Interpretation of Low Latitude Magnetic Anomalies
Magnetic data collected near the earth's magnetic equator is more difficult to interpret than high latitude magnetic data because tha magnetic field intensity at the equator is smaller and because the ambient inducing field direction is horizontal. The low field strength procedures correspondingly smaller anomalies, and the shallow field inclination produces several complications. Anomalies over magnetically susceptible bodies tend to be negative instead of positive, and the anomaly patterns may vary considerably with structural azimuth. Long north-south striking structures such as dikes and contacts can be magnetically invisible, except where the structures are broken or termionate. Equatorial anomalies tend to be stretched in an east-west direction. These latter two conditions can create the illusion of an east-west structural strike, regardless of the true geological strike. Synthetic modeling of magnetic fields produced by simple representations of geologically relevant structures such as ore bodies, pipes, dikes, folds, faults and contacts help the interpreter to better understand the anomaly patterns present in real data. When magnetic remanence is dominant and the remanent field inclination is steep, it may have a strong affect on anomaly patterns. A shallow remanence inclination tends to rotate the induction-only anomaly pattern. Filters of various types - reduction-to-pole, vertical derivates, analytic signal - can enhance interpretation of low latitude data, but all suffer imitations, especially when confronted with north-south striking structures.