Abstract: Disclosed herein are methods and apparatus using one or more toroids to measure formation anisotropy and/or vertical conductivity. In one embodiment, a disclosed method comprises: inducing a current to flow through a conductive surface of a tool body into a surrounding formation; measuring a lateral portion of the current flow; and calculating an anisotropy coefficient from the apparent conductivity so measured. The lateral current flow may be that portion of the current flowing through the tool body surface between two predetermined, axially spaced, circumferences of the tool. In an alternative embodiment, a disclosed logging tool comprises: a substantially cylindrical and conductive tool body, a toroidal transmitter, and a current measuring means. The current measuring means is configured to measure that portion of an induced current that radiates through a predetermined surface of the tool into the formation.

Abstract: A second receiver coil system (45a,45b) at right angles to the primary receiver coil system (40) in an induction logging tool (10) detects the skewness in the logging tool field pattern caused by the presence of assymmetrical formation structures (30). This skew signal, which provides a composite measure of the asymmetry of the formation structures (30), can be used to correct for horns produced in formation conductivity logs when the logging tool sonde (14) traverses interfaces between dipping beds. The skew signal can also be used to determine the dip and strike angles between such dipping beds.

Abstract: A method for measuring properties of earth formations surrounding a borehole and, particularly, properties of the invaded zone of the formations. A short pulse of electrical energy is applied at a transmitter location, and signals are induced at a plurality of receiver locations. The polarities of the initial portions of the signals detected at the receiver locations are used in determining properties of the formation invaded zone, particularly its thickness and its conductivity as compared to the virgin formations.

Abstract: An apparatus and method for measuring properties of earth formations surrounding a borehole and, particularly, conductivity properties of the formations and certain properties of the invaded zone of the formations. In accordance with one embodiment of the invention, a logging device, movable through the borehole, has a transmitter coil and first and second spaced receiver coils mounted thereon. A short pulse of electrical energy is applied to the transmitter coil. Signals are induced in the first and second receiver coils. A conductivity property of the surrounding formations is then determined as a function of the times of occurrence of predetermined characteristics of the signals detected in the first and second receiver coils.

Abstract: The invention is directed to induction logging apparatus and method for obtaining measurements dependent upon the vertical conductivity component of formations surrounding a borehole. The disclosed techniques can also be used to determine horizontal as well as vertical conductivity of the formations, and thereby achieve determination of formation anisotropy. A transmitter coil is provided with its axis substantially perpendicular to the borehole axis. A receiver coil is provided in spaced relation to the transmitter coil. The axis of the receiver coil is also substantially perpendicular to the borehole axis, and is preferably parallel to the axis of the transmitter coil. Means are provided for detecting signals induced in the receiver coil, these signals being a measure of the formation vertical conductivity. In a preferred embodiment of the invention, the detecting means is operative to detect both the resistive and reactive components of the signals induced in the receiver coils.

Abstract: The disclosure is directed to a technique used for determining the mudcake-corrected water-filled porosity of formations surrounding a borehole, and which can also be used for determining the thickness of the mudcake formed on the borehole. Microwave electromagnetic energy is transmitted into the formations. Near and far receiving antennas are employed for receiving microwave electromagnetic energy from the formations. Means responsive to the outputs of the receiving antennas are provided for obtaining a travel-time-dependent differential measurement. Further means, responsive to the output of one of the antennas, are provided for obtaining a travel-time-dependent direct measurement. Finally, means are provided for determining the mudcake-corrected water-filled porosity of the formations from the differential and direct measurements. The mudcake thickness may also be determined from the differential and direct measurements.