U.S. Department of the Interior EROS Data Center

Digital aeromagnetic and Bouguer gravity anomaly data from the Bolivian Altiplano and Cordillera Occidental used to create contour maps for USGS Bulletin 1975

by John W. Cady
U.S. Geological Survey
Federal Center, MS 964
Denver, Colorado 80225
 

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards. Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey.

INTRODUCTION

In 1991 a new aeromagnetic survey was flown for the Servicio Geologico de Bolivia (GEOBOL) and the Bolivian Ministry of Mines and Metallurgy by BGM Airborne Surveys, Inc. The data, which cover an area contiguous to the surveys included in this report, were obtained too late to be included in Bulletin 1975. The digital tapes of flightline data for this 1991 BGM survey, as well as ASCII grids of the data transferred to 1.2 Mbyte diskettes readable on IBM-compatible personal computers (Cady, 1993) are available from the U.S. Trade and Development Agency1.

CONTENTS OF MAGNETIC TAPES

TAPE 1:

Tape 1 is an unlabeled ASCII 9-track tape written at 6250 bpi containing a composite simple Bouguer gravity anomaly grid (1000 meter grid interval) and a composite total field aeromagnetic anomaly grid (500 meter grid interval) used to create Plates 4 and 5 of Bulletin 1975. For most purposes, these are the only data files a user will need. Also included on Tape 1 are: grids of the individual magnetic surveys used to create the composite magnetic grid; edited ASCII “post” file of merged YPFB (Yacimientos Petroliferos Fiscales Bolivianos) and DOD (U.S. Department of Defense, Defense Mapping Agency) gravity station data used to create the composite gravity grid; files containing principal facts of YPFB and DOD gravity data; and Fortran 77 code of selected computer programs that can read most of the above-mentioned files.

All grid and post files are in UTM coordinates, central meridian = -69 degrees (minus sign indicates west longitude), base latitude = -23 degrees (minus sign indicates south latitude). Units are in kilometers with no false easting or northing. The gravity station files indicated by the suffix “.std” are in in a geodetic (latitude and longitude) coordinate system.

1 Address: U.S. Trade and Development Agency
Room 309
SA-16
Washington, D.C., 20523-1602
Attention Orlando Velez.
Telephone (703) 875-4357
Fax (703) 875-4009

There are 18 files on Tape 1. They all have record length = 80, block size = 4000. The files are:

File 1 -  allbou.asc:

ASCII version of merged USGS standard grid file containing simple Bouguer gravity anomaly. Includes a somewhat larger area than Plate 4 in USGS Bulletin 1975. Bouguer reduction density is 2.67 g/cm3. When converted to binary using program ASCII2SF and examined using program IDMAX, the following output describes the grid:

idmax
Enter input file name: allbou.grd
DOD+YPFB Altiplano Bouguer Anom, red.
den=2.67
min-curv
ncol = 550
nrow = 900
nz = 1 x0 = -120.0000
dx = 1.000000
y0 = -20.00000
dy = 1.000000 min = -492.6502
col row 95 568
max = 29.81416
col row 7 356
range= 0.522E+03
277555 dvals;
43.9% of grid defined.
FORTRAN STOP

File 2. bull1975mag.asc:

ASCII version of USGS standard grid file containing merged residual magnetic field (International Geomagnetic Reference Field or other regional removed, see Bulletin 1975). Includes a somewhat larger area than Plate 5 in USGS Bulletin 1975. When converted to binary using program ASCII2SF (file 10) and examined using program IDMAX (file 12), the following output describes the grid (variables are defined in Appendix 1):

idmax
Enter input file name: bull1975mag.grd
Merged Bolivian Aeromagnetic Data for USGS Bulletin 1975
ncol = 748
nrow = 1230
nz = 1 x0 = -15.00000
dx = 0.5000000
y0 = 142.0000
dy = 0.5000000
min = 2.917969
col row 83 685
max = 815.5176
col row 326 496
range= 0.813E+03
550681 dvals;
40.1% of grid defined.
FORTRAN STOP

