Abstract: In one aspect of the invention an induction resistivity tool incorporated into a downhole tool string comprises an outer wall of a downhole component having an outer diameter. At least one induction transmitter assembly is disposed along the outer diameter. The transmitter assembly comprises at least one coil array and at least one induction transmitter coil wound about at least one core. The coil array comprises a plurality of magnetic units and each unit comprising a magnetic field orientation. The magnetic field orientations accumulatively form a Halbach array with an augmented field side and a canceled field side of the array. The transmitter assembly generates an induction signal from outside the outer wall and substantially prevents the signal from entering the outer wall when the transmitter assembly is carrying an electrical current.

Abstract: In one aspect an induction resistivity tool incorporated into a downhole tool string comprises an outer wall of a downhole component comprising an outer diameter and at least one induction transmitter assembly disposed along the outer diameter. The at least one transmitter assembly comprises at least one induction transmitter coil wound about at least one core. The at least one transmitter coil is adapted to project an induction signal outward from the outer wall when the at least one transmitter coil is carrying an electrical current. The transmitter assembly is adapted to create electromagnetic fields that originate the induction signal from outside the outer wall and substantially prevent the signal from entering the outer wall.

Abstract: An antenna structure is separately and independently formed with respect to a downhole tool. The antenna is adapted for easy and rapid deployment on the tool. An independently formed ‘partial ring section’ (or arcuate shaped member) contains an antenna coil. In one embodiment, two or more of the independently formed partial ring sections are placed in a recess on the tubular forming the downhole tool. In another embodiment, the partial ring sections can also be placed in one or more longitudinally distributed sections about the tool. Another embodiment includes partial ring sections forming an antenna with multiple coils of differing orientation to provide directional sensitivity.

Abstract: A multicomponent induction logging tool uses a nonconducting mandrel with a windowed conducting sleeve. This arrangement reduces effects due to tool eccentering and coil misalignment.

Abstract: Multi-component induction measurements are made using a resistivity logging tool in an anistropic earth formation. A subset of the multi-component measurements are inverted to first determine horizontal resistivities. Using the determined horizontal resistivities and another subset of the multi-component measurements, the vertical resistivities are obtained. Results of using the in-phase signals are comparable to those obtained using multifrequency focusing of quadrature signals.

Abstract: A method is disclosed for locating a receiver in a borehole having a conductive liner by inducing an electromagnetic field from a transmitter in a first borehole, detecting at a first location by at least one receiver located in a second borehole having a conductive liner, a first electromagnetic field induced by the transmitter, detecting at a second location by the at least one receiver, a second electromagnetic field induced by the transmitter and calculating a set of geometry coordinates for the first location based on the first electromagnetic field and the second electromagnetic field.

Abstract: An induction logging tool is used on a MWD bottom hole assembly. Due to the finite, nonzero, conductivity of the mandrel, conventional multi frequency focusing (MFF) does not work. A correction is made to the induction logging data to give measurements simulating a perfectly conducting mandrel. MFF can then be applied to the corrected data to give formation resistivities.

Abstract: A method for determining a sonde error in a logging tool comprising a transverse array includes obtaining a first measurement using the transverse array while the logging tool is at a first height from ground in a selected orientation; obtaining a second measurement using the transverse array while the logging tool is at a second height from the ground in the selected orientation; deriving a difference measurement from the first measurement and the second measurement; determining a background signal using the difference measurement and a predetermined function, wherein the predetermined function relates earth signals in a plurality of signals obtained at the second height to a plurality of difference signals between signals obtained at the first height and the second height at a plurality of formation conductivities; and determining the sonde error by subtracting the background signal from the second measurement.

Abstract: Transverse or tilted magnetic dipole antenna systems are used for electromagnetic logging measurements to determine the anisotropic resistivity of subsurface formations. Combined measurements between transverse or tilted antennas, as well as axial antennas, are used to compensate for receiver antenna mismatch in the system. Some embodiments use model-based parametric inversion to estimate the anisotropic resistivity in the presence of invasion and shoulder-bed effects.

