Airborne Hyperspectral Imaging for Monitoring Geothermal Activity Through Vegetation

Remote sensing techniques are used to explore geothermal areas. They can offer spatial, temporal and spectral information to map lithological boundaries and hydrothermal alteration in a fast and cheap manner. However, some geothermal areas are densely covered by vegetation, which can hamper remote sensing monitor efforts for geothermal areas.

Vegetation cover in geothermal areas can reflect the subsurface activity, reacting to interactions between soil’s chemical conditions, heat and gas emissions. An example of such is kanuka (i.e. kunzea ericoides), an endemic shrub of geothermal areas in the Taupo Volcanic Zone (TVZ), New Zealand, which has been used as an indicator species for ground-based geothermal studies. This study assesses the use of airborne hyperspectral and thermal data over the Waiotapu Geothermal Field, TVZ, New Zealand, analysing kanuka shrub surface cover and its spectral response to geothermal activity. To explore the capability in hyperspectral remote sensing for geothermal site mapping and exploration, a series of vegetation indices, including; Anthocyanin Reflectance Index, Atmospherically Resistant Vegetation Index, Moisture Stress Index, Normalised Difference Vegetation Index, Simple Ratio Index, Vogelmann Index and Water Band Index were calculated from narrow bandwidth high-resolution hyperspectral.

The spectral response of vegetation was then analysed to explore the effects of geothermal heat, offering surrogate information on vegetation health. Vegetation indices results were compared against the thermal infrared data by visual interpretation and quantitative analyses, which shows strong spatial correlation among the vegetation cover type and heat distribution. Furthermore, exponential trendlines produced the best fit between vegetation indices and thermal infrared data. This correlation indicates soil temperatures affect the vegetation health (e.g. chlorophyll concentrations, newly forming leaves, water content). This relationship can highlight that there is valuable information in airborne hyperspectral data to complement exploration efforts, such as heat flux mapping. We conclude kanuka shrub has the potential to be employed as a proxy in exploration and monitoring of geothermal areas in New Zealand from remote sensing platforms.