A proposed high-altitude survey of the United States offers an exciting and cost-effective opportunity to collect magnetic-anomaly data. Lockheed Martin Missile and Space Company is considering funding a reimbursable ER-2 aircraft (Figure 1) mission to collect synthetic aperture radar (SAR) imagery at an altitude of about 21 km over the conterminous United States and Alaska. The collection of total and vector magnetic field data would be a secondary objective of the flight. Through this "piggyback approach," the geomagnetic community would inherit invaluable magnetic data at a nominal cost. These data would provide insight on fundamental tectonic and thermal processes and give a new view of the structural and lithologic framework of the crust and upper mantle.
Fig. 1. The ER-2 is the NASA remote sensing version of the U.S. Air Force Lockheed U2-R, which replaced the older U.S. Air Force U2. The superpods and wingtip pods are the proposed locations of the magnetometers.
Magnetic-anomaly data provide important information about regional geology, especially where rock outcrops are scarce or absent, as in offshore economic zones and in covered land areas. They are particularly useful in studying the distribution of magnetic mafic, ultramafic, and plutonic rocks (Figure 2). In short, important insights on the complex geologic history of the crust and possibly the upper mantle (Figure 2) are gained in studying the lateral variations in magnetization across the United States.
Fig. 2. Hypothetical cross section of the continental crust and upper mantle, showing the lateral complexities in the distribution of magnetic mafic, ultramafic and plutonic rocks. Generalized from Fountain and Christensen [1989].
The information contained in a magnetic survey is largely restricted to a specific band of the wavenumber spectrum, with the position of this band and the ability to resolve it being primarily a function of the survey's altitude and size. Although the 20–22 km altitude of the ER mission was established independently of the requirements of geologic or geomagnetic studies, it will be nearly optimal for bridging the gap between existing low-altitude aeromagnetic data (generally at altitudes of 1 km or less) and satellite magnetic data (at altitudes of about 450 km) (Figure 3).
Fig. 3. Amplitude spectra of magnetic surveys flown at three altitudes. Each spectra has been normalized to a maximum of 1. The solid black-shaded area is the spectrum that is expected from a survey flown at 450 km, the nominal altitude of the NASA 1979–1980 Magsat mission. The light-shaded area is the spectrum at 1 km, the altitude of a typical aeromagnetic survey. The medium-gray-shaded area is the spectrum that is expected from the high-altitude aeromagnetic survey. The dashed line is at amplitude of 1/e. Magnetic layer assumed to be 10 km thick. The shapes of the spectra depend only on the depth to the top and thickness of the layer. Thus they are applicable for both total-field and vertical-field surveys.
Because there is little overlap between the spectra from satellite and low-altitude aeromagnetic surveys, a significant part of the spectrum (wavelengths of 200–900 km) is poorly known. This part of the spectrum is critical to crustal studies; it is about the same length scale as major geologic structures, such as the Cascade Range, the Basin and Range province, and the Midcontinent Rift. The high-altitude magnetic-anomaly data will eliminate the gap between wavelengths of 200 and 900 km, and, together with low-altitude aeromagnetic and satellite magnetic data, provide information across the spectrum.
Significant mismatches exist between many survey data sets, some exceeding several hundred nanoteslas—an order of magnitude greater than the amplitudes of magnetic anomalies caused by some of the sources of interest. For surveys of about the size of a 1°×2° quadrangle, a properly conducted high-altitude aeromagnetic survey will significantly reduce data mismatches at survey boundaries and thus greatly expand the utility of the low-altitude magnetic data over a much broader range of wavelengths.
A consistent datum for all aeromagnetic surveys will improve both qualitative and quantitative interpretations. For example, they will be beneficial for geological mapping, particularly where magnetic maps are used to extrapolate observations from outcrop to covered regions, and for quantitative comparisons of magnetic properties of rock units in different parts of the United States. A correctly merged aeromagnetic map of the conterminous United States may be the single most important legacy of the high-altitude mission.
Interpretations of the high-altitude data will provide new insights on the structure and evolution of the crust and possibly the upper mantle. For example, there are few direct methods for characterizing crustal temperatures. Spectral analysis of the high-altitude aeromagnetic data may lead to estimates of crustal depths where elevated temperatures (above about 580°C) cause rocks to lose their magnetic properties. Thus these data could be used to study crustal temperatures and to improve the national heat flow map. The advancement in our understanding of crustal temperatures could benefit many socioeconomic studies. This knowledge could be used to assess geothermal resources and—since the mechanical behavior of the crust is highly temperature dependent—provide new insights on crustal stability related to volcanoes and fault systems.
The large lateral dimensions of the high-altitude aeromagnetic survey will also provide important information about the separation of magnetic fields of crustal and core origin. When the geomagnetic field is expressed as a spherical harmonic expansion, the core and crustal components overlap between harmonic degrees of 12 and 15. It is in this wavelength region where core fields merge with crustal fields. The dimensions of the high-altitude aeromagnetic survey will be several times larger than these wavelengths and thus will be useful in analyzing the overlap between core and crustal fields.
