		Aeromagnetic Surveying in Wisconsin 1996:
			Digital Data Files 


				By



			Stephen L. Snyder 1




	U.S. Geological Survey Open-File Report 00-500

				2000




1 USGS, MS 954, 12201 Sunrise Valley Drive, Reston, VA 20192 
Email: ssnyder@usgs.gov


				Introduction


Over the past 12 years the U.S. Geological Survey has conducted a series of
aeromagnetic surveys in Wisconsin aimed at completing the coverage of
high-quality, moderate-resolution aeromagnetic surveying in the State. The
data from these surveys have proven to be an effective tool for delineating
structures in the Precambrian basement in Wisconsin and have been useful in
the study of the mineral resources in the region. Precambrian basement rocks
rarely crop out in this region because of glacial deposits and Paleozoic
sedimentary cover rocks.  Surveys by the U.S. Geological Survey (USGS) in
1988 (Hittleman and others, 1992), 1996 (Snyder, 1998), 1997-1998 (Daniels
and others, 1998,1999) and 1998-1999 (Bracken and Nicholson, 2000) have
closed gaps in the aeromagnetic coverage of the state of Wisconsin.

The survey flown in 1996 is the focus of this report. Aeromagnetic contour
maps (scale 1:125,000) were previously released as open-file paper maps
(Snyder, 1998).  This report releases the digital data for the same survey.
Both digital flight-line and gridded data are included as well as images
of the aeromagnetic data for the three areas. Facts about this survey and
parameters for the digital files are listed below.


				Disclaimer

This Compact Disc-Read Only Memory (CD-ROM) publication was prepared by an
agency of the U. S. Government. Neither the United States Government nor
any agency thereof, nor any of their employees, makes any warranty,
expressed or implied, or assumes any legal liability or responsibility for
the accuracy, completeness, or usefulness of any information, apparatus,
product, or process disclosed in this report, or represents that its use
would not infringe privately owned rights.  Reference therein to any
specific commercial product, process, or service by trade name, trademark,
manufacturer, or otherwise does not necessarily constitute or imply its
endorsement, recommendation, or favoring by the Government or any agency
thereof.  Any views and opinions of the authors expressed herein do not
necessarily state or reflect those of the Government or any agency thereof.
This report has not been reviewed for conformity with the U.S. Geological
Survey editorial standards.

Although all data published on this CD-ROM have been used by the USGS,
no warranty, expressed or implied, is made by that agency as to the
accuracy of the data and related materials and/(or) the functioning of
the software. The act of distribution shall not constitute any such
warranty, and no responsibility is assumed by the USGS in the use of
these data, software, or related materials.

				Survey Facts

The 1996 Wisconsin aeromagnetic survey was flown between August 1996
and September 1996 in three separate areas. These blocks are outlined
on the index map of Wisconsin and are named: A, B, and C (to see these
images go to the end of this report). The index map shows the location
of this survey and the three areas where these data were collected.
The color-shaded-relief images of the aeromagnetic maps for the three
areas show the magnetic field as if illuminated from the north.  This
south-directed illumination is parallel to the flightline direction,
and minimizes enhancement of "herringbone," or flightline noise.

The survey was flown using a U.S. Geological Survey owned Fairchild
Helio-Porter Short Take-Off and Landing (STOL) aircraft at an average
elevation of 500 feet (152 m) above terrain along 215 flight lines
spaced 1/2-mile (805m) apart in a North-South direction. No tie lines
were flown.  See the "Data Processing" section below for more detail.
The total flight line distance for the three areas was about 6,700 miles
(10,780 km).  Navigational information was gathered in real time using an
on-board Global Positioning System (GPS) with an estimated accuracy of +/-
50 meters.  The airborne magnetometer used was a Geometrics G-813 
proton-precession type with 0.5 NanoTeslas (nT) sensitivity.  Magnetic
values were recorded every 0.5 seconds.

Due to a navigational error in Area B, there is no flight line #12
included in the data in this report.  Instead, flight line #13 was
shifted by 1/2 mile due west, resulting in an overlap between flight
lines #12 and #13.  Therefore, flight line #12 was dropped because
flight line 13 covers a longer distance.  This navigational error
accounts for the one-mile gap in the coverage between flight lines
13 and 14 along longitude 92o 45' west.

Two ground-based Geometrics G-856 proton-precession magnetometers were
used to monitor the daily variations of the Earth?s magnetic field.
One base magnetometer was located at the Hayward Municipal Airport near
Hayward, Sawyer County, WI, and was used to process the airborne data
collected for area A (see Index map).  The second base magnetometer was
located at the Osceola Municipal Airfield near Osceola, Polk County, WI,
and was used to process the airborne data collected for areas B and C
(see Index map).  The aircraft operated out of the Hayward, WI airport
for the duration of the entire survey. 

