IMAGES and CAPTIONS :

Total field aeromagnetic anomaly map of the north-Central United States. Data compiled by David L. Daniels and Stephen L. Snyder of the U.S. Geological Survey from various sources. Most of Minnesota was flown with a line spacing of 400 meters and an elevation (above surface) of 150 meters, whereas much of Wisconsin was flown at line spacings of 400-800 meters and elevations of 150-305 meters. Most of the remaining areas were flown at flight line spacings of 1 mile or wider and at elevations of 305 meters or greater. Following gridding, all data were continued to a common elevation of 305 meters and merged. For more detailed descriptions of the original data sets the reader is referred to the USGS.

Aeromangetic data in Minnesota were acquired by the Minnesota Geological Survey, with support form the Legislative Commission on Minnesota Resources. Aeromagnetic data in Wisconsin were acquired with support from the Wisconsin Geological and Natural History Survey and the U. S. Geological Survey.

First vertical derivative aeromagnetic image of the total magnetic anomaly for the north-central United States. Data compiled by David L. Daniels and Stephen L. Snyder of the U.S. Geological Survey from various sources. Most of Minnesota was flown with a line spacing of 400 meters and an elevation (above surface) of 150 meters, whereas much of Wisconsin was flown at line spacings of 400-800 meters and elevations of 150-305 meters. Most of the remaining areas were flown at flight line spacings of 1 mile or wider and at elevations of 305 meters or greater. Following gridding, all data were continued to a common elevation of 305 meters and merged. For more detailed descriptions of the original data sets the reader is referred to the USGS.

Aeromangetic data in Minnesota were acquired by the Minnesota Geological Survey (MGS), with support form the Legislative Commission on Minnesota Resources. Aeromagnetic data in Wisconsin were acquired with support from the Wisconsin Geological and Natural History Survey and the U. S. Geological Survey.

Bouguer gravity anomaly map of the north-central United States. Data compiled by David L. Daniels and Stephen L. Snyder of the U.S. Geological Survey from various sources. Gravity station coverage over much of Minnesota and Wisconsin is 1-2 miles (1.6-3.2 km); in other areas the coverage is commonly 3-6 miles (4.8-9.6 km) or wider. For more detailed descriptions of the original data sets the reader is referred to the USGS.

Gravity data in Minnesota and Wisconsin were largely acquired by the Minnesota Geological Survey, the Wisconsin Geological and Natural History Survey, and Northern Illinois Univerisity.

Map of the second vertical derivative of Bouguer gravity anomaly for the north-central United States. Data compiled by David L. Daniels and Stephen L. Snyder of the U.S. Geological Survey from various sources. Gravity station coverage over much of Minnesota and Wisconsin is 1-2 miles (1.6-3.2 km); in other areas the coverage is commonly 3-6 miles (4.8-9.6 km) or wider. For more detailed descriptions of the original data sets the reader is referred to the USGS.

Gravity data in Minnesota and Wisconsin were largely acquired by the Minnesota Geological Survey (MGS), the Wisconsin Geological and Natural History Survey, and Northern Illinois Univerisity. Second vertical derivative data have been slightly smoothed by upward continuation 2 kilometers. Note: Reduced data quality and resolution across some state lines and shorelines mainly reflects decreasing station coverage.