Abstract: Systems and methods are provided for directional propagation-type logging measurements not sensitive to dip and anisotropy over a wide frequency range. Estimates of the distance of the logging tool to a bed boundary are obtained using “up-down” measurements with antenna configurations having tilted magnetic dipoles. These estimates are insensitive to anistropy and dip, undesirable complications of directional measurements.

Abstract: The invention relates to a system and methods for locating a fracture in an earth formation. A method for detecting a fracture in an earth formation using an induction tool includes inducing electromagnetic fields using a set of transmitters on the induction tool. A plurality of the transmitters being arranged such that their magnetic moments are transverse or tilted with respect to the tool axis. Voltages induced in a set of receivers on the induction tool are measured, the voltages being induced by eddy currents that result from the electromagnetic fields. A plurality of receivers being arranged such that their axes are transverse or tilted with respect to the tool axis. The transmitters and receivers are spaced apart along the tool axis. The presence of the fracture is determined based on the measured voltages.

Abstract: The invention relates to techniques for locating and determining the orientation of a fracture in an earth formation. Systems and methods for detecting a fracture in an earth formation using a propagation tool include producing electromagnetic fields using a TMD transmitter in the tool; measuring corresponding voltage signals detected with one or more TMD receivers in the tool; determining harmonics from the measured signal responses by shifting the responses (e.g. by 90 degrees) and performing an addition or subtraction using the shifted response. In some embodiments, the second harmonic is processed to determine the fracture orientation.

Abstract: This invention is directed to a downhole method and apparatus for simultaneously determining the horizontal resistivity, vertical resistivity, and relative dip angle for anisotropic earth formations. The present invention accomplishes this objective by using an antenna configuration in which a transmitter antenna and a receiver antenna are oriented in non-parallel planes such that the vertical resistivity and the relative dip angle are decoupled. Preferably, either the transmitter or the receiver is mounted in a conventional orientation in a first plane that is normal to the tool axis, and the other antenna is mounted in a second plane that is not parallel to the first plane. This invention also relates to a method and apparatus for steering a downhole tool during a drilling operation in order to maintain the borehole within a desired earth formation.

Abstract: There is disclosed herein a method of enhancing the vertical resolution of an induction tool, in a manner that may advantageously also reduce undesirable borehole and “negative resistivity” effects. In one embodiment, the method comprises: a) obtaining a vertical magnetic dipole (VMD) response signal from a transmitter-receiver array of antenna elements having magnetic dipoles oriented parallel to a tool axis; b) obtaining a horizontal magnetic dipole (HMD) response signal from a transmitter-receiver array of elements having magnetic dipoles oriented perpendicular to the tool axis; and c) combining the VMD and HMD response signals to obtain a combination response signal. When the relative weights of the VMD and HMD response signals are set as described herein, the combination response signal (and any log calculated therefrom) has a narrow, substantially rectilinear, vertical measurement profile. Further, the combination response signal is relatively insensitive to borehole effects.

Abstract: An improved induction tool for formation resistivity evaluations. The tool provides electromagnetic transmitters and sensors suitable for transmitting and receiving magnetic fields in radial directions that are orthogonal to the tool's longitudinal axis with minimal susceptibility to errors associated with parasitic eddy currents induced in the metal components surrounding the transmitter and receiver coils.

Abstract: The invention relates to techniques for locating and determining the orientation of a fracture in an earth formation. Systems and methods for detecting a fracture in an earth formation using a propagation tool include producing electromagnetic fields using a TMD transmitter in the tool; measuring corresponding voltage signals detected with one or more TMD receivers in the tool; determining harmonics from the measured signal responses by shifting the responses (e.g. by 90 degrees) and performing an addition or subtraction using the shifted response. In some embodiments, the second harmonic is processed to determine the fracture orientation.

