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 are deployed into the cased section of the borehole. One or more additional devices are used to measure the properties of a conductive casing relating to conductivity, thickness and magnetic permeability. A casing coefficient is then calculated that can then be used for the processing of the deep-sensing induction measurements.

Abstract: An apparatus and method for conducting resistivity logging in a borehole. The apparatus may include a housing configured for fluid isolation, an electrical conductor penetrating the housing and a sealing interface. The apparatus may also include at least one of: an electrical component inside the housing and an electrode disposed on the exterior of the housing. The apparatus is configured to allow the electrical conductor to be moved, while maintaining the sealing interface, between an electrically connected state and an electrically disconnected state with at least one of: the electrical component and the electrode. The method may include using the apparatus for resistivity logging operations.

Abstract: The present disclosure relates to methods and systems for detecting electric potential difference in water. A first electrode comprising a first electrode body is configured to be in electrical contact with the water when the device is disposed in the water. A second electrode comprising a second electrode body is configured to be in electrical contact with the water when the device is disposed in the water. An electrical connection exists amongst the first electrode, the second electrode, and a voltage measuring device. At least one of the first electrode body and the second electrode body is formed at least partially of a carbon aerogel material.

Abstract: Cathodic protection (CP) monitoring methods, systems and system components that provide regular (typically daily or more frequent) potential data collection for accurate assessment of CP systems. The methods, systems and components of the invention are used for remote monitoring of passive sacrificial anode CP systems and more particularly for monitoring of CP protection for buried storage tanks. These systems have on-site components for collection of data and transmission of data to remote databases and can employ computer-implemented and/or system operator assessed data processing and interpretation of data to assess protection status of a given structure and further to generate appropriate reports of protection status and collected and processed data.

Abstract: Methods and apparatuses for determining surface wetting of a rock material with changing well fluids. In general, the methods according to the invention include measuring electrical impedance spectroscopy (“EIS”) for a system simulating downhole conditions for the wetting of a surface. Methods and apparatuses for making EIS measurements model double-layer capacitance at a downhole surface in a well, from which the nature and quantification of the wetting of the surface can be inferred.

Abstract: A probe for monitoring electrical conductivity of soil solutions is provided. The probe includes an upper hollow tube section stoppered at one end, a lower hollow tube section capped at its bottom with a porous ceramic material for inserting into soil, a T fitting connecting the other end of the upper hollow tube section in vertical alignment with the lower hollow tube section, said T fitting having an aperture which is stoppered, an electric conductivity (EC) sensor electrode inserted through the stopper of the upper hollow tube section into the lower hollow tube section, and a vacuum tube inserted through the stopper of the aperture of the T fitting extending into the lower hollow tube section. Ground solution can be withdrawn from the probe via the vacuum tube by a vacuum generator and fresh ground solution can be drawn into the probe without removing the sensor probe from the probe.

Abstract: An electrode for enhancing electrical conductivity between an oil or gas field downhole and the surrounding formation. The electrode may serve as a cathode to cooperate with a remote anode to produce an electric field through the formation. The electrode has an electrically conductive, elongated body with a proximal end adapted to be inserted into the production pipe and a distal end adapted to project into the formation. The body of the electrode has a series of radially-projecting flexible filaments. The filaments are composed of an electrically conductive material and are supported by the body so as to be electrically charged. At least one of the filaments project outwardly into engagement with the wall of the downhole. The filaments also project radially outward into the consolidated formation.

Abstract: A current detecting device includes a second fixed part to which the harness is fixed and electrically connected, a bus bar as a resistor inserted between a first fixed part and the second fixed part, a circuit board in which a current detection circuit is mounted, the current detection circuit detecting current flowing through the bus bar based on a potential difference between two points along an energizing direction of the bus bar, and a case accommodating the bus bar and the circuit board. The second fixed part to which the harness is fixed and electrically connected is disposed within a battery upper surface area.

Abstract: To perform surveying of a subterranean structure, an electromagnetic (EM) source array has a plurality of electrodes. Different subsets of the electrodes are dynamically activated to provide corresponding EM radiation patterns to survey the subterranean structure.

