We deployed six Heat Needle probes in and around the Los Azufres geothermal field in Michoacán, Mexico, over a six-month period from September 2014 to March 2015, to assess their functionality for geothermal surveying. The trial was part of Project 23 of the Mexican Center for Geothermal Energy Innovation (CeMIE-Geo): Testing probes for measuring shallow heat flow in geothermal zones. Los Azufres is the second largest and oldest geothermal field in Mexico, with an installed geothermal electric capacity of 247.8 MWe. The geothermal field was chosen for this trial because its underlying hydrothermal system is reasonably well understood. Five Heat Needles were distributed across the geothermal field at spacings between one and four kilometers, with a sixth Heat Needle placed in a location of assumed background heat flow outside the geothermal field. Each Heat Needle precisely recorded thermal gradient in the top 1.10 m of soil at 15-minute intervals for the duration of the trial, and made a single measurement of the thermal conductivity profile of the soil. Combining the thermal gradient records with the thermal conductivity measurements showed the ebb and flow of surface conductive heat flow due to insolation and re-radiation at each site to an absolute precision of about ± 0.02 W/m2. As expected, the observed heat flow pulsed rhythmically each 24-hour period. After filtering the diurnal signal by taking the average of each interval of 96 consecutive records, the resulting mean daily heat flow records drifted by at least 3 W/m2 at each site over the six-month survey, mostly due to the seasonal surface temperature cycle. Fluctuations of 1–2 W/m2 associated with the passage of weather systems were obvious over periods of several weeks. Many of the weather-related fluctuations, however, were strongly correlated between Heat Needle sites, with a detectable mean offset between sites attributed to variations in subsurface heat sources. Rudimentary data processing defined (within a range of ± 0.5 W/m2) positive offsets in the subsurface component of heat flow at four of the sites relative to the background site. Heat flow at the fifth site was not significantly higher than background at the achieved level of precision. The heat flow detected by the Heat Needles and attributed to subsurface sources was comparable to estimates of heat flow from deep geothermal wells in the field. Heat Needles show great potential for delineating shallow thermal features such as outflow pathways or permeable fractures, constraining the total heat flux from a geothermal system, or constraining the lateral extent of deep thermal anomalies with carefully designed surveys.

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