Abstract: An inductive collar locator tool is provided that provides improved sensitivity and precision in locating collars and other discontinuities in a casing string. In a preferred embodiment, the collar locator includes a transmitter coil that produces an oscillating magnetic field, which couples to the casing string proximate the tool. The tool also includes a pair of spaced receiver coils connected to signal processing circuitry which provides an output signal representing the difference in the phase of the magnetic field detected by the receiver coils. The phase difference is relatively constant when the tool is in plain casing pipe, but it changes rapidly when the tool passes through pipe that includes discontinuities, such as joints or collars. The collar locator tool may include an on-board oscillator and digital signaling capabilities. The collar locator is preferably enclosed in a titanium pressure housing that is adapted for through-tubing operations.

Abstract: A retrievable resistivity logging system is provided for obtaining directional resistivity information while drilling (e.g., in an MWD system). The information obtained by this system can be used to determine the proximity of resistivity discontinuities and for use in steering the progress of the wellbore in the presence of geological boundaries of interest. In preferred embodiments, the logging system comprises passive transmitter and receiver electrodes built into subs that are incorporated into the drillstring, and a retrievable instrument cartridge that comprises spring loaded contacts that engage contact surfaces in the subs to connect the electrodes to the cartridge. In some embodiments, the cartridge comprises toroidal transformers coaxial to the cartridge for generating the survey current and for detecting the received current from the receive electrode.