Abstract: A method of detecting a flood front through a subterranean formation, comprising, measuring a distance to the flood front, processing the measured distance using a predictor-corrector filter to determine a corrected distance to the flood front, and adjusting a formation device based on the corrected distance to control a flow of the flood front.

Abstract: An induction transceiver system includes a magnetic core and an insulator surrounding the magnetic core. The system further includes a cylindrically-shaped sleeve substantially surrounding the insulator. The sleeve includes a gap along the length of the sleeve. A potential difference is measured across the gap when receiving, and a current is applied across the gap when transmitting.

Abstract: An electromagnetic sensing system operates either in land environments or in marine environments on the floor of a body of water to measure electromagnetic fields. The sensing system has electrodes that provide capacitive coupling to the local environment where measurements are being made. A new method of deployment in water provides considerable size and weight reductions. The size and weight reductions also facilitate deployment and rapid repositioning on land. The system is particularly beneficial for surveying sites adjacent to bodies of water.

Abstract: Apparatus and methods for detecting stray voltage anomalies in electric fields are provided herein. In some embodiments, an apparatus for detecting an electrical field may comprise: at least one sensor probe for generating data corresponding to an electrical field detected by the at least one sensor probe, wherein the at least one sensor probe comprises at least one electrode; a processor, coupled to the at least one sensor probe, for analyzing the data to identify a voltage anomaly in the electric field; and an indicator, coupled to the processor, for alerting a user to a presence of the voltage anomaly in the electric field.

Abstract: Apparatus and methods for detecting stray voltage anomalies in electric fields are provided herein. In some embodiments, an apparatus for detecting an electrical field may comprise: at least one sensor probe for generating data corresponding to an electrical field detected by the at least one sensor probe, wherein the at least one sensor probe comprises at least one electrode; a processor, coupled to the at least one sensor probe, for analyzing the data to identify a voltage anomaly in the electric field; and an indicator, coupled to the processor, for alerting a user to a presence of the voltage anomaly in the electric field.

Abstract: Apparatus and methods for detecting stray voltage anomalies in electric fields are provided herein. In some embodiments, an apparatus for detecting an electrical field may comprise: at least one sensor probe for generating data corresponding to an electrical field detected by the at least one sensor probe, wherein the at least one sensor probe comprises at least one electrode; a processor, coupled to the at least one sensor probe, for analyzing the data to identify a voltage anomaly in the electric field; and an indicator, coupled to the processor, for alerting a user to a presence of the voltage anomaly in the electric field.

Abstract: Illustrative permanent electromagnetic (EM) monitoring systems and methods have a casing string positioned inside a borehole and equipped with one or more EM transducer modules having a layer of soft magnetic material that substantially encircles the casing string to amplify a signal response of a magnetic field transmitting and/or sensing element. The layer preferably has an axial dimension at least twice an axial dimension of the magnetic field sensing element, with a relative permeability at least twice that of the casing material. The magnetic field transmitting element can be a coil. The magnetic field sensing element can be a coil or a piezoelectric or magnetostrictive element that applies stress to an optical fiber. A well interface system communicates with the one or more EM transducer modules to transmit and/or collect EM signals over time. Additional casing strings may be provided in other nearby boreholes to enable tomographic mapping and monitoring of fluid interfaces in the reservoir.

Abstract: A flexible current and voltage sensor provides ease of installation of a current sensor, and optionally a voltage sensor in application such as AC branch circuit wire measurements, which may require installation in dense wiring conditions and/or in live panels where insulating gloves must be worn. The sensor includes at least one flexible ferromagnetic strip that is affixed to a current sensing device at a first end. The second end is secured to the other side of the current sensing device or to another flexible ferromagnetic strip extending from the other side of the current sensing device to form a loop providing a closed pathway for magnetic flux. A voltage sensor may be provided by metal foil affixed to the inside of the flexible ferromagnetic strip. A clamp body, which can be a spring loaded handle operated clamp or a locking fastener, can secure the ferromagnetic strip around the wire.