Abstract: A coil arrangement and method of collecting and processing data in induction logging of wells is disclosed. The coil arrangement includes three essentially identical coils made of elliptical conductive loops lying in a plane tilted from the axis of the logging tool. Each coil is centered on the same point in space. Data are preferably collected by using the coil arrangement for both transmitter and receiver, and transmitting at three closely-spaced frequencies, one from each of the transmitter coils. The transmitted signals are received and recorded, at which time they are operated on mathematically to create rotation of an after-the-fact virtual sonde for measuring desired earth formation properties.

Abstract: A method for determining the anisotropic resistivity properties of a subterranean formation traversed by a borehole utilizing a multi-component induction logging tool. The method utilizes the measured phase and attenuation signals induced by eddy currents in the formation to determine the azimuthal angle and to create a second set of signals based on the measured signals and the azimuthal angle. The anisotropic resistive properties, as well as the dip angle, are then simultaneously derived from the second set of signals by means of minimizing error functions within an inversion model.

Abstract: The invention relates to a system and methods for locating a fracture in an earth formation. A method for detecting a fracture in an earth formation using an induction tool includes inducing electromagnetic fields using a set of transmitters on the induction tool. A plurality of the transmitters being arranged such that their magnetic moments are transverse or tilted with respect to the tool axis. Voltages induced in a set of receivers on the induction tool are measured, the voltages being induced by eddy currents that result from the electromagnetic fields. A plurality of receivers being arranged such that their axes are transverse or tilted with respect to the tool axis. The transmitters and receivers are spaced apart along the tool axis. The presence of the fracture is determined based on the measured voltages.

Abstract: A transverse induction logging tool having a transmitter and receiver for downhole sampling of formation properties, the tool having a symmetrical shielded split-coil transmitter coil and a bucking coil interposed between the split transmitter coils to reduce coupling of the transmitter time varying magnetic field into the receiver. The tool provides symmetrical shielding of the coils and grounding at either the transmitter or receiver end only to reduce coupling of induced currents into the received signal. The tool provides an insulator between receiver electronics and the conductive receiver housing having contact with conductive wellbore fluid, to reduce parasitic current flowing in a loop formed by the upper housing, feed through pipe, lower housing and wellbore fluid adjacent the probe housing or mandrel. An internal verification loop is provided to track changes in transmitter current in the real and quadrature component of the received data signal.

Abstract: A method and apparatus for a downhole antenna comprising a housing having an outer surface and a longitudinally bored inner cavity, the outer surface having a first slot and a corresponding second slot extending from the outer surface to the longitudinally bored inner cavity; a first removable downhole antenna segment disposed in the first slot in a first transverse cross section of the housing, the first removable downhole antenna segment comprising at least one coil to inductively couple a signal from the longitudinally bored inner cavity to the earth formation. A second removable downhole antenna segment, interchangeable with the first removable downhole antenna segment, disposed in the corresponding second slot, the second slot in a second transverse cross section of the housing, the second removable downhole antenna segment comprising at least one coil to inductively couple a signal from the earth formation to the longitudinally bored inner cavity.

Abstract: A method and apparatus for a downhole antenna comprising a housing having an outer surface and a longitudinally bored inner cavity, the outer surface having a first slot and a corresponding second slot extending from the outer surface to the longitudinally bored inner cavity; a first removable downhole antenna segment disposed in the first slot in a first transverse cross section of the housing, the first removable downhole antenna segment comprising at least one coil to inductively couple a signal from the longitudinally bored inner cavity to the earth formation. A second removable downhole antenna segment, interchangeable with the first removable downhole antenna segment, disposed in the corresponding second slot, the second slot in a second transverse cross section of the housing, the second removable downhole antenna segment comprising at least one coil to inductively couple a signal from the earth formation to the longitudinally bored inner cavity.

Abstract: Methods and apparatus are disclosed for minimizing or eliminating an undesired axial electric current induced along a subsurface borehole in the process of subsurface measurements with transmitter and/or receiver antennas which are substantially time varying magnetic dipoles with their dipole moments aligned at an angle to the axis of the borehole. Some antennas are disposed within the borehole on instruments having a non-conductive support member. One instrument includes a conductive all-metal body with an antenna adapted for induction frequencies. Antenna shields adapted for controlled current flow are also provided with an all-metal instrument. Methods include providing an alternate path for the current along the instrument body. Another method includes emitting a controlled current to counter the undesired current. Another method corrects for the effect of the current using a superposition technique.

