Abstract
The heat-flow studies program has evolved from one in which holes drilled for other purposes (mining and oil exploration, nuclear tests, hydrologic studies, etc.) provided the bulk of the data to a program in which the free holes, while still providing cost-effective and useful data, are being supplemented increasingly by holes drilled specifically for heat-flow determinations at locations where thermal data of high quality are needed, and where nobody else is interested in drilling. Ideally, heat-flow holes should be located in areas with moderate local relief and should be completed so that vertical water movement is inhibited. The most satisfactory test media for heat-flow determinations are crystalline rocks (particularly granites) and unconsolidated sediments; carbonate rocks and volcanic terranes can provide useful heat-flow data, but they present greater challenges both in drilling and interpretation. Drilling techniques have evolved from that of the continuously cored diamond-drilled hole (adapted from mining exploration) to adaptations of the shot-hole and blast-hole techniques used in petroleum exploration, water-well construction, and quarry operations. Spot cores are obtained where necessary to provide specific petrologic, geochemical, and physical data, but primary reliance is placed on ditch samples from rotary or percussion drilling for routine measurements of thermal conductivity andmore » heat production. In shallow (50 to 100 m) holes in low temperature environments, plastic casing is used to maintain access for later temperature measurements. For deeper holes, steel casing is preferred. The annulus between casing and borehole wall in the lowermost 50 to 80 meters of heat-flow holes is routinely grouted off with a specially designed mixture of cement, bentonite, salt, and water to prevent vertical water movement.« less
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