Patent ID: 6854192

Claim:
A method of deriving the true magnitude of the terrestrial magnetic field BZe in the direction of the longitudinal axis OZ of a borehole penetrated by a bottom-hole assembly comprising a drill string including a substantially non-magnetic drill collar to which is attached a drilling bit assembly, said method comprising the steps of measuring the longitudinal magnetic field BZa (the component of the magnetic field B in the direction OZ) at a first predetermined point which is along the length of the drill string at a distance a from a fixed point along the length of the drill string, and measuring the longitudinal magnetic field BZb (the component of the magnetic field B in the direction OZ) at a second predetermined point which is along the length of the drill string at a distance b from said fixed point along the length of the drill string, to provide a longitudinal-position-dependent pair of longitudinal magnetic field measurements BZa,BZb, and calculating BZe from the relationship: BZe =( BZa.a 2 −BZb.b 2 )/( a 2 −b 2 ), on the assumption that a longitudinal magnetic field error is induced by a single notional magnetic pole in the drill string substantially at the attachment of the drill string to the substantially non-magnetic drill collar.