Abstract: An apparatus for performing a nuclear magnetic resonance (NMR) experiment in a borehole penetrating the earth includes: a carrier configured to be conveyed through the borehole; an antenna assembly disposed on the carrier and configured to receive an NMR signal; and an active gain circuit having an input coupled to the antenna and configured to apply gain to the received NMR signal and to provide an output signal comprising NMR experiment data. The apparatus also includes a feedback circuit configured to feed the output signal back to the input of the active gain circuit; wherein the signal fed back to the input of the active gain circuit is out of phase with the received NMR signal and the feedback circuit provides an electrical quality factor Q value of an antenna assembly that is less than the electrical quality factor Q value of the antenna assembly without the feedback circuit.

Abstract: Systems and methods using multi-electrode configurations for characterizing fluids in subterranean formations during various treatment operations are provided. In certain embodiments, the methods comprise: placing a tubular base structure having a multi-electrode configuration disposed thereon in at least a portion of a well bore penetrating at least a portion of a subterranean formation, the multi-electrode configuration comprising at least first and second electrodes with a dielectric layer between the tubular base structure and the electrodes; collecting electromagnetic measurements using the multi-electrode configuration; and processing the electromagnetic measurements to obtain a characterization of at least one fluid in an annulus between the tubular base structure and an inner wall of the well bore. The first and second electrodes of the multi-electrode configuration may be oriented along non-parallel planes and/or positioned at different heights from an outer surface of the tubular base structure.

Abstract: Disclosed embodiments include various borehole imaging tools and methods to provide sensing surface configurations for improving the assessment of anisotropy in layered formations. In at least some embodiments, the borehole imaging tools employ a unitary conductive body to provide a wall-contacting face with grooves that define multiple elongated sensing surfaces. This wall-contacting face can be provided on an extendable sensor pad of a wireline logging tool or embedded on the outer surface of a LWD stabilizer fin. Toroids may be seated in the grooves around each elongated sensing surface for measuring the current flow through each sensing surface. The elongated sensing surfaces can be arranged in pairs as a series of rectangular-shaped grooves that are aligned perpendicularly to each other within each of the pairs to improve detection of anisotropy in thin layered formations and improve the sensitivity to dipping in anisotropic formations.

Abstract: Disclosed herein are electromagnetic logging systems and methods that provide a set of signals that robustly approximate the response of a model tool employing orthogonal triads of point-dipole antennas. One illustrative method embodiment obtains an azimuthally sensitive electromagnetic logging tool's signal measurements as a function of position in a borehole, the tool having at least two spacing distances (d1, d2) between transmit and receive antennas. Orthogonal direct coupling measurements (Vxx, Vyy, Vzz) are derived from the signal measurements and converted into a set of robust signals, the set including: a ratio between Vzz coupling components at different spacing distances, a ratio between Vxx and Vzz coupling components, a ratio between Vyy and Vzz coupling components, and a ratio between Vxx and Vyy coupling components. The set may include an additional robust signal having a ratio between a sum of cross-coupling components Vxz+Vzx or Vyz+Vzy and a sum of orthogonal direct coupling components.

Abstract: Systems and methods provide a mechanism to provide enhanced features for well ranging. Various embodiments may include generating a current profile for a target well, acquiring magnetic signals at a second well, and determining ranging to the target well with respect to the second well using the magnetic signals and the current profile. Additional apparatus, systems, and methods are disclosed.

Abstract: A downhole galvanic logging system including a first transmitter electrode configured to convey an exciter current into a formation and a second transmitter electrode configured to receive a return current from the formation. A first transmission line being coupled to the first transmitter electrode and configured to carry the exciter current, and a second transmission line being coupled to the second transmitter electrode and configured to carry the return current. The transmission lines can be arranged in a twisted pattern. A receiver device is positioned between the transmitter electrodes along an axial length of the downhole galvanic logging system. The receiver device can be configured to detect an electrical signal that is proportional to a resistivity of the formation. The second transmitter electrode can be coupled to one end of the receiver device with the second transmission line coupled to the second transmitter electrode through the receiver device.

