Calibration correction method for magnetic survey tools

Abstract

A method for determining the orientation of the axis of a borehole with respect to an Earth-fixed reference coordinate system at a selected series of locations in the borehole, the borehole having a trajectory, and adapted to receive a drill string, comprising defining a model for the influence of magnetic interference from elements of the drill string on the measurement of components of the Earth's magnetic field in the borehole in terms of an unknown vector, a measurement vector, and a measurement matrix relating the unknown vector and the measurement vector; measuring at two or more selected locations along the borehole trajectory at least one of two cross-borehole components of the Earth's gravity field at the selected locations in the borehole, and two cross-borehole components and an along-borehole component of the Earth's gravity field at the selected locations in the borehole; two cross-borehole components and an along-borehole component of the Earth's magnetic field at the selected locations; computing the elements of the measurement vector and measurement matrix from the measured Earth's gravity and magnetic field components for the selected locations; solving for the unknown vector; computing corrected values for the Earth's magnetic field components using the unknown vector, the model, and the measured Earth's magnetic field components; and determining a value for the azimuthal orientation of the borehole axis using the corrected values for the Earth's magnetic field components and the measured gravity components.