Abstract: A method of determining anisotropy in a borehole is disclosed. An array of measurements along the borehole is obtained and a first depth in the borehole is selected. An arbitrary plane oriented with respect to the borehole at the first depth is designated and an anisotropy for the first depth with respect to the arbitrary plane is determined. The arbitrary plane is repositioned at the first depth and an anisotropy for different positions of the arbitrary plane at the first depth is determined. A minimum anisotropy coefficient with respect to the arbitrary plane at the first depth is identified based on anisotropy for different positions of the arbitrary plane. An anisotropy tensor for the first depth is then identified.

Abstract: A method of monitoring an extent of a hydrocarbon reservoir (2) below the earth's surface (4), includes the steps of applying a current between a bottom current electrode (6) and a top current electrode (8) that is part of a plurality of electrodes that are spaced apart from one another along a part of the earth's surface located above and optionally over the hydrocarbon reservoir. The bottom current electrode is positioned below the hydrocarbon reservoir. The method includes the step of measuring an electrical potential, which is caused by the current, by means of a measurement electrode (10A) that is also part of the plurality of electrodes. The method includes the step of inferring from the measured electrical potential the extent of the hydrocarbon reservoir, in a direction that is oriented substantially parallel with the earth's surface.

Abstract: A marine electromagnetic receiver cable includes a plurality of signal processing modules disposed at spaced apart locations along the receiver cable. A power supply line is connected to each of the signal processing modules and to an electric current source. A current regulation device is connected in the power supply line proximate each signal processing module. The current regulation devices are connected such that an amount of current flowing through the power supply line is substantially constant. At least one electromagnetic receiver is functionally coupled to an input of each signal processing module.

Abstract: A system and method collects at least one measurement within a borehole formed in a formation. The system and method provides a drill collar made of a non-conductive or substantially non-conductive composite material positioned within the borehole. A downhole component is capable of collecting at least one measurement and is embedded within the composite material of the drill collar. The at least one measurement collectable by the downhole component is related to the borehole or the formation about the borehole.

Abstract: The system utilizes electroscan equipment including a voltage source and current meter with a cable having one end grounded and one end having an electric probe thereon sized to fit within an underground pipe. The probe is able to complete an electric circuit back to ground when the probe is adjacent a defect through which electric currents can pass, thus producing varying electric current. A cable reel is provided with portions of the cable supported thereon and with a cable distance sensor coupled to the reel along with the current meter and voltage source in the form of a battery. The current meter and distance sensor transmit wireless signals to an on-site processor, such as a smartphone, for on-site data evaluation. Such unconditioned data is also transmitted to a remote location for conditioning of the data and retransmission of the conditioned data back to the on-site processor.

Abstract: A method and system for monitoring the integrity of a water aquifer is provided. The method and system generally monitors an aquifer for subsurface fractures, fluid intrusion, or water contamination. In one embodiment, the method and system may be utilized before, during, and after contaminating operations to monitor a water aquifer and generate reports detailing the effect of the contaminating operations on the water aquifer. The reports, and associated raw data, may be used as legal documents. For example, in one embodiment, an independent company is responsible for monitoring the aquifer and generating reports, which are then submitted to all interested parties, including the state for regulatory purposes.

Abstract: A substrate includes a first plate member; a plurality of first electrodes provided on the major surface of the first plate member, the first electrodes including at least one electrode for circuit connection and at least one monitor electrode separate from the electrode for circuit connection; a second plate member; a plurality of second electrodes provided on the major surface of the second plate member; a plurality of solder members provided between the first electrodes and the second electrodes for electrical connection therebetween, repeatedly; and a detector for detecting an electrical disconnection between at least one of the monitor electrode and the second electrode.

Abstract: An electrode for electric survey includes a first electrode rod having a tip portion at one end thereof; and a second electrode rod connected to a side surface of the first electrode rod. The second electrode rod is formed at one end thereof with a removing/hook portion perpendicularly connected to the side surface of the first electrode rod, and at the other end thereof with a wire connecting portion for connection with an electric wire, and has a bent portion between the removing/hook portion and the wire connecting portion. The electrode allows convenient connection of electric wires, ensures good contact with the electric wires so as to prevent easy separation of electric wires therefrom, can reduce a time for installation and removal of the electrode and measurement error, and does not necessitate use of electrical tape in installation of the electrode to thereby reduce environmental contamination due to waste tape.

