| 摘要 |
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 (16) and receiver (18, 20) coils and an electromagneti c reflector (38). The reflector (38), which may be a lyer of highly conductive material placed between the coils and the body of the drill collar (12), serves to focus the electromagnetic fields generated and sensed by the tool in the direction away from the reflector (38), thus providing a directional respons e 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 derive d 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 an d follow bed boundaries in geosteering operations.
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