Geothermal measurements in deep-sea drill holes

Abstract

A method of measuring the in situ sediment temperatures in deep bore holes drilled to depths of several hundred meters or more beneath the sea floor has been developed. The technique, as presently used aboard the Deep-Sea Drilling Project (DSDP) drilling vessel Glomar Challenger, involves the emplacement of a temperature sensor, located below a self-contained digital temperature recorder package, a short distance into the undrilled, thermally undisturbed sediment at the bottom of the drill hole. By measuring the in situ temperature at various depths in a single drill hole it is possible to calculate the thermal gradient for various intervals in the hole. This information, in conjunction with thermal conductivity data measured aboard ship on the sediment cores recovered from the drill hole, permits computation of the heat flow through the oceanic crust. Heat flow values measured in deep drill holes in the Indian and Pacific oceans and in the Bering and Red seas are in generally good agreement with the regional geothermal flux as determined by conventional near-surface heat flow measurements, suggesting that the thousands of existent shallow heat flow values are representative of the earth's heat flux. Where multiple downhole temperature measurements made at one site permit calculation of interval heat flow values, there is no consistent indication of a significant vertical increase or decrease in heat flux, such as might be caused by long-term changes in bottom water temperature or the upward migration of interstitial fluids. We note, however, that a more detailed set of temperature measurements in a single hole is required to verify this conclusion. Downhole heat flow values made within a specific physiographic region, such as the Red Sea or the Ninety East ridge, appear to be less variable than, but equal to, the heat flow values calculated using thermal gradient measurements made at shallower depths beneath the sea floor. This observation is in accordance with theoretical considerations which indicate that temperature measurements in deep drill holes are less susceptible than conventional heat flow measurements to the disturbing thermal effects of small-scale surface topography, short-term variations in bottom water temperatures, and local sedimentary processes (slumping, erosion).