Radiometric calibration and analysis of radar sounding observations of ice-sheet subsurface conditions from archival film records

Abstract:

Airborne ice penetrating radar is a key geophysical tool for observing the subglacial environment. Radar sounding can provide direct imaging of englacial and subglacial structures and place constraints on bed conditions at local, regional, and continental scales. Radar sounding data has been collected across much of Antarctica producing records of observations that span more than five decades with the earliest data recorded on optical film and more recent data collected and recorded using modern digital radar systems. A similar archive of data exists for Greenland. This has resulted in distinct data sets, the oldest of which have only recently been directly digitized and released for Antarctica and none of which has been fully calibrated. As a result, only a single proof-of-concept study with a multi-decadal time-series of subsurface conditions from radar sounding data of Antarctica has been produced and none have been produced for Greenland or taken advantage of modern radiometric radar sounding data analysis approaches. To address this, we develop and present radiometric calibrations of and between A-Scope (fast-time traces) and Z-Scope (continuous sounding profile) radar film records. This includes calibrating the noise floor and gain scaling of A-Scope records which are direct (if uncalibrated) recordings of the amplitude of the received radar returns. These A-scope traces are recorded far less frequently (every few kilometers) than the Z-Scope records (every few meters). Unfortunately these Z-Scope records were recorded with a compression scheme (which includes log-detection and then fast-time differentiation) to accombinate the limited dynamic range of optical film. As a result, some of the radiometric information in the Z-Scope records have been lost due to saturation. To overcome these limitations, we present an approach to cross-calibrate and synthesize these types of archival film records into profiles constraining relative returned power and reflectivity with higher radiometric and spatial fidelity than either record type alone. This is the critical development for applying modern quantitative radiometric analysis approaches to the archival film record and for their comparison with modern radar sounding data.