Abstract

We are investigating a new airborne method for measuring surface temperatures that may be useful for identifying thermal anomalies of geologic origin. From Planck's equation we derive the valuable approximation that, for small temperature variations, the radiant emittance is proportional to the emissivity times the absolute temperature to the power of (50/wavelength in ¼m). From this, expressions are obtained for the emitted infrared (ir) radiation measured simultaneously in the 5 and 10 ¼m bands. Ratios of these expressions are shown to have the following useful properties at 288 K: (a) they are insensitive to surface emissivity variations for vegetated terrain, (b) they vary nearly as the 5th power of the surface temperature, and (c) they distinguish emissivity–related from temperature–related effects. We have made preliminary tests of this methodology at a field site in Scipio Center, New York. We have characterized the observed surface temperature variations, the significant effects of soil moisture, and separated out the purely emissivity–related features of vegetated terrain. Cluster analysis served to divide the ir data into groups that behave similarly as a function of the measured soil moisture. Two such distinct terrain groups were identified at the field site. The ir data were corrected for: (a) natural surface emissivity variations, (b) the intervening atmospheric path, and (c) the reflected sky radiation. The corrected surface temperature data were compared with calculated values computed from a model that simulates the surface temperature, using meteorological, hydrological, topographical, and soil thermal input parameters. The simulated mean surface temperatures, 291.9 K (group 1) and 291.6 K (group 2), differed only by, respectively, 0.0 K and 0.1 K from the measured mean surface temperatures. Our preliminary results suggest the potential for developing a new airborne geophysical method for isolating abnormal heat flows. Weak heat flows, about 10–20 times the terrestrial average, have the effect of raising the surface temperature about 0.1–0.2 K. These temperature anomalies would, with the methodology suggested, appear as a residual difference between the measured (corrected) surface temperature and the simulated surface temperature. Such surface temperature differences appear, from our research, to be measurable by airborne ir scanners when data over surface areas of 0.1km2 or larger are averaged. Accordingly, our research appears to support the conclusion that surface temperature enhancements of geophysical origin between 0.1 and 0.2 K can be identified using airborne infrared methods.

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