Abstract: A marine electromagnetic sensor cable system includes a first sensor cable subsystem including at least a first sensor cable segment. The first sensor cable segment includes a plurality of spaced apart electrodes which electrically contact a body of water when the first sensor cable segment is immersed therein, and an electrical conductor coupled to each electrode, each electrical conductor extending from one longitudinal end of the sensor cable segment to the other. The system includes a first signal processing module electrically coupled to a longitudinal end of the first sensor cable segment, and including a voltage measuring circuit electrically connected between two or more electrodes from the first plurality of electrodes. Marine electromagnetic surveys are conducted using the marine electromagnetic sensor cable system.

Abstract: Disclosed are methods and systems that include a multiple-tube electrode assembly. An embodiment discloses an electrode assembly, comprising: a carrier body comprising an elongated support; and electrically conducting surfaces longitudinally spaced along the carrier body, wherein the electrically conducting surfaces are electrically coupled in parallel.

Abstract: The present disclosure relates to methods and apparatuses measuring a property of a material. The apparatus may include a transducer comprising a first electrical conductor, a second electrical conductor, and a piezoelectric component configured to receive the two conductors. The piezoelectric component may include a cavity dimensioned to improve the strength of or reduce stress on an interconnection between piezoelectric component and at least one of the conductors. The method may include using one or more transducers measuring a property of a material. In some embodiments, the material may be an earth formation.

Abstract: A follower amplifier with power supply biased by a controlled voltage source such that the power supply potentials are, for the frequencies of interest, as close as possible to the potential of the follower output. There is proposed a front-end electronic circuit for biopotential and impedance measurements with outstanding performances (very high input impedance and gain very close to unity). Preferably, the explicit guard electrode and the explicit electronic unit at the belt are no longer necessary; all electronics is embedded in units placed directly at the measurement sites. Moreover, the proposed front-end electronic circuit allows a drastic simplification of the cabling and connectors since all units are connected to only one wire (the theoretical minimum) for potential reference and current return. Preferably, this wire does not even require an electrical isolation and can be easily embedded in the textile of a shirt, in a garment, mesh, belt, etc.

Abstract: A system for measuring a resistivity parameter of an earth formation includes: at least one measurement electrode electrically connected to a first electrical source; at least one guard electrode; a shielding electrode interposed between the at least one guard electrode and the at least one measurement electrode, the shielding electrode being electrically connected to a second electrical source independent from the first electrical source, and the guard electrode being electrically connected to a third electrical source independent of the first and second electrical sources; at least one return electrode; insulators positioned between (i) the measurement electrode and the shielding electrode, (ii) the shielding electrode and the guard electrode and (iii) the measurement electrode, the guard electrode and the return electrode; and a processor configured to adjust at least one of the first electrical source and the second electrical source to minimize a current flow through the shielding electrode.

Abstract: Techniques for surface exploration and monitoring are presented. In representative embodiments, a controller can be configured to automatically poll survey probes to obtain identifiers of the probes and determine a relative order the probes. In some embodiments, survey probes can be configured to: (a) collect signals associated with a surface; (b) digitize the signals to form digital data; and (c) store the digital data for later transmission to the survey controller. In some embodiments, survey probes can be configured to: (a) inject current into the surface or (b) provide a return path therefor. Relative positions of survey probes, including those configured to introduce current into a surface as stimulus for geophysical measurements can be automatically determined. Sensor-type survey probes can automatically disconnect from a power conduit while measuring a surface property and operate using an internal source of power when disconnected, to reduce noise.

Abstract: Multimodal geosteering systems and methods are disclosed. Some disclosed tool embodiments include first and second transmitter-receiver arrangements that make geosteering measurements using different forms of energy (such as acoustic and electromagnetic energy) to provide geosteering measurements that at least indicate a boundary direction but may also indicate a boundary distance. Some disclosed method embodiments include: determining a direction to a bed boundary using measurements with different energy types; and adjusting a drilling direction based at least in part on said determination. Combinations of (or selections between) the different measurements may be made based on, inter alia, measurement range, resolution, and contrast. Some disclosed system embodiments include a memory and a processor. The memory stores geosteering display software that configures the processor to generate an image with different regions based on the different types of geosteering measurements.

