Abstract
By removing the effect of the annual temperature cycle with an iterative least-squares method, geothermal gradients at 44 sites were determined from onshore and offshore temperature measurements at depths less than 16 and 2 m, respectively, around the southeastern end of the Salton Sea (water depth <20 m). Onshore temperatures were measured at 1-m intervals in drill holes cased with 0.25-inch PVC pipes and offshore temperatures were obtained with a 2-m lance-like thermal probe (0.50 inch OD) which contains 7 equally spaced thermistor sensors. The gradients range from 0.02 degrees to 1.7 degrees C/m and from 0.72 degrees to 4.3 degrees C/m in the onshore and offshore areas, respectively. Heat fluxes, calculated from an assumed value of heat capacity, and in-situ determined thermal diffusivities and gradients, decrease away from a chain of four Quaternary volcanic domes, near the present shoreline. Superimposed on this general trend are several sites with high heat flux, reflecting local hydrological influence. Corrections for sedimentation, for presence of the lake, or for disturbance around the thermal probe may individually exceed 0.030 degrees C/m (normal geothermal gradient of the upper crust); however, the net effects cannot account for the disparity between the onshore and offshore data sets. Even if the offshore gradients are divided by a factor of 3 to make the two sets of data compatible, these new data (together with data from 22 onshore sites measured previously) indicate the offshore anomaly covers an area at least the size of the onshore anomaly, defined somewhat arbitrarily as an area with heat flux greater than 200 X 10 (super -3) W/m 2 .
