Abstract: A method of estimating standoff in a borehole through an earth formation, includes: disposing a sensor including a plurality of return electrodes and at least one transmitter electrode disposed in a concentric arrangement into the borehole; injecting current, I, into the formation by applying at least two frequencies of alternating current (AC) voltage between at least two of the electrodes; measuring impedance, Z, for each of the frequencies; and using a predetermined relationship between impedance, Z, and standoff, estimating the standoff. A system and a computer program product are provided.

Abstract: A method and system for measuring formation resisitivity is achieved by introducing an electrical signal into the formation. The current of the introduced electrical signal is then sensed, producing an analog voltage signal as a measurement of the formation. The introduced electrical signal is compared with the measured signal of the formation. The comparison is made in a quadrature demodulation device, producing an in-phase demodulated signal. A quasi-direct-current signal is extracted from the in-phase demodulated signal, producing an analog measurement related to the resistivity of the formation.

Abstract: A receiver streamer system for marine electromagnetic surveying includes a first streamer, and a second streamer disposed substantially parallel to and spaced apart from the first streamer. A first pair of electrodes is associated with the first streamer and a second pair of electrodes is associated with the second streamer. Each of the first and second pairs of electrodes is functionally associated with a voltage measuring circuit configured to measure voltage along an inline direction. At least one electrode on each of the first and second streamers is configured and associated with a voltage measuring circuit to make voltage measurements in a cross-line direction.

Abstract: Method for removing air wave noise from shallow water controlled source electromagnetic survey data, using only the measured data and conductivity values for sea water (140) and air. The method is a calculation performed numerically on CSEM data and resulting in an estimate of those data that would have been acquired had the water layer extended infinitely upward from the seafloor. No properties of the sub-sea sediments are used. Synthetic electromagnetic field data are generated for (a) an all water model (141) and (b) an air-water model (146-147) of the survey region. These simulated results are then used to calculate (148-150) electromagnetic field values corresponding to a water-sediment model with water replacing the air half space, which represent measured data adjusted to remove air wave noise.

Abstract: A method for determining electrical properties of a submarine petroleum fluid-bearing formation (1) under overburden geological layers (2) of thickness (s) under a seafloor (3) of a sea. The method includes: *arranging one or more transmitter antennas (9) in the sea, and arranging one or more electromagnetic receivers (8) in the sea, *the sea having a shallow sea depth (d) of 50-350 meters; *using the transmitter antenna (9) transmitting electromagnetic signals (10) of one or more pulses (11) having frequency of very low frequency between 0.01 Hz and 0.

Abstract: A marine electromagnetic sensor cable includes a plurality of sensor modules disposed at spaced apart locations along a cable. Each module includes at least one pair of electrodes associated therewith. The electrodes are arranged to measure electric field in a direction along the direction of the cable. A plurality of spaced apart magnetic field sensors is associated with each module and arranged to enable determining an electric field amplitude in a direction transverse to the direction of the cable from magnetic field gradient.

Abstract: A method and apparatus are provided for detecting and transmitting geophysical data from a plurality of electrodes inserted into the soil utilizing a set of identical dynamically reconfigurable voltage control units located on each electrode and connected together by a communications and power cable. A test sequence is provided in each voltage control unit. Each voltage control unit records data measurements for transmission to a central data collector. Each voltage control unit incorporates and determines its positional relationship to other voltage control units by logging when the unit is attached to the electrode. Each voltage control unit I equipped with a magnetic switch for detecting when they are in contact with the electrode.

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 induction or propagation apparatus for performing measuring the tensor resistivity of a sample of matter includes a non-conductive, generally cylindrical tubular member adapted to receive the matter therein, a triaxial transmitter, at least one triaxial receiver, and an electronic module. The electronic module is adapted to energize the triaxial transmitter, control acquisition of signals by the at least one triaxial receiver, and perform anisotropic measurements and analysis. The triaxial transmitter and the at least one triaxial receiver are disposed on the generally cylindrical tubular member in axial spaced relationship.