Abstract: An imaging tool made includes a pad whose curvature is chosen based on the expected range of borehole radius and the pad size so as to maintain the maximum standoff below a desired value. The curvature may be adjusted using fasteners.

Abstract: A circuit is configured for testing an inrush current of a power supply. The circuit includes a capacitor module, a voltage meter, a semiconductor switch, and a current meter. The capacitor module is connected to an power source for storing electric charge. The voltage meter is connected to the capacitor module for measuring a voltage across the capacitor module. The semiconductor switch is capable of connecting the capacitor module to the power supply and being closed when the voltage across the capacitor module reaches a predetermined value. The current meter is capable of measuring the inrush current at the time the power supply is powered on.

Abstract: A marine sensor streamer includes a jacket covering an exterior of the streamer. At least one strength member extends the length of the jacket. At least one stiffener element extends inside the length of the jacket. The at least one stiffener element includes a gas filling the interior of a flexible, compressible tube and filler elements disposed in the gas. The filler elements have exterior shape and surface roughness such that upon compression of the gas, the filler elements are urged into contact with each other, causing the streamer to become substantially rigid.

Abstract: The invention as disclosed is an autonomous magnetic measurement system for monitoring the background magnetic fields associated with power lines, electronic devices, electronic vehicles and the Earth's background magnetic field. The components of the autonomous magnetic measurement system include a series of three axis analog magnetic sensors, a fluxgate compass, and a programmable micro-controller. The micro-controller receives data from the sensor and compass and is programmed to autonomously detect magnetic signals having particular characteristics of interest.

Abstract: An apparatus for measuring permittivity of a sample. The apparatus includes: a sample chamber including a sealed space portion in which a sample to be measured is put; a pressure adjusting unit for varying pressure by applying water pressure to the space portion of the sample chamber; a permittivity sensor for measuring permittivity of the sample and disposed outside the sample chamber; measurement conducting wires including conductors, installed to contact the sample and connected to the permittivity sensor by using electric wires; and a data logger for storing data relating to permittivity that is measured by the permittivity sensor.

Abstract: Earth formations are characterized by using an array of electrodes which can measure streaming potentials in the formation, and by interpreting the data obtained by the electrodes. The electrodes are placed on a wireline tool, a LWD tool, or in a fixed manner about a completed wellbore. The measured streaming potentials are generated by drilling with an overbearing pressure, slitting the mudcake in a borehole, acid injection, or any of various other manners which causes fluid movement. The data obtained is interpreted to locate fractures, measure formation permeability, estimate formation pressure, monitor drilling fluid loss, detect abnormal pressure, etc. Particularly, a streaming potential voltage transient having a double peak profile signifies the presence of a formation fracture.

Abstract: A method for correcting a measurement of a property of a subsurface material includes: selecting an instrument that includes a test circuit and a separate sensor, the test circuit configured for providing a standard for referencing to generate correction information, the sensor configured for: transmitting an electric signal into the subsurface material; and receiving a data signal from the subsurface material; wherein the test circuit and the sensor are switchably coupled to an electronics unit of the instrument; receiving the electric signal from the test circuit in the electronics unit; using the electronics unit, measuring at least one output characteristic of the instrument; and applying the correction information to the data signal according to the measured output characteristic.

Abstract: An apparatus and method for performing high transverse resolution voltage measurements in downhole logging tools utilized to determine the resistivity of an adjacent portion of a borehole wall. Two current electrodes 508 and 509 are energized by an excitation source to create an oscillatory electric field in a borehole wall. The voltage drop from each voltage electrode in a row of laterally spaced voltage electrodes 604 is measured relative to a shared voltage electrode 606 longitudinally spaced from the row of electrodes. The differential voltage is used in combination with a measured current flow to determine a resistivity value for the borehole. A second row of laterally spaced voltage electrodes 608 having electrodes laterally offset from the electrodes of the first row is included to provide high transverse resolution. A common mode voltage at the shared voltage electrodes is measured and utilized to minimize the resistivity measurement error.

