Thermal imaging of alluvial fans: A new technique for remote classification of sedimentary features

We evaluate the utility of remote thermal image data for mapping geomorphic features and evidence of sedimentary processes on the surfaces of alluvial fans. Prior studies of alluvial fans have made extensive use of visible images and traditional field-based techniques. As a case study demonstration of this technique, we compare thermal images acquired from the ground and a light aircraft (altitude ~ 5000 ft) to a pre-existing ground-based map of features on the Dolomite Fan in Owens Valley, California. Thermal images from the aircraft were acquired at several times of day to determine how the surface temperatures of the alluvial fan rise and fall throughout a diurnal cycle. We have also acquired thermal images from the ground at 5 minute intervals over the course of a diurnal cycle. The aerial thermal images (ground resolution ~ 2 m/pixel) reveal evidence of a variety of sedimentary processes that have acted on the fan surface. These images show spatial–thermophysical patterns associated with clast-rich and clast-poor debris flows, debris-flow levees and the change in particle size at the toe of the fan. The locations of these features in the thermal images match the locations of the features previously mapped by others using traditional ground-based field sedimentology techniques. All debris flows that are exposed at the fan surface are evident in the aerial thermal images, including those that have been heavily weathered and are difficult to observe in visible images. This case study demonstrates that aerial thermal images can be used to provide reconnaissance of an alluvial fan, suggest what sedimentary processes have most recently acted on the surface of the fan, and to prioritize sites for detailed study on the ground.