Monitoring Winter Freezing in a
Silt Soil in southern Manitoba, Canada using Surface DC Resistivity Soundings
Ian J. Ferguson and Gregory A.J.
Desrosiers
The sensitivity of the electrical conductivity
of soil to the temperatures just beneath 0 oC allows the application of
electrical geophysical methods for monitoring soil freezing. Surface DC-resistivity
soundings were performed between over Winter 1996-97 to examine the electrical
response of winter freezing of a silt soil in Winnipeg, Manitoba, Canada
(49o48'30" N, 97o07'22" 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 oC in October to - 18.4 oC in January. The 0 oC 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 500 ohm.m in December when the average temperature of the
frozen layer is -0.5 oC to 1500 ohm.m in February when the average temperature
of the layer is -2 oC. 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.