Abstract
In the 1977 and 1979 geophysical expeditions to the active volcanoes Nyiragongo and Nyamuragira in the Republic of Zaire, detailed geomagnetic surveys were carried out along several routes with about 1 km spacing in and around the volcanoes. Total intensity of the geomagnetic field was measured at about 310 sites by a proton-precession magnetometer. After subtracting the international geomagnetic reference field (IGRF), geomagnetic anomalies were separated into the local anomaly and the regional anomaly which was approximated by two dimensional second degree polinominal. Finally smoothed local geomagnetic anomalies were obtained by the method of two dimensional running mean. These anomalies could be interpreted by geological features such as dykes, lava or rocks at higher or lower temperature than the Curie point, or terrain effects. Local geomagnetic anomalies suggested that the shallow thermally demagnetized zones are near in the sites of recent eruption at Mihaga (erupted in 1954), Murara (1976) and Harakandi (1977) and that the shallow more strongly magnetized dykes exist at the northern part of Goma and the eastern side of Nyiragongo. The thermally demagnetized zones run parallel with the rift axis in this region and also coinside with the regions of volcanic tremors with a longer period and large amplitudes.
