Abstract: A first flux-loop inductive coupler element electrically couples with a second flux-loop inductive coupler element. The first flux-loop inductive coupler element comprises a first ring-like core having high magnetic permeability and a conical-section annular first face transverse to the plane of the first core. The first face has a first annular groove separating a first conical-section larger-diameter face and a first conical-section smaller-diameter face. A first coil is wound within the annular groove. The first and second cores form a low-reluctance closed magnetic path around the first coil and a second coil of the second flux-loop inductive coupler element.

Abstract: A logging tool for use in determining the resistivity of an underground formation surrounding a borehole comprises a mandrel with two transmitters spaced apart thereon, each serving to establish a current in the mandrel and in the underground formation. A series of electrodes are spaced along the body between the transmitters and sensors, located at each electrode, measure radial current flow along a path from the mandrel to the underground formation via a respective electrode. Sensors also provide the axial current flowing along the whole mandrel and at positions corresponding to each electrode. A method of determining the formation resistivity includes the steps of measuring the radial currents R.sub.1 R.sub.2 from the mandrel to the formation via each electrode and obtaining the axial currents M.sub.01 M.sub.02 along the mandrel at each electrode due to each transmitter; measuring the total axial current M.sub.

Abstract: A logging-while-drilling system is disclosed in which a first subassembly is included near the drill bit, and this subassembly comprises a conducting body with measuring means disposed in it and a first toroidal coil antenna disposed on the body, means for coupling a local communication signal generated from the measuring means to the first toroidal coil antenna, and where the measuring means includes means to energize the first toroidal coil antenna with a measurement signal and means mounted on the conductive body to receive the measurement signal after it passes through the formations to be measured.

Abstract: Measurement-while-drilling apparatus is disclosed for determining the resistivity of formations surrounding an earth borehole. An electrically conductive metal body, such as a drill collar on a drill string, is movable through the borehole. A transmitting toroidal coil antenna is disposed on the body and is energized to induce a current which travels in a path that includes the body and the formations. An electrode is disposed on the body, and an electrical signal resulting from said current is measured at the electrode to obtain an indication of the resistivity of the formations. In a disclosed embodiment, one or more electrodes can be mounted on a blade that is mechanically coupled to the body.

Abstract: Apparatus is disclosed for determining the resistivity of formations surrounding an earth borehole. An electrically conductive metal body, such as a drill collar on a drill string, is movable through the borehole. A transmitting toroidal coil antenna is disposed on the body and is energized to induce a current which travels in a path that includes the body and the formations. An electrode is disposed on the body, and an electrical signal resulting from said current is measured at the electrode to obtain an indication of the resistivity of the formations. In a disclosed embodiment, one or more electrodes can be mounted on a blade that is mechanically coupled to the body.

Abstract: A disclosed embodiment utilizes one or more toroidal coil antennas mounted, in an insulating medium, on a drill collar to induce currents which travel in a path that includes the drill collar and earth formations around the drill collar. At least one array of electrodes is provided on at least one stabilizer blade of the drill collar and is utilized to detect currents transmitted by toroidal coil antennas which return via the formations to the electrodes laterally; that is, approximately normal to the axis of the drill collar. Signal traces are obtained from measurements taken at respective ones of the transducers, and a dip characteristic of the formations is determined from the signal traces and from signals representing the rotational orientation of the drill collar.

Abstract: Well logging techniques and apparatus are disclosed for determining formation resistivity at different radial depths of investigation and with greater accuracy than prior techniques, for determining the existence, locations and properties of beds and caves, and for determining changes in the size of a borehole.

Abstract: A well logging apparatus is disclosed including a cylindrical housing with a transmitting antenna and first and second receiving antennas. The antennas are formed on insulating material in recesses in the housing and have a waterproof rubber covering. The receiving antennas have adjacent compartments containing signal conditioning circuits.

Abstract: Well logging techniques and apparatus are disclosed for determining formation resistivity at different radial depths of investigation and with greater accuracy than prior techniques, for determining the existence, locations and properties of beds and caves, and for determining changes in the size of a borehole.