DETERMINATION OF DEPTHS TO CENTROIDS OF 3-DIMENSIONAL SOURCES OF POTENTIAL-FIELD ANOMALIES WITH EXAMPLES FROM ENVIRONMENTAL AND GEOLOGIC APPLICATIONS

A method is developed for determining the depth to the centroid (the g eometric center) of 'semi-compact' sources. The method, called the ano maly attenuation rate (AAR) method, involves computing radial averages of AARs with increasing distances from a range of assumed source cent ers. For well-isolated magnetic anomalies from 'semi-compact' sources, the theoretical AARs range from similar to 2 (close to the sources) t o similar to 3 (in the far-field region); the corresponding theoretica l range of AARs for gravity anomalies is similar to 1 to similar to 2. When the estimated source centroid is incorrect, the AARs either exce ed or fall short of the theoretical values. The levelling-off of the f ar-field AARs near their theoretical maximum values indicates the uppe r (deeper) bound of the centroid location. Similarly, near-field AARs lower than the theoretical minimum indicate the lower (shallower) boun d of the centroid location. It is not always possible to determine usa ble upper and lower bounds of the centroids because the method depends on characteristics of sources/anomalies and the noise level of the da ta. For the environmental magnetic examples considered in this study, the determined deeper bounds were within 4% of the true centroid-to-ob servation distance. For the case of the gravity anomaly from the Bloom field Pluton, Missouri, USA, determination of only the shallower bound of the centroid location (similar to 7 km) was possible. This estimat e agrees closely with the centroid of a previously determined three-di mensional model of the Bloomfield Pluton. For satellite magnetic anoma lies, the method is appropriate only for high-amplitude, near-circular anomalies due to the inherent low signal-to-noise ratio of satellite magnetic anomalies. Model studies indicate that the AAR method is able to place depths within +/- 20-30 km of actual center locations from a 400-km observation altitude. Thus, the method may be able to discrimi nate between upper crustal, lower crustal, and mantle magnetic sources . The results from the prominent Kentucky anomaly are relatively well- resolved (centroid depth similar to 30 km below the Earth's surface). For the Kiruna Magsat anomaly, the deleterious effects from neighborin g anomalies make a determination difficult (possible depth could be be tween 20 and 30 km). The centroid depths are deeper for the Kursk anom aly (similar to 40-50 km). These depths may indicate that magnetic ano malies from the near-surface Kursk iron formations (a known contributo r) and deep crustal magnetic sources could combine to form the Kursk M agsat anomaly. (C) 1998 Elsevier Science B.V. All rights reserved.