				Data Processing

	Intermittent problems with the Osceola Airfield base magnetometer
while data for area B were collected left gaps in the diurnal corrections
for about 20 percent of the area B records.  Data from the Hayward Airport
base magnetometer were used in place of the missing data from the Osceola
Airfield base magnetometer. For the few times when neither base magnetometer
was collecting data, alternate magnetic base data were obtained from the
Glenlea Geomagnetic Observatory, located near Winnipeg, Manitoba, Canada,
about 360 miles (580 km) northwest of the Osceola Airfield.  These other
base data integrated easily into the airborne data; however, some minor
mismatches have inevitably occurred. See Snyder, 1998.

	Because no tie lines were flown, it was impossible to perform any
flight line leveling of the data at the time the data were first processed,
using various USGS-authored UNIX programs.  As a result, visible striping
occurs parallel to some flight lines in the data.  

	The USGS has since obtained PC-based software from Geosoft, Inc.,
Toronto, Ontario, Canada.  Additional processing and noise removal was
carried out using Geosoft's Oasis Montaj software.  The results were
improved leveling of the flight line data.

	The first 13 fields of each ASCII point data file (.dat file - see
below) were imported into a Geosoft database.  Lambert conformal conic
X and Y coordinates were calculated from the geographic coordinates
using a central meridian of 90 degrees West and a base latitude of 0 degrees.
A minimum curvature grid was interpolated from the profile coordinates
using a 0.25 km node interval.  Color shaded-relief magnetic maps were
produced, using a sun azimuth of 90 degrees, as if illuminated from the
east.  This westward illumination is perpendicular to the flight line
direction, which enhances any flight line noise.  This enhancement showed
that several flight lines in Areas B and C needed manual adjustment before
proceeding with Geosoft's decorrugation and microleveling routines.  The
table below lists the values (in nanoTeslas) of flight lines that were
adjusted for Areas B and C.  Area A required no adjustments to any flight
lines.

Area B				Area C

Line #		nT added		Line #		nT added
------		--------		------		--------
57 S		   -4.0		22 N		    4.0
59 S		   -4.0
65 S		   -1.0
66 N		    1.0
67 S		   -7.0
69 S		   -5.0



	The values given above were added to the "Residual Magnetic Intensity"
(field #13), which was used to create the "Adjusted Magnetic Intensity"
(field #14).  This field is then used as the input channel to Geosoft's
decorrugation routine, which applies a sixth-order high-pass Butterworth
filter combined with a directional filter to produce a noise channel,
called "Decorrugated Noise Value" (field #15).  This field will appear as
long stripes along the flight lines when gridded and displayed.  This
channel is then used as the input channel to Geosoft's microleveling
routine, which applies amplitude limiting low-pass filtering to the noise
channel (field #15), which is intended to remove any remaining geological
signal, leaving only the component of line level drift. The line level
drift is then subtracted from the "Adjusted Magnetic Intensity" channel
(field #14) to produce a leveled output channel named "Final Corrected
Residual Magnetic Value" (field #16).  The grid files and images in this
report are produced from this last channel.



				Disc Organization

This CD-ROM disc is structured with 5 directories/folders as follows:

\DATA 		The basic survey measurements are in three ASCII files
(file extension .dat) derived from the Geosoft Oasis Montaj databases.
Each record of each file contains 16 fields of data per measurement,
at a sampling interval of 0.5 second. USGS style DOS binary point files
(.pos) with 9 fields are also given. 

\GRIDS		The point data have been interpolated into binary raster
USGS style grids (file extension .grd) by using a minimum curvature
gridding algorithm using Geosoft Oasis Montaj software. These grids
were produced from the "Final Corrected Residual Magnetic Value"
channel of the databases. See below for description of the data file
format.  ASCII versions of the USGS grids are also given (.agd). USGS
grids and binary point files can be manipulated using USGS (DOS)
software for the PC (Phillips, 1997). This is freeware that is
included in a separate directory on this disc, called \USGS_PF or
may be downloaded from 
ftp://greenwood.cr.usgs.gov/pub/open-file-reports/ofr-97-0725/pfofr.htm.
Two alternate ASCII file formats are also included. These ASCII grids
permit importation into other software packages. Files with extension
.arc can be imported into Arcview 3 - Spatial Analyst, and files with
extension  .gxf  ("Grid Exchange File," Geosoft, Inc. 1998) can be
imported into several programs including ER Mapper and Geosoft Oasis
Montaj.  The X origin (Xo), the Y origin (Yo), the number of columns
and rows of each grid are given in the description of the Grid Files
below.  The origin of the grids is in kilometers from the Central
Meridian and Base Latitude used to project the data. These are also
given below.

\IMAGES	Images of the aeromagnetic data for each of the three
areas covered by the survey are included in this directory and are
given in 5 graphic formats (file extensions .bmp, .eps, .jpg, .pdf and
.tif), derived from Geosoft Oasis Montaj maps.  The index map was created
as a Postscript file using GCLR (unpublished USGS Fortran program), and
associated UNIX programs written by Robert W. Simpson, Jr. of the USGS,
and was later converted into the other graphics formats.  This folder
contains 3 sub-folders (\250k, \IndexMap, and \PageSize) containing the
images.  See the file Readme.txt in this folder for details.

