A NEW FIELD METHOD FOR DETERMINING RELATIVE GRAIN SIZE FROM AERIAL THERMAL IMAGES: COMPARISON OF RATE OF MORNING HEATING AND ∆TMAX METHODOLOGIES FOR MEASURING RELATIVE THERMAL INERTIA AND SEDIMENT GRAIN SIZES

In planetary remote sensing research, the thermal inertia (TI) of sedimentary surfaces can be used as a proxy for grain size. This relationship between TI and grain size has the potential to improve methodologies for sedimentology field work on Earth, specifically in drone-based remote sensing, which continues to open new opportunities for geologic research in remote and inaccessible areas. In terrestrial sedimentology, relative TI is typically calculated from the maximum diurnal temperature change (ΔTmax) of a surface. This method has been shown to be useful in estimating relative grain sizes on a surface but the data collection is time consuming and has logistical obstacles. We designed this study to test a different method of measuring relative TI of sedimentary surfaces by using the rate of morning heating from a diurnal temperature curve as a proxy for grain size. Sabol, et al. demonstrated, using ground-based observations, that rate of heating is related to the TI of the surface. Our work tests the applicability of those observations in analyzing grain size variations over a large field area using drone-based thermal imaging. We conducted a field test of both the morning heating and ΔTmax methodologies along the western shoreline of dry Ivanpah Lake, in SE California, which has a variety of sedimentary units of distinct grain sizes from cobble lags to mud flats. Thermal images were taken using a drone-mounted thermal imaging camera. Thirteen drone flights were conducted between pre-dawn and mid-afternoon in order to capture the full morning heating curve and the full extent of ΔTmax values for different grain sizes and surface materials. Diurnal thermal curves were extracted from the thermal images at locations where we collected sediment samples, in order to link rates of morning surface heating to confirmed sediment grain size. Initial results show that comparing the rate of morning heating works as well as comparing ΔTmax for distinguishing between large differences in grain size, which typically exhibit large variations in apparent TI. The values for the rate of morning heating show distinct differences for sandy material, large cobbles, and indurated material.

Sabol, D. et al. (2006). Int. Symp. of Recent Advances in Quant. Remote Sensing.