WO2002086459A1

WO2002086459A1 - An apparatus and method for wellbore resistivity determination and imaging using capacitive coupling

Info

Publication number: WO2002086459A1
Application number: PCT/US2002/011727
Authority: WO
Inventors: Martin T. Evans; Andrew R. Burt; Albert Alexy; Leonty A. Tabarovsky
Original assignee: Baker Hughes Incorporated
Priority date: 2001-04-18
Filing date: 2002-04-15
Publication date: 2002-10-31
Also published as: EP1390712A4; NO335831B1; GB2392729B; NO20034635L; WO2002086459B1; NO20034635D0; EP1390712A1; GB2392729A; GB0325864D0; CA2444942A1

Abstract

An apparatus for obtaining resistivity parameters of earth formations (7) uses capacitive coupling (5) for injecting measure currents into the formation through a nonconducting mud. In one embodiment, a modulated electrical current is used. Alternatively, multifrequency measurements may be made to obtain the resistivity parameter. In an optional embodiment, the modulation frequency is in the AF range, making it possible to use prior art circuitry designed to reduce cross talk. Measurements may be made either on a wireline or in a MWD configuration.

Description

AN APPARATUS AND METHOD FOR WELLBORE RESISTIVITY DETERMINATION AND IMAGING USING CAPACITIVE COUPLING

Martin Townley Evans, Andrew Richard Burt, Albert Alexy, Leonty Abraham Tabarovsky

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] This invention generally relates to explorations for hydrocarbons involving electrical investigations of a borehole penetrating an earth formation. More specifically, this invention relates to highly localized borehole investigations employing the introduction and measuring of individual survey currents injected into the wall of a borehole by capacitive coupling of electrodes on a tool moved along the borehole within the earth formation.

2. Background of the Art

[0002] Electrical earth borehole logging is well known and various devices and various techniques have been described for this purpose. Broadly speaking, there are two categories of devices used in electrical logging devices. In the first category, a measure electrode (current source or sink) id used in conjunction with a diffuse return electrode (such as the tool body). A measure current flows in a circuit that connects a current source to the measure electrode, through the earth formation to the return electrode and back to the current source in the tool. In inductive measuring tools, an antenna within the measuring instrument induces a current flow within the earth formation. The magnitude of the induced current is detected using either the same antenna or a separate receiver antenna. The present invention belongs to the first category.

[0003] There are several modes of operation that may be used: in one, the current at the measuring electrode is maintained constant and a voltage is measured while in the second mode, the voltage of the electrode is fixed and the current flowing from the electrode is measured. Ideally, it is desirable that if the current is varied to maintain constant the voltage measured at a monitor electrode, the current is inversely proportional to the resistivity of the earth formation being investigated. Conversely, it is desirable that if this current is maintained constant, the voltage measured at a monitor electrode is proportional to the resistivity of the earth formation being investigated. Ohm's law teaches that if both current and voltage vary, the resistivity of the earth formation is proportional to the ratio of the voltage to the current.

[0004] Bir dwell (US Patent 3,365,658) teaches the use of a focused electrode for determination of the resistivity of subsurface formations. A survey current is emitted from a central survey electrode into adjacent earth formations. This survey current is focused into a relatively narrow beam of current outwardly from the borehole by use of a focusing current emitted from nearby focusing electrodes located adjacent the survey electrode and on either side thereof. Ajam et al (US Patent 4,122,387) discloses an apparatus wherein simultaneous logs may be made at different lateral distances through a formation from a borehole by guard electrode systems located on a sonde which is lowered into the borehole by a logging cable. A single oscillator controls the frequency of two formation currents flowing through the formation at the desired different lateral depths from the borehole. The armor of the logging cable acts as the current return for one of the guard electrode systems, and a cable electrode in a cable electrode assembly immediately above the logging sonde acts as the current return for the second guard electrode system. Two embodiments are also disclosed for measuring reference voltages between electrodes in the cable electrode assembly and the guard electrode systems

[0005] Techniques for investigating the earth formation with arrays of measuring electrodes have been proposed. See, for example, the U.S. Pat. No. 2,930,969 to Baker, Canadian Pat. No. 685,727 to Mann et al. U.S. Patent No. 4,468,623 to Gianzero, and U.S. Patent No. 5,502,686 to Dory et al.. The Baker patent proposed a plurality of electrodes, each of which was formed of buttons which are electrically joined by flexible wires with buttons and wires embedded in the surface of a collapsible tube. The Mann patent proposes an array of small electrode buttons either mounted on a tool or a pad and each of which introduces in sequence a separately measurable survey current for an electrical investigation of the earth formation. The electrode buttons are placed in a horizontal plane with circumferential spacings between electrodes and a device for sequentially exciting and measuring a survey current from the electrodes is described.

[0006] The Gianzero patent discloses tool mounted pads, each with a plurality of small measure electrodes from which individually measurable survey currents are injected toward the wall of the borehole. The measure electrodes are arranged in an array in which the measure electrodes are so placed at intervals along at least a circumferential direction (about the borehole axis) as to inject survey currents into the borehole wall segments which overlap with each other to a predetermined extent as the tool is moved along the borehole. The measure electrodes are made small to enable a detailed electrical investigation over a circumferentially contiguous segment of the borehole so as to obtain indications of the stratigraphy of the formation near the borehole wall as well as fractures and their orientations. In one technique, a spatially closed loop array of measure electrodes is provided around a central electrode with the array used to detect the spatial pattern of electrical energy injected by the central electrode. In another embodiment, a linear array of measure electrodes is provided to inject a flow of current into the formation over a circumferentially effectively contiguous segment of the borehole. Discrete portions of the flow of current are separably measurable so as to obtain a plurality of survey signals representative of the current density from the array and from which a detailed electrical picture of a circumferentially continuous segment of the borehole wall can be derived as the tool is moved along the borehole. In another form of an array of measure electrodes, they are arranged in a closed loop, such as a circle, to enable direct measurements of orientations of resistivity of anomalies

[0007] The Dory patent discloses the use of an acoustic sensor in combination with pad mounted electrodes, the use of the acoustic sensors making it possible to fill in the gaps in the image obtained by using pad mounted electrodes due to the fact that in large diameter boreholes, the pads will necessarily not provide a complete coverage of the borehole.

[0008] The prior art devices, being contact devices, are sensitive to the effects of borehole rugosity: the currents flowing from the electrodes depend upon good contact between the electrode and the borehole wall. If the borehole wall is irregular, the contact and the current from the electrodes is irregular, resulting in inaccurate imaging of the borehole. A second drawback is the relatively shallow depth of investigation caused by the use of measure electrodes at the same potential as the pad and the resulting divergence of the measure currents.

