Readme for Open-File Report 2013-1282 Gravity, Magnetic, and Rock Property Data of the Central California Coast Ranges Data Contents The four data files included in Open-File Report 2013-1282 are given as ascii data files on the web site URL http://pubs.usgs.gov/of/2013/1282/. The format of the data files are described below. GRAVITY DATA The gravity data are contained in one file (iso-all2.txt) The horizontal and vertical locations for the stations are on the North American Datum 1927 (NAD27) and the North American Vertical Datum 1929 (NAVD29), respectively. The gravity stations were referenced to the International Gravity Standardization Net 1971 (IGSN71) datum and the reference ellipsoid is the Geodetic Reference System 1967 (GRS67). The free-air anomalies were calculated using a modified version of formulas from Swick (1942). The complete Bouguer anomalies were calculated from the free-air anomalies using the Bouguer correction, terrain corrections, a curvature correction, and a reduction density of 2,670 kg/m3. Isostatic corrections were calculated using an Airy-Heiskanen model of isostatic compensation. For onshore data, Bouguer and isostatic anomalies were also calculated using a density of 2,000 kg/m3. The depth of the crust-mantle boundary was controlled using the following parameters: a crustal thickness at sea level of 25 km, a density contrast of 400 kg/m3 between the crust and mantle, and a crustal density of 2,670 kg/m3. EXPLANATION OF PRINCIPAL FACT FORMAT FOR GRAVITY STATIONS (iso-all2.txt) File iso-all.txt is tab-delimited format. Lines denoted by "\" are header lines that separate data of different sources. Item Explanation STATION_NAME: An alphanumeric combination of up to 8 characters used for station identification. LATD: Degree latitude. LATM: Minute latitude. LOND: Degree longitude. LONM: Minute latitude. ELEV: Elevation in feet. OG: Observed gravity in mGal (-999, unavailable) FAA: Free-air anomaly in mGal. SBA: Simple Bouguer anomaly in mGal (-999, unavailable) ITC: Inner zone terrain correction out to a radius of various distances (see TC CODE) from the station, for a density of 2,67 kg/m3. (-999, unavailable) TTC: Total terrain correction from the station to 166.7 km for a density of 2,670 kg/m3. (-999, unavailable) TC_CODE: Letter denoting the extent of the inner-zone correction, according to the Hayford-Bowie and Hammer templates (D=590 meters; M=2000 meters; F=2290 meters; G=1530 meters; H=2615 meters; U means unknown, but likely D, or unavailable). See Spielman and Ponce (1984) for additional explanation. CBA: Complete Bouguer gravity anomaly reduced for a density of 2,670 kg/m3. ISO: Isostatic residual gravity anomaly values (reduced for a density of 2,670 kg/m3). CBA_200: Complete Bouguer gravity anomaly reduced for a density of 2,000 kg/m3. Only available for onshore data. ISO_200: Isostatic residual anomaly values (reduced for a density of 2,000 kg/m3). Only available for onshore data. Example of format for gravity file BKH 632 35 21.08 119 59.72 2015.0 979567.44 -5.83 -74.55 0.01 0.84 F -74.48 -36.52 -57.25 -19.29 STATION_NAME: BKH 632 LATD: 35 degrees North LATM: 21.08 minutes LOND: 119 degrees West LONM: 59.72 minutes ELEV: 2015.0 feet OG: 979567.44 mGal FAA: -5.83 mGal SBA: -74.55 mGal ITC: 0.01 mGal TTC: 0.84 mGal TC_CODE: F (2290 m) CBA: -74.48 mGal ISO: -36.52 mGal CBA_200: -57.25 mGal ISO_200: -19.29 mGal References Spielman, J.B., and Ponce, D.A., 1984, Handtc, a fortran program to calculate inner-zone terrain corrections: U.S. Geological Survey Open-File Report 84-777, 24 p. Swick, C.A., 1942, Pendulum gravity measurements and isostatic reductions: U.S. Coast and Geodetic Survey Special Publication 232, 82 p. ____________________________________________________________ AEROMAGNETIC DATA Two files contain aeromagnetic information. One file, aeromag_merged_grid.xyz contains total-field anomaly values from a grid of merged aeromagnetic surveys. See report for details. The data in the grid were were interpolated to a rectangular grid with nodes spaced 200 m apart using a minimum curvature algorithm (Briggs, 1974) using a Transverse Mercator projection (central meridian -123.0, base latitude 0.0; scale factor 0.9996). There are three fields in each file, with longitude and latitude (NAD27 datum) being the first two. The third field is total-field anomaly. The second file, lucia_mag.xyz, contains data from the Santa Lucia aeromagnetic survey flown in 2009. The file contains the principal data fields of the aeromagnetic measurements. The first line (marked by a "/") has an abbreviated description for each measurement. The format of the data is described below. Each measurement has a line number, latitude, longitude, date and time information, radar and GPS altitude and various levels of magnetic field measurements. Measurement Description: line * Line number latnad27: decimal degrees latitude coordinate (NAD27 datum) lonnad27: decimal degrees longitude coordinate (NAD27 datum) xwgs84nutm10: meters x UTM zone 10 coordinate ywgs84nutm10: meters y UTM zone 10 coordinate gtime: seconds GPS time, seconds past midnight date: YYYY/MM/DD Date (year/month/day) radarm: meters radar height (meters above ground level) barom: meters barometric altitude galt: meters GPS height above WGS84 geoid diurnal: nT measured base station magnetic field rawmag: nT raw magnetic total field cmag: nT compensated raw magnetic field magdc: nT diurnally-corrected compensated magnetic field maglag: nT lag-corrected magdc magnetic field igrf: nT IGRF model 2005 calculated for actual time and elevation of data acquisition magigrf: nT maglag minus igrf correction plus 54425 nT tielev: nT preliminary leveled total field mag (tie lines only) maglev: nT Preliminary leveled traverse lines (using tielev) magfinal nT final tie-line leveled, decorrugated and microleveled magnetic field References Briggs, I., 1974, Machine contouring using minimum curvature: Geophysics, v. 39, no. 1, p. 39-48. ____________________________________________________________ ROCK PROPERTY DATA Rock property data are contained in the ASCII tab-delimited file all-prop.txt. Lines marked with a "/" separate physical property data from different sources or rock types and give header information. Every data point has a sample id, latitude, and longitude. The horizontal locations for the stations are on the North American Datum 1927 (NAD27). Values of -999 denote that densities (in kg/m3), magnetic susceptibility (in SI), intensity of natural remanent magnetization (in A/m), or depth (in feet for well data) are not available. See header line denoted by "/" for data fields.