Introduction

In many high-latitude rivers, frazil ice production along open reaches sometimes causes blocking of the river course further downstream, beneath the seasonal ice cover, and may eventually lead to flooding and formation of extensive icing on the flood plain (Asvall 1998). When mitigation measures must be taken, rapid mapping of the ice conditions beneath the surface ice is advantageous. Therefore, the Norwegian Water Resources and Energy Directorate have tested the use of ground-penetrating radar (GPR) for this purpose.

The dielectric contrasts between ice and both water and wet sediments are large and clear reflections are expected (Table 16.1). Beneath the surface ice cover, a mixture of water and a variable amount of ice crystals (frazil ice) may be found. It is likely that this mixture of ice and water will cause clutter (chaotic returns from material inhomogeneity) and influence the velocity of a passing electromagnetic signal.

Airborne radar technology has been successfully applied to measure ice thickness on rivers and lakes (e.g. Arcone and Delaney 1987, Arcone 1991, Leconte and Klassen 1991, Arcone et al. 1997), and ground-based surveys of ice thickness have for instance been standardised by Sensors & Software Inc. through designated ‘ice picker’ software for use with their Noggin 500 MHz system. Other aspects of river ice, such as frazil ice and bottom ice, have received less attention, although there is a study by Dean (1977) and detection of frazil ice by airborne radar is also mentioned by Steven Arcone on the CRREL website (www.crrel.usace.army.mil/sid/gpr/Airborne_GPR.html).