Detection of welding in pyroclastic flows with ground penetrating radar: insights from field and forward modeling data

We investigate the potential of ground penetrating radar (GPR) as a tool to delineate welded portions of pyroclastic flows by testing interpretations of field GPR data with numerical modeling. Specifically, we test the hypothesis that regions of changing porosity with depth and regions of constant porosity are characterized by distinct signals in radargrams. We model the propagation of electromagnetic waves through pyroclastic flow deposits with vertical variations in porosity (Φ). The actual Φ profiles are gradational but are represented in our models by a series of 1-cm-thick, homogeneous layers. Each layered porosity profile is converted into a layered K′ profile using an empirical law relating K′ to Φ of dry volcanic rocks that is based on laboratory measurements. The resulting synthetic traces indicate that moderate, gradational changes in welding produce multiple low amplitude crests and troughs rather than strong individual reflections. The lower the peak frequency of the source wavelet, the greater the amplitude of the signal generated and thus the greater the potential signal to noise ratio. Extremely high rates of change in porosity with depth produce distinct, strong reflections. Densely welded or unwelded portions, where porosity is relatively constant, produce sections of zero amplitude. These preliminary results suggest that GPR can be used to map the effects of welding in pyroclastic flows. Specifically, characteristic radar signals can be used to identify zones of changing degree of welding and zones of relatively constant degree of welding.