Dielectric measurements of frozen silt using time domain reflectometry

Abstract

Dielectric properties of ice-rich frozen silt from the permafrost tunnel at Fox, Alaska, have been measured both in the field and in the laboratory, using time domain reflectometry (TDR). Undisturbed field samples obtained with a modified CRREL core barrel were placed in a specially adapted rigid coaxial line mated to the TDR unit. The volumetric water content of the undisturbed samples varied between 65 and 81% and the sample temperature was approximately −7.0°C. The laboratory samples were reconstituted with volumetric water content between 0 and 55%. Temperature was varied between +25° and −25°C. The data were processed to cover the frequency range of 0.05–1.0 GHz. For the undisturbed samples, dispersion tended toward a maximum between 0.3 and 1.0 GHz. The range for the real part of the dielectric coefficient was 3.8–5.3 at the low frequency end, while the imaginary part varied between 0.01 and 0.42 for the entire frequency range. These results from the field studies agree with laboratory measurements and other field observations, indicating that the TDR core barrel sampler is an effective technique for measuring dielectric properties of undisturbed samples. This method could easily be applied for in situ dielectric testing of frozen fine-grained soils and ice.