Abstract: Method and apparatus for estimating a resistivity property of an earth formation in a borehole with oil-based drilling fluid. The method may include estimating the resistivity property using a sensitivity of a real part of a measured impedance to a reactance of a gap between a resistivity sensor and a borehole wall. The real component and the quadrature component of resistivity measurements made by a two-terminal imaging tool in a borehole having an oil-based mud are affected by the tool standoff. It is shown that the sensitivity of the real part of the resistivity measurement to the reactance of the gap can be estimated from the time derivatives of the two components of the measurements. This makes possible a principal component analysis of the data to give an improved image of the formation resistivity.

Abstract: A position sensor for well tools. A position sensor includes an indicator portion including a carrier supporting a resistive element and at least one contact, and a support device for supporting the carrier. Another position sensor includes an indicator portion including a slider for displacing at least one contact relative to a resistive element, the slider being displaceable between first and second limits, and an end load mechanism including a biasing device which exerts a biasing force on the slider as the slider approaches at least the first limit. Another position sensor includes an indicator portion including a resistive element and at least one contact which contacts and displaces across the resistive element, a change in position of the contact relative to the resistive element being indicated by a resistance change as the contact displaces across the resistive element.

Abstract: The present invention provides methods, apparatuses and systems for more effectively and efficiently, directly and indirectly monitoring cement degradation related to carbon dioxide exposure by measuring one or more electrical properties related to resistivity of cement behind a casing in a well.

Abstract: A device and method for measuring electromagnetic signals that can be used to take measurements at or near the sea floor. The device comprises a central housing, a data management system located within the housing and at least two arms extending outwards from the housing. Each arm comprises a flexible elongate sheath attached to the housing, a sensor head, a flexible electrical cable attached to the sheath and connecting the sensor head to the data management system and a rod which is removeably locatable within the sheath. The rod is connectable relative to the housing at one end and connectable relative to the sensor head at the end remote from the housing.

Abstract: Disclosed is a method of evaluating a ground reinforcement effect using 4-D electrical resistivity monitoring, wherein the ground reinforcement is made up on an underground cavity through cement mortar grouting, the method including: (a) installing a survey line for measuring electrical resistivity in a ground reinforcement zone; (b) measuring the electrical resistivity of the ground reinforcement zone through the survey line before grouting mortar, and imaging three-dimensional electrical resistivity distribution in the ground reinforcement zone by applying a three-dimensional electrical resistivity inversion to the measured results; (c) measuring the electrical resistivity of the ground reinforcement zone through the survey line while or after grouting the mortar, and imaging the three-dimensional electrical resistivity distribution in the ground reinforcement zone by applying the three-dimensional electrical resistivity inversion to the measured results; and (d) calculating a change ratio of the electrical

Abstract: An apparatus and method for minimizing the effects of a common mode voltage signal in downhole logging tools utilized to determine the resistivity of an adjacent portion of a borehole wall. Two current electrodes are energized by an excitation source to create an oscillatory electric field in a borehole wall. A voltage drop across a segment of the borehole wall is measured by two voltage electrodes, and the differential voltage is used in combination with a measured current flow to determine a resistivity value for the borehole wall. A common mode voltage in front of the two voltage electrodes is measured and minimized by controlling the excitation source, thereby reducing the resistivity measurement error.

Abstract: An electrode for enhancing electrical conductivity between an oil or gas field downhole and the surrounding formation. The electrode may serve as a cathode to cooperate with a remote anode to produce an electric field through the formation. The electrode has an electrically conductive, elongated body with a proximal end adapted to be inserted into the production pipe and a distal end adapted to project into the formation. The body of the electrode has a series of radially-projecting flexible filaments. The filaments are composed of an electrically conductive material and are supported by the body so as to be electrically charged. At least one of the filaments project outwardly into engagement with the wall of the downhole. The filaments also project radially outward into the consolidated formation.

Abstract: A method of testing the electric field recording of a marine electromagnetic sensor cable including electrodes is provided. The method includes causing current to flow between a pair of first electrodes disposed along the marine electromagnetic sensor cable. The flow of current generates a voltage that is impressed on a pair of second electrodes disposed along the marine electromagnetic sensor cable. A potential difference between the pair of second electrodes is measured. Accuracy of the electric field recording is inferred from the measured potential difference.