Abstract: Various embodiments include methods and apparatus structured to balance electrode pairs (112, 312) of a tool (105, 305) providing voltage equivalence between the electrode pairs. An electrically conductive wire (113, 313) coupling the electrodes of an electrode pair can be arranged such that resistance of the electrically conductive wire does not adversely affect the voltage equivalence. The electrically conductive wire can also be structured to provide temperature independent balancing by arranging the electrically conductive wire with respect to a measuring node (116, 316). Additional apparatus, systems, and methods are disclosed.

Abstract: Multi-array laterolog tool systems and methods acquire a set of array measurements sufficient to provide laterolog tool measurements of differing array sizes. Such systems and method offer multiple depths of investigation while offering greater measurement stability in borehole environments having high resistivity contrasts. In at least some system embodiments, a wireline or LWD tool body has a center electrode positioned between multiple pairs of guard electrodes and a pair of return electrodes. The tool's electronics provide a current from the center electrode to the pair of return electrodes and currents from each pair of guard electrodes to the pair of return electrodes. Each of the currents may be distinguishable by frequency or distinguishable by some other means. This novel arrangement of currents provides a complete set of measurements that enables one tool to simultaneously emulate a whole range of laterolog tools.

Abstract: Various embodiments include apparatus and methods to make resistivity measurements in a borehole using tool having an array of electrodes operable to provide focused currents and measure corresponding voltages to determine resistivity. Tools can be configured with a main electrode having a number of spaced apart electrodes within the main electrode such that the spaced apart electrodes are arranged azimuthally with respect to an axis of the tool. Generation of current from the spaced apart electrodes and control of current from additional electrodes on each side of the main electrode can provide for focused measurements. Additional apparatus, systems, and methods are disclosed.

Abstract: A technique is provided for extracting a substance containing hydrocarbon from a reservoir, wherein the reservoir is applied with thermal energy in order to reduce the viscosity of the substance. As per the technique, at least two conductor loops for the inductive energization are provided as electric/electromagnetic heating elements. Each of the at least two conductor loops has at least two extended conductors, which are guided horizontally inside the reservoir. At least two alternating current generators are provided for electric power, each being connected to a respective conductor loop. The technique involves operating a first of the at least two alternating current generators and at least a second of the at least two alternating current generators synchronously with respect to their frequency and with a fixed phase position in relation to one another.

Abstract: In some embodiments, a method of approximating or determining at least one dimension of at least one underground geological feature in a zone of interest proximate to a well bore includes generating an electric field in the zone of interest while the well bore and geological feature at least partially contain conductive fluid. At least two sensing electrodes are provided in the well bore and configured to detect differences therebetween in electric potential caused by at least one target object in the zone of interest and provide data relating thereto to at least one data processing system. The data processing system(s) approximates or determines the dimension(s) of the geological feature(s) based at least partially upon data provided by the sensing electrodes.

Abstract: A method of approximating or determining at least one dimension or other characteristic of at least one underground geological feature in a zone of interest proximate to a well bore includes generating an electric field in the zone of interest. At least two sensing electrodes are provided in the well bore and configured to detect differences therebetween in electric potential caused by at least one target object in the zone of interest. Proppant containing signal generating devices (SGD) is delivered into the geological feature(s). The SGD generate a detectable signal in response to at least one downhole condition or property. At least one receiver receives the detectable signals and provides data relating thereto. At least one dimension or other characteristic of the geological feature is approximated or determined based at least partially upon data provided by the sensing electrodes and receiver(s).

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: In some embodiments, apparatus useful for determining at least one dimension of at least one geological feature of an earthen formation from a subterranean well bore includes at least two electric current transmitting electrodes and at least two sensing electrodes disposed in the well bore. The electric current transmitting electrodes are configured to create an electric field and the sensing electrodes are configured to detect perturbations in the electric field created by at least one target object.