Abstract: A detector for underwater electromagnetic surveying is described. The detector comprises first, second, third and fourth electrodes which are arranged to define first, second and third electric dipole antennae extending between pairs of the electrodes. Each dipole antennae extends between a pair of the electrodes and the fourth electrode is common to all three dipole antennae. Thus the first electrode is separated from the fourth electrode along a first direction to provide the first dipole antenna, the second electrode is separated from the fourth electrode along a second direction to provide the second dipole antenna, and the third electrode is separated from the fourth electrode along a third direction to provide the third dipole antenna. The electrodes are arranged so that the first, second and third directions are inclined at an angle of between 20 and 70 degrees to a surface on which the detector rests when in normal use.

Abstract: A marine electromagnetic survey cable includes a reference electrode extending substantially along the entire length of the cable. A plurality of spaced apart measuring electrodes is disposed along the cable and each is electrically insulated from the reference electrode. A voltage measuring is circuit functionally coupled between each measuring electrode and the reference electrode.

Abstract: A method of analyzing electromagnetic survey data from an area of seafloor 6 that is thought or known to contain a resistive or conductive body, for example a subterranean hydrocarbon reservoir 12, is described. The method includes providing horizontal electromagnetic field data obtained by at least one receiver 125 from at least one horizontal electric dipole transmitter 22. Horizontal gradients in the electromagnetic field data are determined for a first component of the electromagnetic field data along a first direction and for a second component of the electromagnetic field data along a second direction. The first and second components can be the electric field along the first and second directions, or the magnetic field perpendicular to the first direction and second directions. The gradients are then combined to provide combined response data.

Abstract: Method for conducting an efficient and interpretable controlled-source electromagnetic reconnaissance survey for buried hydrocarbons. While a part of the survey area is being set up for measurement and data are being acquired, data from a nearby part of the survey area, surveyed just previously, are being rapidly processed and analyzed (110). If the analysis shows resistive anomalies of interest in a portion of a survey area, a fine-grid survey is quickly designed for that portion, and that survey is conducted next before moving source and receivers to a more distant part of the survey area.

Abstract: The invention is concerned with electromagnetic exploration of the earth's surface. In a method proposed by the invention, a primary coil is powered to generate a primary field and the primary field is applied to the earth and a receiver, used to detect a secondary field generated by the earth in response to the primary field, is moved over the surface of the earth. According to the invention, Helmholtz coils (14.1, 14.2) are arranged in a predetermined array and are powered in such a manner that they generate, in a volume accommodating the receiver, a magnetic field which serves at least partially to null the magnetic field of the earth. The invention also provides apparatus for carrying out the method.

Abstract: Provided is a method of accurately measuring earth resistance even when the composition state of soil that affects the installation of a metering device for measuring earth resistance is unknown. The method comprises measuring a potential at a first potential electrode point, measuring a potential at a second potential electrode point, calculating a voltage slope corresponding to a change rate of potential difference, using a potential difference between the first and second potential electrode points, and setting a potential point according to voltage slope characteristics and measuring earth resistance. A measurement approximating an accurate value of earth resistance can be performed even when soil composition characteristics are unknown at a site.

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: Phase-sensitive measurements are made by a resistivity imaging tool in a borehole having non-conductive mud in a conductive earth formation at a plurality of frequencies. From the phase sensitive measurements, the formation resistivity can be determined with higher sensitivity than is possible with the single frequency measurements. Tool standoff can also be determined from a knowledge of the mud resistivity and/or dielectric constant. Formation resistivity may also be determined when the effect of formation capacitance cannot be ignored.

Abstract: Impedance measurements made by a galvanic resistivity tool in a borehole in an earth formation are corrected by a factor that depends on the mud conductivity and the mud dielectric constant. Standoff measurements are not necessary.

Abstract: A method and apparatus are provided for detecting and transmitting geophysical data from a plurality of electrodes inserted into the soil utilizing a set of identical dynamically reconfigurable voltage control units located on each electrode and connected together by a communications and power cable. A test sequence is provided in each voltage control unit. Each voltage control unit records data measurements for transmission to a central data collector. Each voltage control unit incorporates and determines its positional relationship to other voltage control units by logging when the unit is attached to the electrode. Each voltage control unit I equipped with a magnetic switch for detecting when they are in contact with the electrode.