Abstract: Methods and related systems are described for measuring naturally occurring electromagnetic fields both at the earth's surface as well as downhole. These fields originate from currents in the ionosphere above the earth, and are the same fields as employed by known magnetotelluric geophysical methods based on surface measurements. Some embodiments are especially useful in horizontal wells that are uncased at depth, although some embodiments are also useful in normal vertical wells that are both uncased or cased with a conductive liner. The method includes receiving downhole electromagnetic survey data of the naturally occurring electromagnetic fields obtained using a downhole receiver deployed at a first location in a borehole. A second set of electromagnetic survey data of the naturally occurring electromagnetic fields is also received that has been obtained using a receiver deployed at a second location.

Abstract: Methods and data acquisition systems enable joint acquisition of seismic and electromagnetic data in a target area using stand-alone digital recorders.

Abstract: A hybrid ground penetrating radar (GPR)/metal detector (MD) head includes a V-dipole GPR antenna and transmit and receive MD coils. One of the MD coils is arranged in a quadrupole configuration with a crossbar, and the V-dipole antenna is perpendicular to the crossbar. The legs of the V-dipole antenna may straddle the crossbar or may be on one side of the crossbar. The MD coils may be fabricated on a printed circuit board, which may be at a non-normal angle with respect to a central axis of the V-dipole antenna.

Abstract: Described herein is a self-powered system for detecting a current spike. The current spike is delivered through a current return network that energizes a resonant circuit to produce an alternating electrical output. The output is rectified by a rectifier into direct output that is then transferred to an integrator circuit. The integrator circuit slowly builds to and dissipates from a voltage threshold corresponding to an output transistor. When the output transistor is triggered by the voltage threshold this trigger is communicated to a fault monitoring software that recognizes the current spike.

Abstract: A method of measuring an electromagnetic field in a formation can include installing an electromagnetic sensor with improved sensitivity, the sensor including multiple optical waveguides and respective multiple materials, and in response to exposure to the electromagnetic field, the materials changing shape in opposite directions. A well system can include an optical electromagnetic sensor which measures an electromagnetic field in a formation, and wherein optical path lengths or phases in optical waveguides of the sensor change both positively and negatively in response to exposure to the electromagnetic field.

Abstract: A method of measuring an electromagnetic field in a subterranean earth formation can include installing at least one electromagnetic sensor in a well, the sensor including an optical waveguide and a material, the material changing shape in response to exposure to the electromagnetic field, and strain in the optical waveguide changing in response to the material changing shape. A well system can include an optical electromagnetic sensor installed in a well, and a transmitter which induces an electromagnetic field in an earth formation. Strain is induced in an optical waveguide of the sensor in response to the electromagnetic field. A method of monitoring an earth formation can include installing an optical electromagnetic sensor in a wellbore which penetrates the formation, and a strain being induced in an optical waveguide of the sensor in response to the electromagnetic field.

Abstract: A vertical receiver antenna device includes first and second receiving, electrode units interconnected by a receiver antenna cable and connected to means for the electromagnetic surveying of electrically resistive targets potentially containing hydrocarbons. The receiver antenna is arranged in a tubular, non-magnetic antenna housing arranged vertically in subsea uncompacted material, and one of the first and second receiving electrode units being connected in a electrically conductive manner to the underlying structure, and the other one of the first and second receiving electrode units being arranged in a portion of the antenna housing distantly from the first one of first and second receiving electrode units.

Abstract: A method and apparatus includes detecting lightning induced electromagnetic pulses and determining a physical property of an underground structure based on the lightning induced electromagnetic pulses. In some embodiments, an apparatus includes an antenna, a low noise amplifier, a processor, cable, and a transmitter. The antenna includes three substantively perpendicular loops of electrical conductors. The processor is configured to condition the amplified signal. The cable is about 100 meters in length and connects the low noise amplifier to the processor. The transmitter is configured to send conditioned data to a data aggregation system.

Abstract: A method and receiver system for identifying a location of a magnetic field source using two horizontally displaced tri-axial antennas. In a preferred embodiment two tri-axial antennas are positioned at opposite ends of a receiver frame. Each antenna detects in three dimensions a magnetic field from a source or transmitter. The receiver is maintained in a horizontal plane and the receiver is moved in the horizontal plane until a flux angle measured at each of the two points is zero so that the receiver is in the vertical plane perpendicular to the axis of the source. The depth and location of the source in three dimensions relative to the receiver is determined using the detected field values. The receiver is moved in a direction defined by a line containing the two points of the receiver until a magnitude of the magnetic field detected at each of the two points is substantially the same so that the receiver is positioned above the source.