Abstract: A method for correcting data from a sensor electrode in a sensor includes determining a potential difference (V) between the sensor electrode and another sensor electrode; from the potential difference (V), determining a by-pass current (Ic) between the sensor electrode and the another sensor electrode; and from the by-pass current (Ic) and a measured current (I) from the sensor electrode, determining a corrected current (J) for the sensor electrode; wherein determining a potential (V1) for the sensor electrode comprises multiplying an intrinsic impedance (Zi1) by the measured current (I). A computer program product and other embodiments are provided.

Abstract: Sequential measurements are made using a two terminal resistivity imaging device wherein the measure electrodes are activated sequentially with the remaining electrodes in a floating mode. This eliminates the hardware requirements for focusing electrodes, prevents galvanic leakage between proximal electrodes and the effects of mutual coupling between circuits including proximal electrodes.

Abstract: A method and system for measuring formation resisitivity is achieved by introducing one or more continuous phase modulated electrical signals into the formation, each signal introduced at a different location. The current of each introduced electrical signal is then sensed, producing an analog voltage signal as a measurement of the formation. Each introduced electrical signal is compared with its respective measured signal of the formation. Each comparison is made in a quadrature demodulation device, producing an in-phase demodulated signal. A quasi-direct-current signal is extracted from each in-phase demodulated signal, producing an analog measurement related to the resistivity of the formation. Each analog measurement is converted into a digital value for further processing. Successive measurements are digitally summed and averaged to increase the signal to noise ratio.

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 surveying instrument performs tracking of a target by controlling an orientation of a measuring unit relative to the target in two different angular directions. The control in one angular direction is based on detected light intensities which are modulated according to a first time pattern, and control of the orientation in the other angular direction is based on detected light intensities modulated according to a second time pattern just different from the first time pattern.

Abstract: An apparatus for logging a formation traversed by a borehole includes a plurality of logging tools adapted for conveyance inside the borehole. The plurality of logging tools includes a tool body, a sensing pad responsive to a density property of the formation coupled to the tool body, a current emitting measure electrode responsive to a lateral resistivity property of the formation incorporated on the sensing pad, a mechanism for urging the sensing pad in contact with a side of the borehole coupled to the tool body, and a pair of mass isolation bands disposed about the tool body to isolate a mass of the tool body adjacent the measure electrode.

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 well logging tool includes an induction array stack disposed on a mandrel; an electronic module stack disposed adjacent the induction array stack along a longitudinal axis of the well logging tool; and a shallow electrode array arranged on a housing disposed around the electronic module stack, wherein the induction array stack comprises a transmitter antenna, a first receiver antenna, and a second receiver antenna spaced apart from each other along the longitudinal axis of the well logging tool, the second receiver antenna being disposed between the transmitter antenna and the first receiver antenna.

Abstract: A mud resistivity measuring device for use in a borehole includes a pair of current electrodes disposed between a pair of guard electrodes. The measure electrodes are carried on a recessed portion of a logging tool so that the measure current flows substantially along the borehole axis and is relatively insensitive to the formation resistivity. A pair of measure electrodes on the logging tool determines a voltage difference caused by the current flow in the borehole.

Abstract: The invention relates to a method and apparatus for studying the resistivity of a geological formation around a borehole fitted with metal casing. An electric current is applied to the casing so as to cause current to leak into said formation at a given level, and said current is shunted by a feedback circuit in contact with the casing on either side of the measurement level, said circuit being organized to ensure that the current flowing along the casing at said level is less compared with the shunt current, the difference between the voltage drops on the adjacent sections of casing situated on either side of the measurement level is determined, and the leakage current (Ifor) is deduced therefrom.

Abstract: A petroleum well (20) for producing petroleum products that incorporates a system adapted to controllably measure a formation resistivity using induction chokes (91-95) to form electrically isolated piping structure sections that can be used as the formation contact electrodes for time-varying current, and methods of producing petroleum products while measuring formation resistivity, are provided by the present invention. The system comprises a first induction choke (91), a second induction choke (92), and a device (50). The first induction choke (91) is located about a piping structure (30) of the well (20). The second induction choke (92) is also located about the piping structure (30) of the well (20), but the first induction choke (91) is distally spaced from the second induction choke (92). The device (50) is located outside of the piping structure (30) and comprises two terminals (71, 72).

