Abstract: A look-ahead logging method includes obtaining an initial resistivity log. The method further includes deriving an initial formation model based on the initial resistivity log. The method further includes estimating deep resistivity measurements from the initial formation model and deriving a reduced-complexity formation model from the estimated deep resistivity measurements. The method further includes collecting actual resistivity measurements and inverting the actual resistivity measurements, using the reduced-complexity formation model, to obtain look-ahead or look-around parameter values. The method further includes displaying the look-ahead or look-around parameter values or storing the look-ahead or look-around parameter values on a non-transient information storage medium.

Abstract: The invention relates to a device for determining petrophysical parameters of an underground formation, comprising at least two electrodes (EL), a means for emitting a frequency-variable electric current (MEC), and a means for measuring electrical resistivity (MRE) in terms of amplitude and phase, two of the electrodes (EL) cooperating with the emission means (MEC) and at least two of the electrodes (EL) cooperating with the means for measuring resistivity (MRE), a means for measuring a difference in electrical potential (MDP) cooperating with at least two of the electrodes (EL). The invention is particularly applicable to oil exploration and development.

Abstract: In a method of processing resistivity log data, two or more resistivity log measurements of differing depths of penetration of an invaded formation are generated. The deepest and shallowest of the resistivity log measurements are combined in accordance with a non-linear combination algorithm that is modulated by a parameter. The value of the parameter used varies in dependence on a comparison between deep and shallow log measurements. The results of the comparison are forward modelled from sample data. A modified log output is generated as a result of combining the deepest and shallowest of the resistivity log measurements in accordance with the non-linear combination algorithm.

Abstract: An apparatus and method may operate to mount one or more communication assemblies relative to the exterior of a casing being placed in a borehole. Each communication assembly is configured to cause electrodes of a fluid sensing assembly to inject currents into fluids in one or more regions of the annulus surrounding the casing, and to obtain excitation responses from electrodes of the fluid sensing assembly, where the excitation responses vary based on electrical properties of fluids in one or more regions of the annulus surrounding the casing. Control is provided to control which electrodes of the fluid sensing assembly will inject currents and generate excitation responses and to control which electrodes of the fluid sensing assembly will sense the currents. Additional apparatus, systems, and methods are disclosed.

Abstract: A cutting element for an earth-boring drilling tool comprises a cutting body having a cutting surface thereon, and a sensor coupled with the cutting surface, the sensor configured to determine resistivity of a contacting formation. An earth-boring drilling tool comprises a bit body and an instrumented cutting element coupled with the bit body. The cutting element includes a cutting body having a cutting surface thereon, and at least one sensor located proximate the cutting surface. The at least one sensor is oriented and configured to determine resistivity of a contacting formation. A method of determining resistivity of a subterranean formation during a drilling operation comprises energizing a sensor of an instrumented cutting element of a drill bit, sensing a return signal flowing on or through the subterranean formation through the instrumented cutting element, and determining a resistivity of the subterranean formation based, at least in part, on the return signal.

Abstract: An apparatus and method for simultaneously obtaining quantitative measurements of vertical and horizontal resistivity and permittivity formation parameters by firing, using at least one transmitter in each of a horizontally and vertically polarized array on opposite sides of a drill collar, signals in the direction of a downhole formation, the fired signals from the transmitters in the arrays being fired simultaneously and engaging the downhole formation. The apparatus and method continues by receiving, using at least one receiver in each of the arrays, signals associated with the fired signals after the fired signals have engaged the downhole formation, where the received signals represent apparent formation data. The apparatus and method further involves determining, using the measured apparent formation data, the true formation data including one or more vertical and horizontal formation parameters.

Abstract: A method for water monitoring about a deviated well is disclosed. The method includes positioning a series of electromagnetic (EM) receivers in a completed deviated wellbore, said receivers being spaced along substantially the length of the well located in a region of a reservoir to be monitored. The method also includes positioning an electromagnetic (EM) source at a first Earth surface location. Then the EM source is activated for a first survey measurement of the reservoir, and an EM field detected at each EM receiver is recorded. The EM source is moved to a second Earth surface location, and activated for a second survey measurement of the reservoir, and an EM field detected at each EM receiver is recorded. From the first and second survey measurements at each of the receivers, an inversion is performed to determine position of water about (and specifically below) the horizontal well.

