640 



histor>', to determine the age at which the rocks were tormed 

 through volcanic activity. 



The Earth's magnetism is a vector field defined by both a 

 magnitude and a direction. Neither the magnitude nor the 

 direction of this field at a point remains constant in time. The 

 Sun's solar wind causes daily, small variations in the 

 geomagnetic field, and the field measured on the surface varies 

 slowly over time, a phenomenon related to core flow and called 

 "secular variation." This variation amounts to a few degrees in 

 the position of the magnetic poles over the course of a century. 

 Thus, the variation between the magnetic and geographic poles 

 had to be mapped periodically in order to correct magnetic 

 compass headings for true North. Because the magnetic field 

 is horizontal at the magnetic equator and vertical at the poles, 

 the angle of the field with respect to horizontal can be used to 

 determine latitude. Historically this was particularly helpful 

 when neither the stars nor Sun could be used because of weather 

 conditions — a particular problem in the past two centuries for 

 Arctic exploration. 



Every few years the geomagnetic field at the surface is published 

 in the current International Geomagnetic Reference Field 

 (IGRF). For the past two centuries, the U.S. Navy has been 

 a major contributor to these important data. This historic 

 contribution will come to an end this year. While the advent 

 of the Global Positioning System (GPS) has certainly lessened 

 reliance on magnetic compasses for navigation, they have not 

 yet been eliminated completely. Even apart from the important 

 scientific applications of geomagnetism to be discussed below, 

 we are, therefore, concerned that funding constraints associated 

 with post-cold-war defense downsizing and changing mission 

 priorities may not allow the Navy to continue this valuable 

 pubUc service. 



At sea a magnetometer is towed sufficiently far behmd a ship 

 to avoid perturbations associated wiUi the magnetic hull. The 

 magnetometer measures the total field of the Earth approximately 

 every 30 seconds.' When the IGRF is subtracted from the total 

 field measured, the resultant anomalies (about I % of the total 

 field) reveal information about the past reversals of the field as 

 recorded in the seafloor rocks. This information led to the 



geophysical discovery of seafloor spreading and the development 

 of die theory of plate tectonics. In general, the Navy's holdings 

 of shipboard magnetics data, largely in the Northern Hemisphere, 

 is the most coherent, systematic, and complete of any survey 

 ever conducted and, as such, i s very important in understanding 

 the past 200 million years of Earth history. 



Figure 8 shows an example of a small section of Earth's 

 magnetic field at relatively high resolution. The data shown in 

 Figure 8 were measured by the Navy's Project Magnet aircraft 

 in 1983 at an altitude of 500 feet and have an north/south track 

 spacing of 3 nm with an along-track sampling of 0.25 nm. The 

 magnetic data shown here depict the local field, with both the 

 global field generated by Earth's core and the regional gradient 

 field having been removed. The stripes on the seafloor represent 

 reversals in the Earth's magnetic field and can be used to outline 

 the detailed tectonic history of oceanic crust. 



Although these data are from the wholly unclassified aircraft 

 surveys of Project Magnet, they serve to illustrate the spatial 

 complexity of such data and to suggest the quantity of 

 geophysical information that is contained in the higher resolution 

 classified NAVOCEANO ship surveys. 



NAVOCEANO's magnetic data are currently stored in simple 

 digital files without software to access or manipulate the data. 

 However these data will be incorporated into the IDBMS 

 before the end of FY 95. This database management system 

 should provide better data accessibility for analyses. There is 

 no existing gridded magnetics database, and constructing one 

 would be a large effort because of the difficulty in reconciling 

 the temporal and spatial variations. There is a plan to acquire 

 scalar magnetic data from the Defense Meteorological Satellite 

 Program (DMSP) S-15 satellite. These data could be used m 

 conjunction widi ship-measured scalar data to produce regional 

 magnetic models. 



b. Accessibility 



Most NAVOCEANO aircraft and satellite magnetics data are 

 in vector form and are unclassified, publicly releasable, and 

 have been turned oxer to National Geophysical Data Center 

 (NGDC) for public distribution. Virtually all of the ship- 



' The siiengih of magnetic fields are today measured in SI units as Testa (T). which is lig/amp/s-. The Earth's magnetic field at the poles is on the order of 

 5 X 10* Teslas or 6 x 10* nanoteslas (nT). A previous measure, used prior to the adoption of SI units, is the gamma, which is equal to a nT 



12 



