Abstract: A method to determine one or more borehole corrected formation properties using measurements made using a logging tool disposed in a borehole penetrating an earth formation is disclosed. The measurements are used to determine an apparent conductivity tensor for the formation and, for a set of parameters, a parameter value for each parameter in a subset of the set of parameters. A parameter value for each parameter in the set of parameters not in the subset is provided and a borehole-inclusive modeled conductivity tensor is computed. The apparent conductivity tensor and the borehole-inclusive modeled conductivity tensor are iteratively used to optimize the parameter values, and the optimized parameter values are used to compute an optimized conductivity tensor. A borehole corrected conductivity tensor is computed using the optimized conductivity tensor, and the borehole corrected formation properties are determined using the borehole corrected conductivity tensor and/or the optimized parameter values.

Abstract: The present disclosure relates to a method to determine the vertical resistivity of a subsurface formation. A downhole logging tool having a plurality of spaced antennas, at least one of which is a transverse antenna, at least two of which are tilted antennas, and at least two of which are axial antennas is provided. Measurements involving the transverse and/or the tilted antennas of the downhole logging tool are obtained. Voltage ratios are formed using the measurements, and conditioning factors are formed by raising the determined voltage ratios not involving the transverse antenna to some arbitrary power. The sum of the exponents of the conditioning factors preferably equals one. A voltage ratio involving the transverse antenna is multiplied by the conditioning factors, and the vertical resistivity of the subsurface formation is determined using the resulting ratio.

Abstract: A system, method and device for interrogating a downhole environment in a borehole beneath a surface includes a source of electromagnetic energy, operable to transmit an electromagnetic signal in the borehole, a sensor module, including a passive resonator having a resonant frequency that varies with changes in the condition in the downhole environment to reflect the electromagnetic signal and generates harmonic frequencies in response to a condition in the downhole environment in the borehole and a detector positionable to receive a returned electromagnetic signal. Embodiments include a filter to pass a harmonic of the resonant frequency of the resonating circuit and circuitry configured and arranged to measure the frequency of the harmonic signal.

Abstract: An apparatus and method for estimating a parameter of interest of an earth formation, particularly relating to borehole logging methods and apparatuses for estimating electrical resistivity properties at multiple depths of investigation. The apparatus may include two or more transmitters for introducing electrical current to the earth formation. The apparatus may include a controller configured to deliver an electrical signal to the two or more transmitters either simultaneously or sequentially. The controller may deliver an electrical signal to two or more transmitters at the same frequency for estimating depth of investigation. The apparatus may include one or more receivers responsive to electric signals from the earth formation at one or more frequencies to provide data from one or more depths of investigation. The method may include steps for using the apparatus to obtain data that may be used to estimate the parameter of interest.

Abstract: An apparatus for estimating a property of an earth formation penetrated by a borehole includes: a carrier configured to be conveyed through the borehole; a transmitter antenna disposed at the carrier and configured to emit electromagnetic energy into the earth formation; a controller configured to control electrical current of frequency f transmitted to the transmitter antenna, wherein the transmitted electrical current is non-sinusoidal having a first section with a uniform positive slope and a second section with a uniform negative slope; a receiver antenna configured to receive a signal from the formation indicative of the property; and a processor configured to receive the signal from the receiver antenna and to estimate the property using the received signal.

Abstract: An apparatus for estimating a property of an earth formation penetrated by a borehole includes: a carrier configured to be conveyed through the borehole; a transmitter antenna disposed at the carrier and configured to emit electromagnetic energy; a direction guidance antenna configured to receive the electromagnetic energy from the transmitter antenna and direct the electromagnetic energy into the formation and to receive a signal from the formation indicative of the property; a receiver antenna configured to receive the signal from the direction guidance antenna; and a processor configured to receive the signal from the receiver antenna and to estimate the property using the received signal.

Abstract: A method of measuring the permittivity and/or conductivity of a multiphase fluid flowing through a conduit is provided. The method includes the steps of measuring the signal from a first electromagnetic transmitter to a first electromagnetic receiver separated by a first distance, measuring the signal from the first electromagnetic transmitter to a second electromagnetic receiver separated by a second distance, measuring the signal from a second electromagnetic transmitter to the first electromagnetic receiver separated by a distance substantially equal to the second distance, measuring the signal from the second electromagnetic transmitter to the second electromagnetic receiver separated by a distance substantially equal to the first distance, and wherein the first and second distances are substantially different.