Abstract: Disclosed is an electro-magnetic tomography including: a modulator configured to modulate a generated transmit reference signal using a pseudo-noise signal; a transmitter configured to transmit the signal modulated by the modulator; a receiver configured to receive a signal transmitted from a transmitter; an amplitude detector configured to compare an amplitude magnitude between the signal received by the receiver and the transmit reference signal to measure a received amplitude; and a demodulator configured to demodulate the signal received by the receiver using the pseudo-noise signal to generate a cyclic signal, compare a phase between the transmit reference signal and the cyclic signal to measure the phase of the cyclic signal, and determine the phase of the received signal based on the phase of the measured cyclic signal.

Abstract: The invention relates to a logging tool (1) for determining properties of a fluid (2) surrounding the tool arranged downhole in a casing (3) comprising a wall (4) and having a longitudinal extension. The logging tool has a substantially longitudinal cylindrical shape with a longitudinal axis and, when seen in cross-section, a periphery (5). Moreover, the logging tool comprises a plurality of electrodes (6) arranged spaced around the longitudinal axis in the periphery of the tool so that the fluid flows between the electrodes and the casing wall and a means for measuring the capacitance between two electrodes in all possible combinations giving n*(n?1)/2 capacitance measurements for n electrodes.

Abstract: The present disclosure relates to determining the attenuation of an electromagnetic signal passing through a conductive material. An antenna is provided and placed in relatively close proximity to the conductive material. An alternating current is passed through the antenna and the impedance of the antenna is measured. The attenuation is determined using the measured impedance. A single frequency measurement may be made, or multiple measurements using different frequencies may be made. Grouped parameters based on properties of the material and the frequency of the current are used to relate the coil impedance to the attenuation. A current frequency for which the ratio of the antenna's resistive part of the impedance to the angular frequency of the current is substantially insensitive to at least one of the parameters is preferred.

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: A method and apparatus for evaluating an earth formation penetrated by a borehole comprises taking resistivity measurements using a plurality of resistivity arrays or pads positioned within the borehole proximal the borehole wall. In one embodiment, pads are spaced apart azimuthally around the perimeter of a tool body, each pad carrying at least one electrode thereon. A sequence of resistivity measurement operations are performed involving sequentially operating each pad, in turn, as a transmitter, with remaining pads and electrodes operated as return electrodes. The sequence preferably involves a succession of adjacent pads around the perimeter of the tool body, resulting in a full rotational (360°) imaging of the formation penetrated by the borehole. In one embodiment, the pads include at least two electrodes spaced vertically apart from one another, such that helical current paths are defined between transmitting electrodes and return electrodes.

Abstract: A flat panel display substrate (FPDS) testing system configured such that prior to testing, the FPDS is loaded into a pallet to prevent breakage, and to provide electrical connections to test pads on the FPDS. The system achieves high throughput by testing FPDSs using one or more charged particle beams simultaneously with the following operations: unloading of already-tested substrates, loading of substrates ready for testing, assembly of pallets, and alignment of electrical contactors to a large number of FPDS test pads. The system design eliminates a prior art X-Y stage, and all moving electrical connections to the FPDS during testing, reducing costs and improving reliability. In one embodiment, the FPDS testing system has three subsystems: a process chamber, loadlock assembly, and pallet elevator; in another embodiment, the functions of loadlock and pallet elevator are combined to reduce system footprint.

Abstract: A process control system and method for use in controlling operation of a process in response to identification of one or more events, where each event is a condition relating to one or more process variables for the process and has a determined association with one or more other events, the system comprising: an event control module which is operative to log from the process the one or more process variables which are attributed to the one or more events, and provide a control indication in response to each identified event, wherein the control indication identifies the event as one of a cause event or an effect event and the one or more associated other events as effect events for a cause event or cause events for an effect event.