Abstract: Measurements made by a resistivity imaging tool in a borehole having non-conductive mud in a conductive earth formation are corrected using the tool standoff. The correction involves removing a calibration signal determined in a medium of high conductivity from the measured impedance. The magnitude and/or the real part of the impedance may be used.

Abstract: Methods and apparatus are provided for controlled source electromagnetic surveying. In particular, a time-varying electromagnetic signal is transmitted at a first location, and an electromagnetic signal responsive to the transmitted signal is received at a second location, wherein the received signal includes a transient response component and a steady-state component. The received signal is continuously recorded during a predetermined time interval, and data corresponding to the transient response component and the steady-state component are extracted from the received signal. The transient response data and the steady-state response data are jointly inverted to identify a subsurface feature, such as resistivity.

Abstract: A method of analyzing electromagnetic survey data from an area of seafloor 6 that is thought or known to contain a resistive or conductive body, for example a subterranean hydrocarbon reservoir 12, is described. The method includes providing horizontal electromagnetic field data obtained by at least one receiver 125 from at least one horizontal electric dipole transmitter 22. Horizontal gradients in the electromagnetic field data are determined for a first component of the electromagnetic field data along a first direction and for a second component of the electromagnetic field data along a second direction. The first and second components can be the electric field along the first and second directions, or the magnetic field perpendicular to the first direction and second directions. The gradients are then combined to provide combined response data.

Abstract: A mixed mode tool uses an inductive source and detects galvanic currents and/or potentials at electrodes in proximity to a borehole wall to produce a resistivity image of the earth formation.

Abstract: The invention concerns a method for determining the position, in a formation (1) containing at least one electrolytic liquid, of an interface (6) in relation to a bore hole (2). One stimulates at a first instant the interface (6) with an excitation signal, one detects at a second instant (t2) a response signal converted by electro-osmosis or electrokinetic coupling effect and, if appropriate, the exication signal after reflection at a third instant (t3). With the three instants and the propagation velocity of the signals, one calculates the distance between the interface (6) and the bore hole (2).

Abstract: A magnetic field generated by a primary winding (1) in which the current (i1) to be measured flows is balanced by a magnetic field of opposing direction generated by a secondary winding (2) in which a compensating current (i2) flows. The device includes a member (3) sensitive to the field resulting from the addition of the magnetic fields of opposing directions to regulate the compensating current (i2) in closed loop mode. According to the invention, the, sensitive member (3) is sensitive only to the direction of the resultant field and, in return, controls the reversal of the direction of circulation of the compensating current (i2) in the secondary winding (2). This sensitive member (3) can be formed by a Hall effect probe with bipolar output signal. The device is suitable for the measurement of a current inautomotive electronics.

Abstract: Device for resistivity soundings on water covered subsurfaces including a Fixed Current Asymmetric Schlumberger electrode configuration with a first and second current electrode and a number of voltage electrodes that are positioned on one line between the current electrodes, and method for measuring the apparent resistivity of water covered subsurfaces using said device and electrode configuration whereby a voltage gradient associated with a generated electrical field between the current electrodes, is measured between the voltage electrodes such that it allows noise recognition and noise filtering by Coupled Electrode Noise Filtering.

Abstract: A current to be applied to a ground is defined as the sum of two sinusoidal waves having first and second frequency components. A signal to be used at the time of synchronous detection is defined as a difference between the sinusoidal waves having the first and second frequency components. As a result, a substance in the ground, and the position and depth of the substance can be specified with high accuracy.

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: The object of the present invention is to realize a low-invasive underground exploration apparatus, system, and method which are capable of specifying the kind, three-dimensional position, and amount of a substance present in the ground. The underground exploration apparatus, system, and method includes: measuring, at multiple points, a high frequency voltage that appears upon conduction of high frequency current through the ground to obtain measurement results at two or more frequency levels; employing an ground model using the finite element method, the boundary element method, an impedance network, or the like to estimate a substance in the ground by changing unknown quantities of the ground model such as local dielectric constant and electric conductivity so as to make an error between the actual measured value and the calculated value smaller; and displaying input information of the ground model and results of the estimation processing two-dimensionally or three-dimensionally.

