The effects of alteration degree, moisture and temperature on laser return intensity for mapping geothermal manifestations

Understanding alteration processes in geothermal systems through remote sensing can help assess the prospectivity of a geothermal field. As a result of which, alteration detection and mapping are vital in the early exploration stage. However, many geothermal surface manifestations, which are evidence of alteration processes, often occur in densely vegetated areas, particularly in tropical regions. Current lithological mapping techniques using passive remote sensing systems can have problems with vegetation canopy completely blocking the signal. LiDAR, an active remote sensing system, can be a solution since it can (partially) penetrate vegetation canopy. It collects information from the ground in 3D-coordinated points with laser return intensity (LRI) values. In this study, we test for the first time the possibility of differentiating hydrothermally altered rocks through their LRI values in a laboratory experiment. We scanned selected altered and unaltered rocks under different moisture and temperature conditions using a terrestrial laser scanner (at 1550 nm wavelength). Our results show LRI values follow the degree of alteration: strongly altered rocks have the highest LRI, unaltered rocks are at the lowest, and weakly altered rocks show an intermediate response. Varying moisture conditions decrease LRI values for all rocks, but the relative LRI order due to alteration remains intact. We recorded no significant temperature effect on LRI values for any of the alteration stages. Our results provide the first evidence that rocks with different hydrothermal alteration phases can be distinguished by their LRI values which opens up the potential for airborne mapping of geothermal surface manifestations with LRI.