Abstract: Calculating resistivity of an earth formation. At least some of the illustrative embodiments are systems including: a first current source current return (CSCR) pair, said first CSCR pair comprises a first current source and a first current return, the first current return receives current from the first current source; a second CSCR pair, the second CSCR pair disposed along a longitudinal axis of said apparatus from the first CSCR pair, the second CSCR pair comprises a second current source and a second current return, and the second current return receives current from the second current source; and a monitor electrode disposed between the first CSCR pair and the second CSCR pair, a distance between the first current source and the monitor electrode is substantially equal to the distance between the second current source and the monitor electrode, and the monitor electrode monitors a first voltage and a second voltage.

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: A method of determining the nature of a submarine or subterranean reservoir having an electromagnetic characteristic is described. The method comprises analyzing data associated with the application of a time varying electromagnetic field from above the reservoir using a data analysis methodology that does not utilize an estimate of the reservoir's geometry in determining one or more characteristics of the reservoir.

Abstract: Method and system for calculating resistivity of an earth formation. At least some of the illustrative embodiments are methods comprising transmitting a current from a first current source, receiving at least part of the current at a first current return, said first current disposed remotely from the first current source, and measuring a first voltage at a monitor electrode, said monitor electrode disposed more remotely from below the first current source than said first current return.

Abstract: An instrument is disclosed for measuring resistivity of Earth formations from within a conductive pipe inside a wellbore drilled through the formations. The instrument includes a plurality of housings connected end to end and adapted to traverse the wellbore. At least one electrode is disposed on each housing. Each electrode is adapted to be placed in electrical contact with the inside of the pipe. The instrument includes a source of electrical current, a digital voltage measuring circuit and a switch. The switch is arranged to connect the source of electrical current between one of the electrodes and a current return at a selectable one of the top of the pipe and a location near the Earth's surface at a selected distance from the top of the pipe, and to connect selected pairs of the electrodes to the digital voltage measuring circuit. The pairs are selected to make voltage measurements corresponding to selected axial distances and selected lateral depths in the Earth formations.

Abstract: A galvanic resistivity tool with one or more measurement electrodes and guard electrodes, and one or more shielding electrodes. The potential difference between the measurement electrodes and the guard electrodes is controlled to minimize current flow between the measurement electrodes and the shielding electrodes. This provides improved focusing and avoids effects of contact impedance.

Abstract: Apparatus for monitoring an underground formation through which a borehole passes, comprises a conductive casing (6) located in the borehole (1) passing through the formation (4); a plurality of sensors (12-1) for measuring an electrical property of the formation (4) located in the borehole (1) outside the casing (6) and electrically insulated therefrom; and an electrical current source (11) positioned in the borehole (1) passing through the formation (4) and arranged to inject a current into the formation (4); the casing (6) having an electrically insulating coating (14) over the surface thereof, at least in the region of the sensors (12-1), and an electrically conductive portion (11) in electrical contact with the formation (4); and a current supply (10) being connected to the casing (6) such that current flows into the formation (4) from the electrically conductive portion (11) which acts as the current source, the sensors (12-1) measuring the electrical property of the formation (4) resulting from such cu

Abstract: An azimuthal resistivity logging tool includes one or more pads for making azimuthal resistivity measurements of earth formation. Current from the pads, guard electrodes and a measure electrode are measured while monitoring the voltage difference between pairs of monitor electrodes. The pads and an adjacent portion of the tool body form a guard electrode. In addition, microelectrodes are provided on the pads for obtaining high resolution resistivity measurements.

Abstract: A multi-mode oil base mud imager for use in non-conductive drilling fluid includes at least one current source—a current return pair, and one or more monitor electrodes not located between the current source and the current return of the current source-current return pair. Where more than one current source-current return pair is included, the tool is capable of multiple depths of investigation into surrounding formation. This data can be used as a gauge to establish the reliability of the data and to determine the resistance of the invaded zone around the borehole, as well as standoff distance of the tool from the borehole wall.

Abstract: A resistivity device for downhole use with a nonconducting mud injects a current at a frequency sufficiently high to pass capacitively through the mud and a mud cake into the formation. The frequency is further selected so that the impedance due to the dielectric constant of the formation is small compared to the formation resistivity. Dual frequency measurements may be used to further minimize the effects of the mud and mud cake resistivity. When multiple frequency measurements are used, frequency focusing may be used to get an estimate of formation resistivity that is independent of near borehole effects.

