Date of Award
Master of Science (MS)
W.D. Gosnold Jr.
A high resolution gravity survey was performed over portions of McLean, Mountrail, and Ward Counties in central North Dakota during the fall and winter of 1990-1991. The study was performed to locate and map Pleistocene sand and gravel aquifers. Well data indicate that aquifers beneath the study area range in thickness from 1 to 60 meters. The Bouguer gravity anomalies resulting from these shallow aquifers was estimated to be on the order of 1 to 2 mgals. Previous studies have focused on available well data and by mapping topographic lows. Due to the inferential nature of these studies, the detailed gravity method was chosen as an attractive and cost-effective method of further defining these aquifers.
Station locations were established at 0.4 km spacings along accessible roads using the Global Positioning Satellite system. Of the 4000 stations that were occupied, approximately 2700 station locations were satisfactorily produced.
The gravity readings were obtained by using three La Coste and Romberg Model G gravity meters. Corrections for meter calibration, solar and lunar tides, instrument drift, station latitude, and elevation were made by the computer program GRAVPAC (LaCoste and Romberg, 1989). The resulting Bouguer gravity anomalies were examined for suspect data. Questionable data were removed.
Strong regional components to the gravity field necessitated the use of filtering methods. These methods included using published data, upward continuation, and wavelength filtering. The upward continuation and. wavelength filtering were performed using the computer program FFTFIL (Hildenbrand, 1983). Both the published data and the upward continuation methods produced reasonable regional anomaly patterns, but difficulties were encountered when attempting to remove the regional field from the Bouguer gravity anomalies. The wavelength filtering method yielded satisfactory results using a filter window of 1.5 km to 10 km. The regional field was compared. With structure contour maps produced from deep oil well data to ensure that an appropriate field had been removed.
A three-dimensional computer modelling program was written to simulate the observed gravity anomalies. Forward modelling was then performed by adjusting density contrast, depth and thickness parameters. Shallow water well data were used to establish initial model parameters. The modelling generated reasonable results which made it possible to infer probable channel locations, depths, and thicknesses. The major limitations of the computer modelling were lengthy processing times and limited geologic data.
The filtered gravity signals show several discernible low anomaly patterns which are interpreted to be stream channel aquifers. Aquifers with significant density contrasts (>0.2 g/cm3) and sufficient areal extent and thicknesses were located by this method.
McDonald, Mark R., "A high resolution gravity survey for the detection of Pleistocene aquifers underlying the Fort Berthold Indian Reservation, North Dakota" (1994). Theses and Dissertations. 196.