Abstract: A method for correcting subterranean resistivity measurements to account for tool bending includes processing at least one coupling component and at least one cross-coupling component in combination with a tool bending angle. Such processing may, for example, remove one or more coupling components from a cross-coupling component. Removal of the coupling component(s) tends to increase the sensitivity of directional resistivity measurements to remote boundaries.

Abstract: Methods and systems for determining whether a tool has been deployed below a drill pipe are provided. A downhole tool can measure various characteristics, which then can be analyzed to determine the likelihood of a tool having been deployed below the drill pipe. For example, density and porosity measurements can be affected by the presence of casing or drill pipe, and thus such measurements can provide an indication of whether the tool has been deployed below the drill pipe.

Abstract: A logging while drilling tool includes a directional resistivity antenna and an antenna shield having. The shield has at least one slot having at least one electrically open end formed therein. The antenna shield may include a base portion and a plurality of spaced apart fingers extending away from the base portion such that the finger ends are electrically isolated from the tool body and from one another. The antenna shield may alternatively include a plurality of spaced apart plates that are electrically isolated from the tool body and from one another. These antenna shields have been found to provide suitable physical protection for sensitive antenna components while at the same time being substantially transparent to both z-mode and x-mode electromagnetic waves.

Abstract: An apparatus configured to evaluate an earth formation that includes a tuned transmitter antenna and an untuned broadband receiver antenna. The transmitter antenna being configured to generate electromagnetic energy and the untuned broadband antenna configured to produce an output responsive to the generated electromagnetic energy. The untuned, broadband receiver antenna is configured reduce to reduce tool complexity, antenna coupling, and cross-talk between conductors to the antenna assemblies. The method includes using the apparatus in earth formation evaluation.

Abstract: An antenna 3 of an electromagnetic probe used in investigation of geological formations GF surrounding a borehole WBH comprises a conductive base 31 and an antenna element 32. The conductive base 31 comprises an opened non-resonant cavity 33. The antenna element 32 is embedded in the cavity 33 and goes right through the cavity. The antenna element 32 is isolated from the conductive base 31. The antenna element 32 is coupled to at least one electronic module via a first 34A and a second 34B port, respectively. The electronic module operates the antenna so as to define either a substantially pure magnetic dipole, or a substantially pure electric dipole.

Abstract: Apparatus having a focused transducer and methods of operating a focused transducer down-hole in a well can provide high resolution downhole imaging. In various embodiments, a focused transducer is used for imaging downhole in a well in which the imaging is based on a seismoelectric effect. In various embodiments, a focused transducer is used for imaging downhole in a well in which the imaging is based on an electroacoustic effect. Additional apparatus, systems, and methods are disclosed.

Abstract: Various embodiments include apparatus and methods of sensing and/or mapping a subterranean reservoir. In various embodiments, arrangements of electromagnetic sensors are disposed relative to a subterranean reservoir such that a signal received by electromagnetic sensors of one arrangement, in response to activation of an electromagnetic transmitter, can be analyzed to map the subterranean reservoir. Additional apparatus, systems, and methods are disclosed.

Abstract: A directional resistivity tool includes a pair of transmitters deployed between at least one pair of receivers. Each of the transmitters and receivers preferably includes collocated z-mode and x-mode antennae. Exemplary embodiments may further include additional receivers, for example, additional pairs of receivers deployed axially about the transmitters or one or more deep reading receivers deployed on one axial end of the transmitters. Tools in accordance with the invention enable directional resistivity measurements to be acquired at multiple depths of investigation using fewer transmitter firings than conventional tools.

Abstract: Various embodiments include apparatus and methods of operation with respect to well logging. Apparatus and methods include a tool having an arrangement of transmitters and receivers to capture a signal from a first region relative to the tool such that signal contributions from a second region relative to the tool are cancelable, based on placement of the transmitters and receivers with respect to each other. Additional apparatus, systems, and methods are disclosed.

Abstract: Methods and related systems are described for making electromagnetic measurements of a subterranean rock formation through a conductive casing of a borehole. A number of transmitters/receivers are deployed into single well having a conductive casing. A magnetic field is induced through the conductive casing and into a surrounding subterranean rock formation, and the magnetic field is detected by the receivers. The receivers have main windings, feedback winding and bucking windings. One of the receiver coils in the tool string is excited at a time, and induced voltages at all of the other receiver coils, either in straight mode, or in feedback mode, are measured.

