Abstract: Well logging apparatus and methods for determining formation resistivity at multiple (>3) depths of investigation. At least one transmitter antenna and at least two receiver antennas are mounted in a logging tool housing, on substantially a common axis. The antennas are untuned coils of wire. Electromagnetic energy is emitted at multiple frequencies from the transmitter into the formation. The receiver antennas, which are spaced apart from each other and from the transmitter, detect reflected electromagnetic energy. Formation resistivity at multiple depths of investigation is determined using only phase differences in the reflected energy at the different frequencies, minimizing false indications of invasion due to mismatch of vertical response with attenuation measurements and also permitting correcting for the effects of varying dielectric constant of the formation.

Abstract: Well logging apparatus and methods for determining formation resistivity at multiple (>3) depths of investigation. At least one transmitter antenna and at least two receiver antennas are mounted in a logging tool housing, on substantially a common axis. The antennas are untuned coils of wire. Electromagnetic energy is emitted at multiple frequencies from the transmitter into the formation. The receiver antennas, which are spaced apart from each other and from the transmitter, detect reflected electromagnetic energy. Formation resistivity at multiple depths of investigation is determined using only phase differences in the reflected energy at the different frequencies, minimizing false indications of invasion due to mismatch of vertical response with attenuation measurements and also permitting correcting for the effects of varying dielectric constant of the formation.

Abstract: NMR logging apparatus is provided which produces a strong, static and homogeneous magnetic field B.sub.0 in a Volume of an adjacent formation on one side of the tool to measure nuclear magnetic resonance characteristics thereof at two different depths of investigations. In the preferred embodiment, the tool has an RF antenna mounted on the outside of the metal body of the tool, directing focused oscillating magnetic fields B.sub.1 at said Volume to polarize or tip the magnetic moments of hydrogen nuclei of fluids within rock pores. The same antenna can be used to receive signals of proton precession in the Volume of interest immediately after transmission of the RF polarizing field B.sub.1. One of the NMR readings is employed to compensate for inaccuracies in the other NMR reading to provide an overall improved NMR log.

Abstract: A transmitter for an induction well logging tool, adapted to be disposed in the borehole of an oil well, develops an output signal for energizing the borehole formation which inherently consists of six separate signal components, the six components including four primary signal components which simultaneously energize the borehole formation inducing in the formation four primary eddy currents. These four primary eddy currents are detected by the array induction tool receiver, the received signals being used to determine the resistivity and conductivity of the borehole formation.