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 of measuring the resistivity of drilling mud in a borehole passing through a terrestrial formation includes the steps of a) inserting a sonde into the borehole, the sonde having an elongate body provided with at least one annular current electrode (A0, A′0) and at least two annular guard electrodes (A, A′, A1, A′1, A2, A′2) situated on either side of the annular current electrode; b) performing computed focusing to simulate an operating mode in which at least one current I0 is emitted into the surrounding formation from the annular current electrode, the current I0 being focused in the formation by emitting two currents I1 and I′1 from the two annular guard electrodes situated on either side of the annular current electrode; and c) producing a signal representative of the resistivity Rm of the drilling mud from the simulated operating mode.

Abstract: An apparatus for determining radial distribution of resistivity of earth formations surrounding a wellbore. The apparatus includes a sonde mandrel having an insulating exterior surface and electrodes disposed on the insulating surface at spaced apart locations. The electrodes are connected to circuits for measuring a focused current resistivity within a predetermined longitudinal span. The electrodes are also connected to circuits for measuring electrical impedance between pairs of electrodes spaced apart at a plurality of different longitudinal spacings. The preferred embodiment includes a voltage measuring circuit interconnected between a pair of electrodes which is positioned between a closest spaced pair of electrodes connected to the impedance measuring circuits, so that a vertical resolution of the impedance measurements can be limited to approximately the axial spacing of the pair of electrodes connected to the voltage measuring circuit.

Abstract: The invention relates to a method of measuring the resistivity of a drilling mud by means of a sonde provided with an annular current electrode (A.sub.0) and with annular guard electrodes (A, A'), current I.sub.0 being emitted into the surrounding formation together with focusing currents I and I'. A signal representative of R.sub.m is produced in response to I.sub.0. Various apparatuses for implementing the method are described. Another method implements a technique of computed focusing. Corresponding apparatuses are also described.

Abstract: An instrument for measuring resistivity of earth formations within azimuthal segments. The instrument includes azimuthally separated measure electrodes placed in contact with the wall of a wellbore penetrating the earth formations. The electrodes are placed in contact with the wall by springs, hydraulics or similar mechanisms. A guard electrode surrounds each measure electrode. The instrument includes a booster current electrode axially spaced apart from the guard electrodes. An insulator axially separates the guard electrodes from the booster electrode. Monitor electrodes are disposed on the exterior of the insulator. A first bucking current circuit is connected to the guard electrodes. The first bucking current constrains the flow of measuring currents from each measure electrode to a path substantially perpendicular to the axis of the instrument. A booster current circuit is connected to the booster electrode.

Abstract: A method for measuring formation resistivity uses a tool including a central current electrode and a series of voltage electrodes arranged in pairs on either side of the current electrode. A series of measurements is made using the current electrode with different numbers of pairs of voltage electrodes maintained at a predetermined voltage so as to allow resistivity in the formation to be determined with different depths of investigation. In this manner these measurements are focused and are relatively unaffected by the borehole or by adjacent layers in the formation. The central current electrode is segmented azimuthally into a series of electrodes, and the current flowing out of each azimuthal segment is measured separately. This can yield resistivity images of the formation surrounding the borehole. The azimuthal imaging capability can be used for stratigraphic or lithologic analysis of the formation and to detect fractures in the rock.

Abstract: A method of determining selected parameters of an earth formation surrounding a borehole by first, obtaining at least one induction logging measurement of the selected parameters in a first predetermined volume of the formation surrounding the borehole having known first radial and vertical dimensions, then obtaining at least one galvanic logging measurement of the identical selected parameters in a second predetermined volume of the formation surrounding the borehole having known second radial and vertical dimensions that overlap the first radial and vertical dimensions of the first predetermined volume, whereby the overlapping volumes form a representative common volume of the formation, and then combining the induction and galvanic logging measurements using an inversion technique to obtain a measurement of the selected parameters of the earth formation surrounding the borehole in the representative common volume of the formation.

