| Abstract: | 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. |
| Genre: | USGS Numbered Series |
| ProdID: | 70038653 |
| Citation Author: | Langenheim, V.E.; Watt, J.T.; Denton, K.M. |
| Citation Contributing Office: | Western Region Geology and Geophysics Science Center - Menlo Park |
| Citation Datum: | NAD27 |
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| Citation Language: | English |
| Citation Larger Work Title: | |
| Citation LatN: | 0351730 |
| Citation LatS: | 0350530 |
| Citation LonE: | -1204230 |
| Citation LonW: | -1205730 |
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| Citation No Pagination: | |
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| Citation Online Only Flag: | Y |
| Citation Phsyical Description: | Map: 47.61 inches x 38.44 inches; Readme TXT; Metadata Folder; GIS Database ZIP |
| Citation Projection: | Universal Transverse Mercator projection, Zone 10 |
| Citation Public Comments: | |
| Citation Publisher: | U.S. Geological Survey |
| Citation Series: | Open-File Report |
| Citation Series Code: | OFR |
| Citation Series Number: | 2012-1080 |
| Citation Search Results Text: | Magnetic map of the Irish Hills and surrounding areas, San Luis Obispo County, central California; 2012; OFR; 2012-1080; Langenheim, V.E.; Watt, J.T.; Denton, K.M. |
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| Citation Year: | 2012 |
| Type: | citation/reference |
| Text: | Magnetic map of the Irish Hills and surrounding areas, San Luis Obispo County, central California; 2012; OFR; 2012-1080; Langenheim, V.E.; Watt, J.T.; Denton, K.M. |
| URL (THUMBNAIL): | http://pubs.er.usgs.gov/thumbnails/ofr_2012_1080.jpg |
| URL (INDEX PAGE): | http://pubs.usgs.gov/of/2012/1080/ |
| Date Other: | Mon, 11 Jun 2012 00:00 -0500 |
| Publisher: | U.S. Geological Survey |
