The distortion of magnetic anomalies near the magnetic equator, where the earth field vector is close to horizontal, is not well understood in all corners of the exploration industry. Unfortunately, a proper correction for anomaly distortion has not been adopted fully. Therefore, many forms of magnetic interpretation ignore or disguise the problem, causing a misleading picture of low-latitude magnetic-line profile or map displays.
Unsatisfactory applications of magnetic surveys in low-latitude areas are particularly disappointing because much regional geologic and tectonic work needs to be per formed in the band near the magnetic equator, where low-cost, easy-access aeromagnetic data should be used. Although forward modeling of magnetic profiles and map grids can be performed with relative success, many of the most common interpretation tools, such as map filtering with color and shaded relief displays and depth estimation by Werner and Euler (particularly 3-D Euler) techniques, are compromised or totally misleading. Figure 1 is a 3-D block model computed at the magnetic pole. Anomaly gradients correspond to block edges or faults. If a color display cannot be created where highs are over the targets and linear trends are above faults, irrespective of strike, the value of magnetic data is lost. Magnetic results must be transformed to appear the same, no matter what the latitude. Otherwise, geologists and seismologists rightfully will continue to eye magnetic data with a certain amount of suspicion.
Proper low-latitude reduction-to-the-pole (RTP) really is a two-stage operation. First, the magnetic grid must be extended carefully, and then a frequencydomain filter can be applied.