Abstract: The present invention provides a method and apparatus for performing resistivity measurements for the purpose of geo-steering with 180-degree azimuth resolution. All the measurements represent a complementary set of data acquired in two operational modes. The directional mode providing sensitivity of the received signals to the azimuth characteristics of the formation. The deep mode provides a large depth of investigation for resistivity determination and bed boundary detection. The directional mode can be implemented using one receiving coil placed in between quadruple type transmitter. The deep mode represents either array induction measurements or multiple propagation resisitivity (MPR) measurements that provide high depth of investigation for resistivity determination and bed boundary detection.

Abstract: This invention is directed to a downhole method and apparatus for simultaneously determining the horizontal resistivity, vertical resistivity, and relative dip angle for anisotropic earth formations. The present invention accomplishes this objective by using an antenna configuration in which a transmitter antenna and a receiver antenna are oriented in non-parallel planes such that the vertical resistivity and the relative dip angle are decoupled. Preferably, either the transmitter or the receiver is mounted in a conventional orientation in a first plane that is normal to the tool axis, and the other antenna is mounted in a second plane that is not parallel to the first plane. This invention also relates to a method and apparatus for steering a downhole tool during a drilling operation in order to maintain the borehole within a desired earth formation.

Abstract: Techniques for implementing antenna configurations with substantially co-located axes are disclosed. A method for constructing co-located antennas includes winding a first antenna on a support, the first antenna having a first magnetic dipole in a first orientation; and winding a second antenna on the support through a first set of openings in the support, the second antenna having a second magnetic dipole in a second orientation, wherein the first orientation is different from the second orientation, and wherein a center of the first magnetic dipole substantially co-locates with a center of the second magnetic dipole.

Abstract: A method is disclosed for generating an image of earth formations penetrated by a wellbore. The method includes generating an initial model of the earth formations using formation resistivity measured by a direct current signal. A response to the initial model of an instrument used to make the direct current resistivity measurements is calculated. The calculated response is compared to the measurements of resistivity. The model is adjusted, and the calculating and comparing are repeated until a difference between the calculated response and measurements reaches a minimum. The adjusted model is refined based on resistivity measurements made using an electromagnetic measuring instrument, and the refined model is constrained using acoustic velocity measurements.

Abstract: Measurements are made with a multicomponent induction logging tool in earth formations in a borehole inclined to earth formations. A combination of principal component measurements is used to determine the horizontal resistivity of the earth formations. The determined horizontal resistivities are used in a model for inversion of other components of the data to obtain the vertical formations resistivities. When multifrequency measurements are available, frequency focusing is used.

Abstract: Shield apparatus are provided for use with logging instruments equipped with transverse magnetic dipole antennas. Flexible dielectric strips and dielectric cylindrical bodies are implemented with conductive elements in various patterns to form Faraday shield structures. The shields provide an extended conductive surface to the instrument support member and redirect axial currents along the borehole, protecting the underlying antennas from the undesired effects of these currents.

Abstract: The invention refers to antenna configurations for electromagnetic logging tools. The antennas have one or more electrical conductors disposed on a dielectric substrate wrapped around a core or disposed directly on the core. The conductor or conductors are arranged such that the antenna has a first magnetic dipole moment substantially perpendicular to a longitudinal axis of the core. The invention also refers to antennas adapted for mutual balancing using interleaved conductive paths or disks to alter the magnetic dipole moments to mutually balance the antenna system.