Abstract: A microresistivity logging tool includes a shield electrode deployed between a guard electrode and a return electrode. A measuring electrode is deployed in and electrically isolated from the guard electrode and first and second potential electrodes are deployed in and electrically isolated from the shield electrode. The tool further includes at least one switch configured to switch the tool between distinct first and second microresistivity measurement modes. The first measurement mode is configured for making microresistivity measurements in conductive (water based) drilling fluid and the second measurement mode is configured for making microresistivity measurements in non-conductive (oil based) drilling fluid, thereby enabling the tool to be utilized in either type of drilling fluid.

Abstract: A method of ocean property forecasting. The method includes generating one of a magnetic field prediction and an electric field prediction between a first observation point and a plurality of uniform ocean cells by generating a vector sum of magnetic field contributions from a plurality of magnetic field vectors and a vector sum of electric field contributions from a plurality of electric field vectors, respectively. The method further includes generating one of a magnetic field forecast and an electric field forecast of the ocean volume at a second observation point in the ocean volume by re-using the magnetic field vectors and electric field vectors and scaling one of the plurality of magnetic field vectors and the plurality of electric field vectors by a plurality of tri-axial equivalent electrical currents with identical geometry to the first observation point.

Abstract: A device, method and system for measuring characteristics of a geologic formation using a floating reference signal having a mud chamber, an electrode disposed within the mud chamber, and an electrically conductive plate disposed within the mud chamber, the plate separated from the electrode. An alternating current source is provided on the electrode, whereby an electric field is be maintained between the electrode and the conductive plate. An opening in the mud chamber allows drilling fluids to pass there through.

Abstract: Disclosed is a method and apparatus for measuring in-situ stress in rock using a thermal crack. The method involves forming a borehole, cooling a wall of the borehole, applying tensile thermal stress, forming a crack in the borehole wall, and measuring temperature and cracking point. Afterwards, the borehole wall is heated to close the formed crack, the borehole wall is cooled again to re-open the crack, and temperature is measured when the crack is re-opened. The in-situ stress of the rock is calculated using a first cracking temperature at which the crack is formed and a second cracking temperature at which the crack is re-opened. Further, the apparatus cools, heats and re-cools the borehole wall, thereby measuring the first cracking temperature, the second cracking temperature, and the cracking point.

Abstract: A sensor cable for surveying. The sensor cable has a housing, which includes one of more electrodes and a conductive gel surrounding the one or more electrodes. The conductive gel is configured to conduct electrical current to the one or more electrodes and keep the one or more electrodes moist. The housing may also have a membrane surrounding the conductive gel, wherein the membrane is configured to hold the conductive gel in contact with the one or more electrodes and permit electric current to flow between a surrounding medium and the conductive gel.

Abstract: To characterize an electrical cable that is deployed in a well, a voltage input is applied to the electrical cable at an earth surface location, where the well extends from the earth surface location. A current response resulting from the voltage input is measured at the earth surface location. At least one parameter of the electrical cable is computed according to the measured current response.

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: Methods and apparatus for determining a presence of a non-conductive coating on electrodes in fluid resistivity measurement devices are described. An example method of determining a presence of a non-conductive coating on electrodes of a fluid resistivity measurement device involves obtaining a first electrical potential value associated with a substantially uncoated condition of first and second electrodes of a fluid resistivity measurement device. The example method also involves measuring a second electrical potential value at a location between the first and second electrodes, comparing the first and second electrical potential values, and determining the presence of a non-conductive coating on at least one of the first and second electrodes based on the comparison.

Abstract: An electrical specific resistivity probing robot is directed to automatically measuring a three-dimensional electrical specific resistivity structure of an underground shallow region by installing a plurality of probing electrodes at regular intervals along a robot-moving caterpillar to measure difference of potentials of the electrodes at a ground plane. The electrical specific resistivity probing robot includes a frame, a driving member installed under the frame, a pair of insulating caterpillars installed to both sides of a lower portion of the frame and driven by the driving member, a plurality of probing electrodes installed along track circumferences of the caterpillars at regular intervals, a slip ring electrically slip-connected to the probing electrodes one by one, and a cable electrically connected to the slip ring.

Abstract: The invention is concerned with a tool for imaging a formation, the tool comprising: a pad capable of alignment adjacent a wall of the formation. The pad has located thereon an imaging electrode for injecting current into the formation. There is a first return electrode separated from the imaging electrode for receiving the injected current. There is at least one recessed electrode that is located at a different distance from the wall of the formation as the other electrodes.