[0010] Yet another drawback with the use of contact devices injecting electrical currents into a wellbore arises when oil-based muds are used in drilling. Oil-based muds must be used when drilling through water soluble formations. An increasing number of present day exploration prospects lie beneath salt layers. Besides reducing the electrical contact between the logging tool and the formation, invasion of porous formations by a resistive, oil-based mud greatly reduces the effectiveness of prior art resistivity imaging devices and conduction-based devices for determination of formation resistivities. This problem is not alleviated by the use of focusing electrodes.

[0011] It would be desirable to have an apparatus and method of determination of formation resistivity that is relatively insensitive to borehole rugosity and can be used with either water based or with oil-based muds. The present invention satisfies this need.

SUMMARY OF THE INVENTION

[0012] The present invention is an apparatus and method for use in a borehole having a substantially nonconducting fluid therein for obtaining a resistivity parameter of an earth formation penetrated by the borehole. At least one measure electrode is capacitively coupled to the earth formation through the nonconducting fluid. A measure current is conveyed into the formation, and by measurements of the current in the electrode and its potential, the resistivity may be determined. A plurality of measure electrodes may be used. With an array of measure electrodes, a resistivity image of the formation may be obtained.

[0013] In one embodiment of the invention, the measure current is a modulated electrical current having a carrier frequency selected to have a low impedance due to the capacitive coupling. An isolator is provided on the logging tool to minimize the cross-talk between the current from a current source and the measure signals. Focusing and guard electrodes may be used with the measure electrodes. The logging tool may be conveyed on a wireline or form part of a bottom hole assembly conveyed on a drilling tubular.

[0014] In an MWD embodiment of the invention, numerous options are available for the disposition of the measure electrodes. They may be on a stabilizer, a non-rotating sleeve, or on a pad. An extension device may be provided to maintain the measure electrode at a specified distance from the borehole wall. When direction sensors are provided, a downhole processor can provide resistivity images without the necessity of having arrays of electrodes with a large number of electrodes.

[0015] In an alternate embodiment of the invention, a modification is made to enable the device to be used with water based muds. The measure electrode is capacitively coupled to the source of modulated electrical current.

[0016] The invention also makes a provision for making multifrequency measurements. When such multifrequency measurements are made, frequency focusing may be used to determine formation resistivity. BRIEF DESCRIPTION OF THE FIGURES

[0017] The present invention is best understood by reference to the following figures wherein like numbers refer to like components, and wherein:

Fig. 1 is a circuit diagram representing a formation resistivity device according to the present invention.

Fig.2 shows a comparison of signals representative of the measure current and the voltage for the circuit of Fig. 1 for a 1 kHz sinusoidal excitation signal.

Fig. 3 shows a comparison of signals representative of the measure current and the voltage for the circuit of Fig. 1 for a 10 kHz sinusoidal excitation signal. Fig. 4 shows a comparison of signals representative of the measure current and the voltage for the circuit of Fig. 1 for a 10 kHz square wave excitation.

Fig. 5 (Prior Art) shows a schematic illustration of a prior art imaging tool in a borehole.

Fig. 6 illustrates a model used for deriving the impedance of an imaging tool. Figs. 7a-7f illustrate the impedance of a measure electrode at a frequency of 1 kHz.

Figs. 8a-8f illustrate the impedance of a measure electrode at a frequency of 10 kHz.

Fig. 9 shows the imaging tool of this invention suspended in a borehole.

Fig. 10 is a mechanical schematic view of the imaging tool.

Fig. 10A is a detail view of an electrode pad. Fig. 11 is a schematic circuit diagram showing the principles of operation of the tool.

Figs. 12a and 12b shows a comparison between a prior art modulated signal and a reverse modulated signal according to the present invention.

Fig. 13 is a schematic circuit diagram of the tool when used with a conducting borehole fluid. Fig. 14 illustrates an alternate embodiment of an electrode pad.

Fig. 15 (Prior art) is a schematic illustration of a drilling system.

Fig. 16 is a schematic illustration of the invention in which resistivity measurements are made at various azimuths

Fig. 17 illustrates the pads on a non-rotating sleeve used for resistivity measurements.

DETAILED DESCRIPTION OF THE INVENTION [0018]In order to gain a proper understanding of the present invention, reference is made to Figs 1 - 17. The description of the present invention starts first with a discussion of the concepts of capacjtive coupling used in the present invention. A multifrequency tool is then discussed, followed by a discussion of embodiments of the invention in which resistivity images of the borehole may be obtained.

[0019] Fig. 1 is a circuit diagram illustrating the methodology of formation resistivity measuring devices. A measure electrode depicted by 3 injects a measure current into a formation denoted by 7 having a resistivity R_t. This current is supplied by a current source 1. The current from the formation returns (not shown) through a return electrode (ground) denoted by 7. Typically, a voltage drop 11 across a resistor 10 in the circuit is used as an indication of the measure current. By measuring the voltage drop 13 between the measure electrode and the return electrode, information is derived about the impedance encountered by the current between the measure electrode and the ground.

[0020] This impedance, as noted above, includes the desired formation resistivity R_t In addition, there is also an impedance 5 between the measure electrode 3 and the formation 7. In water based (conductive) muds, this impedance is almost entirely resistive and is caused by the mud cake and any invasion of the borehole fluid into the formation. However, in oil-based (non conductive) muds, the impedance between the measure electrode 3 and the formation 7 is primarily capacitive, denoted by a capacitance M_c . This capacitance manifests itself in a phase shift between the measure current signal and the voltage drop from the measure electrode to ground. This is seen in Fig. 2 which shows a phase shift between the signals 11' and 13' for a sinusoidal current of 1 kHz. This frequency is typical of prior art formation resistivity measurement devices. The curves in Fig. 2 are normalized independently to emphasize the phase shift: in reality, there could be differences of several orders of magnitude between the two signals.

[0021] Turning now to Fig.3, the signals 11" and 13" for a sinusoidal current of 10 kHz are shown. The phase shift between the two signals is seen to be much smaller. This is due to the fact that at the higher frequency of 10 kHz, the effect of the capacitance is less than at 1 kHz. This suggests that by using higher frequencies, it would be possible to get signals indicative of the formation resistivity. This is confirmed in Fig. 4 which shows the signals 11"' and 13"' for a square wave excitation at 10 kHz. As can be seen, both the signals rise and fall almost instantaneously: this is due to the fact that a square wave contains a lot of high frequencies that are essentially unimpeded by the capacitance of the mud. The use of higher frequencies forms the basis for the present invention as described next.