Abstract: There is disclosed herein a method of enhancing the vertical resolution of an induction tool, in a manner that may advantageously also reduce undesirable borehole and “negative resistivity” effects. In one embodiment, the method comprises: a) obtaining a vertical magnetic dipole (VMD) response signal from a transmitter-receiver array of antenna elements having magnetic dipoles oriented parallel to a tool axis; b) obtaining a horizontal magnetic dipole (HMD) response signal from a transmitter-receiver array of elements having magnetic dipoles oriented perpendicular to the tool axis; and c) combining the VMD and HMD response signals to obtain a combination response signal. When the relative weights of the VMD and HMD response signals are set as described herein, the combination response signal (and any log calculated therefrom) has a narrow, substantially rectilinear, vertical measurement profile. Further, the combination response signal is relatively insensitive to borehole effects.

Abstract: Methods and apparatus are disclosed for minimizing or eliminating an undesired axial electric current induced along a subsurface borehole in the process of subsurface measurements with transmitter and/or receiver antennas which are substantially time varying magnetic dipoles with their dipole moments aligned at an angle to the axis of the borehole. Some antennas are disposed within the borehole on instruments having a non-conductive support member. One instrument includes a conductive all-metal body with an antenna adapted for induction frequencies. Antenna shields adapted for controlled current flow are also provided with an all-metal instrument. Methods include providing an alternate path for the current along the instrument body. Another method includes emitting a controlled current to counter the undesired current. Another method corrects for the effect of the current using a superposition technique.

Abstract: Techniques are provided to transform attenuation and phase measurements taken in conjunction with a drilling operation into independent electrical parameters such as electrical resistivity and dielectric values. The electrical parameters are correlated with background values such that resulting estimates of the electrical parameters are independent of each other. It is shown an attenuation measurement is sensitive to the resistivity in essentially the same volume of an earth formation as the corresponding phase measurement is sensitive to the dielectric constant. Further, the attenuation measurement is shown to be sensitive to the dielectric constant in essentially the same volume that the corresponding phase measurement is sensitive to the resistivity. Techniques are employed to define systems of simultaneous equations that produce more accurate measurements of the resistivity and/or the dielectric constant within the earth formation than are available from currently practiced techniques.

Abstract: A transverse induction logging tool having a transmitter and receiver for downhole sampling of formation properties, the tool having a symmetrical shielded split-coil transmitter coil and a bucking coil interposed between the split transmitter coils to reduce coupling of the transmitter time varying magnetic field into the receiver. The tool provides symmetrical shielding of the coils and grounding at either the transmitter or receiver end only to reduce coupling of induced currents into the received signal. The tool provides an insulator between receiver electronics and the conductive receiver housing having contact with conductive wellbore fluid, to reduce parasitic current flowing in a loop formed by the upper housing, feed through pipe, lower housing and wellbore fluid adjacent the probe housing or mandrel. An internal verification loop is provided to track changes in transmitter current in the real and quadrature component of the received data signal.

Abstract: Systems and methods are provided for directional propagation-type logging measurements not sensitive to dip and anisotropy over a wide frequency range. Estimates of the distance of the logging tool to a bed boundary are obtained using “up-down” measurements with antenna configurations having tilted magnetic dipoles. These estimates are insensitive to anistropy and dip, undesirable complications of directional measurements.

Abstract: A method is disclosed for the determination of the dip angle of anisotropic earth formations surrounding a wellbore. Electromagnetic couplings among a plural of triad transmitters and triad receivers are measured. Each triad transmitter/receiver consists of coil windings in three mutually orthogonal axes. The transmitter coils and receiver coils are oriented such that mutually symmetrical transmitter pairs or mutually symmetrical receiver pairs are equidistant from a centrally located receiver or transmitter, respectively. The measured signals from the created couplings are used to generate initial separate estimates of the dip angle of the formation. The two discrete determination is of dip angle are then averaged to arrive at a more accurate estimate that approaches the true dip angle in the formation.

Abstract: Systems and methods are provided for determining subsurface formation properties using an antenna system disposed within a borehole traversing the formation. A logging system includes a well tool implemented with an antenna system having transverse or tilted magnetic dipoles. One antenna implementation uses a set of three coils having non-parallel axes. Through mechanical or electromagnetic rotation of an antenna about its axis in order to maximize or minimize couplings, the systems and methods are used to estimate formation anisotropic resistivity, providing reduced borehole effects in vertical and small deviation wells. This invention may also be applied in deviated wells when the borehole is perpendicular to the strike direction, as is often the case in wireline logging.