Abstract: Disclosed is an electromagnetic receiver assembly for marine electromagnetic surveying, the electromagnetic receiver assembly comprising an elongated housing and receiver electrodes mounted at separate points along the elongated housing. An embodiment may include an electromagnetic receive assembly that includes an elongated housing, wherein the elongated housing defines an interior chamber. The electromagnetic receiver assembly may further include receiver electrodes configured to be in contact with water when in operation, wherein the receiver electrodes are mounted at separate points along the elongated housing. The electromagnetic receiver assembly may further include sensor electronics disposed in the interior chamber and electrically coupled to the receiver electrodes. The electromagnetic receiver assembly may be configured for deployment on or near a bottom of a body of water.

Abstract: A stabilized field sensor apparatus collects field data, in particular magnetic field data, with reduced motion noise. The apparatus includes a tear drop shaped housing, a tow frame in the housing, a plurality of vibration isolating dampers spaced around the frame, a base assembly mounted to the dampers, a support pedestal having a bottom end fixed to the base assembly and an upper free end, a single spherical air bearing connected to the upper free end of the pedestal, an instrument platform with a lower hollow funnel having an upper inside apex supported on the air bearing for a one point support, principal and secondary gyro stabilizers for maintaining pivotal and rotational stability, and at least one field sensor mounted to the instrument platform for collecting the field data while being stabilized against motion noise including vibration, pivoting and rotation from the base assembly, from the tow frame and from the housing.

Abstract: The invention provides a method and apparatus for the analysis of rocks and rock fragments. A method for the analysis of a rock fragment including: feeding the rock fragment to a microwave irradiation zone of microwave energy generated by a microwave source; gauging energy absorbed by the rock fragment; and correlating rate of change of the gauged energy absorbed by the rock fragment with compositional characteristics of the rock fragment.

Abstract: To perform a marine survey of a subterranean structure, a vertically oriented electromagnetic (EM) source is positioned in a body of water, where the EM source is coincident with an EM receiver. The EM source is activated to cause transmission of EM energy into the subterranean structure. After deactivation of the EM source, an EM field affected by the subterranean structure is measured by the EM receiver. In an alternative implementation, a survey system is provided that has a continuous wave EM source, a main EM receiver, and an auxiliary EM receiver.

Abstract: A receiver coil assembly for performing geophysical surveys, including a hollow outer shell defining a continuous internal passage that forms a loop; a multiturn receiver air coil extending around the continuous internal passage; and a first cored coil comprising multiturn solenoid windings about a ferromagnetic core, the first cored coil having a sensing axis in a different direction than a sensing axis of the air coil.

Abstract: A system for monitoring movement in a subsurface environment, which may be used to determine flow dynamics within a fluid mass such as an ore body, or track subsurface persons or moving assets. A plurality of underground positioning system (UPS) elements in the subsurface environment transmit characteristic signals to a plurality of antennas, which transmit the signals to a data processing apparatus. The system thus determines changes in the positions of the UPS elements to derive an indication of the motion of the fluid mass or the locations and movement of subsurface persons or assets.

Abstract: In one aspect, the present invention provides an imager, preferably portable, that includes a source of electromagnetic radiation capable of generating radiation with one or more frequencies in a range of about 1 GHz to about 2000 GHz. An optical system that is optically coupled to the source focuses radiation received therefrom onto an object plane, and directs at least a portion of the focused radiation propagating back from the object plane onto an image plane. The imager further includes a scan mechanism coupled to the optical system for controlling thereof so as to move the focused radiation over the object plane. A detector optically coupled to the lens at the image plane detects at least a portion of the radiation propagating back from a plurality of scanned locations in the object plane, thereby generating a detection signal. A processor that is in communication with the detector generates an image of at least a portion of the object plane based on the detection signal.

