Temperature effects the resistivity of soils. To adjust the resistivity observed at one Celsius temperature to the approximate resistivity that the same soil would have at another Celsius temperature the following approximation can be used:
Rt2 = Rt1 /(1 + .025( t2 - t1 ))
Freezing reduces the resistivity of soils and rocks. Following is a graph from Geophysics in the Study of Permafrost by Scott, Sellmann and Hunter pulished in Geotechnical and Environmental Geophysics, Volume 1, 1990, page 356
The graph is based on three papers:
Hoekstra, P., Sellman,P.V., and Delaney, A. J., 1975, Ground and airborne resistivity surveys of permafronst near Fairbanks: Geophysics, Volume 40, pages 641-656.
Olhoeft, G.R., 1978, Electrical properties of permafrost: Proc. 3rd International Conf. on Permafrost, National Research Council of Canada, pages 127-131.
Scott, W.J. and Key, A.E., 1988, Earth resistivities of Canadian soils: Can. Elect. Assoc., Montreal. Vol. 1, Main Rep., Vol. 2, Site data.