During the fall and winter of 1990-91, the USGS conducted a high-accuracy gravity survey of about 45 percent of the Fort Berthold Indian Reservation to delineate buried-valley aquifers. Because the depositional model of buried-valley glacial deposits indicates the existence of ancient drainages and the surficial geology of the Reservation in the area of the buried-valleys gives little indication of the underlying material, a high-accuracy gravity survey was undertaken to delineate these features. Small gravity anomalies were expected to be evident at the locations of these ancient drainages because of small density contrasts between the glacial and bedrock materials. Because the anomalies were expected to be so small, their detection would be dependent on very accurate measurements of land-surface elevation at the gravity data-collection stations. GPS technology was used to determine the horizontal and vertical coordinates of nearly 1,800 data stations in an 800 square-mile area of the Reservation north of Lake Sakakawea where glacial material covers the land surface.
The GPS survey was done in the "STOP AND GO" kinematic mode. Static methods were used to determine baseline vectors between USGS standard monument tablets and selected offset points conveniently distributed within the survey area. These offset points were then used as reference monuments. Stations were located along section line roads and were spaced 0.25 miles apart. A maximum baseline length of 14 miles was observed as required to obtain the necessary position accuracy. About 20 percent of the 1,800 different points were reoccupied as a check on data integrity. Based on comparison of GPS reoccupation of the data stations, the vertical and horizontal accuracy of the locations was found consistently to have less than a 4-inch error. The GPS defined coordinates were matched to appropriate gravity measurements, and imported into a geographic information system (UNIX ArcInfo) environment for analysis and interpretation.
Publications:
Balde, M., Aiken, C.V., Hare,J., Ziegler,D., Cates, S.W., (1991) Stop and Go Kinematic GPS for Station Control in a Large Scale Gravity Survey in North Dakota. (Published in the proceedings of the Sixth International Symposium on Satellite Positioning, Ohio State University, Columbus, Ohio, November 8 and 9, 1991.
Balde, M., Aiken, C.V., Hare,J., Ziegler,D., Cates, S.W., (1992) Kinematic GPS Positioning of a Geophysical Survey. (Published as an expanded abstract for the proceedings of the Sixty-second Annual Society of Exploration Geophysicists International Meeting and Symposium, October 25-29, 1992).
Cates, S.W., (1992) High-Accuracy Gravity Data Used for Rapid Delineation Small, Shallow Aquifers in West-Central North Dakota. (Published as proceedings of the Fall 1992 meeting of the American Geophysical Union, October 10,11 and 12, 1992)