Abstract: An apparatus is disclosed for estimating a property of a downhole fluid, the apparatus including but not limited to a test cell that receives the downhole fluid; a swept frequency electromagnetic energy source that emits electromagnetic energy toward the downhole fluid in the test cell; an electromagnetic/mechanical device that is immersed in the fluid and receives the emitted electromagnetic energy, wherein the emitted electromagnetic energy being emitted is swept about a resonant frequency for the electromagnetic/mechanical device; and an electromagnetic energy detector in electromagnetic communication with the electromagnetic/mechanical device immersed in the fluid, the electromagnetic energy detector producing an output signal indicative of the downhole fluid property. A system and method for estimating a property of a downhole fluid are also disclosed.

Abstract: The present disclosure relates to a method to determine a characteristic of a subsurface formation using a downhole logging tool. A downhole logging tool having the ability to make substantially concurrent disparate measurements on the subsurface formation is provided and substantially concurrent disparate measurements on the formation using the downhole logging tool are made. Those measurements are used to solve a system of equations simultaneously and the solution to the system of equations is used to determine the characteristic of the subsurface formation.

Abstract: One method of monitoring a formation core during coring operations can include measuring resistivities of a formation internal and external to a core barrel assembly, comparing the resistivities of the formation internal and external to the core barrel assembly, and determining a displacement of the core into the core barrel assembly, based at least in part on the comparing, while the core is being cut. A formation core analysis system can include multiple longitudinally spaced apart sets of transmitters and receivers which measure resistivity of a core while the core displaces into a core barrel assembly, and multiple longitudinally spaced apart sets of transmitters and receivers which measure resistivity of a formation external to the core barrel assembly while a coring bit penetrates the formation. A speed of displacement of the core may be indicated by differences in time between measurements taken via the different sets as the core displaces.

Abstract: An apparatus for making directional resistivity measurements of a subterranean formation includes a resistivity tool body with a longitudinal axis and an outer surface, a first antenna with two open ends deployed below the outer surface, a second antenna deployed below the outer surface and spaced at an axial distance from the first antenna, and at least one slot formed on the outer surface. A corresponding method for making directional resistivity measurements includes rotating a resistivity tool in a borehole, utilizing a transmitter-receiver antenna group formed in the resistivity tool to process an electromagnetic wave, and computing a resistivity-related measurement from the electromagnetic wave received on the receiver antenna.

Abstract: In aspects, an apparatus for use in a wellbore for determining a property of an earth formation is provided. The apparatus, in one embodiment, may include a tool body including a transmitter configured to induce electromagnetic waves in an earth formation, a receiver configured to provide signals responsive to the induced electromagnetic waves, a conductive member between the transmitter and the receiver extending radially from the tool body and configured to reduce propagation of eddy currents between the transmitter and the receiver when the tool body is in a wellbore and a processor configured to process the signals provided by the receiver to determine the property of the earth formation.

Abstract: The present disclosure relates to a method to determine a formation property of a subsurface formation. A downhole logging tool having two or more antennas, at least one of the antennas being a non-co-located triaxial antenna, is provided. Azimuthally sensitive measurements are obtained using the antennas of the downhole logging tool. The measurements are fitted to a Fourier series having Fourier coefficients and the non-co-location of the triaxial antenna is compensated for by adjusting the Fourier coefficients. Compensated measurements are determined using the adjusted Fourier coefficients and the formation property of the subsurface formation is determined using the compensated measurements. The compensating may use one or more scale factors that depend on an amplitude, phase, and/or frequency of the received signal, and an alignment angle between the triaxial antenna and another of the antennas of the logging tool.

Abstract: An apparatus and method for reducing an electric current leakage in a logging tool, particularly relating to reducing electric current leakage by altering a phase difference between a biasing element and an electric current produced by a first power source. The apparatus may include one or more measure electrodes for imparting electric current into a formation, a current meter, at least one current return electrode, a controller, and a biasing element. The method includes adjusting a phase angle difference between the current leaving the measure electrode and a biasing signal applied to the biasing element from a second power source to reduce the current leakage of the apparatus.

Abstract: A method of passive surveying comprises generating one or more detected signals by passively detecting a signal generated within a subsurface earth formation due to a seismoelectric response or an electroseismic response in at least one porous subsurface earth formation containing at least one fluid, and processing the one or more detected signals to determine at least one property of the subsurface earth formation.