[0022] Fig. 5 is a schematic illustration of a portion of a prior art imaging tool suitable for use with the method of the present invention. Shown is a borehole 51 that is filled with a borehole fluid (drilling mud). A mud-cake 53 is formed between the borehole fluid and the formation 55. The tool comprises one or more measure electrodes 59 carried on a conducting pad 57. In the illustration, only two electrodes are shown. As discussed below, the actual number of electrodes may be much larger and they may be arranged in an array. The electrodes 59 are separated from each other by insulator 61. For simplifying the illustration, additional insulation between the electrodes 59 and the pad 51 is not shown.

[0023] In prior art imaging tools, the pad functions as a guard electrode and is maintained at a potential related to the potential of the measure electrodes. As would be known to those versed in the art, due to the presence of the guard electrode and the current flowing into the formation therefrom, the current from the measure electrodes flows in current paths such as that shown by I and is prevented from diverging due to the focusing current F from the guard electrode. Optionally, additional focusing electrodes may be used (not shown). Focusing electrodes are known in the prior art but a specific embodiment using focused electrodes for a resistivity imaging tool is discussed below. The current flowing from the measure electrode is related to the potential V and the impedance of the electrical circuit in which the measure currents flow. [0024] When a device such as that shown in Fig. 5 is used with a water-based drilling mud, the impedance of the mud and the mudcake is relatively small compared to the impedance of the formation. As would be known to those versed in the art, at the frequencies used in prior art devices, the formation impedance is primarily resistive and from a knowledge of the potential V and the measure current I, the formation resistivity can be derived.

[0025] On the other hand, in oil-base mud, the measured impedance of individual measure electrodes severely depends on the mud cake parameters. In addition, an oil film on the pad surface may completely eliminate the electrical contact between pad and formation.

[0026] The size of a measure electrode is associated with the tool spatial resolution. Usually, the measure electrode radius is in the range of 1 to 2 mm that creates a very large ground resistance. For example, a 2 mm measure electrode on a typical pad device has the ground resistance of 10,000 Ω in a 1 Ω-m formation or 10 M Ω in a 1,000 Ω-m formation. This illustrates the technical challenge of producing a high definition image in a resistive environment

[0027] There are several possible ways to overcome the physical limitation of DC imaging in oil-base mud. One approach that has been used is to change composition of oil-base mud to increase the mud cake conductivity. The present invention relies on increasing the frequency to produce capacitive coupling between pad and formation.

[0028] Turning now to Fig. 6, the impedance of the measure electrode is derived. We consider a model consisting of two conductive layers 103, 105 enclosed between insulating half-space at the top 101 and a perfect conductor at the bottom 107. From the upper boundary, a uniform current is injected with the surface density, J_s. A measure electrode of any shape may be studied by cutting out an appropriate area 109 from the injection plane. The upper half-space 101 represents a borehole filled with oil-base mud. The conductor 107 at the bottom is a current sink. In reality, at a certain distance, depending on the focusing conditions, current lines diverge. This provides a finite value for the measure electrode's K-factor. To simplify modeling, we introduce a. parallel current flow. We can change the K-factor by placing the current return (perfect conductor) at different distances from the borehole. It is well known that the K-factor of a cylindrical volume with a cross section, S, and length, L, is defined by the following equation:

where S(l) is the cross-sectional area at a distance / along the current path.

[0029] The mud cake 103 is characterized by a conductivity σ„ permittivity e, and thickness h,. Similarly, the formation 105 is characterized by a conductivity σ₂, permittivity €₂ and thickness h₂. The complex conductivities of the mudcake and formation are given by

V = σ_x + iωε_x (2) and v₂ = σ₂ + iωε₂ (3) respectively, where ω- 2 τtf(f being the frequency).

[0030] Denoting by E, and E₂ the electric field in the mud cake and the formation and by Fthe potential difference between the measure electrode and the current return (ground on Fig. 1), the following equations result: J_B = Vj Ej S (current injected through the electrode) (continuity of current) and Ej h, +E₂ h₂ =V (overall voltage).

This gives

[0031] Introducing the electrode impedance, we finally obtain

The first term on the right hand side in eq. (5) represents the impedance of the mud cake while the second term represents the impedance of the formation. At low frequencies (ω -» 0), the measured impedance depends primarily on the mud cake conductivity and the formation conductivity, i.e., it does not depend upon the dielectric constant of the mud cake and the formation. However, if the mud is oil based ( mud cake is resistive), then the measured impedance may become so large that it would be virtually impossible to inject any current into the formation.

[0032] Eq. (5) indicates that we can reduce the mud cake impedance by increasing the frequency ω. This can be done by selecting the frequency such that ωε_x » σ (6)

While reducing the mud cake impedance, we must also maintain the frequency such that the second term in eq. (5) depends mostly oh the formation conductivity σ₂. This leads to the condition ωε₂ « σ₂ (7).

Combining eqs. (6) and (7) gives the results

σ_x σ

— « ω « — (8). ε_x ε₂

In an oil-based mud, both inequalities in eq. (6) must be satisfied because σ_x « σ Under these conditions, eq. (5) may be written in the form

Eq. (9) can be written in the form

Z = 9t (Z) + 3(Z) (10) where 9.(Z) and S§ (Z) are the real and imaginary (inphase and quadrature) parts of the impedance given by

[0033] The following points may be noted about eq. (11) ( the real part of the impedance):

1. The first term depends on formation conductivity and does not include dielectric permittivity. It exactly represents the resistivity reading in the absence of mud cake. 2. The second term contains only mud cake properties. Importantly, it is inversely proportional to the second power of the frequency.

3. The second term may be eliminated in two different ways. The first way is to use a high frequency. The second way to eliminate the second term is by combining measurements are two different frequencies. This is given by the following equation:

[0034] Turning now to eq. (12), the quadrature (out of phase) component of the impedance, the following points may be noted.

1. With the frequency increase, the formation contribution (the second term) becomes more significant.

2. While dominating, the formation signal retains dependence on the formation dielectric constant. This introduces undesirable uncertainty in the process of interpretation.

3. Due to eq. (8) the out of phase component is typically small compared to the in phase component.

[0035] The points noted above are brought out in Figs. 7 - 8 which show exact relationships derived from eq. (5). Calculations were done for an electrode radius of 2 mm, K factor of 12,000m^"1 , and a relative dielectric constant of 10 for both the mud and the formation. The relative dielectric constant is the ratio of the permittivity of a medium to that of free space.

[0036] Referring now to Fig. 7a, the abscissa is the formation resistivity in Ωm and the ordinate is the 9t(Z). Values are plotted for a frequency of 1 kHz. Three curves are shown for mud cake resistivities of 10 kΩm, 100 kΩm and 1000 kΩm and a mud cake thickness of 0.1 mm.. As can be seen, the 3t(Z) depends not only on the formation resistivity but also on the resistivity of the mud cake.

