Although a great variety of interpretation techniques for basement depth determination has been developed during the past two or three decades, the half-slope and straight-slope methods are still popular due to their simplicity and general reliability in manual interpretation and are widely used in oil exploration work (Nettleton, 1976). The half-slope and straight-slope rules are derived for a particular set of geologic/geophysical conditions and care should be taken in applying them in a more general way. For example, the half-slope method of Peters (1949) was derived for magnetic anomalies over vertical dikes with vertical polarization. The straight-slope method uses the horizontal projection of the straight-line part of the steepest gradient at the inflection point on the anomaly curve as the depth estimator. This rule is purely empirical because mathematically there is no straightline part on the anomaly curve. Vacquier et al. (1951) made an exhaustive study of the straight-slope method and presented several depth indices measured on different flanks of anomalies due to prismatic bodies.