| Abstrak/Abstract |
Remote detection of hydrothermally altered grounds in geothermal exploration becomes challenging when these altered grounds are situated in densely vegetated areas (e.g. tropical regions). Traditional remote detection using spaceborne imagery often fails to detect these altered grounds through gaps in the canopy due to the dataset's limited spatial resolution. Airborne LiDAR can be an alternative source of information for their detection from its laser return intensity (LRI) values. The link between alteration degree and LRI values has been demonstrated under laboratory conditions but is yet to be shown on airborne data. In this work, we investigate the applicability of our laboratory results to airborne LRI data for alteration detection. We use an airborne LiDAR dataset with a point density of at least 12 points per square meter. The dataset was acquired in the early dry season by a Leica ALS70 scanner (at 1064 nm) from a nominal 1000 m above ground level over the same area in Bajawa, Indonesia, where rock sampling for the laboratory study occurred. We compare the airborne LRI values within a 3-m radius (due to hand-held GPS uncertainty) of each ground sampling location with the laboratory LRI values of the corresponding rock sample. Our results show that the trends of the airborne and laboratory-based LRI data in relation to the alteration intensity are strikingly similar, although the data were acquired by LiDAR devices with different wavelengths, and the airborne LRI values were not corrected to absolute ground reflectance. While the link between alteration intensity and airborne LRI is strong for the sample locations, the airborne LRI patterns become less predictive of alteration when we look at the entire area. Our investigation shows that other parameters than alteration alone influence the LRI values of soil surface returns, such as varying topographic [removed]i.e. slope and aspect), soil moisture, and vegetation. These influences create ambiguity around the link between alteration degree and LRI values, which needs further investigations to quantify and ultimately eliminate them. Although additional work on controlling and reducing these variables is required, the consistent LRI trend for alteration degree presented here opens up the possibility of LRI-based alteration mapping from airborne platforms. |