[0037] Fig. 7b is similar to Fig. 7a except that the mud cake thickness is 0.5 mm. Differences between Fig. 7b and Fig. 7a show that the 9. (Z) is also dependent upon the mud cake thickness. Fig. 7c is a plot of the absolute value of the electrode impedance for a mud cake thickness of 0.1 mm.

[0038] Turning now to Fig. 7d, a plot of the dual frequency impedance determined by eq. (13) for a mud cake thickness of 0.1 mm is shown. The dual frequency values were obtained using measurements at 1 kHz and 2 kHz respectively. Fig. 7e shows the results of dual frequency measurements for a mud cake thickness of 0.2 mm. Finally, Fig. 7f shows a plot of the ratio of 9.(Z) to S (Z).

[0039] In summary, Figs. 7a - 7f explain why measurements made by conventional resistivity imaging tools do not work with oil based muds: the measured impedance for audio frequency signals depends on many factors other than the formation resistivity.

[0040] Turning now to Figs. 8a - 8f, a completely different picture emerges. The figures are similar to Figs. 7a - 7f with the significant difference that the operating frequency is now 1 MHz (compared to 1 kHz in Figs. 7a - 7f). For a relatively thin mud cake (Fig. 8a), the 9t(Z) is primarily dependent upon the formation resistivity except for extremely conductive formations where some dependence upon the mud cake resistivity is noted. The effect is more noticeable for a thicker mud cake (0.5 mm in Fig.8b). The amplitude of the impedance (Fig. 8c) shows little variation with mud cake resistivity but does exhibit a nonlinear dependence upon the formation resistivity. The dual frequency measurements (Fig. 8d, 8e) show that the measured impedance is substantially independent of mud cake thickness and resistivity and further exhibits the desirable property of being linearly related to the formation resistivity. The present invention takes advantage of the fact that at high frequencies (≥ 1MHz or so), the effect of the mud cake and the mud impedance may be ignored for all practical purposes.

[0041] The dual frequency solution given by eq. (13) is a special case of multifrequency focusing. In an alternate embodiment of the invention, measurements are made at a plurality of frequencies ω, , ω₂ , ω₃ . . . . ω_m . As disclosed in United States Patent 5,703,773 to Tabarovsky et al., the contents of which are fully incorporated herein by reference, the response at multiple frequencies may be approximated by a Taylor series expansion of the form:

In a preferred embodiment of the invention of the number m of frequencies CO is ten. Using the measurements at the m frequencies, the quantities s₀ , s_1/2 , s_3/2 are determined. In eq.(12), n is the number of terms in the Taylor series expansion. This can be any number less than or equal to m. The coefficient s₃₂ of the ω^3a term (ω being the square of A:, the wave number) is generated by the primary field and is relatively unaffected by any inhomogeneities in the medium surround the logging instrument, i.e., it is responsive primarily to the formation parameters and not to the borehole and invasion zone. In fact, the coefficient s₃₂ of the ω^3/2 term is responsive to the formation parameters as though there were no borehole in the formation. This frequency focusing method has been shown to give reliably consistent results even when there is a significant invasion of the formation by borehole fluids. In one embodiment of the invention, a processor controls the signal generator to provide a measure current at a plurality of frequencies. The processor then performs a frequency focusing of the apparent conductivity at the plurality of frequencies to obtain the coefficients s_3/2. This is then used as an estimate of the formation conductivity.

[0042]Turning now to embodiments of the present invention suitable for resistivity imaging, Fig. 9 shows an imaging tool 110 suspended in a borehole 112, that penetrates earth formations such as 113, from a suitable cable 114 that passes over a sheave 116 mounted on drilling rig 118. By industry standard, the cable 114 includes a stress member and seven conductors for transmitting commands to the tool and for receiving data back from the tool as well as power for the tool. The tool 110 is raised and lowered by draw works 120. Electronic module 122, on the surface 123, transmits the required operating commands downhole and in return, receives data back which may be recorded on an archival storage medium of any desired type for concurrent or later processing. The data may be transmitted in analog or digital form. Data processors such as a suitable computer 124, may be provided for performing data analysis in the field in real time or the recorded data may be sent to a processing center or both for post processing of the data.

[0043]Figs. 10a and 10b are schematic external views of a borehole sidewall imager system. The tool 110 comprising the imager system includes resistivity arrays 126 and, optionally, a mud cell 130 and a circumferential acoustic televiewer 132. Electronics modules 128 and 138 may be located at suitable locations in the system and not necessarily in the locations indicated. The components may be mounted on a mandrel 134 in a conventional well known manner. The outer diameter of the assembly is about 5 inches and about fifteen feet long. An orientation module 136 including a magnetometer and an accelerometer or inertial guidance system may be mounted above the imaging assemblies 126 and 132. The upper portion 138 of the tool 110 contains a telemetry module for sampling, digitizing and transmission of the data samples from the various components uphole to surface electronics 122 in a conventional manner. If acoustic data are acquired, they are preferably digitized, although in an alternate arrangement, the data may be retained in analog form for transmission to the surface where it is later digitized by surface electronics 122.

[0044]Also shown in Fig. 10a are three resistivity arrays 126 (a fourth array is hidden in this view^ Referring to Figs. 10a and 10b, each array includes measure electrodes

141a, 141b, . . . 141n for injecting electrical currents into the formation, focusing electrodes 143a, 143b for horizontal focusing of the electrical currents from the measure electrodes and focusing electrodes 145a, 145b for vertical focusing of the electrical currents from the measure electrodes. By convention, "vertical" refers to the direction along the axis of the borehole and "horizontal" refers to a plane' perpendicular to the ve_tical.

[0045] In a preferred embodiment of the invention, the measure electrodes are rectangular in shape and oriented with the long dimension of the rectangle parallel to the tool axis. Other electrode configurations are discussed below with reference to Fig. 14. For the purpose of simplifying the illustration, insulation around the measure electrodes and focusing electrodes for electrically isolating them from the body of the tool are not shown.

[0046] Other embodiments of the invention may be used in measurement- while- drilling (MWD), logging-while-drilling (LWD) or logging- while-tripping (LWT) operations. The sensor assembly may be used on a substantially non-rotating pad as taught in U.S. Patent 6,173,793 to Thompson et al., having the same assignee as the present application and the contents of which are fully incorporated herein by reference. he sensor assembly of the present invention may also be used with rotating sensors as described in Thompson. These embodiments are discussed below with reference to Figs. 15 - 17. The sensor assembly may also be used on a non- rotating sleeve such as that disclosed in United States Patent 6,247,542 to Kruspe et al, the contents of which are fully incorporated here by reference.