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 sensor element especially adapted for use in borehole resistivity logging, having an outside surface comprising conductive portions and insulating portions distributed in a determined pattern, said conductive and insulating portions being made respectively of a material that is hard and electrically conductive and of a material that is hard and electrically insulating, each of said materials being deposited in the form of a layer.

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: A logging method and apparatus for investigating the geometrical characteristics of a borehole comprises an array of circumferentially spaced apart azimuthal current electrodes (Aaz.sub.i) disposed on a sonde (21), and an annular current electrode (A) longitudinally spaced apart from the array of azimuthal electrodes. In a method of investigating the geometrical characteristics of the borehole, currents (Ic.sub.i) are emitted between the azimuthal electrodes and the annular electrode (A). Monitor electrodes (M, Maz.sub.i) are respectively associated with the current electrodes (A, Aaz.sub.i). The potential differences (Vc.sub.i) between annular monitor electrodes (MAz.sub.i) are detected. In response to the detected potential differences, output signals (Rc.sub.i) are generated indicative of the distances between the sonde and the wall of the borehole in a plurality of directions around the sonde.

Abstract: The apparatus includes an array of azimuthal current electrodes (Aaz.sub.i) that are circumferentially spaced apart from one another on a body together with two annular guard electrodes (A2) disposed on opposite sides of the array of azimuthal current electrodes (Aaz.sub.i). Measurement currents (Iaz.sub.i) are emitted by the current electrodes (Aaz.sub.i) and focusing currents are emitted by the guard electrodes (A2). Azimuthal monitor electrodes (Maz.sub.i) are associated with respective ones of the azimuthal current electrodes (Aaz.sub.i). Two annular monitor electrodes (M3, M4) are disposed on opposite sides of the array of azimuthal current electrodes (Aaz.sub.i). For focusing purposes, a servo-control system controls the emission of the currents so as to substantially cancel the potential difference detected between the short circuited monitor electrodes (M3, M4) and each of the azimuthal monitor electrodes (Maz.sub.i). In response to the measurement currents (Iaz.sub.i), output signals (Raz.sub.

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: A device for detecting and sensing the boundaries between differing rock strata is provided. This device utilizes electrical conductivity or resistivity to detect and measure the boundary layer. The device of the present invention may be used with a "measurement-while-drilling" (MWD) tool, using either wire-line or wireless data communication with a surface receiver. Moreover, the resistivity or conductivity detection device of the present invention may also be used alone or in conjunction with gamma radiation sensors. The present invention utilizes at least a pair of electrodes which occupy a small area and are located on one side of the tool aligned with the longitudinal axis thereof. Thus, in drilling a well along a specific formation stratum (which may be horizontal or inclined), the present invention will act to indicate to the driller whether the drill bit is approaching a stratum boundary, and in which direction that boundary lies.

Abstract: An in-situ method of determining the pore size distribution, capillary pressure curve, and permeability of a formation using induced polarization logging measurements. The induced polarization (IP) voltage decay curve is measured opposite the formation of interest and the voltage decay curve is decomposed into a series of relaxation times related to the pore size distribution. The capillary pressure curve and the permeability are approximated from the pore size distribution. The measurements can also be made in the frequency domain using various frequencies to obtain the variation in phase shift with frequency. The variation in phase shift with frequency can be correlated with previous core measurements to obtain the pore size distribution, capillary pressure curve and formation permeability.

Abstract: The stimulus signal for an induced polarization logging system is presented to the formation intermittently, alternately being applied and removed as a square wave of alternating polarity, and the response signal amplification level is time gated with the stimulus signal.

Abstract: A well logging instrument for measuring the resistivity of relatively thin formation beds. A well logging instrument includes an elongated body member, serving as a single guard electrode, and a point current emitting measure electrode recessed therein. The instrument is lowered into a borehole and urged into contact with the wall of the borehole. A measure current emanates from the measure electrode into the formation for determining the resistivity thereof.

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.