SURVEYING OF SUBTERRANEAN MAGNETIC BODIES FROM ADJACENT OFF-VERTICAL BOREHOLE

摘要

A method and apparatus for performing a surveying operation to locate, a target subterranean ferromagnetic body having remanent or impressed magnetization. The surveying operation is conducted from an off-vertical borehole adjacent the target magnetic body using magnetic field sensing apparatus, and involves determining the range and direction of the subterranean target with respect to the location of the magnetic field sensing apparatus. Target direction is determined by measuring three magnetic field components and resolving the measured components into a resultant vector. Target range is determined by measuring total magnetic field intensity and target body magnetic field intensity gradient in the direction of the off-vertical borehole. Both static and time-varying fields may be detected by the subsurface apparatus. The methods and apparatus disclosed may be used in such diverse areas as the location of ore deposits, guidance systems for drilling off-vertical wells to intersect a previously drilled well, and location metallic objects underwater.