[0047] For a 5" diameter assembly, each pad can be no more than about 4.0 inches wide. The pads are secured to extendable arms such as 142. Hydraulic or spring- loaded caliper-arm actuators (not shown) of any well-known type extend the pads and their electrodes against the borehole sidewall for resistivity measurements. In addition, the extendable caliper arms 142 provide the actual measurement of the borehole diameter as is well known in the art. Using time-division multiplexing, the voltage drop and current flow is measured between a common electrode on the tool and the respective electrodes on each array to furnish a measure of the resistivity of the sidewall (or its inverse, conductivity) as a function of azimuth.

[0048] Turning now to Fig. 11, a circuit diagram showing the principles of operation of the tool is given. A source of electrical power 151 produces an electrical current that is provided to the measure electrodes. In one embodiment of the invention, the apparatus is intended for use with oil based drilling mud and the capacitor 157 depicts the capacitive coupling between a measure electrode such as 141a in Fig. 10b and the formation 113 in Fig. 9. The electrical current flows through the formation that has an equivalent impedance of Z_f and returns to the current source 151 through an equivalent capacitor 159 representing the coupling between the formation and the diffuse return electrode, typically the body of the tool. The measurement of the voltage drop across a resistor 153 is used as an indication of the current flowing to a measure electrode. Other methods for measurement of the current in the measure electrode may also be used. Such methods would be known to those versed in the art and are not discussed here. In a preferred embodiment of the invention, the value of the resistor 153 is Ik Ω. The impedance of the rest of the return path in the body of the tool can be ignored.

[0049] Still referring to Fig. 11, a voltage detector 161 measures the voltage difference between the measure electrode and the diffuse return electrode and controls the current at the current generator to maintain a constant voltage. In this case, the output of the current measuring circuit serves as a measure signal. Alternatively (not shown), the output of the current measuring circuit 155 is used to maintain a constant current and the output of the voltage detector is used as a measure signal. As still another alternative, both the voltage detected by the voltage detector 161 and the current measured by the current measuring circuit 155 are used as measure signals.

[0050] Selection of the size of the measure electrode and the operating frequency is based upon several considerations. One important consideration is that the impedance of the formation must be substantially resistive at the operating frequency so that the currents in the measure electrode are indicative of the formation resistivity and substantially unaffected by its dielectric constant. Based upon typical values of formation dielectric constant such as that disclosed in United States Patent 5,811,973 issued to Meyer et al, the operating frequency should be less than 4 MHz. As mentioned above, a preferred embodiment of the present invention uses a measuring current at a frequency of 1MHz. A second consideration is that the impedance (i.e., resistance) of the formation be greater than the impedance of the rest of the circuit of Fig. 11. Another consideration is the desired resolution of the tool. A reasonable resolution for a useful imaging tool is approximately 3 mm. in the horizontal and vertical directions.

[0051] The impedance of the equivalent capacitance 159 and the body of the tool may be ignored at 1MHz since the equivalent capacitor has an enormous area comparable to the size of the tool. The capacitance of 157 is a function of the dielectric constant of the borehole fluid, the area of the electrode, and the stand-off between the electrode and the borehole wall. Formation resistivities encountered in practice may range between 0.2 Ω-m and 20,000 Ω-m. As noted above and discussed below, the present invention makes use of focusing electrodes so that, in general, the effective dimensions of the formation that are sampled by an electrode are less than the actual physical size of the electrodes. Based upon these considerations, and the requirement that a plurality of electrodes must fit on a single pad, in a preferred embodiment of the invention as shown in Figs 10a, 10b, the individual measure electrodes 141a, 141b . . . 141n have a width of 8 mm. and a length of between 20 - 30 mm. This makes it possible to have eight electrodes on a single pad. The corresponding value of the capacitance 157 is then typically between lpF and 100 pF. At the lower value, the impedance of the capacitance 107 at 1MHz is approximately 160 kΩ and at the higher value approximately 1.6kΩ.

[0052] In the present device, the focusing electrodes 145a, 145b are of some importance as they perform a significant amount of focusing. Denoting by Fthe potential of the measure electrodes 141a, 141b. . . the electrodes 145a, 145b are maintained at a potential of V + δ. The body of the pad is maintained at a voltage V± e. The pad functions as a guard electrode and prevents divergence of the measure current until the current has penetrated some distance into the formation. This makes it possible to get deeper readings. A typical value of the voltage is 5 volts while typical value of <5and c are 500 μV and 100 μV, with € being less than δ. Since little focusing is needed in the horizontal direction, the side focusing electrodes 143a, 143b are maintained at substantially N volts. Those versed in the art would recognize that the device could also function if all the voltages were reversed, in which case, the voltages mentioned above as typical values would be magnitudes of voltages.

[0053] With the potentials of the measure electrodes, the focusing electrodes and the pads as discussed above, the current from the current source 151 in Fig. 11 will be focused down to square blocks approximately 8 mm. on the side. The operating frequency of the present device is typically 1 MHz, compared to an operating frequency of 1.1 kHz for the device of the ' 431 application.

[0054]Those versed in the art would recognize that a considerable amount of crosstalk would normally be generated between the current flowing to the measure electrodes from the electronics module 138 and the measure signal(s) returning from the measure electrodes carrying information about the voltages and/or currents of the electrodes. The measuring electrodes are preferably isolated from the electronics module by an isolator section such as 137 that is preferably between 2'6" and 15' long. Cross-talk between conductors (not shown) over such distances would be quite large at an operating frequency of 1MHz would overwhelm the measure signal(s) indicative of the formation resistivity.

[0055] This problem is addressed in the present invention by modulating the current output of the generator at 1.1 kHz. The result is that the current traveling down conductors in the isolator section and into the formation is a 1MHz current modulated at 1.1 kHz. A demodulator (not shown) is provided in the voltage measuring circuit so that the return signal to the electronics module 138 is a 1.1kHz signal. This makes it possible to use substantially the same hardware configuration as in prior art devices designed to substantially attenuate the cross-talk.

[0056]To further reduce the effects of cross-talk, instead of conventional amplitude modulation of the currents, an inverse modulation is used. Conventional amplitude modulation is given by a current i(t)

i(t) = Cos(ω_mt)Cos(ω_ct) ₍₁₎ where ύ)_m is the modulating signal frequency (1.1 kHz) and ω_c is the carrier frequency (1MHz). The inverse modulation of the present invention uses a modulation of the form

.ι( = (l - aCos(ω ))Cos(ω_ct) (2) where a is small compared to 1. The result is that the current output of the generator 151 is substantially at 1MHz with an amplitude close to unity at all times. This makes the cross-talk substantially independent of the magnitude of the measure current. Substantially the same result may be obtained in alternate embodiments of the invention by using frequency or phase modulation of the 1 MHz carrier signal.