File 3. dodm3920pypfbm14.asc:

ASCII version of USGS post file containing gravity data used to make the gravity grid in File 1. Can be converted to a binary post file (see Appendix 2) using program A2POS. The file name has the following derivation: “dod”--DOD/Defense Mapping Agency Gravity Library; “m3920”--minus DOD project 3920 which was deleted because it conflicted with YPFB gravity stations; “pypfb”--plus YPFB; “m14”--14.0 mGal subtracted to convert to the new Potsdam gravity datum (Cady and Wise, 1992).

The file contains fewer gravity stations than files dodgrav.std and ypfbgrav.std from which it was created. DOD project 3920 was eliminated as described above, and other duplicate or erroneous stations were eliminated from the post file.

When converted to binary using program A2POS and examined using program XYZMAX, the following information is revealed about the file:

xyzmax enter filename:
dodm3920pypfbm14.pos
post file input with 8 data channels
enter starting and ending channel numbers:
1 8
# 1 min= -107.828 max= 1393.44 range= 1501.27
# 2 min= -58.7039 max= 1541.38 range= 1600.08
# 3 min= -385.780 max= 753.340 range= 1139.12
# 4 min= -536.420 max= 83.8000 range= 620.220
# 5 min= 0.000000E+00 max= 5982.59 range= 5982.59
# 6 min= 0.000000E+00 max= 0.000000E+00 range= 0.000000E+00
# 7 min= 0.000000E+00 max= 0.000000E+00 range= 0.000000E+00
# 8 min= 0.000000E+00 max= 0.000000E+00 range= 0.000000E+00
59933 logical records in the file
FORTRAN STOP

File 4. scgabmag.asc

ASCII version of USGS standard grid file containing residual magnetic field of 1962-63 SGCAB survey. Flown 150 m above terrain. SCGAB removed a regional trend instead of the IGRF. Hand-digitized by Patterson, Grant, and Watson. (In the grid names and header records, SGCAB is inadvertently misspelled “scgab”.) When converted to binary using program ASCII2SF and examined using program IDMAX, the following output describes the grid:

idmax
Enter input file name: scgabmag.grd
SCGAB aeromag -636nT.
Not continued upward.
spline2d
ncol = 298
nrow = 448
nz = 1 x0 = 210.0000
dx = 0.5000000
y0 = 329.0000
dy = 0.5000000
min = 24143.45
col row 116 59
max = 24587.53
col row 65 135
range= 0.444E+03
74336 dvals;
44.3% of grid defined.
FORTRAN STOP

File 5. scgabmaguc.asc 

ASCII version of USGS standard grid file containing residual magnetic field of 1962-63 SGCAB survey. Flown 150 m above terrain and continued upward 1 km to a nominal elevation of 1.150 km above terrain. 636 nT were subtracted from data values to merge with YPFB- AeroService survey. When converted to binary using program ASCII2SF and examined using program IDMAX, the following output describes the grid:

idmax
Enter input file name: scgabmaguc.grd
SCGAB mag up-cont 1km above “150m” flt lines,
-636nt gmask
ncol = 298
nrow = 448
nz = 1 x0 = 210.0000
dx = 0.5000000
y0 = 329.0000
dy = 0.5000000
min = 24268.77
col row 117 59
max = 24473.63
col row 197 82
range= 0.205E+03
74336 dvals;
44.3% of grid defined.
FORTRAN STOP

