LIMITATIONS ON DIGITAL FILTERING OF THE DNAG MAGNETIC DATA SET FOR THE CONTERMINOUS UNITED-STATES

The magnetic data set compiled for the Decade of North American Geolog y (DNAG) project presents an important digital data base that can be u sed to examine the North American crust. The data represent a patchwor k from many individual airborne and marine magnetic surveys. However, the portion of data for the conterminous U.S. has problems that limit the resolution and use of the data. Now that the data are available in digital form, it is important to describe the data limitations more s pecifically than before. The primary problem is caused by datum shifts between individual survey boundaries. In the western U.S., the DNAG d ata are generally shifted less than 100 nT. In the eastern U.S., the D NAG data may be shifted by as much as 300 nT and contain regionally sh ifted areas with wavelengths on the order of 800 to 1400 km. The worst case is the artificial low centered over Kentucky and Tennessee produ ced by a series of datum shifts. A second significant problem is lack of anomaly resolution that arises primarily from using survey data tha t is too widely spaced compared to the flight heights above magnetic s ources. Unfortunately, these are the only data available for much of t he U.S. Another problem is produced by the lack of common observation surface between individual pieces of the U.S. DNAG data. The height di sparities introduce variations in spatial frequency content that are u nrelated to the magnetization of rocks. The spectral effects of datum shifts and the variation of spatial frequency content due to height di sparities were estimated for the DNAG data for the conterminous U.S. A s a general guideline for digital filtering, the most reliable feature s in the U.S. DNAG data have wavelengths roughly between 170 and 500 k m, or anomaly half-widths between 85 and 250 km. High-quality, large-r egion magnetic data sets have become increasingly important to meet ex ploration and scientific objectives. The acquisition of a new national magnetic data set with higher quality at a greater range of wavelengt hs is clearly in order. The best approach is to refly much of the U.S. with common specifications and reduction procedures. At the very leas t, magnetic data sets should be remerged digitally using available or newly flown long-distance flight-line data to adjust survey levels. In any case, national coordination is required to produce a consistent, high-quality national magnetic map.