[0057] Figs. 12a and 12b show a comparison between a prior art modulated signal and a reverse modulated signal according to the present invention. A carrier signal

161 having a carrier frequency has its amplitude modulated by a lower frequency modulating signal 143. As can be seen, the level of amplitude of the modulated signal goes to zero whenever the modulating signal goes to zero at times such as 165. A reverse modulated signal is shown in Fig. 12b with a carrier signal 161' and a modulating signal 163'. This modulated signal always has a significant current flowing. The advantage of using such a reverse modulated signal is that the cross talk is substantially unaffected by the level of the modulating signal.

[0058] In an alternate embodiment of the invention, the measure signal(s) is sent through an optical fiber. When an optical fiber is used for the purpose, there will not be any cross talk between the current conveyed through the isolator section and the measure signal. Modulation of the current is then not necessary.

[0059] In an alternate embodiment of the invention, the principles described above are used when the measure electrodes are not part of an array of electrodes. With a single electrode, measurements indicative of the resistivity of the formation may be obtained. With a plurality of azimuthally distributed electrodes, such output measurements may be processed using prior art methods, such as those used in dipmeters, to obtain information relating to the dip of formations relative to the borehole. When combined with measurements of the borehole orientation and tool face orientation, such relative dip information may be further processed to give estimates of absolute dip of the formations.

[0060] Another embodiment of the present invention may be used with water based muds. The equivalent circuit for this embodiment is shown in Fig. 13. It is identical to Fig. 11 except that the gap between the measure electrode and the formation is a conductive gap denoted by the points 209 - 211 and a return gap denoted by 219-221. An additional capacitor 207 may be incorporated into the circuit. The operation of the device is substantially unchanged from that used for non-conducting muds. The conductive paths through the mud shunts any effect of the capacitance of the tool stand-off.

[0061] Such an arrangement has been used in the past with contact electrodes for resistivity measurements or resistivity imagers. The function of an internal capacitor in such prior art circuits has been solely for the purpose of blocking any extraneous currents emanating from sources external to the measure circuit from entering the amplifiers and distorting the operation of such prior art apparatus. Other methods have also been used for compensating for such extraneous currents. However, the particular embodiment utilizing an external capacitor constructed from instrument electrode plate, conductive earth formation plate and drilling mud dielectric, with high frequency, modulated measure currents such as are used in the present invention and depicted in Fig. 13 have not previously been used.

[0062] The resolution of the devices disclosed above is substantially equal to the dimensions of the focused current at a depth where the current from the measure electrode has the smallest dimensions. Those versed in the art would recognize that if lower resolution is acceptable, the focusing electrodes may be eliminated. In such a device, the beam of measure current is only guarded or constrained to flow substantially outward from the surface of the measure electrode, as in prior art non- focused conductive mud devices, by the pad (or guard electrode) being maintained at substantially the same voltage as the measure electrode.

[0063] Alternatively, other configurations of the electrodes on a measuring pad may also be used. Fig. 14 shows an arrangement in which five circular measure electrodes 303a, 303b . . . 303e are located on a pad 301. Each measure electrode is surrounded by an associated focusing electrode 305a, 305b . . . 305e with insulation 307a, 307b. .

. 307e therebetween. For simplifying the illustration, the insulation between the guard electrodes and the pad 301 is not shown.

[0064]The invention has further been described by reference to logging tools that are intended to be conveyed on a wireline. However, the method of the present invention may also be used with measurement-while-drilling (MWD) tools, or logging while drilling (LWD) tools, either of which may be conveyed on a drillstring or on coiled tubing.

[0065] FIG. 15 shows a schematic diagram of a drilling system 410 having a drilling assembly 490 shown conveyed in a borehole 426 for drilling the wellbore. The drilling system 410 includes a conventional derrick 411 erected on a floor 412 which supports a rotary table 414 that is rotated by a prime mover such as an electric motor (not shown) at a desired rotational speed. The drill string 420 includes a drill pipe 422 extending downward from the rotary table 414 into the borehole 426.. The drill bit 450 attached to the end of the drill string breaks up the geological formations when it is rotated to drill the borehole 426. The drill string 420 is coupled to a drawworks 430 via a Kelly joint 421, swivel, 428 and line 429 through a pulley 423. During drilling operations, the drawworks 430 is operated to control the weight on bit, which is an important parameter that affects the rate of penetration. The operation of the drawworks is well known in the art and is thus not described in detail herein.

[0066] During drilling operations, a suitable drilling fluid 431 from a mud pit (source) 432 is circulated under pressure through the drill string by a mud pump 434. The drilling fluid passes from the mud pump 434 into the drill string 420 via a desurger 436, fluid line 328 and Kelly joint 421. The drilling fluid 431 is discharged at the borehole bottom 451 through an opening in the drill bit 450. The drilling fluid 431 circulates uphole through the annular space 427 between the drill string 420 and the borehole 426 and returns to the mud pit 432 via a return line 435. A sensor S_t preferably placed in the line 438 provides information about the fluid flow rate. A surface torque sensor S₂ and a sensor S₃ associated with the drill string 420 respectively provide information about the torque and rotational speed of the drill string. Additionally, a sensor (not shown) associated with line 429 is used to provide the hook load of the drill string 420.

[0067] In one embodiment of the invention, the drill bit 450 is rotated by only rotating the drill pipe 452. In another embodiment of the invention, a downhole motor 455 (mud motor) is disposed in the drilling assembly 490 to rotate the drill bit 450 and the drill pipe 422 is rotated usually to supplement the rotational power, if required, and to effect changes in the drilling direction.

[0068] In the embodiment of FIG. 15, the mud motor 455 is coupled to the drill bit 450 via a drive shaft (not shown) disposed in a bearing assembly 457. The mud motor rotates the drill bit 450 when the drilling fluid 431 passes through the mud motor 455 under pressure. The bearing assembly 457 supports the radial and axial forces of the drill bit. A stabilizer 458 coupled to the bearing assembly 457 acts as a centralizer for the lowermost portion of the mud motor assembly. [0069] In one embodiment of the invention, a drilling sensor module 459 is placed near the drill' bit 450. The drilling sensor module contains sensors, circuitry and processing software and algorithms relating to the dynamic drilling parameters. Such parameters preferably include bit bounce, stick-slip of the drilling assembly, backward rotation, torque, shocks, borehole and annulus pressure, acceleration measurements and other measurements of the drill bit condition. The drilling sensor module processes the sensor information and transmits it to the surface control unit 440 via a suitable telemetry system 472.

