Mapping of modern pyroclastic deposits with ground penetrating radar: experimental, theoretical and field results

This thesis explores the utility of ground-penetrating radar (GPR) in mapping and characterizing young pyroclastic deposits. In particular, laboratory measurements examine the relationship between porosity and dielectric constant of volcanic rocks. Simulations and field studies demonstrate how GPR data can be used to both image, and quantify, relative porosity variations in pyroclastic deposits. The laboratory results (Chapter 2) indicate a strong and definite relationship between total porosity and dielectric constant of dry, felsic to intermediate volcanic rocks. The trend formed by these data is remarkably tight and coherent, especially considering the samples derive from five deposits and two volcanoes. Chapter 3 models the propagation of a radar wave through a welded pyroclastic flow deposit of variable porosity using the porosity-dielectric constant relationship from Chapter 2. Although porosity changes are gradational, the deposit generates reflections. Distinctive signals correspond to portions with essentially constant porosity or areas where porosity changes with depth at a moderate rate. GPR data were collected on sections of pyroclastic fall and welded and unwelded pyroclastic flow deposits in Central Oregon (Chapter 4). Airfall and pyroclastic flow deposits can be distinguished on the basis of their distinct geophysical character. For example, the same characteristic signals for regions of changing porosity with depth and regions of constant porosity, which were recognized in the modeling (Chapter 3), are seen in the GPR data. The results indicate that GPR can be used to map the intensity of welding and separate zones of uniform welding from zones of variable welding. Velocity analysis of common midpoint (CMP) surveys is shown to aid in mapping facies variations, as well as, converting travel times into depths in radargrams. GPR is also found to be useful in delimiting deposit thicknesses and geometries away from exposure.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat