The advantage of using analytic signal techniques to determine magnetic parameters from magnetic anomalies is the independence of magnetization direction. In addition, using the amplitude ratio method to determine depths to magnetic sources can avoid tedious operations with several characteristic points and reduce interference effects. A drawback of this method, however, is the assumption that near-surface structures can be characterized adequately by step models. To improve mapping resolution, we have extended the amplitude ratio method to allow both step-like and dike-like structures. A criterion is constructed that discriminates between maxima from dike-like or step-like structures and significantly improves near-surface structural mapping. This avoids a bias in geological interpretation caused by the original assumption that all structures can be characterized by step models. Synthetic data and real data acquired offshore northern Taiwan demonstrate the effectiveness of this method.