Abstract: In an electrical measurement apparatus and method for measuring an electrical characteristic of an earth formation, such as in a hole drilled for the exploration for hydrocarbons, a detection portion has an operational amplifier operating in voltage follower mode, with a measurement resistor in the feedback loop. The detection portion will typically include a plurality of sensor elements, and the operational amplifier can be switched between the sensor elements. The apparatus also includes a measurement portion adapted to measure the current flow through the measurement resistor, the calculation portion including an integrator.

Abstract: Using micro-resistivity techniques, an image of the wall of a borehole can be obtained. A downhole tool stack is provided with plural imaging tools, wherein each imaging tool has a set of pads that deploy radially outward to contact the borehole wall to obtain the image thereof. As the pads deploy outwardly, gaps are formed between the pads. The pads of one tool are circumferentially offset relative to the pads of the other tool so that more complete circumferential coverage of the borehole wall is obtained during the imaging process. Furthermore, an adaptive polling technique is utilized to telemeter the large amounts of data generated by the two imaging tools to the surface. Data is obtained from the imaging tools by polling at a particular rate, which rate is modified depending upon the amount of data generated by the imaging tools.

Abstract: A small diameter tool for measurement of resistivity of geological rock formations, including a complex auxiliary electrode structure which is adjustable in electrode length between deep and shallow mode measurement operations.

Abstract: The present invention describes a method and apparatus to control the effect of contact impedance on a formation resistivity measurement during a logging-while-drilling operation. The control of contact impedance is accomplished by maintaining a substantially zero difference in potential between two monitor electrodes positioned on the resistivity logging tool near a current electrode. The tool can employ a ring electrode configuration and/or a button electrode, configuration. The ring electrode configuration incorporates two pairs of ring monitor electrodes on each side of a ring current electrode. The button electrode configuration incorporates. a monitor electrode, surrounded by a current electrode, surrounded by a second monitor electrode. Insulation gaps are positioned between each electrode to separate the electrodes. A variable current is supplied to the current electrode in each configuration to maintain the same potential at each monitor electrode.

Abstract: A method and apparatus for determining the geometrical characteristics of a well, including insertion of a sonde of elongate shape into the well, the sonde including azimuth electrodes (A.sub.azi), an annular current electrode (A.sub.0), and at least one annular current return electrode (A). The annular current electrode (A.sub.0) emits a current I.sub.0, which flows essentially in a column of mud situated inside the well and returns to the annular current return electrodes. Signals are induced in response to the current I.sub.0 from which can be determined the geometrical characteristics of the well. Alternative embodiments include an apparatus which is symmetrical about the azimuth electrodes. High resolution measurements are obtained by computed focusing. In a first effective operating mode, currents i.sub.1 and i'.sub.1 are emitted from guard electrodes A.sub.1 and A'.sub.1 located on either side of the current electrode (A.sub.0,A'.sub.0), the current I.sub.

Abstract: A capacitive measuring device with a measuring electrode (3) arranged some distance from a counter-electrode (8), a dielectric material for analysis is arranged between the electrodes (3, 8), and a measuring current (IC) is supplied to the measuring electrode (3) and the capacitance-dependent capacitor voltage (UC) is measured and evaluated.

Abstract: The apparatus includes an annular current electrode (Ao) mounted on a body and guard electrodes (A2, A'2) disposed on the body above and below the annular current electrode. A first measurement current (Io) is emitted by the central current electrode (Ao) and focusing currents are emitted by the guard electrodes (A2, A'2). A first output signal representative of the deep resistivity of the formations is generated in response to the first measurement current (Io). One of the guard electrodes (A2) includes two longitudinally spaced-apart portions, and an array of azimuthal current electrodes (Aazi.sub.i) that are circumferentially spaced-apart from one another is disposed between these two portions. Second measurement currents (Aaz.sub.i) emitted by the azimuthal current electrodes (Aaz.sub.i) are focused longitudinally by the currents emitted by the said two portions of the guard electrode (A2). Second output signals are generated in response to the second measurement current (Iaz.sub.

