3-DIMENSIONAL INTERPRETATION OF THE KTB-GRAVITY AND MAGNETIC-ANOMALIES

A combination of a closely spaced surface gravity survey with a high-r esolution helicopter aeromagnetic survey as well as borehole gravity a nd magnetometer measurements allowed a detailed three-dimensional (3-D ) modeling of the anomalies at the KTB drill site. The models could be constrained by new evidence from a 3-D seismic survey and by structur al geology and petrophysical data from drill cores and cuttings. The s ource body for the positive gravity anomaly consist of high-density me tabasite. The vertical derivative of the Bouguer anomaly does not rese mble the aeromagnetic anomaly in all areas, indicating that parts of t he metabasites are more or less nonmagnetic. Surprisingly and confirmi ng the observation in other deep drill holes into continental crystall ine basement rocks, pyrrhotite is the dominant magnetic mineral below a depth of about 300 m. Magnetite mainly occurs in the depth intervals 360-520 m and 7300-7900 m. The lower interval causes the anomalous ve rtical gradient of 60 nT/km for the geomagnetic field. The occurrence of strongly magnetic minerals in the borehole down to about 3000 m cor relates with the lithology, while in the deeper parts it is more relat ed to fissures and fault zones where chemical processes (reduction/oxi dation) are active.