VNIR-SWIR infrared (imaging) spectroscopy for geothermal exploration: Current status and future directions

Infrared spectroscopy (IRS) is a method used to identify minerals based on their spectral response to infrared light. Spot-based IRS (also known as ‘reflectance spectroscopy’) has frequently been used as an analytical method by the geothermal industry, while infrared imaging spectroscopy (IRIS) has only recently been introduced. Research applying IRIS to the geothermal industry is still limited to academic trials, but sufficient progress has been made in algorithm development and application cases to make this technology ready for uptake by the geothermal industry. In contrast, the mineral exploration industry has embraced IRIS for a number of years and is driving its development forward. In this paper, we review the work that has been done in the geothermal industry with spot-based and imaging IRS, as well as review relevant examples from the mineral exploration realm to look for pathfinders for future uses of the method within the geothermal industry. The review focuses on the application of visible to near infrared (VNIR) and short-wave infrared (SWIR) spectroscopy, as the application of long-wave infrared (LWIR) spectroscopy in the geothermal industry is still very limited. To cater for a wide range of audiences, we will explain the background of infrared spectroscopy as well as the commonly used geothermal index minerals and analytical techniques typically employed by the geothermal industry. Our review shows that IRS has a higher sensitivity in identifying kaolinite (along with its degree of crystallinity), ammonium-bearing minerals, and chemical variations of (spectrally-active) minerals compared to other methods. The ability of IRIS to obtain spectra with high spatial resolution enables the method to identify: (1) less common minerals, and (2) the potential to distinguish smectite and illite formed as interlayered minerals from those formed as two different grains. The latter identification typically cannot be done using spot-based IRS. Other parameters that can only be seen in IRIS include spatial relationships amongst minerals and more robust mineral abundance estimates. The mineral exploration industry has demonstrated that applying IRIS can go beyond just identifying minerals. It can also successfully identify host lithologies on intensely altered rocks, as well as identify and extract the position of veins and veinlets. The latter information is important for geothermal exploration, particularly to indicate permeability and cross-cutting relationships amongst alteration minerals. The IRIS applications that have been demonstrated by the mineral exploration community potentially represent the future trend in the geothermal industry. Spot-based IRS has already proven its added value in assisting geothermal exploration and exploitation. With the latest instruments and algorithm developments in place, imaging IRS is now on the brink of demonstrating its value to the geothermal industry.