Water Resources of North Dakota

High-Accuracy Gravity Data Used for Rapid Delineation of Small, Shallow Aquifers in West-Central North Dakota

By S. W. Cates, C. Aiken, M. Balde, and D. Ziegler

Abstract: Fall Meeting American Geophysical Union, 1992

Aquifers composed of glacial and preglacial fluvial sediments constitute a major component of the water resources of the Fort Berthold Indian Reservation in west-central North Dakota. The areal extent and potential yield of aquifers underlying a 2,000-km2 area are not well understood. Global Positioning System (GPS) surveying was used to obtain locations and elevations of gravity stations for a high-accuracy gravity data set used for initial delineation of the aquifers. Geometry and density characteristics of known aquifers indicated that the expected gravity anomalies would be small, ranging between 0.5 and 1.5 mgal. The magnitude of the gravity anomalies required extremely accurate vertical positioning to produce sufficiently accurate Bouguer gravity values. The desired accuracy of the Bouguer gravity values and the size of the study area necessitated positioning of more than 2,700 gravity stations. GPS technology was used to rapidly survey the stations, and vertical and horizontal accuracy was achieved that varied less than 5 cm and was better than the theoretical positioning requirements. The gravity data set was analyzed using Fast Fourier Transform operations and inverse modeling techniques. Results from this study indicate that GPS technology can be used with accurate Bouguer gravity values to delineate small gravity anomalies over a large area at about one-eighth of the cost of using conventional techniques. Furthermore, primary analysis of the data indicates modern potential-field geophysical techniques can be used to delineate small, shallow aquifers.

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