•  About Gravity Anomaly Maps
•  Examples of Gravity Anomaly Maps

About Gravity Anomaly Maps
Gravity anomaly maps depict the difference between theoretical computed gravity values and observed gravity values for a region of the earth's crust. The difference between these values may be positive or negative. P. Bouguer first observed this anomaly in 1735, noting that the deviation of a plumb-line in gravitational observations was less than a calculated value. He attributed the anomaly to isostatic compensation resulting from a deficiency of mass in the earth's crust where it underlies mountain ranges.

The anomaly is mapped with isolines (lines of equal value) that represent gravity (isogals). The contour interval uses milligals (1/1000th of a gal) as gravitational units of measure. A gal is a unit of acceleration named after Galileo, equivalent to 1 cm/sec/sec, but is too large to measure the earth's gravitational field anomalies and so it has been divided into milligals to facilitate measurement. Bouguer anomalies are calculated for a sea-level datum using density values of 2.67 grams per centimeter cubed for rock density and 1.00 grams per centimeter cubed for water.

Because gravity anomaly maps depict differences in the density of subsurface materials, they are commonly used in mineral exploration and assessment. Gravity contours are overlaid on bedrock geology base maps, so regional subsurface geology is also depicted on these maps.

Gravity Anomaly maps of Wisconsin have been produced at several scales of detail, covering different parts of the state. As with other maps, gravity anomaly maps show the greatest amount of gravity survey detail (ie. smallest isogal contour interval) on large scale maps. These maps are generally produced at scales of 1:1,000,000 (1"= 16 miles), 1:500,000 (1"= 8 miles), and 1:250,000 (1"= 4 miles).

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Examples of gravity anomaly maps for Wisconsin include:

•  Simple Bouguer gravity map of
Minnesota and northwest Wisconsin
Compiled by C. Craddock, H.M. Mooney, and V. Kohelmainen in 1966-67 and published in 1970 by the Minnesota Geologic Survey and the University of Minnesota (Map M-10, University of Minnesota miscellaneous map series). This map is published at a scale of 1:1,000,000, with a Bouguer gravity contour interval of 10 milligals. Gravity values for this region range from 60 to 90 milligals. Gravity measurement base stations are represented as point symbols, with all additional sources of gravity observations listed on the map as well.


•  Bouguer gravity anomaly map of Wisconsin
Compiled by C.P. Ervin and S. Hammer, published by the University of Wisconsin Extension and the United States Geologic Survey in 1974. All public domain geophysical data for Wisconsin was compiled for this map, and surveys were taken from gravity stations tied to the World Relative Gravity Reference Network. Map scale is 1:500,000 (1"= 8 miles), with a Bouguer gravity contour interval of 5 milligals. This map is produced in two adjoining sheets that cover the north and south portions of the state. A small part of Michigan's Upper Peninsula (near Green Bay) is also included. The Wisconsin share of Lake Superior and Lake Michigan basins is extensively covered.


•  Bouguer gravity anomaly map and geologic interpretation of the Iron River 1 x 2 quadrangle, Michigan and Wisconsin"
Compiled by J.S. Klasner and W.J. Jones in 1989. The scale of this map is 1:250,000 (1" = 4 miles), with a Bouguer gravity contour interval of 2 milligals. The base map uses USGS geophysical data from 1958 and 1967. Gravity stations are represented by point symbols that indicate two levels of precision. Gravity anomaly values range from 40 to -80 milligals over this region. All gravity data were tied to the International Gravity Standardization Net 1971 (Morelli 1974) and the Geodetic Reference System 1967 (International Association of Geodesy, 1971). This is one of the most current gravity anomaly maps produced for Wisconsin and includes several figure insets that display isometric data, gravity trend surface, residual gravity, regional tectonic features, gravity and magnetic profiles, and gravity and geologic profiles.

•  Simple Bouguer gravity map, Northeast sheet
Compiled by C.P. Ervin, K. Tuftee, and J. Funck in 1983. Map scale is 1:250,000 and it is published in black and white.


•  Simple Bouguer gravity map, Superior sheet
Compiled by M.D Thompson, C.P. Ervin, M.L. Czechanski, M.G. Mudrey Jr., and B.A. Brown in 1991. Map scale is 1:250,000.

The Wisconsin Geological and Natural History Survey (WGNHS) sells gravity anomaly maps of Wisconsin. Prices range from $2.00 to $10.00, depending on the level of detail of the map and accompanying report.