Shallow-hole (<13 m) temperature measurements made at various depths and/or times may yield reliable values of geothermal gradient and thermal diffusivity if the groundwater table is shallow (a few meters) such that the effect of time-dependent moisture content and physical properties is negligible. Two numerical methods based on nonlinear least-squares curve fitting are derived to remove the effect of annual temperature wave at the ground surface. One method can provide information on the gradient and diffusivity as a function of depth while the other gives average value over the depth interval measured.Experiments were carried in six test holes cased with 2 cm OD PVC pipes in the Salton Sea geothermal field. A set of 5 to 7 thermistors was permanently buried inside the individual pipes with dry sand. Consistent gradient determinations have been obtained with both numerical methods from six monthly observations. By linearly extrapolating the depths to the 100 degrees C and 200 degrees C isotherms from the calculated gradients and mean ground temperatures, we have found good agreement with the nearby deep-well data for four holes. Discrepancy is found for two holes, one of which is located near the field of CO 2 mud volcanoes and the other near the volcanic Red Hill, reflecting complicated local hydrologic conditions.

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