledge of the location of subbottom ice‐bonded permafrost is essential in planning major infrastructure projects such as pipelines in the Arctic. A combination of refraction and shallow reflection seismic surveys and drilling has been considered the most effective means of permafrost detection in offshore environments. Field experiments recently carried out on the Beaufort Shelf by the Geological Survey of Canada indicate that multifrequency horizontal‐loop electromagnetic (HLEM) measurements are a fast and cost‐effective alternative. During test surveys, 60 to 80 soundings were completed in a day. Analysis of HLEM field data shows that a four‐layer model is appropriate for the local geologic conditions. The first layer in the model, sea ice, is highly resistive (5000 Ω ⋅ m). The second layer consists of seawater and seawater‐saturated sediments (estimated resistivity 1 Ω ⋅ m). Because their true resistivities are not significantly different, it was not possible to separate the two highly conductive layers by inversion. The target of the survey, subbottom ice‐bonded permafrost, was the third layer in the section (resistivity 5000 Ω ⋅ m). The bottom layer is formed by unfrozen sediments (average resistivity 2 Ω ⋅ m). Comparison with seismic and drilling results indicates that detection of ice‐bonded permafrost by the HLEM method is reliable; the outlines of its upper boundary given by seismic and HLEM measurements are in good agreement. At the only borehole available, unconstrained inversion of HLEM data provided an accurate estimate of permafrost depth and thickness, but there is not enough drilling information to objectively assess the reliability of the technique.