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Preliminary Aeromagnetic Anomaly Map of California

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Preliminary Aeromagnetic Anomaly Map of California PRELIMINARY AEROMAGNETIC ANOMALY MAP OF CALIFORNIA By Carter W. Roberts and Robert C. Jachens Open-File Report 99-440 1999 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY 1 INTRODUCTION The magnetization in crustal rocks is the vector sum of induced in minerals by the Earth’s present main field and the remanent magnetization of minerals susceptible to magnetization (chiefly magnetite) (Blakely, 1995). The direction of remanent magnetization acquired during the rock’s history can be highly variable. Crystalline rocks generally contain sufficient magnetic minerals to cause variations in the Earth’s magnetic field that can be mapped by aeromagnetic surveys. Sedimentary rocks are generally weakly magnetized and consequently have a small effect on the magnetic field: thus a magnetic anomaly map can be used to “see through” the sedimentary rock cover and can convey information on lithologic contrasts and structural trends related to the underlying crystalline basement (see Nettleton,1971; Blakely, 1995). The magnetic anomaly map (fig. 2) provides a synoptic view of major anomalies and contributes to our understanding of the tectonic development of California. Reference fields, that approximate the Earth’s main (core) field, have been subtracted from the recorded magnetic data. The resulting map of the total magnetic anomalies exhibits anomaly patterns related to the distribution of magnetized crustal rocks at depths shallower than the Curie point isotherm (the surface within the Earth beneath which temperatures are so high that rocks lose their magnetic properties). The magnetic anomaly map has been compiled from existing digital data. Data obtained from aeromagnetic surveys that were made at different times, spacings and elevations, were merged by analytical continuation of each set onto a common surface 305 m (1000 ft) above terrain. Digital data in this compatible form allows application of analytical techniques (Blakley, 1995) that can be used to enhance anomaly characteristics (e.g., wavelength and trends) and provide new interpretive information. This report in PDF format (file name OF99-440.PDF) and other geophysical studies may be found on the Internet site of the Geophysical Unit in Menlo Park at: http://wrgis.wr.usgs.gov/docs/gump DATA REDUCTION The magnetic anomaly map was compiled from a group of magnetic surveys with diverse specifications (table 1). The 123 aeromagnetic surveys were flown with flight-line spacings ranging from 0.4 km (1/4 mi) to 10 km (6 mi) and either in a drape mode (approximately constant elevation above ground surface) or in a level mode (constant Barometric altitude). In all cases the total field component was used. To produce a coherent magnetic database, the following procedures were applied to the individual surveys: 1. The International Geomagnetic Reference Field (IGRF), updated to the date that the survey was flown, was removed from each survey to generate an initially consistent set of residual data. 2 2. All surveys were then gridded at a 1-km spacing (Lambert Conformal Conic projection, central meridian 120°, base latitude 0°) and continued on a common datum of 305 m (1000 ft) above the ground surface. Surveys not collected at this height were mathematically modified to approximate the magnetic field that would have been measured at the 305-m common datum (Cordell, 1985). 3. The individual surveys were compared in areas of overlap, datum shifted as necessary to give the best fit, and finally merged into a single grid using a minimum curvature algorithm (Webring, 1982) to smoothly fill across gaps of about 2 km between surveys. The overall precision of the anomaly values is difficult to estimate, mainly because of the diversity of surveys used to construct the map. Very rugged topography and concerns about safety caused significant departures from the ideal flight surface on many draped surveys. Because of the wide 10-km (6-mi) spacing of surveys 6150 and 6184, expressions of near- surface sources may have been misrepresented or not detected because of lack of resolution. Continuing of the data to the common reference level of 305 m above the ground can also introduce errors in the anomaly map. The final data set is judged suitable to gain new insights into regional geologic features at scales of 1:250,000 or smaller. Note: This and other recent USGS publications in the Western Region may be found on the World Wide Web at: http://geopubs.wr.usgs.gov. See also http://wrgis.wr.usgs.gov. ACNOWLEDGMENT We wish to thank Pat Hill (USGS Denver) for providing most of the data sets and detailed information about each survey. Without her help this compilation would have been very difficult to accomplish. REFERENCES Abrams, G.A., 1992, Aeromagnetic map of the San Jose area, California: