Abstract: An electromagnetic logging apparatus for investigating properties of an earth formation surrounding a borehole comprises an electromagnetic energy transmitter and four receivers supported by a support member, a measurement channel, and a multiplexer for selectively coupling the measurement channel to the receivers. The measurement channel comprises an amplitude processing channel including a highly gain-stabilized amplifier and gain control circuit; a phase processing channel including a highly phase-stabilized amplifier; and a circuit for measuring the amplitude and phase. The gain of the gain-stabilized amplifier is controlled by the gain control circuit in a closed loop feedback arrangement, while the gain of the phase-stabilized amplifier is controlled by the same gain control circuit without the closed loop arrangement.

Abstract: Apparatus for communicating electromagnetic energy between a logging tool and a surrounding medium comprise a support member, a radiating/receiving element disposed on one surface of a nonconductive dielectric material having a shape compatible with the shape and dimensions of the support member, a ground plane disposed on the other face of the dielectric opposite the radiating/receiving element and a predetermined distance therefrom, and impedance means, a short circuit in one embodiment, electrically connecting an end of the first element to the second element. The dielectric, radiating/receiving element, and ground plane are carried by the support member, which may be a mandrel or a pad. Various embodiments particularly suitable for use in certain applications and with certain kinds of logging tools are described.