Abstract: A method that involves developing an electromagnetic property model of a near surface area and using this electromagnetic property model and electromagnetic data acquired using one or more electromagnetic transmitters located above the near surface area and one or more electromagnetic receivers located within a wellbore to determine one or more electromagnetic properties of a subsurface area. Also an electromagnetic data acquisition system that includes one or more electromagnetic transmitters capable of being operated on the earth's surface, one or more first electromagnetic receivers capable of being operated within a wellbore and capable of receiving signals from the one or more electromagnetic transmitters, and one or more second electromagnetic receivers capable of being located on the earth's surface near the one or more electromagnetic transmitters and capable of receiving signals from the one or more electromagnetic transmitters. Related embodiments are also described.

Abstract: A survey apparatus for surveying a subterranean structure includes an electromagnetic (EM) sensing element to measure an EM field received from the subterranean structure, and a sample collector activatable to collect a sample of soil.

Abstract: Structure for deep MWD resistivity measurements suitable for both geo-steering and measurements ahead of the drill bit is disclosed. The structure is capable of extracting information about resistivity of formation in the presence of conductive drill. In one embodiment, ahead-of-bit sensing is achieved via transient electromagnetic measurements when current is switched off in the transmitter loop and the transient signal is measured in the receiver loops placed at some distances from the receiver. To reduce a parasitic effect of the currents in the drill three-coil bucking system with magnetic nonconductive shield in combination with a highly conductive copper shield are applied. Specifically, a short (less than 1 m) copper cover and short (less than 0.2 m) ferrite cover are placed in the vicinity of transmitting and receiving coils. A bucking operation followed by an inversion is performed on received signals to obtain data reflecting parameters of the surrounding subsurface formations.

Abstract: To perform a marine survey of a subterranean structure, a vertically oriented electromagnetic (EM) source is positioned in a body of water, where the EM source is coincident with an EM receiver. The EM source is activated to cause transmission of EM energy into the subterranean structure. After deactivation of the EM source, an EM field affected by the subterranean structure is measured by the EM receiver. In an alternative implementation, a survey system is provided that has a continuous wave EM source, a main EM receiver, and an auxiliary EM receiver.

Abstract: A method and apparatus includes detecting lightning induced electromagnetic pulses and determining a physical property of an underground structure based on the lightning induced electromagnetic pulses. In some embodiments, an apparatus includes an antenna, a low noise amplifier, a processor, cable, and a transmitter. The antenna includes three substantively perpendicular loops of electrical conductors. The processor is configured to condition the amplified signal. The cable is about 100 meters in length and connects the low noise amplifier to the processor. The transmitter is configured to send conditioned data to a data aggregation system.

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: Methods and related systems are described for measuring naturally occurring electromagnetic fields both at the earth's surface as well as downhole. These fields originate from currents in the ionosphere above the earth, and are the same fields as employed by known magnetotelluric geophysical methods based on surface measurements. Some embodiments are especially useful in horizontal wells that are uncased at depth, although some embodiments are also useful in normal vertical wells that are both uncased or cased with a conductive liner. The method includes receiving downhole electromagnetic survey data of the naturally occurring electromagnetic fields obtained using a downhole receiver deployed at a first location in a borehole. A second set of electromagnetic survey data of the naturally occurring electromagnetic fields is also received that has been obtained using a receiver deployed at a second location.

Abstract: A method that involves developing an electromagnetic property model of a near surface area and using this electromagnetic property model and electromagnetic data acquired using one or more electromagnetic transmitters located above the near surface area and one or more electromagnetic receivers located within a wellbore to determine one or more electromagnetic properties of a subsurface area. Also an electromagnetic data acquisition system that includes one or more electromagnetic transmitters capable of being operated on the earth's surface, one or more first electromagnetic receivers capable of being operated within a wellbore and capable of receiving signals from the one or more electromagnetic transmitters, and one or more second electromagnetic receivers capable of being located on the earth's surface near the one or more electromagnetic transmitters and capable of receiving signals from the one or more electromagnetic transmitters. Related embodiments are also described.

Abstract: A communications system including: a plurality of sub-surface portable transceivers each including: a digital data source, a modulator coupled to the digital data source, a power amplifier coupled to the modulator and a loop antenna inductively coupled to the power amplifier; and a base transceiver including: an electrically insulated ferrite core antenna for receiving magnetic signals, an electric di-pole antenna for receiving ambient noise, a noise canceller coupled to the insulated ferrite core antenna and electric dipole antenna and responsive to the electric-dipole antenna to filter noise from signals received via the insulated ferrite core antenna, a demodulator coupled to the noise canceller and a decoder coupled to the demodulator.

