Readme for Data Files

The digital data and text of this open-file report are available from the
following U.S. Geological Survey URL:  

https://doi.org/10.3133/ofr20191128

Data Contents

The four files included in Open-File Report 2019–1128 are given as ascii, comma-delimited data files.  All locations are referenced to the NAD27 horizontal datum and are in geographic coordinates.

The file gravity_data.csv contains gravity measurements.  Each gravity measurement has a name, location (latitude and longitude), elevation (NGVD29 datum), an observed gravity value, followed by free-air, Bouguer, and isostatic anomalies and terrain correction values. A header line separates previously collected data from data collected for this report.

The data are on the IGSN71 gravity datum and the reference ellipsoid is the Geodetic Reference System 1967 (GRS67). The free-air gravity anomalies were calculated using standard formulas (Telford and others, 1976). The Bouguer, curvature, and terrain corrections were applied to the free-air anomaly at each station to determine the complete Bouguer gravity anomalies at a reduction density of 2670 kg/m3 (Plouff, 1977). An isostatic correction was then applied to remove the long-wavelength effect of deep crustal and/or upper mantle masses that isostatically support regional topography (Jachens and Roberts, 1981).

Item					Explanation

NAME            ..................Station Name--An alphanumeric combination of 
				           up to 8 characters used for station  
                                       identification

latd              ...................Degree Latitude North
latm             ...................Decimal Minute 

lond             ...................Degree Longitude West
lonm            ...................Decimal Minute

elev_ft         ...................Elevation in feet (NGVD29 datum)

obs_grav      ...................Observed gravity, in mGal

faa              ....................Free-air anomaly, in mGal

sba            .....................Simple Bouguer anomaly, in mGal

itc             ....................Inner terrain correction for a density of
				        2,670 kg/m3, in mGal.  See Code column for 
                                    radius of correction.

ttc            .....................Total terrain correction from the station to
				         166.7 km for a density of 2,670 kg/m3, in 
 				          mGal.

code            ...............Letter denoting the zone (radius) of the 
                                  correction, "D"= 0.59 km; "G" = 3.52 km; 
                                  "M"= 2.0 km.

cba            ..................Complete Bouguer anomaly reduced for a
				          density of 2,670 kg/m3, in mGal.

iso             ...................Isostatic residual anomaly,in mGal,assuming an
				        Airy model for isostatic compensation of
				        topographic loads.  This model assumes a
			 	        a crustal thickness of 25 km, a topographic
				        load density of 2,670 kg/m3, and a density 
				        contrast across the base of the model crust
				        of 400 kg/m3.  Isostatic correction would be 
                                   difference between CBA and ISO values.

NAME              latd       latm         lond       lonm    elev_ft obs_grav       fas          sba           itc          ttc   code             cba          iso
PB1104	34	54.75	-117	42.71	2738.4	979462.12	-5.79	-99.19	0.12	0.39	M	-99.77	-16.54	

—————————————————————————————————————————————————————————
Two files contain well information taken from Cromwell and others (in press), adding the depth to bedrock from the gravity inversion.  basement_wells.csv contains well information for those wells interpreted to intercept bedrock.  basin_wells.csv contains well information for those wells interpreted to have bottomed in basin-fill deposits.  Both files contain well identification, latitude, and longitude.

basement_wells.csv
Well_ID          ………………well name—alphanumeric combination used for well identification
latitude          ………………decimal degree latitude (north)
longitude        .……………..decimal degree longitude
well_depth_to_bedrock…..depth in feet relative to ground surface of bedrock intercept by well
model_depth_bedrock…….depth in feet relative to ground surface of bedrock intercept modeled by gravity inversion

      Well_ID                                             latitude                             longitude           well_depth_to_bedrock     model_depth_bedrock
009N008W6H01S	34.90529282	-117.756414	459	453.332	

basin_wells.csv
Well_ID          ………………well name—alphanumeric combination used for well identification
longitude        .……………..decimal degree longitude
latitude          ………………decimal degree latitude (north)
total_depth    ………………total depth of well in feet 
model_depth_bedrock…….depth in feet relative to ground surface of bedrock intercept modeled by gravity inversion

     Well_ID                                             latitude                             longitude                            total_depth        model_depth_bedrock
009N006W18K01	-117.5529	34.8736	46	102.6723	

—————————————————————————————————————————————————————————

The file containing grid values of the modeled depth to bedrock is depth_to_bedrock.csv.  It contains longitude, latitude, and depth_to
bedrock in feet relative to the ground surface.  The values are spaced 250 m apart.
                                                    longitude                                                        latitude                                        depth_to_bedrock
-118	34.7489	-5919.8701	

Cromwell, G., Fenton, N.C., and Ely, C.P., in press, Select borehole data for Edwards Air Force Base and vicinity, Antelope Valley, California:  U.S. Geological Survey Data Release, https://doi.org/10.5066/P90N3MQQ.

Jachens, R.C., and Roberts, C.W., 1981, Documentation of a Fortran program, 
'ISOCOMP', for computing isostatic residual gravity:  U.S. Geological Survey 
Open-File Report 81-574, 26 p.

Plouff, Donald, 1977, Preliminary documentation for a FORTRAN program to 
compute gravity terrain corrections based on topography digitized on a 
geographic grid:  U.S. Geological Survey Open-File Report 77-535, 45 p.

Telford, W.M., Geldart, L.O., Sheriff, R.E., and Keyes, D.A., 1976, Applied
Geophysics:  New York, Cambridge University Press, 960 p.