Empirical observations relating near‐surface magnetic anomalies to high‐frequency seismic data and Landsat data in eastern Sheridan County, Montana

Abstract

Our empirical synergistic correlations of aeromagnetic and seismic data and a Landsat lineament interpretation revealed lineations on the magnetic map that have expression on seismic sections. We observed a conjugate set of northwest‐southeast and northeast‐southwest trending magnetic lineaments (zones which offset and truncate near‐surface magnetic anomalies). We believe these OZs (offset zones) represent lateral faults in a wrench‐fault system. Lateral offsets appear to be 100s of meters to a few kilometers (fractions of a mile to a few miles). We observed a direct correlation between OZs and vertical faults in seismic data. Faults on seismic sections extend from near the surface to near the seismic basement. The faults are most pronounced in the Upper Cretaceous reflectors and seem to disappear with depth. Fault throws are inconsistent (reversing throw across faults). OZs trend northeast‐southwest in the north half of the study area and both northeast‐southwest and northwest‐southeast in the south half. The OZ direction of northeast‐southwest in the north half of the survey is confirmed with seismic data. The northwest‐southeast seismic line contains numerous faults and the northeast‐southwest seismic line contains few faults. Most northeast‐southwest faults do not appear to reach seismic basement and are not seen in an interpretation of the magnetic basement. In two cases, northwest‐southeast OZs and correlative Landsat lineaments coincide with mapped magnetic basement faults. These magnetic basement faults can be seen in seismic data too. Faults trending northwest‐southeast may represent Precambrian faults reactivated during the Laramide Orogeny. Movement along these faults possibly generated the northeast‐southwest faults. Most oil fields have an associated near‐surface magnetic anomaly. Other near‐surface magnetic anomalies occur over obvious, untested (in 1985), seismic character or amplitude anomalies in seismic events which correlate with producing intervals in the oil fields. This synergistic correlation is the most important single observation from our study. Different data types and interpretation techniques identified the same geologic trends and prospective geographical areas. This fundamentally important information is often lost in bickering over which filter or processing technique to use or in arguments over which data type is “more important” than others. Further, if the synergistic correlation of data types were not done, the importance of the anomalous features in each individual data type may not have been recognized.