New computer techniques permit the calculation of AFMAG and VLF anomalies in the vertical magnetic field component for situations which cannot be conveniently modeled with analog models. Conductivity and geometry of the 'ore' zone, the overburden, and the bedrock can all be varied independently; although all must have the same strike direction. Model results accurate to about 10 percent can be obtained at negligible cost.One example shows how a decrease in overburden resistivity from 100 ohm-m to 10 ohm-m can obliterate a distinct AFMAG anomaly. An increase of overburden thickness from 20 m to 50 m has the same effect. This study shows the importance of obtaining some direct overburden resistivity data before ruling out areas which show no anomalies.