Three‐dimensional interpretation of the KTB gravity and magnetic anomalies

Abstract

A combination of a closely spaced surface gravity survey with a high‐resolution helicopter aeromagnetic survey as well as borehole gravity and 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 structural geology and petrophysical data from drill cores and cuttings. The source body for the positive gravity anomaly consist of high‐density metabasite. The vertical derivative of the Bouguer anomaly does not resemble the aeromagnetic anomaly in all areas, indicating that parts of the metabasites are more or less nonmagnetic. Surprisingly and confirming the observation in other deep drill holes into continental crystalline 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 vertical gradient of 60 nT/km for the geomagnetic field. The occurrence of strongly magnetic minerals in the borehole down to about 3000 m correlates with the lithology, while in the deeper parts it is more related to fissures and fault zones where chemical processes (reduction/oxidation) are active.