Improvements in Shallow (Two-Meter) Temperature Measurements and Data Interpretation

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Conference Paper: Improvements in Shallow (Two-Meter) Temperature Measurements and Data Interpretation

The Great Basin Center for Geothermal Energy has been working on improvements in shallow (two-meter) temperature surveys in two areas: overcoming limitations posed by difficult ground conditions with the use of a portable rock drill, and improvements in temperature measurements and interpretations Previous 2-meter temperature surveys conducted by the Great Basin Center for Geothermal Energy have been limited to areas that were not excessively rocky. This limitation has been overcome by the use of a self-contained gas powered pavement breaker/rock drill capable of penetrating extremely rocky ground including bedrock. With the use of the rock drill completion of approximately 10 temperature probe holes in an 8 hour field day is possible. Large variations in surface albedo (reflectivity of a surface to sunlight) can cause differences in the temperatures measured at a depth of 2 meters of up to several degrees centigrade. A fieldbased method of correcting two-meter temperatures for the effects of albedo was utilized at Columbus Marsh, Nevada. ASTER satellite imagery was used to independently estimate albedo so that solar heating effects could be filtered from the temperature data, with a resultant noticeable improvement in the definition of geothermal anomalies. To better understand factors effecting shallow (two-meter) temperature measurements and improve the ability to predict thermal anomalies, long term monitoring of a number of sites at Desert Queen, Nevada were conducted in background and high heat flow areas to study seasonal effects on temperature at two meters. Sites were also selected to study influences on shallow temperature measurements related to geological and solar radiation factors specifically, slope orientation, ground composition and albedo. In addition to measuring a single temperature at a depth of two meters, multiple sensors were used to measure temperatures at 1, 1.5 and 2 meter depths for calculation of shallow temperature gradients. Results of the long term studies indicate that differences between thermal anomalies and background areas are smaller during the coldest part of the seasonal cycle which at a two meter depth is approximately two months after the coldest average air temperature (February- March). Nevertheless, the difference between thermal anomalies and background areas is roughly proportional throughout the year. The effects of albedo appear to be seasonally dependent and have less influence in the colder parts of the season. The slope orientation, north verses south facing slope, displays expected trends of southern exposures being warmer than northern. Measurements of shallow gradients were found to be useful in distinguishing thermal anomalies from background areas and should help remove solar radiation effects. The shallow temperature gradient in background areas is lower than in thermally anomalous areas. The gradient of background areas is also more likely to be negative (decreasing with depth) during the spring and summer as the sun heats the upper part of the soil profile. The temperature gradient profile is useful for discriminating anomalous zones from background areas when the study areas are more topographically complex and corrections for solar radiation factors such as albedo and slope orientation become important.

Chris Sladek, Mark F. Coolbaugh and Christopher Kratt

GRC Annual Meeting; Reno, NV; 2009/10/04

Geothermal Resources Council, 2009

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Chris Sladek,Mark F. Coolbaugh,Christopher Kratt. 2009. Improvements in Shallow (Two-Meter) Temperature Measurements and Data Interpretation. In: Transactions. GRC Annual Meeting; 2009/10/04; Reno, NV. Davis, CA: Geothermal Resources Council; p. 535–541