Abstract: A ground conductivity meter that includes a transmitter coil and a receiver coil that are horizontally spaced apart from each other, and a conductivity meter controller connected to the transmitter and receiver coils and including an electronic storage element and at least one processor, the conductivity meter controller being operative to: determine a first conductivity reading in dependence on signals from the receiver coil when the transmitter coil and receiver coil are positioned a predetermined distance above a ground surface in one of a vertical dipole orientation or a horizontal dipole orientation; determine a second conductivity reading in dependence on signals from the receiver coil when the transmitter coil and receiver coil are positioned the predetermined distance above the ground surface in the other of the vertical dipole orientation or horizontal dipole orientation; calculate a correction factor in dependence on the first and second conductivity readings and store the correction factor in the s

Abstract: A method of tracking seismic activity through monitoring sunspots and sun disturbances and correlating them with a developed model in order to map out specific locations where an earthquake will consequently appear is provided (FP). The method of comprises mapping coordinates of electromagnetic disturbances on the sun, calculating the coordinates while taking into consideration Earth/sun geometry and relative positions (FP). These coordinates are consequently mapped to a location and time on earth wherein the location and time predicts the location and approximate time of the earthquake, which will consequently occur on earth.

Abstract: An apparatus and method of determining location of an object hidden from view. The apparatus includes an imaging tool for detecting hidden objects. The imaging tool includes a housing including a first end and a second end, a display supported by the first end of the housing, and a tracking device supported by the second end of the housing. The imaging tool also includes a transmitter supported by the housing and operable to transmit electromagnetic radiation toward a hidden target, an analysis module supported by the housing and operable to analyze feedback data related to the interaction between the target and the electromagnetic radiation, and an image module operable to receive data from the analysis module to generate an image on the display.

Abstract: A survey apparatus for surveying a subterranean structure includes an electromagnetic (EM) sensing element to measure an EM field received from the subterranean structure, and a sample collector activatable to collect a sample of soil.

Abstract: A system for the electronic detection of military threats includes an electromagnetic (EM) gradiometer for detecting secondary EM waves scattered from objects illuminated by a primary EM wave existent in a local area. A right magnetic dipole antenna and a left magnetic dipole antenna are included in the EM gradiometer for receiving the secondary EM waves and able to reject the primary EM wave. A synchronizer derives a reference signal to the EM gradiometer from the primary EM wave. A synchronous detection processor makes measurements of the phase and amplitude of the secondary EM waves. A graphical user interface (GUI) is used to display information about the location of objects assessed to be threats in the local area that were computed from the measurements of the phase and amplitude. A vehicle may be used to transport the EM gradiometer over the local area during periods the objects are being radio illuminated.

Abstract: A method of optimising electromagnetic surveying comprising applying current to a current source, receiving a signal at one or more voltage receivers and recording the signals received, characterised by varying one or more acquisition parameters as a function of the source-receiver separation.

Abstract: A ground conductivity meter that includes a transmitter coil and a receiver coil that are horizontally spaced apart from each other, and a conductivity meter controller connected to the transmitter and receiver coils and including an electronic storage element and at least one processor, the conductivity meter controller being operative to: determine a first conductivity reading in dependence on signals from the receiver coil when the transmitter coil and receiver coil are positioned a predetermined distance above a ground surface in one of a vertical dipole orientation or a horizontal dipole orientation; determine a second conductivity reading in dependence on signals from the receiver coil when the transmitter coil and receiver coil are positioned the predetermined distance above the ground surface in the other of the vertical dipole orientation or horizontal dipole orientation; calculate a correction factor in dependence on the first and second conductivity readings and store the correction factor in the s

Abstract: Measurements made with an induction logging tool are processed to provide a resistivity model of fluid invasion of the formation. Up to five zones can be determined over a radial distance of about 0.6 m. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

Abstract: A method for tracking a sinusoidal electromagnetic signal in noisy data using a Kalman filter or other tracking algorithm. The method is useful for controlled source electromagnetic surveying where longer source-receiver offsets can cause the source signal to decay significantly and be difficult to retrieve from magnetotelluric or other electromagnetic background.

