Use of airborne thermal imaging to quantify heat flux and flow rate of surface geothermal fluids at Pilgrim Hot Springs, Alaska

Measuring the temperature and flow rate of hot springs is important for quantifying convective heat losses from low to moderate temperature geothermal systems. When coupled with sub-surface measurements of convective and conductive heat flow (e.g. with shallow-temperature probes, temperature gradient drilling), total surface heat losses from a geothermal system may be estimated, which can be used in assessment of undeveloped resources or monitoring of existing developments. A novel approach to estimating the heat flux and flow rate of surface geothermal waters using airborne thermal imagery is presented that has been applied and validated at Pilgrim Hot Springs near Nome in Alaska. Over this study site thermal imagery was acquired using a broadband FLIR camera during two airborne surveys undertaken in fall 2010 and early spring 2011. Calibrated surface temperature data for surface hot springs and pools was input into a thermal budget model that calculates the geothermal heat input on a pixel-by-pixel basis by accounting for radiative, evaporative/sensible, short-wave solar, and long-wave atmospheric heat fluxes. The sum of geothermal heat fluxes for geothermal waters was used to estimate the outflow rate of hot springs based upon values of the specific enthalpy of water at the average spring temperature (81°C, derived from field measurements) and average temperature of non-geothermal surface waters (measured from the thermal images). The results of applying these methods to fall and spring airborne thermal data from Pilgrim Hot Springs yield geothermal heat fluxes supporting surface geothermal fluids of 3.28 MW, and 3.80 MW respectively that correspond to outflow rates from hot springs of 192.39 and 198.04 gpm (gallons per minute). These estimates are considerably higher than previous field based results (~2 MW or 100 gpm) derived from direct measurement of the flow rate of the main hot pool at the site. This outcome demonstrates the value of the synoptic coverage of airborne thermal imaging that can map all sources of hot fluids including small lower temperature springs and diffuse outflow. This work has demonstrated the potential of airborne thermal imaging for providing estimates of heat flux associated with hot spring outflow in a rapid and repeatable manner.