Help understanding radar data in subglacial lake complex beneath Devon Ice Cap

Question

The BBC News article Isolated lakes found beneath Canadian ice sheet links to the open access Science Advances article Discovery of a hypersaline subglacial lake complex beneath Devon Ice Cap, Canadian Arctic.

The discovery is based on radar "soundings" of the Devon Ice Cap in Nunavut, Canada.

I'm having trouble understanding the nature of the evidence for liquid water as stated in the article. I understand that there will be a relatively strong reflection at an interface where the dielectric constant changes, and that this is expressed in dB in some of the plots, but I don't understand the blue "blobs" or the axis label Spec. content shown.

I've cropped some of the plots and rearranged them below to help highlight the "blue blobs" below.

Also it looks like the potential lakes are associated with dips in the underlying bedrock contour, but I can't tell if the lakes are sitting on top of the bedrock, or at an intermediate elevation above it with ice both above and below them, and I don't understand at all what the axis label Hydrolic head (m) in other plots means. And why does the top surface of the ice look relatively flat in the radar images, but rise and fall hundreds of meters in the ice surface and bed plots? Did the plane fly at a fixed distance with respect to the top ice surface rather than a fixed elevation?

below: Figure 1 from here.

Answer 1

In the figure: The top panel is the radar gram measured in depth below ice surface (m). It is flat because when you do the measurement it starts from the surface which is 0m. The distance from the aircraft to the surface would have been relatively constant but the plot starts from the air-ice boundary at 0m.

The middle panel is a cross section measured in m above sea level showing the ice surface and bedrock (black), the hydraulic head (blue) relative to sea level. The hydraulic head is calculated by working out the pressure of the ice sitting on top of the bedrock. It is basically the water table of the ice sheet. Whats important is the differences in hydraulic head (gradient) which can tell you if the water is moving or not.

The bottom panel is the reflectivity (black) and specularity (blue) of the basal surface. Specularity is a ratio of reflected to scattered energy. Simply put, high reflectivity means the boundary is wet and high specularity means the boundary is flat (Ice-Water), and low specularity means its rough (Ice Rock).