Abstract: The invention relates to a logging method and apparatus using passive azimuthal electrodes. The apparatus includes an array of azimuthal electrodes (M'az.sub.i) that are circumferentially spaced apart from one another on a body together with two annular guard electrodes (A2) disposed on either side of the array of azimuthal electrodes (M'az.sub.i). The guard electrodes (A2) emit currents. Two annular monitor electrodes (M3, M4) are associated with the annular guard electrodes (A2). The potential difference (.DELTA.Vaz.sub.i) between the interconnected monitor electrodes (M3, M4) and each of the azimuthal electrodes (M'az.sub.i) are detected. In response to the detected potential differences, output signals (R'az.sub.i) representative of the resistivity of the formations in a plurality of directions around the axis of the borehole are generated.

Abstract: A logging method for measuring the resistivity of formations having a borehole passing therethrough, in which a logging sonde is suspended inside the borehole by means of a cable, the sonde comprising electrodes for injecting alternating currents into the formations, these electrodes comprising a central electrode A.sub.0 and focusing electrodes, together with a return electrode B and a potential reference electrode N, one of which is placed above the sonde and the other of which is placed at the surface, and producing a resistivity log from the current I.sub.0 emitted by the central electrode and the potential difference between the sonde and the reference electrode. The method further comprises the step of producing a second log of the resistivity with the return and potential reference electrodes being placed as for the first log and under conditions such that the investigation depth is different from that applicable to the first log. The invention is applicable to correcting the Groningen effect.

Abstract: The invention relates to a logging method and apparatus for determining the electrical resistivity of an earth formation traversed by a borehole. The resistivity is determined using a measuring pad which has a central electrode operable to deliver a measuring current. The measuring current is focused radially by means of two focusing systems: an active focusing system which focuses the measuring current in a first direction; and a passive focusing system which focuses the measuring current in a second direction perpendicular to said first direction.

Abstract: An apparatus and a method are disclosed wherein simultaneous electrical well logs can be made with a plurality of focused currents emitted from a plurality of electrodes located along a single equipotential surface of a housing. The focused currents are generated in response to a single voltage signal from a single power supply connected between the housing and a single current return electrode. A voltage reference is located on a cable which suspends the housing and the current return electrode in a well borehole, and a voltage detection circuit detects a voltage offset between the voltage reference and the housing.

Abstract: A method and apparatus are described for reducing "Groeningen" effect and other anomaly induced errors in resistivity measurements of an earth formation. Such errors are typically noted during resistivity measurements of an earth layer and are generally referred to as "Groeningen" effect or "casing" effect. A sonde having a plurality of voltage measuring electrodes and current emitting electrodes is provided and suspended in a borehold by means of a conductive cable. The sonde is attached to the conductive cable by means of a length of insulated cable and two reference electrodes are disposed above the sonde. A survey current and at least two focusing currents are emitted from the sonde at alternate times or on alternate frequencies and various voltages induced by each current are measured and utilized to calculate a corresponding set of transfer impedances for that current.

Abstract: In a well logging sonde having a sonde body and wall engaging pad, the sonde body comprises two conductive sections spaced from one another by an insulating section. The pad and sonde are maintained in a relative longitudinal position such that the focussing effect from the upper sonde section is relatively weak or negligible for current emitted from the lower pad face and relatively strong for current emitted from the upper pad face. The current is returned through the lower sonde section. Additionally, the current electrodes are provided with particular individual characteristics that are defined to optimize the signal-to-noise ratio of the respective measurements. The pad may be provided with voltage electrodes on a lower insulating region of its face to allow for correction of the measurements for the resistivity of the borehole mud. The pad may be provided with additional current electrodes by which the application of the pad to the borehole wall may be monitored.

Abstract: Discloses a two-way telemetry system and method for transmitting data taken at the bottom of a well bore to the earth's surface with one part of the telemetry circuit being the metallic drilling string and with another part of the telemetry circuit being a large volume of the earth formation surrounding the well bore between the drill bit and the earth's surface.

Abstract: In a dual guard type resistivity logging apparatus, the improvement having the preferred and illustrated form of a short guard electrode pair for making shallow resistivity measurements in a formation is disclosed; a long guard electrode pair is also incorporated for making deep resistivity measurements. The long and short guard pairs are driven by separate oscillators operating at the same frequency or at different frequencies. An electronic switch driven at a low frequency selectively connects the current emitting electrodes to a current return electrode for both long and short guard pair electrodes. The electronic switch empowers one measuring system to operate while the other is inoperative. The switching frequency at is typically of 20-40 Hertz.