Abstract: Disclosed is an apparatus for estimating a property of an earth formation penetrated by a borehole. The apparatus includes a carrier configured to be conveyed through the borehole. A first transmitter electrode and a second transmitter electrode are disposed at the carrier and configured to inject an electrical current into the earth formation. A first measurement electrode and a second measurement electrode are disposed at the carrier and configured to measure the electrical current to estimate the property of the earth formation. A first bucker amplifier is coupled to the first measurement electrode and a second bucker amplifier is coupled to the second measurement electrode, wherein the first and second bucker amplifiers are configured to equalize electrical potentials of areas in front of the first and second measurement electrodes.

Abstract: An apparatus and method for estimating a parameter of interest of an earth formation involving an electromagnetic transient response of an earth formation and a two thin-sheet conductor model of the earth formation. The method may include generating an electromagnetic transient response in the earth formation, generating a signal indicative of the response, and estimating the at least one parameter of interest using the signal. The method may also include estimating a boundary distance using the at least one parameter of interest. The apparatus may include at least one antenna configured to generate the electromagnetic response in the earth formation and at least one processor configured to estimate the at least one processor based on the electromagnetic transient response.

Abstract: A downhole induction resistivity assembly that comprises a downhole tool string component. The tool string component comprises an induction transmitter. The transmitter is adapted to induce an induction field in the surrounding formation. A first induction receiver is spaced apart from the transmitter and is adapted to measure the induction field. A magnetic field generating mechanism is disposed circumferentially adjacent the transmitter and adapted to guide the transmitter's signal into the formation.

Abstract: In various embodiments, apparatus and methods are provided to determine formation resistivity associated with a well. Measurements taken using sub-arrays of a tool at different distances of investigation can be used to determine formation resistivity, where the sub-arrays are arranged to make azimuthal related measurements. Separations readings related to resistivity can be generated from signals received from different directions and can be analyzed to characterize validity of a measurement reading. Additional apparatus, systems, and methods are disclosed.

Abstract: A method for determining a formation type within a wellbore includes forming the wellbore such that the wellbore intersects fractures within a formation. Hydrocarbons are produced from the formation. An induction tool is disposed into the wellbore after producing the hydrocarbons. A resistivity of a portion of the formation is measured with the induction tool. The resistivity of the portion of the formation is compared with a known formation type. The formation type of the portion of the formation is determined based on the resistivity. A characteristic of the formation type is output.

Abstract: An electromagnetic antenna for Measurement-While-Drilling (MWD) applications is disclosed. The antenna can include several array elements that can act alone or together in various measurement modes. The antenna elements can be disposed in tool body recesses to be protected from damage. The antenna elements can include a ferrite plate crossed or looped by independent current carrying conductors in two or more directions forming a bi-directional or crossed magnetic dipole. Although disclosed as a MWD system conveyed by a drill string, basic concepts of the system are applicable to other types of borehole conveyance.

Abstract: A module for use as a subsea electronic module of a subsea well, comprises an input (7) for receiving signals from a sensor arrangement which senses at least one parameter of a well, the module including a plurality of separate, different electronic circuits (9), each of which is selectable to be in communication with said input and each of which is adapted to co-operate with a respective, different form of such a sensor arrangement.

Abstract: An antenna array can be mounted on a flexible substrate and connected by a flexible interconnect to an integrated circuit such as a radio frequency front end. The antenna array can be mounted in a device housing that includes radio frequency interference (RFI) shielding. The antenna array is aligned with and next to an area of the housing that is not shielded against RFI.

Abstract: The present disclosure relates to a method to produce an image of a subsurface formation using directional measurements. A downhole logging tool having one or more transmitters and one or more receivers, and being capable of making directional measurement, is used to measure the voltage in a particular receiver due to a particular transmitter for one or more transmitter/receiver pairs, at least one of those voltage measurements being a directional measurement. The complex (phasor) voltage recorded on a receiver coil is divided by the complex voltage recorded at another reference receiver coil. Alternatively, we can use the ratio of a receiver voltage at a particular rotation angle of the tool divided by the voltage on the same receiver when the tool has rotated by an angle of 180 degrees. The information in those ratios is combined to produce images of the resistivity of the subsurface formation surrounding the tool.