A consistent, high-quality survey could lead to a better understanding of the statistical aspects of the crustal field over the United States, which should lead to improved core-crustal separations worldwide. The importance of the high-altitude magnetic data to core-crustal separation studies will be enhanced if the ER-2 mission occurs during the Oersted magnetic satellite mission scheduled to launch in June 1997. Moreover, the regional crustal field defined by the high-altitude magnetic survey can be added to the improved core field to provide magnetic models needed in higher precision positioning, such as directional drilling by oil and gas exploration companies or testing missile guidance systems in U.S. firing ranges.
There are many other geologic and geomagnetic applications of a high-altitude aeromagnetic survey. It could provide a better understanding of the continent/ocean transition, new insights on regional tectonic problems such as constraints on basin evolution and regional controls on mineralization, and mineralogical implications for the deep crust and mantle. A continent-wide tectonic perspective could be gained if the results were merged with those of Canada's high-altitude (4 km) magnetic survey. For the geomagnetic field, the survey could improve U.S. magnetic charts and what is known about the magnetic field in littoral areas, and assure the quality of small amplitude and short-wavelength satellite magnetic anomalies.
An ad-hoc committee was formed to determine the feasibility of also acquiring quality magnetic-anomaly data. Magnetic data collected last November during six test flights with a cesium total field magnetometer indicate that the ER-2 is an appropriate platform for magnetic measurements. In December, magnetic specialists met at the Ames Research Center in California to address the rationale and operational plan for the ER-2 magnetic survey and the resources necessary to accomplish the mission [Hildenbrand et al., 1996]. The workshop participants concluded that the instrument package (Figure 1) should include two potassium magnetometers (one in each wingpod), a cesium magnetometer in a wing superpod, and a high-precision vector magnetometer in the other wing superpod.
Although the ER-2 is an appropriate platform for collecting total-field magnetic data, further tests are needed to establish the anticipated quality of the vector magnetic data. Additional studies and test flights are needed to evaluate methods to overcome diurnal variations, to isolate observed DC shifts, and to design optimum compensation/calibration flight maneuvers. The Ames magnetic workshop also addressed the resources needed to install the magnetometers and to collect, process, and distribute the magnetic-anomaly data. Although most of the mission costs would be funded by Lockheed Martin, costs related to the collection of the magnetic data are not. These additional costs include equipment purchase and magnetometer installation; processing and distribution of the magnetic data; flight time for test flights, mission tie lines, and compensation flight maneuvers; and flight time to collect offshore data.
The U.S. Geological Survey funded the November test flights and the Ames workshop, and it plans to commit people to assist in planning the mission and processing the data. Many other scientists from the geomagnetic community are supporting this effort. However, funding sources for the remaining additional costs have yet to be identified. The estimated 122 flight hours, primarily for tie lines and for compensation and calibration maneuvers, may decrease if new test flights indicate that tie lines may be unnecessary or that the planned number of maneuvers for compensation and calibration can be reduced. The 140 flight hours to collect offshore data could also be reduced by eliminating less critical offshore coverage. Due to limited budgets, the ER-2 mission will require combining resources from a consortium of federal and state agencies, private industry, and academic institutions.
Two recent reports call for acquiring consistent magnetic-anomaly data at a high-altitude over the United States [National Research Council, 1993; U.S. Magnetic-Anomaly Data Task Group, 1994]. The U.S. Geodynamics Committee of the National Research Council formed the working groups that produced these two reports. Copies can be obtained from Charles Meade, Board of Earth Sciences and Resources, National Research Council, 2001 Wisconsin Ave. NW, Washington, D.C. 20007 (e-mail cmeade@nas.edu). Comments or advice regarding any aspect of the high-altitude magnetic survey will be valuable in the formulation of a strategic plan. We encourage interested parties to send comments to Tom Hildenbrand, U.S. Geological Survey, Mail Stop 989, 345 Middlefield Road, Menlo Park CA 94025 (e-mail tom@usgs.gov).
Fountain, D. M., and N. I. Christensen, Composition of the continental crust and mantle: A review, in Geophysical Framework of the Continental United States, edited by Pakiser, L. C. and W. D. Mooney, Geol. Soc. of Am. Mem., 172, 711, 1989.
Hildenbrand, T. G., et al., Workshop proceedings: Rationale and preliminary operational plan for a high-altitude magnetic survey over the United States, U.S. Geol. Surv. Open File Rep. 96-276, 55 pp., 1996.
Malliot, H.A., Digital terrain elevation mapping system, in Proceedings of the 1996 IEEE Aerospace Applications Conference (IEEE 96CH35904), Vol. 4, Snowmass at Aspen, Colo., Feb. 3–10, pp. 91–105, 1996.
National Research Council, The National Geomagnetic Initiative Report, National Academy Press, Washington D.C., 246 pp., 1993.
U.S. Magnetic-Anomaly Data Set Task Group, Rationale and operational plan to upgrade the U.S. magnetic-anomaly data base, NASA Report, Washington, D.C., 25 pp., August 1994.
