ONE CENTURY AND COUNTING: THE USE OF GEOPHYSICAL METHODS TO EXPLORE THE UNSEEN GEOLOGY OF MINNESOTA

For over a century, various organizations have used geophysical methods to explore the bedrock geology of Minnesota, most of which is hidden beneath glacial sediments. At the beginning of the last century, magnetic dip needle surveys were extensively used in the search for iron ore which, among other things, led to the discovery of the Cuyuna Iron Range in 1904. Following a successful regional-scale aeromagnetic program by the U. S. Geological Survey (USGS), the Minnesota Geological Survey (MGS), in conjunction with the USGS and other organizations, conducted state-wide programs of high-resolution aeromagnetic surveying and ground-based gravity coverage. These state-wide databases, along with a rock-properties database, form core elements to all Precambrian bedrock mapping programs of the MGS. In addition, model studies of gravity and magnetic data have been effectively used to investigate upper crustal structure.

Seismic and magnetotelluric surveys have been successfully used to investigate the entire crustal section. Beginning with refraction studies in the 1960’s, seismic methods have been used to estimate crustal thickness and composition. More recently, seismic reflection profiling connected with the COCORP and GLIMPCE projects have been effective in imaging the crustal framework of the Archean Superior Province and the Mesoproterozoic Midcontinent Rift, respectively. Magnetotelluric studies have been used to help define Paleoproterozoic basins and shear zones.

Geophysical methods have also been used for near-surface investigations. Detailed seismic refraction and reflection profiling have been used in some engineering and ground-water investigations, as have electrical methods. Electrical and electromagnetic surveys have been used in numerous mineral exploration projects over the last few decades, as have detailed gravity and magnetic surveys. Recently the MGS has been using passive seismic methods to help map the thickness of the glacial sediments.

 Much geophysical work remains to be done in the region. Doubtlessly this will include bedrock and crustal studies, but because glacial sediments contain one of Minnesota’s most valuable resources--water--it is likely that more geophysical effort will be devoted to characterizing these sediments, instead of looking through them.