Abstract: Methods and related systems are described for making an electromagnetic induction survey of a formation surrounding a cased section of a borehole. An electromagnetic transmitter and/or receiver is deployed into the cased section of the borehole. Electromagnetic survey measurements and impedance measurements relating to impedance of the transmitter and/or receiver are made while deployed in the section of the borehole. Compensation is made for the attenuation amplitude and/or phase in the electromagnetic survey measurements due to the conductive casing. The compensation is based on the impedance measurements and equivalent circuit parameters. The impedance measurements are correlated with numerical modeling results of a purely inductive electromagnetic transducer, and calculated equivalent circuit parameters are combined with the numerical modeling results.

Abstract: A probe for measuring electromagnetic properties of a subsurface formation includes a pad having a face adapted for engagement with a borehole wall, a pair of transmitting antennas, and two pairs of receiving antennas. The transmitting antennas and receiving antennas are mounted in the face of the pad, and each of the two pairs of receiving antennas include a first and second antenna distributed on opposite sides of the pair of transmitting antennas. The probe further includes an electronic arrangement having a transmitter module and a receiver module. The transmitter module is arranged to operate the pair of transmitting antennas in either a broadside mode or an endfire mode by applying an excitation signal modulated by two or more frequencies, and the receiver module is arranged to determine an attenuation and a phase shift of each reception signal provided by each receiving antenna relatively to the excitation signal.

Abstract: Apparatus and methods for determining the rock solid (matrix) dielectric permittivity for subterranean rocks, such as carbonate rocks, are described. According to some embodiments, this is accomplished by linking the matrix permittivity to the detailed chemical composition of the rock. The linking function is defined by coefficients for each component that can be determined by inversion in a laboratory calibration process such that the function compensates for the permittivity contribution of rock components that may be undetectable through downhole logging procedures.

Abstract: A measuring head for use in electrical measurements conducted in holes drilled in the ground comprises an elongated body, adaptable in the hole and comprising an electrode, electrically connecting the measuring head with its surroundings, for transmitting an electrical signal between the measuring head and its surroundings. In accordance with the invention, the measuring head comprises insulating means, placed on the body on both sides of the electrode along the length of the hole in order to form a measuring area, electrically insulated from other parts of the hole, around the electrode placed in the hole.

Abstract: The present disclosure relates to determining attenuation factors relating to an electromagnetic signal passing through, a conductive material seen by a real sensor. A sensor is provided and disposed proximate to the material. An alternating current is passed through the sensor and the impedance of the sensor is measured. The impedance of an ideal coil is obtained from the measured impedance using electromagnetic modeling combined with a circuit analysis of the coil impedance, and the attenuation factors for the real coil in straight or feedback mode are determined by electromagnetic modeling of casing attenuation factors and impedance of an ideal coil combined with equivalent circuit modeling of the sensor transfer functions. The attenuation factors seen by the real sensor may be determined in real-time or post-survey. The material may be magnetic or non-magnetic.

Abstract: The disclosure provides an apparatus and method for estimating one or more formation properties downhole. The apparatus includes a tool body that has a longitudinal axis. The tool body has a number of metallic pillars that are attached to or are an integral part of the tool body. Antenna wires may be positioned on the tool body between the pillars in a plurality of orientations to provide a multidirectional antenna. An insulating material is used to cover the antenna wires. A high magnetic permeability material is placed between the antenna wires and the tool body. The antenna may be configured to operate as a transmitter or as a receiver for a resistivity sensor or an NMR sensor.

Abstract: Disclosed herein are electromagnetic resistivity logging systems and methods that employ an antenna configuration having at most two transmitter or receiver antenna orientations that rotate relative to the borehole. The measurements made by this reduced-complexity antenna configuration enable the determination of at least seven components of a coupling matrix, which may be determined using a linear system of equations that express the azimuthal dependence of the measurements. For increased reliability, measurement averaging may be performed in azimuthally spaced bins. The coupling matrix components can then be used as the basis for determining logs of various formation parameters, including vertical resistivity and anisotropy.

Abstract: A method and a downhole tool determine one or more parameters of a formation traversed by a borehole where at least a portion of the formation has substantially parallel boundaries. A tool is disposed in the borehole that includes a transmitter having a dipole moment at an angle ?T with respect to a longitudinal axis of the tool, a receiver having a dipole moment at an angle ?R with respect to the longitudinal axis of the tool and a rotational position indicator. The transmitter-receiver pair transmits an electromagnetic signal while rotating the tool, receives the electromagnetic signal to produce a measured signal, and determine(s) the formation parameters for the portion of the formation having substantially parallel boundaries based on the measured signal.

