USING A WEB-BASED GEOGRAPHICAL INFORMATION SYSTEM TO ACCESS GEOPHYSICAL LOGS AND THE TOP OF AQUIFERS AND CONFINING UNITS IN THE MISSISSIPPI EMBAYMENT

As the demand for ground-water increases, so do the needs of water-resource managers for reliable data for determining the depth to the tops of aquifers and confining units. Depths to the top of aquifers and confining units were interpreted from more than 2,700 geophysical logs from across eight states, including Alabama, Arkansas, Illinois, Kentucky, Louisiana, Mississippi, Missouri, and Tennessee, and stored digitally. These depths were then used to construct digital surfaces for the top of 10 Cenozoic-age formations within the Mississippi embayment for an area approximately 70,000 square miles. A spatial interpolation method of inverse distance-weighting was used in a Geographical Information System (GIS) to create each digital surface. Each digital surface was constrained to land surface in the outcrop area of each formation. A statistical analysis, based on the difference between the geophysical log data points and the interpolated digital surfaces, was conducted for each digital surface. Standard deviation values ranged from 22 feet to 64 feet for all digital surfaces.

The web-based GIS uses open-source software and the digital surfaces developed as part of the Mississippi Embayment Regional Aquifer Study to display, upon request, a stratigraphic section based upon a user-defined location. Such information is often required by water resource managers who need a quick, reliable method for determining the aquifer associated with a ground-water users' well screen. For example, flow meters are now required on any well screened in an aquifer that has been identified as a “sustaining aquifer” by the Arkansas Natural Resources Commission (ANRC). There is sometimes a question about what primary aquifer the well is screened, and the web-based GIS allows water-resource managers, such as ANRC, to have a dynamic means for accessing the digital surfaces to determine the primary aquifer. A user can select a well location on a map, enter a screen or well depth, and a virtual well and stratigraphic column are created for that location. Also, the locations and images of nearby geophysical logs and previously published structure maps are available from the web site to assess the confidence in the web-based GIS.