1. Simple Bouguer gravity map of Minnesota, St. Paul sheet, M-48 Maps University Of Minnesota Public content 1980. Chandler, Val W., Ervin, C. Patrick, and Van De Voorde, B.W. Bouguer gravity anomaly map (anomaly related to different densities of rocks in the upper crust, Bouguer anomaly is a corrected difference between ... Minnesota Geological Survey.