Abstract: A current sensor measures an electrical current flowing in well casing or other magnetic structure. The current sensor can be installed in a tool for performing electromagnetic (EM) induction surveying in a wellbore lined with an electrically conductive casing. The tool includes an EM element to transmit or receive a magnetic field through the casing. The measured current using the current sensor can be used to relate the change of casing effect on the EM element (e.g., an induction receiver) placed inside the well casing for performing the EM induction survey.

Abstract: A method for electromagnetic exploration includes imparting a first electromagnetic signal into subsurface formations and imparting a second electromagnetic signal into the formations substantially contemporaneously with imparting the first electromagnetic signal. The first and second electromagnetic signals are substantially uncorrelated with each other. A combined electromagnetic response of the formations to the first and second imparted electromagnetic signals is detected. A response of the formations to each of the first and the second imparted signals is determined from the detected response.

Abstract: Disclosed is an apparatus for estimating a property of an earth formation penetrated by a borehole. The apparatus includes a carrier configured to be conveyed through the borehole. A first transmitter electrode and a second transmitter electrode are disposed at the carrier and configured to inject an electrical current into the earth formation. A first measurement electrode and a second measurement electrode are disposed at the carrier and configured to measure the electrical current to estimate the property of the earth formation. A first bucker amplifier is coupled to the first measurement electrode and a second bucker amplifier is coupled to the second measurement electrode, wherein the first and second bucker amplifiers are configured to equalize electrical potentials of areas in front of the first and second measurement electrodes.

Abstract: A method for detection of underground anomalies including in a system of distributed antennas (10), which are leaky transmission lines, disposed in 16 boreholes (12) formed in a ground, a transmitter (14) being connected to one of the antennas (10), and a receiver (16) being connected to another of the antennas (10), injecting an electromagnetic pulse into one of the antennas (10), wherein the pulse gradually leaks out, and wherein if a speed of propagation in the line in which the pulse is injected is faster than a speed of propagation in the ground, a shock wave is transmitted through the ground, called a transmitted signal, and received as a received signal at another of the antennas (10), and wherein an underground anomaly diffracts the shock wave, resulting in a detectable disturbance in the received signal, and locating the anomaly as a function of a time delay of the disturbance relative to the transmitted signal.

Abstract: A downhole induction resistivity assembly that comprises a downhole tool string component. The tool string component comprises an induction transmitter. The transmitter is adapted to induce an induction field in the surrounding formation. A first induction receiver is spaced apart from the transmitter and is adapted to measure the induction field. A magnetic field generating mechanism is disposed adjacent on either or both sides of the transmitter and adapted to guide the transmitter's signal into the formation. A second induction receiver is disposed in close proximity to the magnetic field generating mechanism and is adapted to measure the magnetic field generated by the mechanism.

Abstract: An electromagnetic energy transmitter is provided for borehole to surface electromagnetic (BSEM) surveying. The transmitter is lowered into a well borehole to a desired location in a well borehole to emit electromagnetic energy for the surveys. The transmitter affords more accurate measurements of its position in the well borehole. The transmitter also senses pressure and temperature conditions at its borehole location to provide indications of detected potential problems in the well from the survey or other sources, and to reduce risks and enhance safety and quality of the operations.

Abstract: Method for improving frequency-domain (1505), controlled-source electromagnetic data readability and interpretability in hydrocarbon prospecting by mapping a scaled amplitude or the relative amplitude (or phase) of the electromagnetic field in a scaled offset?scaled frequency plane (1510). The preferred mapping space uses the offset times the square-root of the frequency as the X-axis and the square-root of the frequency as the Y-axis. The preferred way to scale the amplitude of the electric field is to multiply it by the frequency to the power of ?1.5. Resistive anomalies in the data may be identified by comparing negative offset data (or data relative to a reference) to positive offset data (1515). The location and size of a potentially hydrocarbon-bearing resistive body may be estimated from the position of the anomaly on the map (1520).

Abstract: A rotating, transversely magnetized, magnet on a drill collar induces magnetization in a casing of a preexisting well. A coil rotating synchronously with the magnet produces a current at twice the frequency of rotation and having an amplitude that depends upon the distance from the magnet to the preexisting well. Alternatively, a variable magnetic field is produced in the casing using a switchable magnet.