Estimation of snow wetness using multi-offset ground penetrating radar : towards more accurate estimates of snow water equivalent

Measurements of snow water equivalent (SWE) constitute an important input to hydrological models used to predict snowmelt runoffs. The new generation of such models use distributed snow data, including distribution of SWE, as input, and rely on it for calibration and validation. Using ground penetrating radar (GPR) from snowmobiles or helicopters is one of the methods to estimate SWE, and it allows for covering large areas in a short period of time. However, the accuracy offered by GPR is detrimentally affected by the presence of liquid water in the snow. This is a problem since when a snowpack is at its peak, and therefore of the largest interest, it has quite often started to melt so there might be liquid water in the snowpack. The present work is an attempt to solve this problem for SWE estimates made by multi-offset GPR operated from a snowmobile. The main idea is to use radar data already available, and to utilize, in addition to two-way travel time, radar wave attenuation, which both depend on snow wetness. Thus obtained liquid water content of a snowpack can be used to get more accurate estimates of SWE.Using radar wave attenuation to obtain liquid water content requires the relationship between liquid water content and electrical conductivity, which has to be established experimentally. The results of several series of experiments, first establishing this relationship for a specific salt content, and then confirming that variation in salt content does not significantly affect it, are presented in this work.However, there remains another problem to be solved. Attenuation caused by energy dissipation in the snow can only be determined from measured radar wave amplitude if losses due to reflection at the snow/ground interface are known. Since a multi-offset GPR system is in fact an array of antennas, several measurements can be made at each point with radar waves reflecting from the ground with different angles of incidence. It should therefore be possible to calculate angle-dependent reflectivity from radar wave amplitudes using Snell's law and one of Fresnel equations. However, applicability of this method in the presence of measurement errors has to be verified. Initial experiments point to problems due to antenna ring-down from the direct wave interfering with the reflected wave, so further tests of the method should be conducted, or ultimately another method to determine reflectivity of the snow/ground interface should be found. Theoretical and experimental results presented in this thesis lead to the conclusion that when SWE is estimated with a multi-offset GPR system, radar wave amplitudes, available in radar data, can be used to establish liquid water content of a snowpack and hence improve the accuracy of SWE estimates, provided that the problem with establishing reflectivity of the snow/ground interface is solved.Godkänd; 2009; 20091123 (nilgra); LICENTIATSEMINARIUM Ämnesområde: Tillämpad geofysik/Applied Geophysics Examinator: Docent Angela Lundberg, Luleå tekniska universitet Tid: Torsdag den 17 december 2009 kl 13.00 Plats: F 531, Luleå tekniska universite