MOTION COMPENSATION APPARATUS AND METHOD OF GYROSCOPIC INSTRUMENTS FOR DETERMINING HEADING OF A BOREHOLE

摘要

A method and apparatus is disclosed for measuring motion signals of gyroscopesin downhole instruments used to determine the heading of a borehole. Anillustrativeembodiment of the invention includes a measuring-while-drilling system whichmayexperience motion even while the drill string is suspended in rotary tableslips when theheading of the drill string is being determined. Accelerometer andmagnetometer dataalong three orthogonal axes of a measurement sub are used to obtain unitgravitationalvectors ~ at a first time and at a second time and unit magnetic vectors ~ atthe first timeand the second time. The difference between the two unit gravitational vectorsat thedifferent times, .DELTA.~, and the difference between the two unit magneticvectors at thedifferent times, .DELTA.~, are used along with the unit vectors ~ and ~ andthe difference in time.DELTA.t to determine the rotation vector of the probe ~ which has occurredduring such timedifference. The vector representing the rotation of the earth, ~ is thendetermined bysubtracting ~ from the vector ~ from three gyroscope instruments placed alongthe axesof the measurement sub. The heading of the drill string is determined from thegravitational vector and the earth rotation vector.