REDUCTION OF TERRAIN‐INDUCED AEROMAGNETIC ANOMALIES BY PARALLEL‐SURFACE CONTINUATION; A CASE HISTORY FROM THE SOUTHERN SAN JUAN MOUNTAINS OF COLORADO

A method for reduction of terrain‐induced anomalies in aeromagnetic data collected at constant or variable elevation has recently been developed by Bhattacharyya and Chan (1977a). The method utilizes an equivalent‐source approach to continue the anomalous field data to a reference surface parallel to the terrain, thereby attenuating the topographic effect. This approach to the reduction of terrain effects requires no assumption about physical properties or distribution of causative bodies. We present a test of the method in the Chama‐southern San Juan Mountains wilderness study area of southwestern Colorado, a region of substantial topographic relief (exceeding 1600 m) and extensive volcanic cover. This study shows that in many places in this area terrain effects contribute appreciably to the inaccuracy in geologic interpretation of the original aeromagnetic data. Comparison of interpretations based on original and revised data shows that some anomalies are shifted with respect to the original data by as much as 6 km, while others are not. The process now permits the interpreter to easily separate terrain‐induced lows from lows caused by hydrothermal alteration. In one example, a northern extension of a magnetic low thought to be caused by topography was shifted and aligned with several small zones of hydrothermal alteration. Another magnetic low incursion previously thought to be alteration‐caused was removed entirely. Several terrain‐induced highs were removed or combined, and several lows associated with valleys became relatively more prominent, leading to significant reevaluation of earlier interpretations. Several of these revisions have possible economic consequences. We show that this technique cannot be duplicated by filtering or upward continuation, and in regions of significant topographic relief the value of this new method to exploration geophysicists may be substantial.