The Marmara region of northwestern Turkey is characterized by frequent manifestations of anomalously high temperature gradients in the underground, thermal springs, and steaming grounds. Within an interdisciplinary project, our research group measured temperature profiles in many drillholes in this region, using a digital temperature logging tool. Temperatures are measured by a PT 100 sensor; the 2 km wireline and its winch are mounted on a trailer; the data acquisition system is freely movable, weighs about 40 kg, and is transported by jeep along with a 4.7 kW power generator. This system consists of a controlling unit for the winch drive, a high-precision electrical resistance meter with a sensitivity of 100 ppm Ohm and a 286 MS-DOS PC as the recording unit. All parts of the equipment are commercially available. The depth interval between two measurements is 10 cm and maximum logging speed is 5 m/min. Using this temperature tool we were able to detect convection cells in a borehole in northern Switzerland. These cells, about 1 m in length, have a temperature amplitude range of about 0.01-0.04 degrees C. Repeated measurements of boreholes in Switzerland and in northwestern Turkey show, for the entire length of the wells, a maximum difference of 0.05-0.1 degrees C. The measured temperature profiles show different distortions (nonlinearity) as a result of the strong influence of groundwater circulation. The observations are checked for constancy in time (repeated measurements after one year). Using different models of groundwater movement, local heat flow can be calculated even from such disturbed temperature logs. These models are presented and discussed with respect to their reliability.