U.S. Geological Survey Miscellaneous Field Investigations Map MF-2191, scale 1:100,000. Abrams, G.A., Kucks, R.P., and Bracken, R.E., 1991, Aeromagnetic map of Palo Alto and vicinity, California: U.S. Geological Survey Open-File Report 91-336, scale 1:62,500. Aero Service Corp., 1981, Airborne gamma-ray spectrometer and magnetometer survey Ukiah Quadrangle, California: U.S. Department of Energy, Grand Junction Office Report GJBX-390(81), 2 vols. Aero Service Corp., 1981, Airborne gamma-ray spectrometer and magnetometer survey Weed Quadrangle, California: U.S. Department of Energy, Grand Junction Office Report GJBX-391(81), 2 vols. Aero Service Corp., 1981, Airborne gamma ray spectrometer and magnetometer survey Chico Quadrangle, California: U.S. Department of Energy, Grand Junction Office Report GJBX-407(81), 2 vols. 3 Aero Service Corp., 1981, Airborne gamma-ray spectrometer and magnetometer survey Eureka Quadrangle, California: U.S. Department of Energy, Grand Junction Office Report GJBX-409(81), 2 vols. Aero Service Corp., 1981, Airborne gamma-ray spectrometer and magnetometer survey Susanville Quadrangle, California: U.S. Department of Energy, Grand Junction Office Report GJBX-410(81), 2 vols. Aero Service Corp., 1981, Airborne gamma-ray spectrometer and magnetometer survey Redding Quadrangle, California: U.S. Department of Energy, Grand Junction Office Report GJBX-411(81), 2 vols. Andreasen, G.E., and Petrafeso, F.A., 1963, Aeromagnetic map of the east-central part of the Death Valley National Monument, Inyo County, California: U.S. Geological Survey Geophysical Investigations Map GP-428, scale 1:62,500. Andreasen, G.E., Pitkin, J.A., and Petrafeso, F.A., 1964, Aeromagnetic map of the Long Beach- Santa Ana area, Los Angeles and Orange Counties, California: U.S. Geological Survey Geophysical Investigations Map GP-464, scale 1:48,000. Andreasen, G.E., Pitkin, J.A., and Petrafeso, F.A., 1964, Aeromagnetic map of eastern Los Angeles and vicinity, California: U.S. Geological Survey Geophysical Investigations Map GP-465, scale 1:48,000. Andreasen, G.E. Pitkin, J.A., and Petrafeso, F.A., 1964, Aeromagnetic map of western Los Angeles and vicinity, California: U.S. Geological Survey Geophysical Investigations Map GP-466, scale 1:48,000. Blakely, R. J., 1995, Potential theory in gravity and magnetic applications: Cambridge University Press, 441 p. Brown, R.D., Jr., Grimes, D.J., and Leinz, Reinhard, 1981, Mineral resources of the Snow Mountain Wilderness Study area, California, with a section on Interpretation of aeromagnetic data by Andrew Griscom and R.D. Brown, Jr.: U.S. Geological Survey Bulletin 1495, 48 p., scale 1:62,500. California Division of Mines and Geology, 1978, Aeromagnetic map of the Modoc area, California: California Division of Mines and Geology Open-File Report 78-13A, scale 1:250,000. California Division of Mines and Geology, 1978, Aeromagnetic map of the north half of the Great Valley, California: California Division of Mines and Geology Open-File Report 78-13D, 2 sheets, scale 1:250,000. California Division of Mines and Geology, 1978, Aeromagnetic maps of the north coastal area, California: California Division of Mines and Geology Open-File Report 78-13E, 11 sheets, scale 1:62,500. Cordell, Lindrith, 1985, Techniques, applications and problems of analytical continuation of New Mexico Aeromagnetic data between arbitrary surfaces of very high relief, in International Meeting on Potential Fields in Rugged Topography Proceedings, Abstract with Programs: Institute de Geophysique Universite de Lausanne, Bulletin 7, p. 96-101. Couch, R.W., 1982, Maps showing total-field Aeromagnetic anomalies and topography of the Cascade Mountain Range, northern California: U.S. Geological Survey Open-File Report 82-198, 2 sheets, scale 1:250,000. Couch, Richard, and Gemperle, Michael, 1982, Aeromagnetic measurements in the Cascade Range and Modoc Plateau of northern California, report on work done from June 1, 1980, to November 30, 1980: U.S. Geological Survey Open-File Report 82-932, 26 p. 4 Duffield, W.A., Weldin, R.D., 1976, and Mineral resources of the South Warner Wilderness, Modoc County, California, with a section on Aeromagnetic data, by W.E. Davis: U.S. Geological Survey Bulletin 1385-D, 31 p., scale 1:48,000. Fox, R.C., 1978, Low-altitude Aeromagnetic survey of a portion of the Coso Hot Springs KGRA, Inyo County, California: U.S. Department of Energy Report IDO-1601-2, 19 p., 6 pls. GeoLife, Inc., 1979, Aerial radiometric and magnetic survey, Trona National Topographic Map, California: U.S. Department of Energy, Grand Junction Office Report GJBX-065(79), 2 vols. GeoLife, Inc., 1979, Aerial radiometric and magnetic survey, Death Valley National Topographic Map, Nevada and California: U.S. Department of Energy, Grand Junction Office Report GJBX-164(79), 2 vols. Grauch, V. J., 1984, Aeromagnetic map of the Eagle Mountains area, Riverside County, California: U.S.
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