Cost-effective Vegetation Anomaly Mapping for Geothermal Exploration

Vegetation anomalies are sometimes found over hydrothermal convection systems, where they form because of mineralization, anomalous soil gas, heat, and/or other abnormal conditions. These anomalies are often difficult to delineate without expensive biogeochemical or hyperspectral surveys. This study uses inexpensive Landsat Thematic Mapper (TM) data to delineate vegetation anomalies potentially related to a hydrothermal convection system. Past work by the authors has shown that vegetation anomalies can be affected by seasonality and that plant stress signals tend to increase toward the end of the growing season. Therefore, inexpensive multitemporal TM data were analyzed to determine if anomalies could be mapped based upon this assumption. Two data sets were acquired in early and late summer 1986 for this purpose. These data were processed to determine the amount of vegetation change over this period, the logic being that the areas showing the greatest change would be those stressed beyond normal seasonal expectations. To accomplish this, the TM data were (1) georectifed and coregistered, (2) processed to remove haze, (3) used to produce vegetation indices, and then (4) used to determine the relative change in vegetation through time. The results clearly delineated several areas showing change beyond what one would expect from normal seasonal variation. One of the anomalies was near the Bonnett Geothermal Power Plant at Sulphurdale, Utah. Field checking several other areas, mapped as anomalies, revealed over-grazing by cattle to be the probable causal agent. Although this method makes no distinction between causal agents of plant stress, its low cost, ease of use, and usefulness in limiting the extent of targets, especially when used with ancillary exploration data, can make it useful for reconnaissance exploration programs.