Patent ID: 8407005

Claim:
Gradient tensor induction magnetic field measuring apparatus for measuring a gradient tensor induction magnetic signal of a transient magnetic field in a wellbore to be used to determine a conductivity gradient of an anisotropic earth formations penetrated by the wellbore, the apparatus comprising, a), c) and d); or b), c), and d); or a), b), c), and d), wherein: a) a tri-axial electromagnetic induction transmitter formed by three mutually orthogonal transmitter coils, and a pair of tri-axial electromagnetic induction receivers, each formed by three mutually orthogonal receiver coils, separated by a first distance, wherein the first distance between the two tri-axial electromagnetic induction receivers is smaller than a distance L between the tri-axial transmitter and a center point of the first distance, in order that conditions are fulfilled such that gradients in a direction of a line defining the first distance are measured of different components of an induction tensor formed by the magnetic fields induced in the earth formation; b) a pair of tri-axial electromagnetic induction transmitters, separated by a first distance, and a tri-axial electromagnetic induction receiver, wherein the first distance between two tri-axial electromagnetic induction transmitters is smaller than the distance L between the tri-axial receiver and a center point of the first distance, in order that conditions are fulfilled, based on a reciprocity principal, such that gradients in a direction of a line defining the first distance are measured of different components of an induction tensor formed by the magnetic fields induced in the earth formation; c) means for receiving voltages induced in said receiver coils; and d) means for measuring a difference between the said voltages in the different pairs of the receivers, or due to different pairs of transmitters.