README The digital data of this open-file report are available from the following URL: http://dx.doi.org/10.3133/ofr20151186 DATA CONTENTS The database includes an ASCII file containing new aeromagnetic data for the Sacramento Delta area, an ASCII file containing new aeromagnetic data from the Red Bluff area, an ASCII file containing new aeromagnetic data from the Chico area, an ASCII file with gridded values of the combined aeromagnetic survey data, and two ASCII files with point locations of the local maximum horizontal gradient derived from the aeromagnetic data. Three digital datasets consist of three files (chico_mag.xyz, redbluff_mag.xyz, sacramento_mag.xyz) The chico_mag.xyz contains new aeromagnetic data for the central Northern Sacramento Valley area of California. The file contains the principal data fields of the aeromagnetic measurements, with the first line (marked by a "/") that has an abbreviated description for each measurement. Files should be imported as generic ASCII flat file databases, with white-space separators (particularly for software like Oasis Montaj). Total number of flight lines for the Chico survey is 206; this number includes 32 tie lines. Note that this survey includes two separate areas, a larger eastern area and a smaller western area. Tie lines cross the gap between the two. The format of the data is described below. Each measurement has a line number, latitude, longitude, date information, radar and GPS altitude and various levels of magnetic field measurements. Measurement Description for the Chico aeromagnetic survey: line * Line number lon_27: decimal degrees longitude coordinate (NAD27 datum) lat_27: decimal degrees latitude coordinate (NAD27 datum) X_84: meters x UTM zone 10 coordinate (WGS84 datum) Y_84: meters y UTM zone 10 coordinate (WGS84 datum) date: YYYY/MM/DD Date (year/month/day) fiducial * fiducial time radar_alt: meters radar height (above ground level), corrected for lag cbaro: meters barometric altitude (above mean sea level), corrected for drift dgpsz: meters Differentially corrected GPS height basemag1: nT measured base station magnetic field for base station 1 (39 31'01.3" -122 13'0") basemag1_ed: nT ground base station 1 edited for cultural disturbance basemag2: nT measured base station magnetic field for base station 2 (39 30'35.3" -122 12'47.9") basemag2_ed: nT ground base station 2 edited for cultural disturbance mbu: nT raw magnetic total field (uncompensated, lower tail) mbc: nT compensated raw magnetic field (lower tail) mbc_lag: nT lagged, compensated total-field magnetic mbc_dc_ed: nT diurnally-corrected compensated magnetic field from basemag1_ed maglev: nT tie line leveled total field magnetic igrf2010: nT IGRF model 2010 updated to 2014/12/12 and measured altitude mag_level_igrf nT final leveled residual magnetic (mag_level minus igrf2010) vmi nT Longitudinal vector magnetic field vmv nT Vertical vector magnetic field vmt nT Transverse vector magnetic field vmtf nT Vector magnetic total field The second digital dataset, redbluff_mag.xyz contains new aeromagnetic data for the Red Bluff area in the northern Sacramento Valley of California. The file contains the principal data fields of the aeromagnetic measurements, with the first line (marked by a "/") that 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. Files should be imported as generic ASCII flat file databases, with white-space separators (particularly for software like Oasis Montaj). Total number of flight lines is 262; this number includes 28 tie lines. Measurement Description for the Red Bluff aeromagnetic survey: line * Line number lon_27: decimal degrees longitude coordinate (NAD27 datum) lat_27: decimal degrees latitude coordinate (NAD27 datum) x_84: meters x UTM zone 10 coordinate (WGS84 datum) y_84: meters y UTM zone 10 coordinate (WGS84 datum) GPStime: HH:MM:SS.ss GPS time (hours:minutes:seconds) date: YYYY/MM/DD Date (year/month/day) radar_alt meters radar height (meters above ground level), edited and despiked baro meters barometric altitude (above mean sea level), edited and despiked baro_lev meters barometric altitude (above mean sea level), leveled to GPS gpsz meters differentially corrected GPS altitude diurnal nT measured base station magnetic field at Red Bluff airport (40 9'3.35" -122 14'56.9") maga nT raw magnetic total field, tail cmaga nT compensated raw magnetic field igrf nT IGRF value at data point (altitude and day) using 2010 IGRF model mag_igrf nT lag, diurnally corrected total-field magnetic minus igrf magfinal nT leveled, igrf-corrected total-field magnetic anomaly The third digital dataset, sacramento_mag.xyz contains new aeromagnetic data for the Sacramento Delta area in the central and southern Sacramento Valley of California. The file contains the principal data fields of the aeromagnetic measurements, with the first line (marked by a "/") that 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. This survey also includes vector magnetic field measurements. Note that this dataset contains a few small gaps in coverage over certain cultural areas (for example, Folsom prison). Files should be imported as generic ASCII flat file databases, with white-space separators (particularly for software like Oasis Montaj). Total number of flight lines is 285; this number includes 20 tie lines) Measurement Description for the Sacramento Delta aeromagnetic survey: line * Line number lon_27: decimal degrees longitude coordinate (NAD27 datum) lat_27: decimal degrees latitude coordinate (NAD27 datum) x_84: meters x UTM zone 10 coordinate (WGS84 datum) y_84: meters y UTM zone 10 coordinate (WGS84 datum) GPStime: HH:MM:SS.ss GPS time (hours:minutes:seconds) date: YYYY/MM/DD Date (year/month/day) radar_alt meters radar height (meters above ground level) baro meters barometric altitude (above mean sea level)d gpsz meters differentially corrected GPS altitude diurnal nT measured base station magnetic field at Vacaville airport (38 22'25" -122 57'46") rawmag nT raw magnetic total field, tail cmag nT compensated raw magnetic field maglag nT lag-corrected magnetic field magdc nT diurnally-corrected compensated magnetic field mag_igrf nT lag, diurnally corrected total-field magnetic minus igrf value at data point (altitude and day) using 2010 IGRF model magfinal nT leveled, igrf-corrected total-field magnetic (nT), total field magnetic constant applied back into data An ASCII file of the gridded detailed aeromagnetic data, mag_grid.xyz, contains total-field anomaly values from a grid of combined aeromagnetic surveys. See map 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. Latitude and longitude are given because they are more universal; to obtain 200 m cell size, reproject geographic coordinates to UTM coordinates. In the area where the northern part of the Chico survey overlaps with the Red Bluff survey, data from the Chico survey was retained because the Chico survey was flown closer to the nominal height above terrain. Holes in coverage were filled by interpolation. Two additional files are also included that contain the horizontal maximum gradients calculated using the method described in Blakely and Simpson (1986) on the grid that encompasses the entire merged grid. mag_grad_big.dat contains those gradients larger than the mean gradient; mag_grad_small.dat contains those gradients that are smaller than the mean gradient. See report for details of the measurements. The format of the file is longitude, latitude, and magnitude of horizontal maximum gradient. Locations are on the NAD27 datum. References Blakely, R.J., and Simpson, R.W., 1986, Approximating edges of source bodies from magnetic or gravity anomalies: Geophysics, v. 51, p. 1494-1498. Briggs, I., 1974, Machine contouring using minimum curvature: Geophysics, v. 39, no. 1, p. 39-48.