QUATERNARY AQUIFER MATERIAL INVENTORY AND 3-D MODEL CONSTRUCTION FOR THE FARGO-MOORHEAD REGION (NORTH DAKOTA AND MINNESOTA) USING GIS BASED GEOLOGIC CROSS SECTIONS

The Minnesota Geological Survey and the Minnesota Department of Natural Resources constructed 215 geologic cross sections to locate potential water resources for the greater Fargo-Moorhead area (North Dakota and Minnesota). Over 130 aquifers and potential aquifers were identified and mapped within the study area. Portions of 12 buried ice margins at various stratigraphic levels were mapped suggesting trends that may be useful for future exploration and mapping. This work was done under contract to the U.S. Department of the Interior, Bureau of Reclamation in support of their Red River Valley Water Supply Project

Water-well cross sections were constructed using ArcView 3.3, ArcGIS 9.0, and some custom extensions. Thirty-four regional cross sections were constructed in the 38,000 square kilometer study area. The cross sections are 230 km in length and are constructed every 5 km. The project water-well database consisted of about 30,000 drillers logs and about 60 Quaternary stratigraphic control sites. Water-well logs and surface geology were displayed along the locus of each cross section greatly enhancing our ability to make correlations. Four smaller areas, ranging from 1,300 to 3,200 km2 in area, were chosen for detailed analysis. East-west cross sections were constructed in these areas using 0.5 to 3 km spacing.

Creating these cross sections in ArcView allowed aquifer and till boundaries to be correlated by overlaying shapefiles in the same view window. A series of points for each stratum were extracted from the shapefile lines and converted into X, Y, Z values using an ArcView script. These points were then used as control points for surface construction using GoCAD software. In addition, cross sections were scanned and registered in three-dimensional space in order to check surface locations and to add additional control points to constrain surface dimensions. Errors or inconsistencies between cross sections were easily identified and adjusted in three-dimensional space. Visualization in three dimensions also provides for qualitative and quantitative assessment of the model uncertainty by showing or measuring the distance between portions of the model and control points used to construct it.