\ACROBAT	Adobe Acrobat software can be used for viewing PDF files.
The latest version can also be downloaded from the web at this URL: http://www.adobe.com/products/acrobat/readermain.html.

\USGS_PF	The USGS potential field software package (Phillips, 1997).
Installation is described in the readme files.

				File Facts

Point Files 

File Name		Bytes		Records
Wisc96_A.dat	    17,784,984       105,863
Wisc96_A.pos	     4,446,248       105,863
Wisc96_B.dat	    36,809,976       219,107
Wisc96_B.pos	     9,202,496       219,107
Wisc96_C.dat	    12,677,784        75,463
Wisc96_C.pos	     3,169,448        75,463


ASCII Point Data File (.DAT) Format:

Each record represents one measurement recorded at one-half second flight
intervals. The record length is 165 bytes. The "Final Corrected Residual
Magnetic Value" field is the residual total magnetic intensity data with
the Definitive Geomagnetic Reference Field (DGRF) removed and corrected
for flight line errors. The grid files are constructed from this field.

Field	Format     	Field Name						Units
No.
-----	------		-----------		 				-----   
1	A8		Flight-Line number, Direction			
2	F9.5		Longitude						Decimal Degrees
3	F11.5		Latitude						Decimal Degrees
4	I9		Fiducial
5	F10.3		Date (Year.Julian Day)				YYDDD
6	F10.1		Time					          	Seconds of the day
7	F11.2		Radar Altimeter					Feet
8	F11.2		Barometric Altimeter					Feet
9	F11.2		Raw Magnetic Field Intensity			NanoTeslas
10	F9.2		DGRF Base Magnetic Correction			NanoTeslas
11	F11.2		Total Field Magnetic Intensity			NanoTeslas
12	F11.2		DGRF Correction					NanoTeslas
13	F11.2		Residual Magnetic Intensity (DGRF removed)	NanoTeslas
14	F11.2		Adjusted Magnetic Intensity				NanoTeslas
15	F11.2		Decorrugated Noise Value				NanoTeslas
16	F11.2		Final Corrected Residual Magnetic Value		NanoTeslas





USGS Binary Post File (.POS) Data Fields:

Field	Description					Units
------	---------------				-------
1	Flight-line number, Direction
2	Longitude 					Degrees 
3	Latitude 					Degrees 
4	Final Corrected Residual Magnetic Value	Nanoteslas 
5	Total Field Magnetic Intensity		Nanoteslas 
6	Radar Altimeter 				Feet 
7	Barometric Altimeter 			Feet 
8	Fiducial 
9	Year.Julian Day 				YYYY.DDD


				Grid Files

	Residual Total Magnetic Intensity Binary Grid Files, USGS Grid Format

Grid File		Rows	Columns	Xo (km)	Yo (km)
Wisc96_A.grd		190	353		-123.75	5395.75
Wisc96_B.grd		280	402		-224.75	5310.25
Wisc96_C.grd		241	165		-197.50	5282.75

	Grid Parameters (.grd, .agd, .gxf, .arc)

X, Y units:                    Kilometers
Grid node interval:            0.25 kilometers
No-Data Value (.grd, .agd):    10E+38
No-Data Value (.gxf):          -99999.99
No-Data Value (.arc):          -99999.99
Ellipsoid:                     Clark 1866
Horizontal Datum:              1927 North American Datum 
Projection:                    Lambert Conformal Conic
Standard Parallels:            33 degrees North and 45 degrees North
Central Meridian:              90 degrees West
Base Latitude:                 0 degrees


				References Cited

Bracken, R.E., and Nicholson, S.W., 2000, Aeromagnetic Surveying in
Wisconsin 1998-99: Digital Data Files: U.S. Geological Survey Open-File
Report 99-527, 1 CD-ROM.

Daniels, D.L., Snyder, S.L., Nicholson, S.W., and Cannon, W.F., 1998,
New aeromagnetic surveys in Wisconsin by the U.S. Geological Survey:
Institute on Lake Superior Geology, 44th Annual Meeting, May 6-10, 1998,
Minneapolis, Minnesota, pp. 62-63.

Daniels, D.L., Nicholson, S.W., and Cannon, W.F., 1999, Aeromagnetic
Surveying in Wisconsin 1997-98: Digital Data Files: U.S. Geological
Survey Open-File Report 99-28, 1 CD-ROM.

Geosoft, Inc., 1998, GXF Grid eXchange File Revision 3.0, DRAFT 6: 
Geosoft, Inc., Toronto, Ontario, Canada.

Hittleman, A.M., Buhmann, R.W., Racey, S.D., Chandler, V.W., 1992,
Aeromagnetics Earth System Data, Minnesota Region: National Geophysical
Data Center, CD-ROM, diskettes, and User's Manual.

Phillips, J.D., 1997, Potential-Field Geophysical Software for the PC,
version 2.2: US Geological Survey Open-File Report 97-725 34 p.

Snyder, S.L., 1998, Aeromagnetic map of part of northwestern Wisconsin
and adjacent areas: U.S. Geological Survey Open-File Report 98-228,
Scale 1:125,000, 2 sheets.
