A practical approach to the filtering of airborne magnetic data in the Cobar region of NSW

Abstract

Since 1981 CRA Exploration (CRAE) has flown in excess of 43,000 line km of airborne magnetics in the Cobar district. The data, which were acquired from a number of surveys, are being used to assist in the exploration for base metals and gold. Examples of the targets sought are the Elura lead-zinc-silver deposit, the CSA copper deposit and the Cobar gold deposits which all have associated magnetic features. A 1974 aeromagnetic survey flown at 90 m terrain clearance gave a discrete 40 nT response over the Elura orebody. Such an anomaly could be detected in areas of low magnetic relief within the detailed surveys. Unfortunately significant parts of these surveys are affected by the variable responses of maghemite concentrations in shallow channels prevalent in the Cobar district which could mask small discrete anomalies. To test this, a process of upward and downward continuation was applied to the original 90 m data set. These data sets were added to a grid containing a maghemite channel response. The resulting grids were then filtered in an attempt to extract the original data. The results suggest that even without filtering an Elura anomaly could be detected under a maghemite channel to a depth of about 200 m. Using a Butterworth low pass filter it was considered that at depths to 400 m below a channel, an Elura anomaly could be recognised. The paper concludes that this practical approach is applicable to other data sets that may require filtering.