Patent ID: 7937221
Filing Date: 2011-05-03
Classification: G01V

Abstract:
1. 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 e, f and any or any combination of a)-b), wherein: a) at least one transmitter coil and a pair of closely located parallel tri-axial EM induction receivers, each formed by three mutually orthogonal receiver coils, separated by a small vector, Δr, wherein a distance between the two parallel tri-axial EM induction receivers is much smaller than a distance L between the transmitter and a center of the vector Δr, connecting the two receivers, wherein conditions are fulfilled that the gradients are measured of different components of the magnetic field induced in the earth formation; b) at least one receiver coil and a pair of closely located parallel tri-axial EM induction transmitter coils, separated by a small vector, Δr, wherein a distance between the two parallel tri-axial EM induction transmitters is much smaller than a distance L between the receiver and a center of the vector Δr, connecting the two transmitters, wherein conditions are fulfilled, based on a reciprocity principal, that gradients are measured of different components of the magnetic field induced in the earth formation; e) means for receiving voltages induced in said receiver coils; and f) means for measuring a difference between the said voltages in the different pairs of the receivers, or due to the different pairs of the transmitters.