Remanent magnetism as a contributor to some aeromagnetic anomalies

An investigation of some aeromagnetic anomalies in N.-central Montana shows that remanent magnetization is the dominant factor in anomalies over some rocks and a contributing factor in anomalies over others. In volcanic rocks, remanence is commonly S.-seeking down and has an intensity approximately 10 times the induced intensity of magnetization. Remanence is shown to be the dominating factor in anomalies over volcanic rocks by the agreement between the profiles of an observed anomaly over a volcanic rock body and an anomaly calculated from remanence data for the same body. In intrusive rocks, the remanence is N.-seeking down and comparable in intensity to the induced intensity of magnetization. That remanence also contributes to the anomalies over some intrusive rocks is shown by the agreement between the profiles of an observed anomaly over an intrusive body and an anomaly calculated from the resultant of induced and remanent magnetizations. A consistent relationship between the axis of the anomaly, or direction between the anomaly high and low, and the horizontal direction of magnetization suggests that the direction of the anomaly axis can be used to indicate the horizontal direction of magnetization in this area.