Remote detection of geothermal alteration from LiDAR return intensity data

Traditionally, remote detection of geothermally altered ground relied on optical imagery. The images' coarse spatial resolution often fails to detect key altered ground situated in densely vegetated areas. Light detection and ranging (LiDAR) can be an alternative to help their detection with the Laser Return Intensity (LRI). In this study, we investigate the detection of hydrothermal alteration using LRI values by comparing the results of fieldwork, laboratory, and airborne survey. We took rock samples from the geothermal field of Bajawa, Flores Island, Indonesia. These samples were scanned using a terrestrial laser scanner (at 1550 nm) under controlled moisture and temperature conditions. Our laboratory results show a pattern that strongly altered rocks can be distinguished by their higher LRI from unaltered rocks, even in different conditions. Next, we used the coordinates of the lab samples to select LiDAR points from an airborne dataset (at 1064 nm). We calculated the average of LRI values within the radius of 3 meters for every sample point (due to the accuracy of the hand-held GPS). Although the two scanner systems use different wavelengths and the airborne system is not corrected for absolute ground reflectance, the trends of the LRI values in the terrestrial and airborne datasets are strikingly similar. However, if the field sampling area increases, the variability of LRI values will also increase and become less separable. Although careful processing is required, the airborne LRI values can detect the alteration. Thus, it opens the possibility of LRI application for airborne hydrothermal alteration mapping.