GEOTHERMAL INVESTIGATIONS IN BRAZIL

Abstract

A georhermal project was initiated in 1975 to delineate the regional distribution of geothermal gradients and terrestrial heat flow and, at the same time, to provide information related to such problems as underground mine ventilation, and movements of subsurface water. Borehole and mine temperature measurements were made with a high sensirivity electrical resistance thermometer (thermistor), attached by cable to a Wheatstone resistance bridge. Temperature disturbances due to drilling operations, variation in surface temperature, undergiound air and water movements,topographic relief, and complex geological structures are illustrated and evaluated. Temperature gradients are reported for 85 boreholes and two underground mines, located in t-9 different geographic sites of the Precambrian shield of Brazil, the Phanerozoic Parana Basin, and the paleocene alkaline intrusion of Pocos de Caldas. Individual gradients range from 6°C/km, in the early Proterozoic quartzites of Jacobina (Bahia), to 42°C/km obtained in the southern portion of the Parana Basin (Butia, R.G.S.). There is an increasing gradient pattern for decreasing tectonic age, from the low average gradients of 11-12°C/km for the Transamazonic (2600-1800 Ma) and Espinhaco (1800-1300 Ma) folding, l6°C/km for the Brazilian folding (900-550 Ma), to the strikingly-higher 28°C/km for the sediments of the Parana Basin, and 30°C/km for the magmatism of Pocos de Caldas. Heat flow determinations will be made at each site following the measurements of the thermal conductivity of borehole cores.