Abstract: An electromagnetic survey sensing device includes at least two electrodes disposed at spaced apart locations. An electrical to optical converter is electrically coupled to the at least two electrodes. The converter is configured to change a property of light from a source in response to voltage imparted across the at least two electrodes. The device includes an optical fiber optically coupled to an output of the electrical to optical converter, the optical fiber in optical communication with a detector.

Abstract: A system for detecting precursor seismic electromagnetic waveforms.

Abstract: A logging radar system and method for measuring propped fractures and down-hole formation conditions in a subterranean formation including: a radar source; an optical source; an optical modulator for modulating an optical signal from the optical source according to a signal from the radar source; a photodiode for converting the modulated optical signal output from the optical modulator to the source radar signal; a transmitter and receiver unit; and a mixer. The transmitter and receiver unit receives the source radar signal from the photodiode, transmits the source radar signal into the formation and receives a reflected radar signal. The mixer mixes the reflected radar signal with the source radar signal to provide an output. This technology can be used to describe all fractures connected to the wellbore and differentiate between the dimensions of the two vertical wings of a propped fracture.

Abstract: An aerial electronic system for detection of surface and underground threats comprises an electromagnetic (EM) gradiometer flown aloft over the possible ground and underground threats to a convoy. The EM gradiometer is disposed in a Styrofoam torpedo shaped pod that is towed in flight behind an airplane. An illumination transmitter and loop antenna mounted to the airplane radiate a primary EM wave that travels down to the ground surface and penetrates beneath. Frequencies of 80 KHz to 1 MHz are selected according to whether the targets are laying on the surface or deeply buried. Detonation wire pairs, buried cables and pipes, and other conductors will re-radiate a secondary wave that can be sensed by the EM gradiometer. A reference sample of the transmitter signal is carried down a fiberoptic from the airplane to the towed pod. This signal is used in the synchronous detection to measure the secondary EM wave phase.

Abstract: A device for detecting a weather condition, particularly a tornado. The device includes a receiver module configured to receive electromagnetic signal from an atmosphere, particularly electromagnetic signal in the range of 1 MHz to 100 MHz. The device also includes a processing module coupled to the receiver module and configured to generate frequency spectrum data corresponding to the received electromagnetic signal, such as by performing a Fast Fourier Transform operation. Frequency spectrum data corresponding to the weather condition desired to be detected is stored in a stored data module. The device also includes a comparison module for comparing, such as by performing a cross-correlation operation, the generated frequency spectrum data and the stored frequency spectrum data in order to determine whether the weather condition is present in the atmosphere.

Abstract: An aerial electronic system for detection of surface and underground threats comprises an electromagnetic (EM) gradiometer flown aloft over the possible ground and underground threats to a convoy. The EM gradiometer is disposed in a Styrofoam torpedo shaped pod that is towed in flight behind an airplane. An illumination transmitter and loop antenna mounted to the airplane radiate a primary EM wave that travels down to the ground surface and penetrates beneath. Frequencies of 80 KHz to 1 MHz are selected according to whether the targets are laying on the surface or deeply buried. Detonation wire pairs, buried cables and pipes, and other conductors will re-radiate a secondary wave that can be sensed by the EM gradiometer. A reference sample of the transmitter signal is carried down a fiberoptic from the airplane to the towed pod. This signal is used in the synchronous detection to measure the secondary EM wave phase.

Abstract: A device for detecting a weather condition, particularly a tornado. The device includes a receiver module configured to receive electromagnetic signal from an atmosphere, particularly electromagnetic signal in the range of 1 MHz to 100 MHz. The device also includes a processing module coupled to the receiver module and configured to generate frequency spectrum data corresponding to the received electromagnetic signal, such as by performing a Fast Fourier Transform operation. Frequency spectrum data corresponding to the weather condition desired to be detected is stored in a stored data module. The device also includes a comparison module for comparing, such as by performing a cross-correlation operation, the generated frequency spectrum data and the stored frequency spectrum data in order to determine whether the weather condition is present in the atmosphere.

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.