Abstract: In one embodiment the invention comprises a system for generating an image of an Earth formation surrounding a borehole penetrating the formation. Resistivity of the formation is measured using a DC measurement, and conductivity and resistivity of the formations is measured with a time domain signal or AC measurement. Acoustic velocity of the formation is also measured. The DC resistivity measurement, the conductivity measurement made with a time domain electromagnetic signal, the resistivity measurement made with a time domain electromagnetic signal and the acoustic velocity measurements are combined to generate the image of the Earth formation.

Abstract: Methods and apparatus are disclosed for canceling or eliminating borehole eccentricity effects on a formation resistivity measurement obtained with transmitter and/or receiver antennas which are substantially time varying magnetic dipoles with their dipole moments aligned at an angle to the axis of the borehole. Various apparatus are configured with a plurality of antennas having tilted or transverse magnetic dipole moments, at least one current sensor, means for conducting alternating current through one or more of the antennas, and means for calculating a scaling factor from signal measurements and for scaling the alternating current with the factor. One method includes scaling an alternating current and passing said current through one or more antennas to obtain the resistivity measurement.

Abstract: Measurements are made with a multicomponent induction logging tool in earth formations in a borehole inclined to earth formations. A combination of principal component measurements is used to determine the horizontal resistivity of the earth formations. The determined horizontal resistivities are used in a model for inversion of other components of the data to obtain the vertical formations resistivities. When multifrequency measurements are available, frequency focusing is used.

Abstract: This invention is directed to a downhole method and apparatus for simultaneously determining the horizontal resistivity, vertical resistivity, and relative dip angle for anisotropic earth formations. The present invention accomplishes this objective by using an antenna configuration in which a transmitter antenna and a receiver antenna are oriented in non-parallel planes such that the vertical resistivity and the relative dip angle are decoupled. Preferably, either the transmitter or the receiver is mounted in a conventional orientation in a first plane that is normal to the tool axis, and the other antenna is mounted in a second plane that is not parallel to the first plane. This invention also relates to a method and apparatus for steering a downhole tool during a drilling operation in order to maintain the borehole within a desired earth formation.

Abstract: Methods and apparatus are disclosed for minimizing or eliminating an undesired axial electric current induced along a subsurface borehole in the process of subsurface measurements with transmitter and/or receiver antennas which are substantially time varying magnetic dipoles with their dipole moments aligned at an angle to the axis of the borehole. Some antennas are disposed within the borehole on instruments having a non-conductive support member. One instrument includes a conductive all-metal body with an antenna adapted for induction frequencies. Antenna shields adapted for controlled current flow are also provided with an all-metal instrument. Methods include providing an alternate path for the current along the instrument body. Another method includes emitting a controlled current to counter the undesired current. Another method corrects for the effect of the current using a superposition technique.

Abstract: Techniques are provided to transform attenuation and phase measurements taken in conjunction with a drilling operation into independent electrical parameters such as electrical resistivity and dielectric values. The electrical parameters are correlated with background values such that resulting estimates of the electrical parameters are independent of each other. It is shown an attenuation measurement is sensitive to the resistivity in essentially the same volume of an earth formation as the corresponding phase measurement is sensitive to the dielectric constant. Further, the attenuation measurement is shown to be sensitive to the dielectric constant in essentially the same volume that the corresponding phase measurement is sensitive to the resistivity. Techniques are employed to define systems of simultaneous equations that produce more accurate measurements of the resistivity and/or the dielectric constant within the earth formation than are available from currently practiced techniques.

Abstract: A resistivity tool and associated method include measuring the phase shift of an electromagnetic signal at different locations. Preferably, this is accomplished by a resistivity tool having three receivers. The difference of the phase shifts at the different locations indicates with precision the depth in a borehole of a bed boundary. The preferred resistivity tool also yields improved radial resolution.

