Mapping porosity variation in a welded pyroclastic deposit with signal andvelocity patterns from ground-penetrating radar surveys

Ground-penetrating radar (GPR) is used to both image and quantify porosity variations in a variably welded pyroclastic flow deposit. Characteristic ra dar signals for nonwelded (constant, high porosity) and welded (porosity lo wer and variable) zones are identified by comparison of radar signals to ex posed stratigraphy. A moderate rate of change in porosity with depth genera tes abundant, unresolvable reflections. A relatively constant porosity resu lts in a flat, zero-amplitude response. Lastly, a discrete jump or extremel y high rate of change in porosity (abrupt at scale of radar wavelength) can produce a strong, distinct reflection. Common-midpoint (CMP) survey data a re analyzed to determine relative radar velocity patterns in the pyroclasti c flow. Changes in radar velocity are linked to changes in relative porosit ies that are attributed to differential welding. Our analysis shows that we lding causes substantial reductions in radar velocity. Moisture in subsurfa ce stratigraphy also strongly affects velocity. Therefore, we advocate the interpretation of radar data in terms of relative changes in porosity. Our results also suggest that, in areas of rapid facies changes, multiple CMP s urveys are required for accurate conversions of travel time to depth.