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 standares. Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey. TEXT FOR 9-TRACK TAPE, 6/18/93, p. 1 INTRODUCTION As part of a two-year assessment of the geology and mineral resources of the Altiplano and Cordillera Occidental, Bolivia, aeromagnetic and gravity data were acquired from various sources and merged into digital data sets. These were published as contour maps (Plates 4 and 5 of Cady and Wise, 1992) in the final report of the study, USGS Bulletin 1975. With approval of the Bolivian Ministry of Mines and Metallurgy, this report, consisting of this text and two 9- track ASCII magnetic tapes, makes the digital data available to the public. For information about sources of data and their processing, see Cady and Wise (1992). 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. TEXT FOR 9-TRACK TAPE, 6/18/93, p. 2 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 TEXT FOR 9-TRACK TAPE, 6/18/93, p. 3 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 Channel 1 is the x-coordinate (east-west) in UTM kilometers. Channel 2 is the y-coordinate (north-south) in UTM kilometers. Channel 3 is the free air anomaly in mGal. Channel 4 is the simple Bouguer anomaly in mGal. Channel 5 is the elevation in meters above mean sea level. Channels 6-8 are zero. TEXT FOR 9-TRACK TAPE, 6/18/93, p. 4 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 TEXT FOR 9-TRACK TAPE, 6/18/93, p. 5 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 TEXT FOR 9-TRACK TAPE, 6/18/93, p. 6 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 67330308 -9 390 -683970 12800 7814281 355 -9 500 -684000 12190 7814266 78371586 -92400 -631500 10000 7814133 78371486 -95570 -63 350 11740 7816291 67330309 -95640 -674940 13500 7813905 67330311 -95840 -674870 15250 7813496 78371078 -95848 -674860 15000 7813461 Key: Station latlat longlong elev og-900000 degmin deg min m*10 mGal*100 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=.01 381 61 40-152912 -691586383910 7733247 580 61 40-152921 -69 348389697 7733635 579 61 40-152928 -69 364394845 7732685 380 61 40-152933 -691572384339 7733285 578 61 40-152947 -69 359398102 7732092 379 61 40-152952 -691552384188 7733448 378 61 40-152971 -691533384448 7733393 Key: Station latlat longlong elev og-900000 degmin deg min m*10 mGal*100 1 2 3 4 Col: 12345678901234567890123456789012345678901234567890123 The preceding 4 non-blank lines are a key to the columns and fc TEXT FOR 9-TRACK TAPE, 6/18/93, p. 7 n 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 The ASCII text of this written report. TEXT FOR 9-TRACK TAPE, 6/18/93, p. 8 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: Item Format Description id a6 Identification ifid i7 Fiducial number iyr i5 Year itime i6 Time flat f8.4 Latitude (degrees) flon f9.4 Longitude (degrees) radar f4.0 Radar altimeter (meters above surface) baro f5.0 Barometric altitude (meters above sea level) tf f8.2 Total magnetic field (nanoteslas) rtf f8.2 Reduced total mag. field (IGRF removed) nT APPENDIX 1 Specifications of usgs standard grid (Cordell and others, 1992): 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). Header record (23 4-byte words): id: 56 ascii characters of identification (character*56). pgm: 8 ascii characters identifying creating program (sometimes); character*8. TEXT FOR 9-TRACK TAPE, 6/18/93, p. 9 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). Data record. Each data record contains one row of scalar, real-valued data. The first word should contain the row coordinate, but in some programs this is not adhered to. In the data of this report, the first column is always "0". Subsequent words contain data: f(1,j), f(2,j)...f(ncol,j), for the j-th row of data. As an example of Fortran grid io: 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) All grids are rectangular. Areas within the grid containing no data are flagged by dummy values (DVALS), normally 1.0e+38. APPENDIX 2 Specifications of post file (after Cordell and others, 1992): LINE AND POINT DATA: 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 TEXT FOR 9-TRACK TAPE, 6/18/93, p. 10 *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. As an example, input and output of post files can be stated in Fortran as: character*8 id real *4 x,y,f(6) read or write (..) id,x,y,f APPENDIX 3 Specifications for USGS standard gravity file: Columns Format Description 1-8 a8 Alphanumeric identifier 9-10 i2 Year 11-13 i3 Julian date 14-15 i2 Time, hours, 24 hour clock 16-17 i2 Time, minutes 18-21 f4.1 Time zone, positive west from Greenwich TEXT FOR 9-TRACK TAPE, 6/18/93, p. 11 Mean Time, eg. MDT = 6.0 22-24 f3.0 Latitude degrees, positive north 25-28 f4.2 Latitude minutes, eg. 5999 -> 59.99 29-32 f4.0 Longitude degrees, positive east 33-36 f4.2 Longitude minutes, eg. 5999 -> 59.99 37-42 f6.2 Elevation in meters, eg. 123395 -> 1233.95 or 37-42 f6.2 Elevation in feet, eg. 040484 -> 4048.4 43-44 a2 Alphanumeric state or country code 45-51 f7.2 Observed gravity in milligals - 900,000.00 52-57 f6.2 Inner-zone terrain correction in milligals 58-61 f6.2 Outer-zone terrain correction in milligals 64-69 f6.2 Field of user's choice 70-75 f5.3 Tide correction, milligals 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. TEXT FOR 9-TRACK TAPE, 6/18/93, p. 12