North-Central Section - 39th Annual Meeting (May 19–20, 2005)

Paper No. 10
Presentation Time: 11:20 AM


ELLINGSON, Jonathan B., Minnesota Department of Natural Resources, Div of Lands and Minerals, 2300 Silver Creek Road NE, Rochester, MN 55906, ANDERSON, Heather E., Minnesota Department of Natural Resources, Div of Lands and Minerals, 500 Lafayette Road, Box 45, Saint Paul, MN 55155-4045 and MELCHERT, Glenn, Minnesota Department of Natural Resources, Div of Lands and Minerals, 1525 Third Ave E, Hibbing, MN 55746,

When Ground Penetrating Radar (GPR) and Two-Dimensional Electric Resistivity Imaging (2-D ERI) are used in conjunction, both the shallow and deep characteristics of an aggregate deposit can be modeled. GPR can provide very detailed information about the overburden thicknesses and produce high-resolution pseudo cross-sections showing the internal structure and characteristics of near surface deposits (<20 feet). When evaluating aggregate deposits deeper than the GPR can penetrate, 2-D ERI should be used. 2-D ERI provides resistivity profiles (cross-sections) that can be used to evaluate aggregate deposits to a depth of 185 feet or more.

Two case studies were completed to evaluate these techniques. GPR was evaluated by collecting several transects in and around an active gravel pit and directly comparing the cross-sections with the material exposed at the site. Several GPR “cross-sections” were collected to determine if GPR could be used to accurately model the overburden, sand and gravel thickness, depth to bedrock, and fluvial structures; as well as determine the maximum depths GPR can be used in different geological conditions. The application of 2-D ERI was evaluated by comparing the results of a surface resistivity study to detailed geological logs of rotosonic drill cores. Twelve geophysical profiles were completed; each profile was 900 feet long and reached a maximum depth of 185 feet. Rotosonic drill holes were then placed on 10 of the 12 geophysical lines to test the accuracy and application of the 2-D ERI.

GPR worked very well for modeling the overburden thicknesses, determining the depth to shallow bedrock (<10 feet), and characterizing shallow sand and gravel deposits less than 10-12 feet. 2D-ERI worked very well for defining the horizontal and vertical extent of the deep deposit. The 2-D ERI profiles showed the differences between gradational and sharp contacts and between granular (sands and gravel) and non-granular (silts and clays) material. When these two methods are used together, along with drilling, the overburden, aggregate thicknesses, continuity, anomalous areas, and textural characteristics of the deposit can be modeled with a high degree of accuracy. Both GPR and 2-D ERI are also non-intrusive methods that can be used in environmentally sensitive areas without disturbing the surface.