Illinois, Indiana, and Ohio magnetic and gravity maps and data

Online link https://geo-nsdi.er.usgs.gov/metadata/digital-data/321/metadata.faq.html
Description Aeromagnetic surveys of Illinois, Indiana, and Ohio, merged to form seamless grids across the state. Gravity anomaly data are provided as well.
Originators Daniels, David L.; Kucks, Robert P.; and Hill, Patricia L.
Publication U.S. Geological Survey Data Series 321

Related topics

States or counties

Overview of attributes

Aeromagnetic anomaly grid cells are given in nanoteslas, and values
are expected to range from -400 to 400.
Gravity anomaly grid cells are given in milligals.  Values are
expected to range from -64 to 0 for complete Bouguer anomaly and
from -40 to 20 for isostatic anomaly.
Grids are provided in Geosoft's binary (proprietary) format with
georeference information stored in an ancillary file and as TIFF
images with georeference transformation information stored in
an ancillary .tfw "world" file.

http://pubs.usgs.gov/ds/321/ilinoh_desc.htm