Browsing by Author "Montonen, Markku"

Magnetic properties of Kevitsa intrusion are studied in this thesis. The goal is to find a reason for a negative magnetic anomaly observed in the center of dunite layer of the Kevitsa intrusion. This thesis concentrates mainly on petrophysical measurements made from drill hole samples, but also previous geophysical measurements from the study area are used. The first approximation is that the negative magnetic anomaly is caused by a strong remanent magnetization which has a direction that opposes geomagnetic field and induced magnetization. This kind of remanent magnetization is most likely formed during a reversed polarity period of geomagnetic field. An approximately 55 meters thick layer of strong remanent magnetization can be observed in drill hole KV297, which is located on the negative surface-magnetic anomaly. The average Königsberger ratio in the samples from this layer is 4.2 and the average inclination of remanent magnetization is -42.4 degrees. Declination of remanent magnetization stays unknown, because none of the samples were directional. One can also observe two distinct magnetic layers in the dunite layer of drill hole KV297. The lower one of these magnetic layers extends to approximately 160 meters. Both of these layers are strongly magnetized, but only the upper one has a strong remanent magnetization. In drill hole KV200 one cannot observe a strong remanent magnetization, but this drill hole is not located on the negative surface-magnetic anomaly either. However, the samples from the drill hole KV200 have a strong magnetic susceptibility. Based on these observations one can conclude that the dunite layer of the Kevitsa intrusion is overall strongly magnetic, but only the center part of it has a strong remanent magnetization. This layer of strong remanent magnetization is the cause of negative magnetic anomaly observed on the surface. The direct mathematical models, which are based on the measured petrophysical data, are consistent with the magnetic anomaly observed on the surface. In the models the declination of remanent magnetization is set to 240 degrees.