File 6. ypfbasmag.asc

ASCII version of USGS standard grid file containing residual magnetic field of 1988-1989 YPFB-AeroService survey. IGRF updated to time of survey removed by contractor. This is the best of the data sets used in Bulletin 1975. It is the reference to which the other surveys are merged in bull1975mag.grd. When converted to binary using program ASCII2SF and examined using program IDMAX, the following output describes the grid:

run idmax
Enter input file name: ypfbasmag.grd
YPFB-AeroService aeromag from the Bolivian Altiplano
min-curv
ncol = 378
nrow = 906
nz = 1 x0 = 85.00000
dx = 0.5000000
y0 = 142.0000
dy = 0.5000000
min = 24053.65
col row 302 375
max = 24703.34
col row 302 381
range= 0.650E+03
133553 dvals;
61.0% of grid defined.
FORTRAN STOP

File 7. ypfbpraklamag.asc

ASCII version of USGS standard grid file containing residual magnetic field of 1967-68 YPFB-Prakla survey. Hand digitized by Esther Sandoval-Castellanos in USGS Branch of Geophysics. When converted to binary using program ASCII2SF and examined using program IDMAX, the following output describes the grid:

run idmax
Enter input file name: ypfbpraklamag.grd
Hand-digitized YPFB-Prakla aeromag +446nT
min-curv ncol = 598
nrow = 1136
nz = 1 x0 = -15.00000
dx = 0.5000000
y0 = 189.0000
dy = 0.5000000
min = 24002.92
col row 83 591
max = 24836.29
col row 326 403
range= 0.833E+03
498247 dvals;
26.7% of grid defined.
FORTRAN STOP

File 8. dodgrav.std

USGS standard (ASCII) gravity file containing principal facts for 8,162 gravity stations obtained from the Defense Mapping Agency gravity library. The first 10 lines of dodgrv.std follow:

DOD Bolivia Gravity, Lat. 9-23 South, Long. 56-73W.
Converted using new DOD2STD prog. by JCady 10/31/90
2g-xxx dodbol
elev=m
gu=.01

                          1                  2                  3                  4
Col: 12345678901234567890123456789012345678901234567890123

The preceding 4 non-blank lines are a key to the columns and variables in the file. For information about the complete format of a USGS standard gravity file, see Appendix 3.

File 9. ypfbgrav.std

USGS standard (ASCII) gravity file containing principal facts for 55,178 gravity stations obtained from YPFB. For specifications of standard gravity file, see Appendix 3. The first 10 lines of dodgrv.std follow:

YPFB Bolivia Altiplano gravity.
Converted to USGS .std format
2g-xxx ypfb
elev=m
gu=.01

                          1                  2                  3                  4

The preceding 4 non-blank lines are a key to the columns and variables in the file. For information about the complete format of a USGS standard gravity file, see Appendix 3.

File 10. ascii2sf.for

Fortran program to convert from ASCII version of USGS standard grid to binary version. For specifications of standard grid file, see Appendix 1.

File 11. sf2ascii.for 

Fortran program to convert from binary version of USGS standard grid to ASCII version. For specifications of standard grid file, see Appendix 1.

File 12. idmax.for

Fortran program to print identification, grid parameters, and minimum and maximum of USGS standard grid.

File 13. pos2a.for

Fortran program to convert USGS binary post file to ASCII format. For specifications of post file, see Appendix 2.

File 14. a2pos.for

Fortran program to convert USGS ASCII post file to binary. For specifications of post file, see Appendix 2.

File 15. xyzmax.for

Fortran program to determine range of variables of USGS binary post file. (Also used for “xyz” files, not used in this data release.)

File 16. dod2std.for

Fortran program to convert DOD gravity station data to USGS standard gravity format. Included to show file dodgrav.std was created.

File 17. readstd.for

Simple program to read and count the number of stations on the limited versions of standard gravity files used on this tape. A user can modify this program to convert standard gravity files to another desired format.

File 18. readme.doc

TAPE 2:

Tape 2 is an unlabeled ASCII 9-track tape written at 6250 bpi. It contains three files of magnetometer flight-line data provided by AeroService, Inc., from the 1988-1989 YPFB-AeroService aeromagnetic survey. The files are not organized by geographic area--each file contains flight lines that cover large parts of the survey area. The 6250 bpi tape provided contains data originally supplied by AeroService on three separate 1600 bpi tapes.

