Method to detect deviations from a wellplan while drilling in the presence of magnetic interference

This invention relates generally to method and apparatus for drilling a wellbore in a sub-surface formation, to enable avoidance of physical interference with an existing magnetic source, or magnetically responsive source such as well casing in the formation; more particularly, it concerns comparison and/or use of information derived from an existing magnetic source in the formation, from a wellbore plan, and from an actual wellbore being drilled, to enable such avoidance. A method for drilling a wellbore to assure the avoidance of a substantially adjacent known magnetic source comprising the steps of obtaining survey data for the path or trajectory of a known magnetic source; preparing a wellplan comprising a path or trajectory for a second wellbore to be drilled, from the survey data and the wellplan, computing expected values of one or more parameters of the magnetic field to be encountered along a second wellbore during drilling operations, in association with drilling a second wellbore, measuring components of a magnetic field encountered and computing one or more magnetic field parameters at a series of survey points along the wellbore path or trajectory; and determining if measured values differ systematically from expected values, thereby indicating a deviation from the planned wellpath.