[0070] Fig; 16 shows an embodiment of the invention in which sensors mounted on stabilizers of a drilling assembly are used to determine the resistivity of the formation. One or more of the stabilizers 1033 is provided with a recess 1035 into which a sensor module 1054 is set. Each sensor module 1054 has one or more measure electrodes 1056 for injecting measure currents into the formation as described above. As discussed above, the body of the sensor module is maintained at approximately the same potential as the measure electrode to operate as a guard electrode. Optionally, focusing electrodes may be provided as discussed above.

[0071] In a measurements while drilling environment, there is usually a small gap between the stabilizer and the borehole wall (not shown): the diameter of the drill bit

(not shown) conveyed on the drilling tubular 1040 is greater than the outer diameter as defined by the stabilizers. The operation of the stabilizers would be known to those versed in the art and is not described further here. When used with a nonconducting fluid in the borehole, the gap defines the capacitance 107 discussed above". If necessary, extendable arms (not shown) may be provided to keep the gap within acceptable limits. When used with a conducting borehole fluid, the size of the gap is not critical. An electronics module 1052 at a suitable location is provided for processing the data acquired by the sensors 1056.

[0072] FIG. 17 illustrates the arrangement of the sensor pads on a non-rotating sleeve. This is similar to an arrangement of sensors taught by Thompson though other configurations could also be used. Shown are the drilling tubular 1260 with a non-rotating sleeve 1262 mounted thereon. Pads 1264 with one or more measure electrodes 1301 are attached to sleeve 1262. The mechanism for moving the pads out to contact the,,borehole, whether it be hydraulic, a spring mechanism or another mechanism is not shown. The shaft 1260 is provided with stabilizer ribs 1303 for controlling the direction of drilling.

[0073] Data may be acquired using the configuration of either Fig. 16 or Fig. 17 while the well is being drilled and the drillstring and the measure electrodes thereon are rotating. In a MWD environment, telemetry capability is extremely limited and accordingly, much of the processing is done downhole. Processing of the data in the present invention is accomplished using the methodology taught in Thompson et al. The resistivity measurements are made concurrently with measurements made by an orientation^ensor (not shown) on the drilling assembly. As the resistivity sensor rotates in the borehole while it is moved along with the drill bit, it traces out a spiral path with known depths and azimuths. The depths are determined either from data telemetered from the surface or by using at least two axially space apart measure electrodes to give a rate of penetration. In one embodiment of the invention, the downhole^δprocessor uses the depth information from downhole telemetry and sums all the data within a specified depth and azimuth sampling interval to improve the S/N ratio and to reduce the amount of data to be stored. A typical depth sampling interval would be one inch and a typical azimuthal sampling interval is 15° . Another method of reducing the amount of data stored would be to discard redundant samples within the deptl_5and azimuth sampling interval. Further details of the processing method may be found in the teachings of Thompson et al.

[0074] While the foregoing disclosure is directed to the preferred embodiments of the invention, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope and spirit of the appended claims be embraced by the foregoing disclosure.

Claims

1. An apparatus for use in a borehole for obtaining a resistivity parameter of an earth formation penetrated by the borehole, the borehole having a substantially nonconducting fluid therein, the apparatus comprising: (a) - at least one measure electrode capacitively coupled to the earth formation through said nonconducting fluid, said at least one measure electrode conveying a measure current into the formation; and (b) a device responsive to least one of (i) the current in the at least one measure electrode, and, (ii) a voltage of the at least one measure electrode for producing a measure signal representative of the resistivity parameter.

2. The apparatus of claim 1 wherein the at least one measure electrode further comprises a plurality of measure electrodes and wherein said resistivity parameter further comprises a resistivity image.

3. The apparatus of claim 1 wherein the at least one measure electrode further comprises an array of measure electrodes.

4. The apparatus of claim 1 wherein said measure current comprises a modulated current provided from a source thereof, the apparatus further comprising a demodulator for demodulating said measure signal and producing a demodulated signal therefrom, and an isolator section for isolating the at least one measure electrode from the source of the measure current.

5. The apparatus of claim 1 further comprising at least one focusing electrode in proximity to the at least one measure electrode, said at least one focusing electrode focusing the measure current into the formation.

6. The apparatus of claim 5 further comprising a guard device for maintaining focusing of said measure current in the formation.

7. The apparatus of claim 1 wherein said apparatus is conveyed on a wireline.

8. The apparatus of claim 1 wherein said apparatus is a Measurement- while- drilling (MWD) apparatus that is part of a bottom hole assembly (BHA) conveyed on a drilling tubular.

9. The MWD apparatus of claim 8 wherein the at least one measure electrode is carried on a body of a sensor module deployed in a recess on a stabilizer of the BHA.

10. The MWD apparatus of claim 9 wherein the at least one measure electrode comprises at least two measure electrodes spaced apart in an axial direction of the BHA.

11. The MWD apparatus of claim 9 wherein the at least one measure electrode is electrically isolated from said body of the sensor module and wherein said body is maintained at substantially the same potential as the at least one measure electrode.

12. The MWD apparatus of claim 8 further comprising an extension device for moving the at least one measure electrode to maintain a specified distance between the at least one measure electrode and a wall of the borehole.

13. The MWD apparatus of claim 8 further comprising: (i) a telemetry device for receiving depth information from an uphole controller, (ϋ) a directional sensor for making measurements related to the orientation of the at least one measure electrode, (iii) a processor for determining the resistivity parameter from the measurements made by the directional sensor, the depth information, and said measure signal.

14. The MWD apparatus of claim 8 wherein said measure current is a modulated current, the apparatus further comprising a source of a modulated electrical current coupled to said at least one measure electrode.

15. The MWD apparatus of claim 14 wherein the modulated electrical current has a carrier frequency and a modulating frequency substantially less than said carrier frequency.

16. The MWD apparatus of claim 15 further comprising an isolator section between the current source and the at least one measure electrode, said isolator section including conductors carrying said modulated current to the measure electrode and said demodulated measure signal from the at least one measure electrode.

17. The apparatus of claim 1 wherein said measure current has a frequency selected to make an impedance caused by a dielectric constant of the substantially nonconducting fluid to be substantially less than a resistivity of said nonconductive fluid.

18. The apparatus of claim 17 wherein said at least one measure electrode is conveyed on and isolated from a conducting pad, said conducting pad being maintained at a potential sufficient to maintain focusing of said measure current into the formation.

19. The apparatus of claim 17 wherein the at least one measure electrode comprises an array of measure electrodes.

20. The apparatus of claim 17 wherein said frequency is further selected to make an impedance caused by a dielectric constant of said formation substantially less than a resistivity of said formation.

21. The apparatus of claim 17 further comprising a processor for determining from said measure signal said resistivity parameter of the formation.

22. The apparatus of claim 17 further comprising at least one of (i) a wireline, and, (ii) a drilling tubular, for conveying said downhole tool into the borehole.

