REDUCTION OF WELLBORE POSITIONAL UNCERTAINTY WITH A NEW GEOMAGNETIC IN-FIELD REFERENCING TECHNIQUE

A new technique for estimating geomagnetic parameters at or close to t he rigsite is described and shown to allow reduction in the errors ass ociated with main-field model predictions of geomagnetic field paramet ers. Valid reduction in the magnitude of certain directional error-ter m values becomes possible, resulting in significant reduction in wellb ore positional uncertainty for magnetic-survey systems. These can then approach levels normally reserved for high-accuracy gyros. Wider appl ication of the use of geomagnetic measurements to enhance both the acc uracy performance and quality control of magnetic surveys is now much more feasible because continuous measurements at the drilling site can effectively be replaced by taking a ''snapshot'' to establish local b aseline geomagnetic conditions. Then, with remote, fixed observatories subsequent variations at the drillsite can be determined. A single se t of measuring equipment could service the needs of many drilling proj ects, and the feasibility of development of the capability for offshor e application is now considerably improved.