Retrievable resistivity logging system for use in measurement while

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. A plurality of receiver electrodes may be employed to provide simultaneous directional resistivity information from several directions around the wellbore, which may be used for detecting the presence of a nearby contrasting formation. Methods are provided for using this logging system to determine the direction and proximity of a contrasting resistivity layer, for determining apparent homogeneous resistivity of the formations surrounding the system, and for determining the dip and strike of formations intersected by the wellbore.