Abstract: A method and apparatus for evaluating the resistivity of earth formations surrounding a borehole, particularly high-contrast thin-layer formations or at high dip angles. The method involves positioning a pair of transmitters and a pair of receivers within the borehole, the receivers or transmitters adhering to specific spacing limitations, alternately transmitting electromagnetic energy of a particular frequency and receiving voltage data associated with the transmitted energy. Multiple voltage data are acquired and a representation of a resistivity or conductivity profile is created from a formulated difference of the data from a particular depth and/or neighboring depths. The apparatus forms part of a well logging system including a well tool adapted to be moveable through a borehole. The apparatus being coupled to the well tool and adapted with means to input voltage data developed by the receivers disposed on the well tool.

Abstract: An azimuthally tunable resistivity measurement tool is provided. In one embodiment, the tool comprises a set of three skewed receiver antennas and a transmitter antenna. The three skewed receiver antennas are oriented in equally-spaced azimuthal directions to give them preferential sensitivity in those directions. The transmitter antenna transmits a radio-frequency signal that propagates through the formation surrounding a borehole. The signals from the three receiver antennas can be measured and combined to synthesize the signal that would be received by a virtual antenna oriented in any desired direction. Accordingly, virtual receivers oriented perpendicular to the tool axis and with variable azimuthal orientations can be synthesized. The orientation of such a virtual receiver that has a maximized receive signal amplitude can be used to identify the direction of a nearby bed boundary, and the maximized amplitude can be used to estimate the distance to the boundary.

Abstract: This invention is directed to a downhole method and apparatus for simultaneously determining the horizontal resistivity, vertical resistivity, and relative dip angle for anisotropic earth formations. The present invention accomplishes this objective by using an antenna configuration in which a transmitter antenna and a receiver antenna are oriented in non-parallel planes such that the vertical resistivity and the relative dip angle are decoupled. Preferably, either the transmitter or the receiver is mounted in a conventional orientation in a first plane that is normal to the tool axis, and the other antenna is mounted in a second plane that is not parallel to the first plane. Although this invention is primarily intended for MWD or LWD applications, this invention is also applicable to wireline and possible other applications.

Abstract: A method is provided for determining an electric conductivity of an earth formation formed of different earth layers, which earth formation is penetrated by a wellbore containing a wellbore fluid, is provided. The method includes the steps of: lowering an induction logging tool into the wellbore to a location surrounded by a selected one of the earth layers, the tool having a magnetic field transmitter effective to induce magnetic fields of different frequencies in the earth formation, and a magnetic field receiver effective to receive response magnetic fields and to provide a signal representative of each response magnetic field, at least one of the transmitter and the receiver having a plurality of magnetic dipole moments in mutually orthogonal directions.

Abstract: A method for determining the 2-dimensional distribution of horizontal and vertical electrical conductivities of earth formations surrounding a wellbore using measurements made by a transverse electromagnetic induction well logging instrument. A model is generated of the axial distribution of the horizontal and vertical conductivities, from induction signals acquired by the instrument using two-frequency alternating current. The model is generated by calculating an initial estimate of the conductivity distribution and axially inverting the estimate with respect to the measurements made using the two-frequency alternating current. Shoulder correction is applied to measurements made by the instrument using single-frequency alternating current. An estimate of the radial distribution of the conductivities is generated from the shoulder corrected induction signals acquired using the single-frequency alternating current.

Abstract: A method of measuring the conductivity of earth formations penetrated by a wellbore. The method comprises selectively passing an alternating current through transmitter coils inserted into the wellbore. Each of the transmitter coils has a magnetic moment direction different from the magnetic moment direction of the other ones of the transmitter coils. The alternating current includes a first and a second frequency. The amplitude at the first frequency has a predetermined relationship to the amplitude at the second frequency. The relationship corresponds to the first and the second frequencies. The method includes selectively receiving voltages induced in a receiver coil having a sensitive direction substantially parallel to the axis of the corresponding transmitter coil through which the alternating current is passed.