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Hare, J.T., 1998

High resolution geodetic techniques for monitoring fluid levels over time

Bibliographic Reference

Hare, J.T., 1998, High resolution geodetic techniques for monitoring fluid levels over time: University of Texas, Dallas, Ph.D. dissertation, 180 p.


In the first study, a novel surveillance technique is developed in which surface gravity observations are used to monitor the progress of a gas cap waterflood in the 8,200 ft (2,500 m) deep Prudhoe Bay reservoir, Alaska. This cost-effective method requires that high-precision gravity surveys be repeated every 3 to 5 years. Differences in the gravity field with time reflect changes in the reservoir fluid density distribution. A preliminary field test at Prudhoe Bay indicates survey accuracy of 5 to 10 µ Gal can be achieved for gravity data using a modified Lacoste & Romberg 'G' type meter or Scintrex CG-3M combined with Global Positioning System (GPS) positioning. Forward gravity modeling of a suite of reservoir simulations of the proposed waterflood predicts variation in surface measurements of 100 µ Gal after 5 years of injection, and 180 to 250 µ Gal after 15 years. A constrained, least-squares method is used to invert synthetic gravity data for subsurface density distributions. The modeling procedure has been formulated to allow testing of the models for sensitivity to gravity sampling patterns, noise characteristics, and various constraints on model parameters such as density range, total mass, and model moment of inertia. Horizontal feature resolution of the waterflood is about 5,000 ft (1,520 m) for constrained inverse models from synthetic gravity with 5 µ Gal standard deviation noise. Results of the modeling indicate that inversion of time-lapse gravity data is a viable and promising technique for monitoring reservoir gas cap waterfloods. In the second study, the problem of how to estimate ancient lake levels from the geomorphology of remnant shoreline terraces is investigated. High-resolution GPS-controlled topographic data from around the highstand shoreline of Pleistocene Lake Lahontan in western Nevada provide the means for isolating coherent terrace features that are related to the paleoshoreline level. Determination of an unambiguous point or lineation for elevation measurement is complicated by erosional degradation, and the fact that the terraces were irregular at the time of formation. To address this problem, local high-resolution Digital Elevation Models (DEMs) are generated at six sample terrace sites using a variety of survey techniques, including conventional total station, rapid static GPS, and new real-time kinematic GPS methods. The data are tied to a regional framework of absolute geodetic control. A signal processing method is developed that uses derivative filters for geomorphic feature recognition and averaging for noise reduction. Propagation of errors related to surveying, geoid estimation, and terrace feature estimation indicates that tectonic displacements on the order of one-half meter should be resolvable. This work establishes the foundation for analysis of the terraces on a regional scale by using a high resolution DEM of the entire shoreline trace around the perimeter of Lake Lahontan. (Abstract shortened by UMI.)

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