23. The apparatus of claim 6 wherein the at least one measure electrode is maintained at a first electrical potential, the at least one focusing electrode is maintained at a second potential having a magnitude greater than the magnitude of the first potential and the guard device is maintained at a third electrical potential substantially equal to the first electrical potential.

24. An apparatus for use in a borehole for obtaining a resistivity parameter of an earth formation penetrated by the borehole, the apparatus comprising: (a) at least one measure electrode capacitively coupled to a source of a modulated electrical current, said at least one measure electrode conveying a measure current into the formation; and (b) a device responsive to least one of (i) the current in the at least one measure electrode, and, (ii) a voltage of the at least one measure electrode for producing a measure signal representative of the resistivity parameter

25. The apparatus of claim 24 wherein the at least one measure electrode further comprises a plurality of measure electrodes and wherein said resistivity parameter further comprises a resistivity image.

26. The apparatus of claim 24 wherein the at least one measure electrode further comprises an array of measure electrodes.

27. The apparatus of claim 24 further comprising a demodulator for demodulating said measure signal and producing a demodulated signal therefrom, and an isolator section for isolating the measure electrode from the source of the measure current.

28. The apparatus of claim 24 further comprising at least one focusing electrode in proximity to the at least one measure electrode, said at least one focusing electrode focusing the measure current into the formation.

29. The apparatus of claim 24 further comprising a guard device for maintaining focusing of said measure current in the formation.

30. A method of obtaining a resistivity parameter of a n earth formation penetrated by a borehole having a substantially nonconducting fluid therein, the method comprising: (a) using at least one measure electrode capacitively coupled to the earth formation through said nonconducting fluid for conveying a measure current into the formation; and (b) using a device responsive to least one of (i) the current in the at least one measure electrode, and, (ii) a voltage of the at least one measure electrode for producing a measure signal representative of the resistivity parameter.

31. The method of claim 30 wherein the at least one measure electrode further comprises a plurality of measure electrodes and wherein said resistivity parameter further comprises a resistivity image.

32. The method of claim 30 wherein the at least one measure electrode further comprises an array of measure electrodes.

33. The method of claim 30 wherein said measure current comprises a modulated current, the method further comprising: (i) using a demodulator for demodulating said measure signal and producing a demodulated signal therefrom, and (ii) using an isolator section for isolating the at least one measure electrode from a source of the measure current.

34. The method of claim 30 further comprising using at least one focusing electrode in proximity to the at least one measure electrode for focusing the measure current into the formation.

35. The method of claim 34 further comprising using a guard device for maintaining focusing of said measure current in the formation.

36. The method of claim 30 further comprising conveying said at least one measure electrode into the borehole on a wireline.

37. The method of claim 30 further conveying said at least one measure electrode into the borehole on a bottom hole assembly (BHA) conveyed on a drilling tubular.

38. The method of claim 37 wherein the at least one measure electrode is carried on the body of a sensor module deployed in a recess on a stabilizer of the BHA.

39. The method of claim 37 wherein the at least one measure electrode comprises at least two measure electrodes spaced apart in an axial direction of the BHA.

40. The method of claim 38 wherein the at least one measure electrode is electrically isolated from said body of the sensor module and wherein said body is maintained at substantially the same potential as the at least one measure electrode.

41. The method of claim 38 further comprising using an extension device for moving the at least one measure electrode for maintaining a specified distance between the at least one measure electrode and a wall of the borehole.

42. The method of claim 38 further comprising: (i) using a telemetry device on the BHA for receiving depth information from an uphole controller, (ii) using a directional sensor on the BHA for making measurements related to the orientation of the at least one measure electrode, (iii) using a processor for determining the resistivity parameter from the measurements made by the directional sensor, the depth information, and said measure signal.

43. The method of claim 38 wherein said measure current is a modulated current, the method further comprising operatively coupling a source of the modulated electrical current to said at least one measure electrode.

44. The method of claim 43 wherein the modulated electrical current has a carrier frequency and a modulating frequency substantially less than said carrier frequency.

45. The method of claim 44 further comprising using an isolator section between the current source and the at least one measure electrode for isolating the at least one measure electrode from the source of the modulated current.

46. The method of claim 30 further comprising selecting a frequency for said measure current for making an impedance caused by a dielectric constant of the substantially nonconducting fluid to be substantially less than a resistivity of said nonconductive fluid.

47. The method of claim 46 wherein said at least one measure electrode is conveyed on and isolated from a conducting pad, said conducting pad being maintained at a potential sufficient to maintain focusing of said measure current into the formation.

48. The method of claim 46 wherein the at least one measure electrode comprises an array of measure electrodes.

49. The method of claim 46 further comprising selecting the frequency for making an impedance caused by a dielectric constant of said formation substantially less than a resistivity of said formation.

50. The method of claim 46 further comprising a processor for determining from said measure signal said resistivity parameter of the formation.

51. The method of claim 30 further comprising using one of (i) a wireline, and, (ii) a drilling tubular, for conveying said at least one measure electrode into the borehole.

52. The method of claim 46 further comprising: (i) repeating steps (a) and (b) of claim 30 at at least one additional frequency, and (ϋ) frequency focusing apparent conductivities derived from said measured signals at said frequency and said at least one additional frequency.

53. The method of claim 52 wherein said frequency focusing further comprises representing each measured signal by a Taylor series expansion.

54. The method of claim 53 wherein said resistivity parameter is related to a coefficient of an (Jⁿ term in said Taylor series expansion.

55. The method of claim 35 wherein the at least one measure electrode is maintained at a first electrical potential, the at least one focusing electrode is maintained at a second potential having a magnitude greater than the magnitude of the first potential and the guard device is maintained at a third electrical potential substantially equal to the first electrical potential.

56. A method of obtaining a resistivity parameter of an earth formation penetrated by a borehole, the method comprising: (a) using at least one measure electrode capacitively coupled to a source of a modulated electrical current for conveying a measure current into the formation; and (b) using a device responsive to least one of (i) the current in the at least one measure electrode, and, (ii) a voltage of the at least one measure electrode for producing a measure signal representative of the resistivity parameter

57. The method of claim 56 wherein the at least one measure electrode further comprises a plurality of measure electrodes and wherein said resistivity parameter further comprises a resistivity image.

58. The method of claim 56 wherein the at least one measure electrode further comprises an array of measure electrodes.

59. The method of claim 56 further comprising using a demodulator for demodulating said measure signal and producing a demodulated signal therefrom, and an isolator section for isolating the measure electrode from the source of the measure current.

60. The method of claim 56 further comprising at least one focusing electrode in proximity to the at least one measure electrode, said at least one focusing electrode focusing the measure current into the formation.

61. The method of claim 56 further comprising a guard device for maintaining focusing of said measure current in the formation.