Magnetic map of the Irish Hills and surrounding areas, San Luis Obispo County, central California

Open-File Report 2012-1080
By: , and 



A magnetic map of the Irish Hills and surrounding areas was created as part of a cooperative research and development agreement with the Pacific Gas and Electric Company and is intended to promote further understanding of the areal geology and structure by serving as a basis for geophysical interpretations and by supporting geological mapping, mineral and water resource investigations, and other topical studies. Local spatial variations in the Earth's magnetic field (evident as anomalies on magnetic maps) reflect the distribution of magnetic minerals, primarily magnetite, in the underlying rocks. In many cases the volume content of magnetic minerals can be related to rock type, and abrupt spatial changes in the amount of magnetic minerals can be related to either lithologic or structural boundaries. Magnetic susceptibility measurements from the area indicate that bodies of serpentinite and other mafic and ultramafic rocks tend to produce the most intense magnetic anomalies, but such generalizations must be applied with caution because some sedimentary units also can produce measurable magnetic anomalies. Remanent magnetization does not appear to be a significant source for magnetic anomalies because it is an order of magnitude less than the induced magnetization. The map is a mosaic of three separate surveys collected by (1) fixed-wing aircraft at a nominal height of 305 m, (2) by boat with the sensor at sea level, and (3) by helicopter. The helicopter survey was flown by New-Sense Geophysics in October 2009 along flight lines spaced 150-m apart and at a nominal terrain clearance of 50 to 100 m. Tie lines were flown 1,500-m apart. Data were adjusted for lag error and diurnal field variations. Further processing included microleveling using the tie lines and subtraction of the reference field defined by International Geomagnetic Reference Field (IGRF) 2005 extrapolated to August 1, 2008.

Study Area

Publication type Report
Publication Subtype USGS Numbered Series
Title Magnetic map of the Irish Hills and surrounding areas, San Luis Obispo County, central California
Series title Open-File Report
Series number 2012-1080
DOI 10.3133/ofr20121080
Year Published 2012
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Western Region Geology and Geophysics Science Center
Description Map: 47.61 inches x 38.44 inches; Readme TXT; Metadata Folder; GIS Database ZIP
Country United States
State California
County San Luis Obispo
Other Geospatial Irish Hills
Datum NAD27
Projection Universal Transverse Mercator projection, Zone 10
Scale 24000
Online Only (Y/N) Y
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Additional publication details