High-Accuracy GPS and Gravity Surveys in North Slope, Alaska

Tianyou Chen, John Ferguson, Carlos Aiken, Jennifer Hare and Jerry Brady

Abstract:	Between 1994 and 2001, four high-accuracy GPS surveys have been carried out on the North Slope of Alaska supporting micro-gravity surveys. The goal has been to determine whether a surface gravity/GPS survey could be used to map density changes in a deep hydrocarbon reservoir due to water injection in the gas cap. A water injection program might last for decades and failure of the water to move in the subsurface as planned could result in a serious loss of oil production. Due to various environmental and logistical problems, which are peculiar to the Arctic, it is not practical to monitor the water flood by drilling wells. A time differenced or “4D” gravity survey was proposed as a cost effective alternative. Simulations of the water flood have indicated a requirement for sub 10 micro-Gal noise level in the gravity and 1 to 2 cm accuracy in elevation for about 500 stations. In the Arctic environment it is impossible to establish any kind of permanent monument to facilitate resurveys (many of the stations are on seasonal sea ice!). GPS control is the only practical way to establish repeatable stations. Local monuments used for differential GPS measurements are re-established during each survey with respect to distant permanent (CORS network) control. Once a station is originally established the use of high- precision Real Time Kinematic (RTK) GPS is necessary for its recovery. The latitude and the time of year of the survey also make the attainment of centimeter-level accuracy more challenging due to satellite geometry and ionospheric activity. In the Arctic measurement of gravity at the micro-Gal level is also problematic. The fundamental resolution of modern gravity measurement is near 1 micro-Gal, but wind generated seismic noise, ice movement and other phenomena must be countered by refined procedures. Three fundamentally different gravity measuring devices, The LaCoste and Romberg Super-G meter, Scintrex CG- 3M meter and Micro-g A-10 absolute meter were used in the winter at temperatures sometimes below -40 degrees F. The GPS antenna was mounted on top of the instruments for a constant known position relative to the gravity sensor. About 30 stations were established in 1994 (and about 15 more in later years). Results after multiple occupations, over 1 to 3 year intervals, indicate that the required accuracy can be attained by this combination gravity/GPS survey techniques.
Published in:	Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001) September 11 - 14, 2001 Salt Palace Convention Center Salt Lake City, UT
Pages:	532 - 563
Cite this article:	Chen, Tianyou, Ferguson, John, Aiken, Carlos, Hare, Jennifer, Brady, Jerry, "High-Accuracy GPS and Gravity Surveys in North Slope, Alaska," Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001), Salt Lake City, UT, September 2001, pp. 532-563.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In