1. Airborne Electromagnetic Surveys for 3-D Geological Mapping Jared D. Abraham and James C. Cannia

2. Project Sponsors North Platte Natural Resources District South Platte Natural Resources District Twin Platte Natural Resources District Nebraska Environmental Trust Nebraska Geological Survey

3. Project Contributors Jared D. Abraham James C. Cannia Bruce D. Smith Burke J. Minsley Paul A. Bedrosian Steven S. Sibray Lyndsay B. Ball Michaela R. Johnson Maria Deszcz-Pan J. Alton Anderson Brian H. Imig

4. Study Location and Groundwater Models in the Area

5. Motivation for Enhancing Hydrogeologic Frameworks Difficult models, failure to converge/calibration issues due to poor geologic control Poorly estimated bedrock topography despite large amounts of borehole and surficial geologic data Groundwater quality will be better understood with new interpretations of geology Understanding of groundwater age-dating information can be improved and used to calibrate model with new interpretation

6. Geology and Lithology A. Iron-stained sediments B. Manganese-stained sediments C. Siltstone clasts ~1.5 feet in diameter Cross bedded, fluvial, coarse grained sediments of the Pliocene Broadwater Formation Photo taken ~1.5 miles north of Big Springs, Nebraska, facing west A. Tertiary Brule fractures B. Unfractured Tertiary Brule Photo taken along summit road between tunnels 1 and 2, Scottsbluff National Monument, facing east.

7. Airborne Electromagnetic Survey SkyTEM Time Domain EM System Fugro Resolve Frequency Domain EM System

8. Location of Airborne EM Surveys

9. Inversion of EM data

10. Electrical Properties High Resistivity – Red (80 to 500 ohm-m) sand or gravels (aquifer) Low Resistivity – Blue (5 to 30 ohm-m) clays and silts (aquitard) Many combinations exist

11. Interpretation of EM Inversions (displayed as 2-D Section)

12. 3-D EM Inversions and Interpretations

13. Refined Base of Aquifer Contours

14. Results of Interpretation (displayed as 2-D section)

15. Selected Sections from 3-D Model

16. Impact of 3-D Model Refined 3-D hydrogeological model Refined estimate of saturated thickness Refined estimate of flow paths Refined estimation of surface and groundwater interaction

17. Selected Sections from the 3-D Geological Model

18. Change in Saturated Thickness New Saturated Thickness Previous Saturated Thickness

19. For the area of the airborne geophysical surveys, an additional ~3.7 million acre-feet of water in storage was identified

20. CONTACT INFORMATION Jared D. Abraham(303) 236-1318 e-mail: jdabraha@usgs.gov USGS Crustal Imaging and Characterization Team (303) 236-1213 Denver Federal Center, Bldg. 20, Box 25046, MS 964, Denver, CO 80225-0046 http://crustal.usgs.gov/ James C. Cannia(402) 416-5321 e-mail: jcannia@usgs.gov USGS Nebraska Water Science Center (402) 328-4100 5231 South 19 th St., NE 68512-1271 http://ne.water.usgs.govLincoln