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Description
The potential represented by areas of accessible geothermal energy is now being actively explored. The method of exploration employed at present requires the expenditure of large sums of money and damages the environment of the area under investigation. The development of a less expensive and nondamaging passive heat flow measuring system is desirable. Heat flow is measured by determining the difference in temperature across a thickness of material and determining the thermal conductivity of that material. In order to measure heat flow in the earth, a test hole must be drilled to allow measurement of the temperature gradient and to obtain a core sample for laboratory thermal conductivity measurements. By constructing an instrument of known thermal conductivity and with the capability of sensitive differential temperature measurement across itself, it would be possible to measure heat flow out of the earth. Thermocouples, connected in series and alternating between the top and bottom of the instrument, can provide a sensitive differential temperature measurement device called a thermopile. Since the heat flow out of the earth (q) is proportional to the earth's thermal conductivity (Ke) and the temperature difference (dt) e over a given thickness (dz) or q/unit area = Ke (dt/dz) and the output of a thermopile (Etp) is proportional to the temperature difference (dt) across the instrument and the thermoelectric power of the system (e) (where n is the number of thermocouple junctions) or Etp = e • n • (dt) then, by adding the instrument to the earth's system, the thermopile output is directly proportional to the earth's heat flow; the only thing that changes is dt. Instruments based on this principle were manufactured for this project and were tested in two envirionments. Punta Banda, Ensenada, Mexico, has an area of hot springs which provided an area of high heat flow for initial testing and instrument handling practice. The heat flow in this area was easily measurable; however, results were not replicable due to ground water fluctuation. The instrument sensitivity is one millivolt of output per one and one-half heat flow units (_cal/ cm2-sec.). Adjustments to the transducers were made to damp out the wind's effects and testing was begun in the Imperial Valley at the Mesa Geothermal Anomaly east of Holtville, California. In this area of more subtle heat flow variation, it was discovered that the non-replicability of measurements between two locations was due to small microclimate and time constant differences which could not be avoided. The solution to this problem will either be the statistical handling of large numbers of data points in order to average out the microclimate effects, or the possibility of artificially "burying" the transducers (to a level at which the diurnal effects are damped out) by covering them with insulation equivalent to the insulating parameters of the desired thickness (depth) of soil.