No detailed specifications are available for the data on the tape, and the notes I made when processing the data in 1990 are cryptic. The tapes can be read to disk using an ASCII tape copy program. Examination of the ASCII data should indicate where header information ends and flight-line data begins. Each record contains the following data items and formats:

APPENDIX 1

GRID DATA: (Referred to as “standard file” in some of the programs). Gridded input and output to all the programs are in standardized binary grid format, often with file-name suffix “.grd”. Currently, the standard grid applies only to real-valued scalar data in rectangular cells. The grid file consists of a header record followed by one record for each row of data. Row 1 is the first row stored. Origin is in the lower left (southwest) corner, starting at row 1, column 1. Row numbers increase upward (northward); column numbers increase to the right (eastward).

id: 56 ascii characters of identification (character*56).
pgm: 8 ascii characters identifying creating program (sometimes); character*8.
ncol: number of columns (integer).
nrow: number of rows (integer).
nz: number of words per data element. Normally nz = 1. (integer).
x0: position of first column of data, in km (real*4).
dx: delta x, spacing interval of columns, in km (real*4).
y0: position of first row of data, in km (real*4).
dy: delta y, spacing interval of rows, in km (real*4).

read or write (..) id,pgm,ncol,nrow,nz,x0,dx,y0,dy do
10 j=1,nrow
10 read or write (..) y,(f(i,j),i=1,ncol)

APPENDIX 2

The post file is a binary file containing, for each data point, an identification, the x and y coordinates, and 6 independent variables, sometimes referred to as channels. Post files have no header. (The data in File 3 are an ASCII version of a post file that can be converted to a binary post file using program A2POS.) The fields of each record are:
 
id: Identification of the data point, such as line number for profile or flight-line data, or station number for gravity data. Any 8 ascii characters (character  *8). In the case of flight-line data, the cardinal and intermediate flight directions can be indicated by two- letter code after the line number, e.g.“L100NW ”.

x: the x coordinate, normally decimal degrees of longitude or projected easting in km. (real*4).

y: the y coordinate, normally decimal degrees of latitude or projected northing in km (real*4).

f1,f2,...f6: Six dependant variables, each real*4. Some can be dummies. With gravity data, the sequence convention is: free-air anomaly; Bouguer anomaly including sum of inner and outer zone terrain corrections, if listed in fields 4 and 5, below; elevation in feet or meters; inner zone terrain correction; outer zone terrain correction; and observed gravity in mGals minus 980000.0. With magnetic data the fields occurring after the 8-character id, x and y location values are: total field residual (geomagnetic reference field removed); total field; height above terrain (meters) barometric altitude (meters); fiducial number; and year and Julian day in format XX.XXX.

character*8 id
real *4 x,y,f(6)
read or write (..) id,x,y,f

APPENDIX 3

Specifications for USGS standard gravity file:

REFERENCES

Cady, J.W., and Wise, R.A., 1992, Gravity and magnetic studies, in U.S. Geological Survey and Servicio Geologico de Bolivia, Geology and mineral resources of the Altiplano and Cordillera Occidental, Bolivia, with a section on Application of economic evaluation to deposit models, by Donald I. Beiwas and Robert G. Christiansen: U.S. Geological Survey Bulletin 1975, p. 56-62, plates 4 and 5.

Cady, J.W., compiler, 1993, Grids of aeromagnetic survey flown in the Bolivian Altiplano and Cordillera Occidental, Bolivia, by BGM Airborne Surveys, Inc. in 1991: U.S. Geological Survey Administrative Report (to U.S. Trade and Development Agency), two 1.2 Mbyte IBM format diskettes.

Cordell, Lindrith, Phillips, J.D., and Godson, R.H., 1992, U.S. Geological Survey Potential-Field Geophysical Software Version 2.0, U.S. Geological Survey Open File Report 92-18.