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5. CRREL Report

Title:	Effects of seasonal changes and ground ice on electromagnetic surveys of permafrost
Authors:	Arcone, Steven A. Delaney, Allan J. Sellmann, P. V. (Paul V.)
Keywords:	Electromagnetic ground resistivity technique Seasonal variations Fairbanks (Alaska) Ground ice Ice Permafrost Frozen ground Frozen soils Electromagnetic prospecting Prudhoe Bay
Publisher:	Cold Regions Research and Engineering Laboratory (U.S.) Engineer Research and Development Center (U.S.)
Series/Report no.:	CRREL report ; 79-23.
Description:	CRREL Report Abstract: The performance of surface impedance and magnetic induction electromagnetic subsurface exploration techniques was studied seasonally at various sites in Alaska where permafrost and massive ground ice occurred. The surface impedance method, which uses radiowaves in the LF and VLF bands, and the magnetic induction method, which uses low-frequency magnetic induction fields, distinguish subsurface materials by the electrical resistivity of the materials. The methods used have greatest sensitivity within about 20 m of the surface and are, therefore, most applicable for shallow subsurface investigations. The selection of study sites was based on anticipated contrasts in electrical resistivity between ground ice and adjacent earth materials. A magnetic induction instrument, using a separation of 3.66 m between the transmitter and receiver antennas, in general was able to detect near-surface zones of massive ice and to provide data regarding permafrost distribution in both the Fairbanks and Prudhoe Bay areas. At this antenna separation, the depth of magnetic field penetration was sufficient to include mainly the zone containing maximum contrasts in resistivity between ground ice and other earth materials. In the Fairbanks area, contrasts in this zone were greatest in late winter when the seasonally thawed surface layer was completely frozen. When thawed, this layer usually becomes more conductive and often masks the deeper resistivity contrasts. In the Prudhoe Bay area, maximum ground resistivity contrasts were detected in late summer when shallow subsurface temperatures had risen sufficiently to permit resistivity contrasts between the massive ice and the ice-rich ground to appear. Use of the surface impedance method between 200 and 400 kHz revealed resistivity anomalies that qualitatively agreed with massive ice zones in both the Fairbanks and Prudhoe Bay areas. However, in some cases the results were misleading, most likely because of the plane wave assumptions used for data interpretation. On the basis of the studies presented, it is concluded that the magnetic induction method was generally superior to the surface impedance method for delineating permafrost and zones of massive ground ice because the magnetic induction fields were local and well defined and because antenna loop spacing and orientation could be varied to help discriminate subsurface features. Also, it is concluded that the optimum time of year to perform electromagnetic surveys at shallow depths is not necessarily when the electromagnetically absorbing surface layer is frozen. At this time of year, ground temperatures may be so low in some geographic regions that resistivity contrasts between frozen earth and massive ice are eliminated. Therefore, in planning a survey, careful attention must be paid to the geological and thermal aspects of a site or region.
Rights:	Approved for public release; distribution is unlimited.
URI:	http://hdl.handle.net/11681/9110
Appears in Collections:	CRREL Report