

          Aeromagnetic Surveying in Wisconsin 1998-99:
                      Digital Data Files
         by Robert E. Bracken and Suzanne W. Nicholson
         U.S. Geological Survey Open-File Report 99-527



DATA DESCRIPTION



Navigating on this CD

The data products have been placed on this disk in a digital
directory structure that allows specific information and data
objects to be found quickly.  Beginning at the entry point (the
main folder), choose among only three sub-folders and examine the
contents of the folder you select.  Keep following this procedure
until you find the desired information or data object.  This
process is facilitated if you are using an IBM-style PC computer
with a Windows operating system and Word or Adobe compatible
reading software.  In this case, you can navigate among the
read_Nth. files via a system of hyper-links.   Begin by opening
the read_1st. file in the main folder.  The rest should be
self-explanatory.

the pdf_pgm\ folder contains a program for reading .pdf files.  A
pdf reader will allow you to take advantage of the hyper-linked
system.



Survey Specifications

The flight lines were nominally spaced 800 meters apprt and
trended North-South with a flight altitude of 304 meters above
ground level.  Control-lines were spaced 12500 meters apart and
trended East-West.  However, in the southeast corner of the
survey area surrounding Milwaukee, the flight lines were
East-West and the control-lines North-South.

The magnetic data were sampled every 0.1 seconds using a
Geometrics G-822a cesium sensor and a High-Sense frequency
counter, with a resultant resolution of about 0.005 nT per
sample.  Data samples were effectively taken every 6 meters of
ground distance.

Radar, Barometric, and GPS altitudes were recorded and are
included in the line data set found elsewhere on this CD.  Base
magnetometer data were also collected and are supplied as an
independent channel in the line data set.  Surveying was
disallowed during the relatively few periods of greater
geomagnetic activity.

Navigation and position recovery were accomplished using a Global
Position System (GPS) and differential corrections.  The expected
horizontal accuracy is on the order of 3 meters.

Additional details may be found in the appendix, which contains
the logistics reported provided by the contractor, High-Sense
Geophysics Limited.



Survey Time-frame, Location, and Extent

The survey was flown between September 15, 1998 and February 21,
1999.  During this time 74,000 line kilometers of magnetic data
were collected.

Excluding three previous survey areas, this survey filled the
southern one-third of the state of Wisconsin being bounded on
East by Lake Michigan, on the South and West by the Wisconsin
state line, and on the North by the 44th parallel, but also
included a substantial North-East extension over Green Bay.  The
index map shows a color map of the complete survey area
superimposed on a black and white reference map.



Digital Data Products

The original digital data products included 14 line-data files
sampled at 1/10 second intervals and 2 gridded data sets, one of
magnetic data and the other of radar altitude data.  This CD
contains those data products plus a 1-second winnowed line-data
set and Images of a shaded shaded relief map made from the
magnetics grid.  Following are short descriptions of each:

Line Data, 1/10-second:  An ASCII data set consisting of 14 files
that have been compressed.  The compressed data files, summaries
of the contents of each file, and a channel and format
description is given on this CD in the folder named cmpressd\.
Additionally, programs and instructions for uncompressing the
files (Bracken, 2000) on various computer types are included
under cmpressd\cmprpgms\.  The uncompressed data set will take
about 2 Gb of disk space with each file averaging 150 Mb.  The
formatting of the ascii files is such that "free-form" reads with
a fortran program are difficult; specific formats are necessary.

Winnowed Line Data, 1-second:  The original 1/10-second data was
winnowed by a factor of 10 to a 1-second interval.  These files
are included in the winnowed\ folder, taking about 200 Mb.  They
may be viewed immediately with a good ascii editor.  A
description of the winnowing process is included in the folder.
Their summaries, and channel and format description are the same
as the original data.

Gridded Data:  Two large gridded data sets were provide by the
contractor, one the the final leveled magnetic data, the other of
radar altimeter data.  Both grids are provided in Grid Exchange
format and the magnetics grid has been translated to various
other formats as well.  They may found in the grid\ folder along
with descriptions of the more common formats.  Additionally, A
usgs standard file (grid) has been included.  This file format
may be used directly in the USGS potential-field geophysical-
processing software (Phillips, 1997) also included on this CD in
the procpgms\ folder.

Imaged Map:  A map was produced from the magnetics grid, and
images are provided in various formats for 800, 400, and 200
meter pixel sizes.  These may be found in the reports\images\
folder.