Abstract: A method and apparatus are described for detecting the presence of an anomaly, such as the Groningen effect, whereby an error in a resistivity measurement of an earth formation layer is noted when such layer is located below a highly resistive bed. The resistivity is measured with a resistivity measuring sonde with the aid of a reference electrode located between the sonde and a conductive sheath around the cable from which the sonde is suspended in a borehole. A phase sensitive signal is measured and used to detect the presence of an error. In one embodiment, the out-of-phase potential of an electrode is recorded alongside a resistivity measurement as a function of depth. In another embodiment, the out-of-phase potential deviation from a normal value is determined and used to derive a corresponding in-phase potential deviation from which a measurement of the magnitude of the error in the resistivity measurement is made. Other embodiments are shown and described.

Abstract: A dual resistivity and induction apparatus for the investigation of earth formations traversed by a borehole, comprises an electrode system, supported on a support member, comprising a central electrode and five pairs of electrodes respectively short-circuited and aligned symmetrically about the central electrode at increasing distances therefrom; the central electrode and the electrodes of the fourth and fifth pairs are called current electrodes, the electrodes of the first, second and third pairs are called voltage electrodes; a source of alternating current at a first frequency, f.sub.1, is coupled between the electrodes of the fifth and fourth pairs; a source of alternating current at a second frequency, f.sub.2, is coupled between the electrodes of the fifth pair and an electrode at electrical infinity with respect to the electrode system; a circuit arrangement coupled to the electrodes of the fourth pair for generating current at f.sub.

Abstract: Method and apparatus for reducing the dynamic range of measure signals encountered in simultaneously obtaining two focused logs of formation resistivity with different depths of investigation and means for providing the required electrical connections of the electrode system. The apparatus includes an electrode system having a central electrode, a near and far pair of current electrodes, respectively shortcircuited, and aligned symmetrically on both sides of the central electrode, and a near and far pair of measure electrodes, respectively shortcircuited, and aligned symmetrically between the central electrode and the near pair of current electrodes. A signal detected on the near measure electrode pair at a preselected frequency and a first and second signal are combined to produce first and second combined voltages which are used in the production of a first and second frequency, f.sub.1 and f.sub.2, respectively.

Abstract: In the representative embodiment of the invention disclosed herein, an electrical logging tool is arranged to include an axial body around which a plurality of annular electrode members and insulating members are alternately stacked to achieve a particular configuration of formation-measuring electrodes. Fluid seals are arranged between the annular members and the stack is compressed between the ends of the axial body for defining an oil-filled chamber within the tool body.

Abstract: Apparatus for investigating subsurface earth formations traversed by a borehole comprising a system of electrodes consisting of a central electrode A.sub.0 and four pairs of electrodes M.sub.1 -M'.sub.1, M.sub.2 -M'.sub.2, A.sub.1 -A'.sub.1, A.sub.2 -A'.sub.2 respectively short-circuited and aligned symmetrically on both sides of the electrode A.sub.0.This apparatus comprises arrangements which, for a frequency .function..sub.2, establish a potential gradient between electrodes A.sub.1 -A'.sub.1 and A.sub.2 -A'.sub.2. A source of alternating current of frequency .function..sub.1 is connected between the electrodes A.sub.1 -A'.sub.1 and A.sub.2 -A'.sub.2 and a source of alternating current of frequency .function..sub.2 between a surface electrode B and electrodes A.sub.2 -A'.sub.2. The potential difference between electrodes M.sub.1 -M.sub.2 and M'.sub.1 -M'.sub.2 is maintained substantially at zero by circulating a current between electrodes A.sub.1 -A'.sub.1 and electrode A.sub.0.

Abstract: A resistivity device for downhole use with a nonconducting mud injects a current at a frequency sufficiently high to pass capacitively through the mud and a mud cake into the formation. The frequency is further selected so that the impedance due to the dielectric constant of the formation is small compared to the formation resistivity. Dual frequency measurements may be used to further minimize the effects of the mud and mud cake resistivity. When multiple frequency measurements are used, frequency focusing may be used to get an estimate of formation resistivity that is independent of near borehole effects.