Some problems in interpreting low latitude magnetic surveys

Near the earth's magnetic equator, the ambient magnetic field is almost horizontal, is oriented approximately north-south, and has a field intensity of between 25000 and 40000 nT, about one-half the intensity at the magnetic poles. The magnetic equator lies within 10° of the earth's geographic equator. The decreased equatorial field intensity causes local magnetic anomalies at low latitudes to have smaller magnitudes than those produced by similar structures at high latitudes. The horizontally oriented inducing field often creates complicated anomaly pattems that can be difficult to interpret, particularly for European or North American geoscientists whose experience may derive exclusively from latitudes above 40°. Furthermore, the north-south orientation of the horizontal inducing field means that a long north-south striking magnetic structure may show no anomaly at all, except at the north and south termination of the structure. In this abstract we illustrate using real data and computer models some of the complexities involved in interpreting low latitude magnetic survey data collected over human-made structures commonly encountered in surveys undertaken for environmental or engineering purposes.