Analysis of Alternate Borehole Survey Systems

Conventional borehole survey technology, involving the use of either magnetic or gyroscopic instruments, provides at best a lateral position uncertainty of 1% of measured well depth. The conventional tools also exhibit severe accuracy degradation in inclined boreholes. Preliminary test programs have indicated that an order-of-magnitude improvement over conventional technology can be achieved by adapting modern inertial navigation and guidance techniques to the development of an improved gyroscopic survey instrument. The objective of this paper is to analyze the performance characteristics of certain gyroscopic tools currently in development and outline major system error sensitivities. Mathematical error models have been developed for several different gyroscopic tools, which can be grouped into three general categories: Gyrocompass, Attitude Reference System and Inertial Navigation System. These models are used as the basis for a series of computer error simulations which characterize the statistical survey accuracy of the various tool designs as a function of time and displacement. The analysis also identifies the major operational limitations of each design and establishes the sensitivities to key error sources.