WELL LOGGING

field technique used in mineral exploration to analyze the geologic formations penetrated by a drill hole. When electrical instruments are employed, the technique involves the recording of variations in the natural electrical properties of subsurface formations. Changes in the porosity, fluid content, or lithology (rock character) of the formation are indicated as variations in resistivity or conductivity. A driller's log records occurrences during drilling of such factors as the pump pressure, the action of the tools, and the rate of drilling. In core logging, samples of the core are extracted from the drill hole and subjected to physical and chemical tests. Borehole camera logging is employed when the condition of the rock strata must be analyzed before major drilling can be undertaken. In this method a camera is lowered into a pilot hole. The camera can take either black-and-white or colour photographs or can transmit television images to a monitor on the surface, where they are then photographed. Radioactivity logging is the method in which natural or induced radioactivity in subsurface formations is measured and recorded. Variations in the radioactive gamma rays emitted from formations reveal the character of the rock because of the presence of uranium, thorium, or potassium minerals. Visual wall logging, as the name implies, is the actual examination of the wall of the hole by a geologist. Cuttings-analysis logging involves cuttings recovered from the drilling fluid; when combined with other logging methods, it can furnish valuable data.