Transient electromagnetics for permafrost

Abstract

Transient electromagnetic (TEM) soundings were carried out with a Geonics EM-37 instrument at more than forty sites in Alaska, primarily along a line from Reindeer Island to Glennallen, to investigate its application to the study of permafrost. Procedures were developed for correcting TEM sounding data for the effects of transmitter turnoff time. Best fit geoelectric models of horizontally layered earth were derived by computer inverse modeling, using program NLSTCI (Anderson, 1982), and used to interpret the soundings in terms of the thickness of permafrost at each site. The interpretations indicate permafrost thicknesses vary substantially between sounding sites on land, although the general trend of thicker permafrost at more northern latitudes is evident. Under favorable circumstances, the depth to the base of ice-bearing permafrost may be resolved within $\pm$10%. Soundings taken over sea ice indicate that the thickness of the thawed sediments overlying ice-bearing permafrost can be determined, the subsea permafrost is multilayered beyond about 3 km offshore, but that the TEM system used may not resolve the base of ice-bearing subsea permafrost in this warm, high-salinity, and multilayered environment. An anomalous, double-sign reversal was obtained at a site in the Kuparuk region which was successfully modeled using a complex resistivity of the Cole-Cole type. The model parameters indicate that this unusual signature may be related to the known deposits of gas hydrates beneath the site suggesting that deep deposits of gas hydrates may be detectable from the ground surface using the TEM method. Finally, it is noted that TEM soundings for permafrost are most productively performed in a line or grid tied to sites with known subsurface lithology so that modeling parameters may be constrained to physically reasonable values.