Coastal Subsea Permafrost and Bedrock Observations Using DC (Direct Current) Resistivity

摘要

Measurements were made at several New England coastal sites and at three sites in Prudhoe Bay, Alaska, to evaluate dc resistivity techniques for mapping resistive seabed features: Bedrock and Subsea permafrost. The field studies employed the four-probe Wenner array technique, with electrode separations up to 50 m. The New England sites were selected to simulate permafrost conditions to help establish a feeling for the range of apparent resistivity in areas of subsea permafrost and for information on the vertical and lateral resolution of the technique. At Prudhoe Bay, offshore measurements were made with a floating cable, and inland measurements were made using electrodes driven into the ground. These observations indicate that the electrical properties of permafrost beneath the coastal bluff and adjacent tundra are rapidly modified by coastal erosion and periodic flooding during storms. Maximum apparent resistivity at the water's edge was around 50 Ohm-meters, and at distances greater than 100 m from shore all values were less than 20 Ohm-meters. Modeling supported by the drilling data permitted an interpretation of the position of the top of ice-bonded subsea permafrost. Real resistivities for the ice-bonded permafrost ranged from 200 to 1000 Ohm-meters. The techniques and equipment used for this study appear to have applications for studies in shallow coastal waters where permafrost is not more than 30 m below the seabed and where depths do not exceed 6 to 7 m. Keywords: Electrical resistance measurement; Bedrock mapping; Coast permafrost; Marine geophysics; Marine resistivity; Permafrost mapping; Subsea permafrost; Direct current. (EDC)