Ground-penetrating radar in glaciological applications
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Ice-radar is a very useful tool for the study of glaciers and ice sheets. Whereas the radar applications discussed in other chapters, like satellite-borne SAR (Chapter 9), mostly focus on the surface of the ice, applications of ground-penetrating radar (GPR) try to peer into and through the ice. This is achieved by investigating not just a single pulse reflected from the medium, but a whole time series of reflections. This chapter is not intended to present a complete overview of GPR, but just to provide the reader with the necessary background to the subject in order to clearly understand the main focus of the chapter, which is the measurement of ice thickness as a tool to determine the ice volume stored by glaciers and their temporal variations. We will start by presenting the fundamentals of radio-wave propagation in ice, followed by a description of the main radar systems used in glaciology and the key elements of a typical radar system. We will then explain how such systems are used in fieldwork and outline the main steps of radar data processing. Next we will give an overview of the applications of GPR in glaciology, and finally, we will discuss in more detail how ice volume and bed topography are determined from radar-measured ice thickness, paying particular attention to the error estimates. We will split the applications of radar in glaciology into two main types: those that are principally concerned with the internal properties of the ice body, i.e. the full snow/firn/ice column from glacier surface to its base, and those more directed at basal properties, for which one focuses on the interface between the ice body and the underlying bed. We will pay more attention to internal layering and density/water content. The former, because of its use as a tool to determine accumulation rates, and the latter because of their influence on radio-wave velocity and hence on ice thickness and volume estimates, and also for the role of density in the volume to mass conversion.