Monitoring Winter Freezing in a Silt Soil in Southern Manitoba, Canada Using Surface DC Resistivity Soundings

Abstract

The sensitivity of the electrical conductivity of soil to the temperatures just beneath 0 °C allows the application of electrical geophysical methods for monitoring soil freezing. Surface DC‐resistivity soundings were performed over Winter 1996–97 to examine the electrical response of winter freezing of a silt soil in Winnipeg, Manitoba, Canada (49 °48′30″ N, 97 °07′22″ W). Temperature measurements were made over a 2.7 m deep section during this time interval providing a semi‐continuous soil‐temperature profile. Additional geophysical measurements including seismic refraction, terrain conductivity, and time‐domain electromagnetics were completed to characterize the lateral and vertical variations in the soil properties. During the recordings the surface temperature in Winnipeg decreased from 5.6 °C in October to in January. The 0 °C isotherm reached a depth of 15 cm in mid‐November and a depth of 75 cm in late‐January. The freezing produced a two‐order of magnitude increase in the electrical resistivity of the soil which could be detected by Wenner DC‐resistivity soundings with electrode spacings of less than 3 m. Geophysical inversion of the resistivity soundings provides an image of the time‐varying electrical‐resistivity structure of the soil. When the frozen layer is modelled as a single layer of uniform resistivity, it increases in thickness from around 20 cm in December to 45 cm in February. These values represent estimates of the minimum thickness of the true layer. The resistivity of the frozen layer increases from in December when the average temperature of the frozen layer is to in February when the average temperature of the layer is This increase is probably due to increasing ice content in the layer. The results suggest the DC‐resistivity method provides a useful method for monitoring temporal and spatial changes in the frozen soil layer.