Abstract: A method and apparatus for minimizing direct coupling between transmitters and receivers on a downhole logging tool are disclosed. One embodiment includes a transmitter, a bucking device, and a plurality of receivers, where signals may be directly coupled from the transmitter into the receivers and signals may be indirectly coupled from the transmitter into the receivers through the formation and borehole environment. The bucking device minimizes the magnitude of the signals that are directly coupled from the transmitter into multiple receivers within the plurality. By varying the current in the bucking device, the bucking device minimizes the magnitude of the directly coupled signal of each receiver in the plurality, and each receiver may utilize a common bucking device.

Abstract: A method and apparatus for the rapid tomographic measurement of conductivity distribution in a sample in which current excitations or voltage excitation is applied to the sample via electrodes or the like and potential differences or magnetic field strengths association with those excitation fields are measured and analyzed, e.g. by a Fourier analysis.

Abstract: The invention relates to a device for measuring an electrical parameter through electrically-conductive casing, said device including a current injection electrode (3) and a measurement electrode (7) on one side of the wall of the casing (1). According to the invention, the injection electrode (3) is associated via a first magnetic coupler (4) with a current source (2) and the measurement electrode (7) is associated via a second magnetic coupler (6) with a measurement device (5) situated on the other side of the wall.

Abstract: The invention relates to a method of mapping subsurface resistivity contrasts by making multichannel transient electromagnetic (MTEM) measurements on or near the earth's surface using at least one source, receiving means for measuring the system response and at least one receiver for measuring the resultant earth response. All signals from the or each source-receiver pair are processed to recover the corresponding electromagnetic impulse response of the earth and such impulse responses, or any transformation of such impulse responses, are displayed to create a subsurface representation of resistivity contrasts. The invention enables subsurface fluid deposits to be located and identified and the movement of such fluids to be monitored.

Abstract: A method and apparatus for the rapid tomographic measurement of conductivity distribution in a sample in which current excitations or voltage excitation is applied to the sample via electrodes or the like and potential differences or magnetic field strengths association with those excitation fields are measured and analyzed, e.g. by a Fourier analysis.

Abstract: A method is provided for detecting and locating anomalies in soil subjacent to an existing structure. Electrodes are placed into the soil both outside the structure and through the structure's foundation. Electrical resistivity data is collected using these electrodes. This electrical resistivity data is analyzed to create an electrical resistivity map or model. Inspection of the map or model can allow the operator to detect and locate resistivity anomalies which may correspond to geological anomalies in the soil. In alternative embodiment, one or more soil samples are taken from the soil adjacent to the existing structure. Laboratory resistivity measurements are performed on the soil samples to provide a set of baseline resistivity data. A resistivity map or model is produced as described above. This resistivity map or model is compared to the baseline values provided by the laboratory resistivity measurements of the soil samples to detect and locate resistivity and geological anomalies.

Abstract: A method of monitoring a fluid front movement is provided. The method includes: determining at least two techniques for monitoring the fluid front movement; determining a configuration of monitoring sensors, corresponding to the at least two monitoring techniques, from a joint sensitivity study of the at least two techniques; acquiring data with the monitoring sensors; and monitoring the fluid front by joint inverting the data.

Abstract: A method for detecting malfunctions in a flowmeter includes the measuring of the receive signal VIN output by a transducer. A characteristic of the receive signal is compared to a predetermined reference characteristic VREF and the peak voltage VPK of the receive signal is stored. An alarm signal VAL is generated when a trigger characteristic VDEC of the receive signal is less than the predetermined reference characteristic. A threshold voltage VTH is defined, proportional to the peak amplitude VPK of the receive signal in such a manner that VTH=K×VPK where K is a factor depending on the transducer. The receive signal is compared with the threshold voltage and a conditioned output signal VOUT is generated in a first state when the receive signal is greater than the threshold voltage, and in a second state when the receive signal is less than the threshold voltage.

