NEED FOR A U.S. HIGH-ALTITUDE MAGNETIC SURVEY

High-altitude magnetic data over the U.S. (15 km altitude) are needed as a reference field to properly level the U.S. low-altitude magnetic data set (0.3 km altitude) but will also provide new insights on fundamental tectonic and thermal processes and give a new view of the structural and lithologic framework of continental and offshore regions. The low-altitude magnetic database was constructed from a patchwork of over 1,000 surveys acquired over a period of 50 years. Significant mismatches exist between many survey data sets, some exceeding several hundred nT. Thus, in the recently compiled national low-altitude magnetic database, the accuracy of anomaly wavelengths greater than roughly 180 km (the average magnetic survey size) is suspect. A correctly merged, low-altitude magnetic database, using high-altitude magnetic data as a reference field, may be the single most important legacy of a high-altitude magnetic mission, as it will greatly expand the utility of our invaluable U.S. magnetic data.

A high-altitude aeromagnetic data set also has considerable independent scientific value. Based on our understanding of the magnetic properties of the lithosphere, the high-altitude magnetic data will clearly aid in the solution of a broad range of applied Earth science issues related to: the conundrum of long-wavelength magnetic anomalies; geologic and tectonic processes, for example, crustal accretion and evolution; thermal and mechanical properties of the lithosphere; societal concerns, such as localization of favorable areas for mineral, energy, and thermal resources; and mitigation of earthquake and volcanic hazards. The wavelength band of a high-altitude survey will be particularly helpful in studying the lower crust--its composition, structure, and thermal regime--and large geologic/tectonic structures, such as basement terranes. In addition, these results will provide significant new constraints for geological interpretation of complementary regional topographic, seismic, gravity, and heat flow data. The year 2004 may offer an exciting and cost-effective opportunity to acquire a high-altitude magnetic data set, as a national mission is presently being planned to collect IFSAR imagery but also total and vector magnetic field data (a secondary mission objective) at an altitude of about 15 km.