A directional
induction logging tool is provided for
measurement while drilling. This tool is preferably placed in a side pocket of a
drill collar, and it comprises
transmitter and
receiver coils and an electromagnetic reflector. The reflector, which may be a layer of highly conductive material placed between the coils and the body of the
drill collar, serves to focus the electromagnetic fields generated and sensed by the tool in the direction away from the reflector, thus providing a directional response to formation
conductivity with a relatively high depth of investigation. In preferred embodiments of the invention, magnetically permeable cores are placed within the coils to concentrate the magnetic fields that pass through them. Circuitry is described for balancing the mutual
inductive coupling of the coils by injecting a
direct current signal through one or more of the coils, which alters the magnetic permeability of the core material. The magnitude of the
direct current required to achieve a balanced condition may be derived from the quadrature phase component of the return
signal. Circuitry is also provided for generating a transmitted
signal and for
processing the return signals, including digital-to-analog conversion circuitry for providing
digital data for transmission to the surface. This tool may be employed to provide real-time directional
conductivity information that may be used to detect and follow
bed boundaries in
geosteering operations.