Abstract: The invention relates to a device for assisting in the measurement of the earth resistance comprising a ground rod with a wire coiled up on a reel or the like, and electrical connecting means for connecting the wire to the ground rod. To provide such a device which is practical to handle, simple, small, and capable of being produced at low costs, it is provided for the reel or the like to be rotatably mounted on the ground rod. A further improvement results in that the ground rod comprises an offset portion in whose region the reel or the like is rotatably mounted. Preferably, the electrical connecting means between ground rod and wire comprise sliding rings on the rotation axis, the reel or the like. For an rapid exchangeability of the reel, the latter may be mounted on the ground rod by a snap connection.

Abstract: A method of imaging an object contained in a medium, having a specific impedance which is different from the specific impedance of the medium, comprising applying current to the medium at various locations at a surface of the medium, extracting current at other locations, detecting voltages produced by the current which has passed through the medium from the surface of the medium at various other locations, successively determining a location and shape and conductivity of the object with increasing accuracy by processing values of the detected voltages, determining a region in the medium in which the object is located from values of the detected voltages which are within upper and lower threshold values, applying acceleration procedures to the conductivities within the region in the course of iterative refinement of these values in the course of an imaging procedure, subsequently restricting further determination of the location of the object with increasing accuracy to voltages obtained from the region of th

Abstract: Sensors are permanently placed in the ground near observation and injection wells in order to passively and continuously monitor the status of seawater advance toward fresh water aquifers near coastal cities as well as the status of fresh water injected into the injection wells. Such sensor devices are installed in the ground and electrically connected to surface acquisition equipment that would, without human intervention, transmit acquired data to a centralized facility for processing and interpretation. Various types of sensors can be used: the sensors used for general reservoir monitoring and/or the sensors used for leak detection, soil heating, and temperature mapping. Alternatively, a special type of sensor can be designed and provided for the purpose of monitoring the status of seawater advance toward fresh water aquifers.

Abstract: For transient-current testing of an electronic circuit, a differentiating current measuring device is arranged for measuring an undershoot voltage for each of a series of current pulses controlled in the circuit. In particular, the device is executed in integrated circuit technology and simulates a differentiating current probe. Furthermore, it may have calibration for imparting an offset voltage to each undershoot voltage of the series. This calibrates an actual potential of the simulation and produces for a correct Device Under Test in each cycle a substantially uniform undershoot voltage.

Abstract: The invention is directed to an apparatus and method for locating objects in a body through the mapping and imaging of the conductivity profiles of such objects by applying a force to the object and/or body and measuring certain characteristics of the body responsive to the application of force. In accordance with a preferred embodiment, the force applied to the object and body is in the form of an electrical voltage or current such that electrical potentials, currents, and magnetic fields are generated throughout the subsurface site. The voltage, current, or magnetic field is then measured at the surface or the boundary of the body. An estimate of subsurface conductivity is then made and a conductivity profile generated by minimizing a loss function. Preferably, the loss function is in the form of the sum square of the differences between measurement values and a set of computed values based on a gradient approximation technique.

Abstract: A method for determing the potential in a three-dimensional domain containing a source domain of source particles and a target domain of target particles involves assigning the source particles into source particle clusters and the target particles into target particle clusters, assigning a collection point to each of the clusters, and determing the inverse distance from each particle to the collection point of its respective cluster. A pseudo-particle representative of each particle within the cluster is defined at the collection point of each cluster in accordance with the determined inverse distances and the charge-like measure of each of the particles. The potential, and/or its gradient, at any of the target particles is then determined from the position and the charge-like measure of the associated pseudo-particle.

Abstract: Resistivity mapping apparatus includes a tow mechanism for use by a human operator in moving the mapping apparatus over a surface and measuring the resistivity of soils of the surface. The tow mechanism includes a generally triangular shaped tow ring which is engageable by the operator's battery belt, harness, or pack-frame, the tow ring having two opposing hooks at a vertex of the tow ring for receiving a loop from a tow line. The tow line engages the tow ring and a cable depressor weight which keeps the resistivity mapping apparatus in close proximity to the earth as the apparatus is towed. A communication cable in the tow line preferably comprises an optical fiber which transmits signals from the receiver to a data logger carried by the operator.

