README The digital data of this open-file report are available from the following U.S. Geological Survey URL: http://pubs.usgs.gov/of/2013/1024/i/ DATA CONTENTS The database includes an ASCII file containing new helicopter aeromagnetic data for the Leach Basin area, an ASCII file containing new aeromagnetic data from the Soda Lake region, an ASCII file with gridded values of helicopter data merged with preexisting detailed aeromagnetic survey data, two ASCII files with point locations of the local maximum horizontal gradient derived from the aeromagnetic data, and one ASCII file with magnetic susceptibility data. Two digital datasets consist of two files (LeachMag.xyz and SodaMag.xyz) The LeachMag.xyz contains new helicopter aeromagnetic data for the Leach Basin in the northern part of the Fort Irwin National Training Center, 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. These data were flown at the same time as the airborne electromagnetic data. The format of the data is described below. Each measurement has a line number, latitude, longitude, radar and GPS altitude and various levels of magnetic field measurements. Measurement Description: Line * Line number lat decimal degrees latitude coordinate (NAD27 datum) lon decimal degrees longitude coordinate (NAD27 datum) galtf meters Height above sea level Ralt_lag meters radar height (meters above ground level) balt meters barometric altitude lalt meters laser altitude magU_raw nT raw magnetic total field magU_filt nT raw magnetic field filtered to remove spikes and corrected for lag of ~0.1 seconds basemag nT measured base station magnetic field magU nT diurnally corrected magU_filt magnetic field IGRF nT Calculated value from IGRF model maguf nT IGRF-corrected magU magnetic field The other digital dataset, Sodalake_mag.xyz contains new aeromagnetic data for the Soda Lake region in the eastern part of the Fort Irwin National Training Center, 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. Measurement Description: Line * Line number lat decimal degrees latitude coordinate (NAD27 datum) lon decimal degrees longitude coordinate (NAD27 datum) date YYYY/MM/DD Date (year/month/day) galt meters Height above sea level radarm meters radar height (meters above ground level) barom meters barometric altitude rawmag nT raw magnetic total field cmag nT compensated raw magnetic field maglag nT lag-corrected magnetic field (magdc) diurnal nT measured base station magnetic field magdc nT diurnally-corrected compensated magnetic field maghdg nT heading-corrected magnetic field tielev nT leveled magnetic field using tielines maglev nT microleveled magnetic field igrf nT IGRF model 2005 calculated for actual time and elevation of data acquisition magigrf nT maglag minus igrf correction plus 54425 magfinal nT final tie-line leveled, decorrugated and microleveled magnetic field 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. An ASCII file of the gridded detailed aeromagnetic data, mag_hires_grid.xyz, contains longitude, latitude and magnetic field values from a rectangular grid with a spacing of 250 m. The projection of the grid is Transverse Mercator with a central meridian of -117.0, a base latitude of 0.0, and a scale faults of 0.9996. Locations are on the NAD27 datum. Note that the grid contains regions of nodata, which reflects the data coverage. Magnetic Susceptibility Measurements An additional file named mag_susc.txt contains magnetic susceptibility measurements from hand samples in the map area. Data are listed as tab delimited values. Magnetic susceptibilities were measured using a Geophysica KTÜ5 susceptibility meter and are reported to 0.01 x 10-3 SI units. The Geophysica KT-5 calculates volume susceptibility by assuming the sample shape is an infinite half-space. The instrumentÍs ability to measure magnetic susceptibility is affected by surface roughness, weathering, and sample size, all of which can result in an underestimation of a sample's true susceptibility. The magnetic susceptibility values reported represent the highest reading on the sample. Blakely, R.J., and Simpson, R.W., 1986, Approximating edges of source bodies from magnetic or gravity anomalies: Geophysics, v. 51, p. 1494-1498.