Abstract: Sensors are permanently placed in the ground near observation and injection wells in order to passively and continuously monitor the status of seawater advance toward fresh water aquifers near coastal cities as well as the status of fresh water injected into the injection wells. Such sensor devices are installed in the ground and electrically connected to surface acquisition equipment that would, without human intervention, transmit acquired data to a centralized facility for processing and interpretation. Various types of sensors can be used: the sensors used for general reservoir monitoring and/or the sensors used for leak detection, soil heating, and temperature mapping. Alternatively, a special type of sensor can be designed and provided for the purpose of monitoring the status of seawater advance toward fresh water aquifers.

Abstract: The invention is a method and apparatus for initiating an azimuth controlled vertical hydraulic fracture in unconsolidated and weakly cemented soils and sediments using active resistivity to monitor and control the fracture initiation and propagation. Separate or overlapping treatment walls and containment barriers can be created by controlling and monitoring the propagation of fractures in the subsurface. A fracture fluid is injected into a well bore to initiate and propagate a vertical azimuth controlled fracture. The fracture fluid is energized to conduct electrical current while the fracture propagates through the ground. A series of electrical resistivity monitors measure the electrical conductivity of the fracture fluid in real time against the background conductivity of the formation.

Abstract: A geological data acquisition system (1) for obtaining data from a geological body (2). System (1) includes a plurality of nodes (3) for simultaneously obtaining respective first measurement signals representative of predetermined characteristics of body (2) at a corresponding plurality of equally spaced apart locations adjacent to the top surface (4) of body (2). First memory means (5) are disposed at each node (3) for selectively storing first data representative of the first measurement signals. A processing center (7) is remote from nodes (3) and selectively obtaining the first data from each memory means (5).

Abstract: When prospecting for geological formations, a feeding electrode 2 is grounded in the centre of a circle formed by uniformly grounded feeding electrodes 3. The feeding electrodes 3 are connected by lengths of supply lines 4 to current generators 1 via regulators 5. A first stage of investigations involves a preliminary sounding. Current pulses are applied from the generators 1 to the lengths of supply lines 4 which are connected to diametrically opposite feeding electrodes 3. A current pulse period is synchronized with a reference signal received by a receiver 7 from the global radio navigation system. Upon turning the current off, a signal of a field magnetic component transient is measured by a meter 8 that is coupled to a magnetic component sensor 9.

Abstract: A method is provided for detecting and locating anomalies in soil subjacent to an existing structure. Electrodes are placed into the soil both outside the structure and through the structure's foundation. Electrical resistivity data is collected using these electrodes. This electrical resistivity data is analyzed to create an electrical resistivity map or model. Inspection of the map or model can allow the operator to detect and locate resistivity anomalies which may correspond to geological anomalies in the soil. In alternative embodiment, one or more soil samples are taken from the soil adjacent to the existing structure. Laboratory resistivity measurements are performed on the soil samples to provide a set of baseline resistivity data. A resistivity map or model is produced as described above. This resistivity map or model is compared to the baseline values provided by the laboratory resistivity measurements of the soil samples to detect and locate resistivity and geological anomalies.

Abstract: A method and apparatus for measuring impedance of grounding system wherein a transient electric current is injected at the ground system under test while measuring the a transient ground potential difference, as well as the injected electric current. The ground potential difference is measured with respect to several points on the earth's surface at small distances from the test system. These measurements are processed by computer software which filters noise, corrects voltage and current transducer errors, and estimates the test system ground potential rise with respect to remote earth. Correction algorithms are used to remove irregularities in the frequency response of the voltage and current transducers and analog filters. The filtered and corrected measurements are used to estimate the ground impedance as a function of frequency. For this purpose, a parameter estimation procedure is used which takes into consideration the geometric arrangement of the voltage and current probes.