Abstract:
An apparatus and method for reducing crosstalk and other noise sources in downhole tools is disclosed. Due to confined space in the downhole tool environment and the fact that the transmit path utilizes significantly more electrical power than the receive path, electromagnetic noise easily couples between adjacent circuitry. The specification discloses a phase-reversal element that can selectively allow the received signal to pass unaffected, or which can cause a phase reversal of the received signal. A digital signal processor samples the received signal, which after receipt but before sampling has noise induced thereon. Thereafter, the digital signal processor samples the phase reversed received signal, which likewise has the noise induced thereon after receipt but before sampling. Subtracting the first sampled signal from the second produces a resultant signal in which the noise is substantially reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The preferred embodiments of the present invention are generally directed to reducing crosstalk and other noise sources in downhole tools. More particularly, the preferred embodiments are directed to implementing a phase-reversal element that enables an electromagnetic resistivity tool&#39;s circuitry to quantify and/or nullify crosstalk and other noise sources. 
     2. Background of the Invention 
     In the exploration and production of hydrocarbons in an underground reservoir or formation, it is often desirable to have downhole parameters and information readily available. For example, when exploring a formation by drilling, it is desirable to know formation properties in order to determine if a reservoir has been encountered. Methods and apparatuses for determining and measuring downhole parameters are well known in the art, and may include generating an electromagnetic stimulus using transmit circuitry that penetrates the formation while the formation&#39;s response to the electromagnetic stimulus is acquired with receive circuitry. 
     Due to confined downhole conditions, the electromagnetic transmit and receive circuitry are often located in close proximity to each other. Additionally, because of formation attenuation, the receive circuitry operates at a lower power level (e.g. several orders of magnitude) than that of the transmit circuitry, causing almost any electromagnetic interference (EMI) generated by the transmit circuitry to affect the operation of the receive circuitry drastically. This contamination of EMI between the transmit and receive circuitry leads to a condition commonly know as crosstalk. 
     Current trends in attempting to solve crosstalk and other noise related problems in downhole applications include separating the transmit and receive circuitry physically, but this may lead to, among other things, redundant power circuitry. Filtering circuitry that the transmit/receive circuits share (e.g. power supplies, reference oscillators, etc.) has also been attempted, but it does not provide the desired performance. Shielding the affected circuit may also be employed in effort to reduce crosstalk, however, this is often relatively expensive and does not provide adequate performance. 
     Thus, there is a need for an apparatus and method to reduce crosstalk in downhole tools that is both cost effective and also provides improvements in the desired level of performance. 
     BRIEF SUMMARY OF SOME OF THE PREFERRED EMBODIMENTS 
     The problems noted above are solved in large part by a phase-reversal element employed so that the phase of the signal coming from a receive sensor, which represents the formation&#39;s response to the electromagnetic stimulus, may be reversed prior to being processed by the receive circuitry. As such, the phase-reversal element is preferably located in close physical proximity to the sensor so that the maximum amount of crosstalk between the transmit circuitry and the receive circuitry is reduced by subsequent digital signal processing circuitry. In the preferred embodiments, the signals processed by subsequent circuitry are a composition of the original signal measured by the sensor, followed by a phase-reversed version of the same signal. The receive circuitry then applies the remainder of the analog and digital signal processing to the resultant signal, where the crosstalk may be both quantified and nullified digitally. 
     In another embodiment, a phase-reversal element may be employed at any point in the receive path circuitry. In this manner, the subsequent digital signal processing circuitry may quantify and nullify any crosstalk or noise that comes after the phase-reversal element. For example, if it is known that the signal conditioning circuitry is the most susceptible to crosstalk, then the phase-reversal element may be employed just before the signal conditioning circuitry. Therefore, if it is undesirable to implement the phase-reversal element in close proximity to the sensor, placing it proximate to elements that are known to be more susceptible than others may be an alternative embodiment. 
     The disclosed devices and methods comprise a combination of features and advantages which enable it to overcome the deficiencies of the prior art devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more detailed understanding of the preferred embodiments, reference is made to the accompanying Figures, wherein like parts throughout the drawings are marked with the same reference numerals: 
     FIG. 1 shows a downhole logging tool; 
     FIG. 2 shows a block diagram of an electrical system for downhole measurement; 
     FIG. 3A shows an exemplary signal as received by a receiver; 
     FIG. 3B shows the received signal of FIG. 3A including crosstalk and without any correction; 
     FIG. 3C shows an exemplary control signal from a programmable phase control element; 
     FIG. 3D shows a phase cycled version of the signal of FIG. 3A using a phase-reversal element; 
     FIG. 3E shows the signal of FIG. 3D including crosstalk and ready for digital quantification and nullification; 
     FIG. 4 shows an embodiment of implementing a phase-reversal element using solid state devices; and 
     FIG. 5 shows an embodiment of implementing a phase-reversal element using switches. 
    
    
     NOTATION AND NOMENCLATURE 
     Certain terms are used throughout the following description and claims to refer to particular system components. As one skilled in the art will appreciate, oilfield service companies, and tool manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. 
     In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. Also, the term “couple” or “couples” is intended to mean either an indirect or direct electrical or mechanical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical or mechanical connection, or through an indirect electrical or mechanical connection via other devices and connections. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention relate to methods and apparatuses that employ phase cycling in reducing crosstalk in downhole electromagnetic resistivty tools, and may also find application in other tools, such as acoustic tools. The present invention is susceptible to embodiments of different forms. There are shown in the drawings, and herein will be described in detail, specific embodiments of the present invention with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that illustrated and described herein. It should be understood that the phase-reversal element, as described below, itself does not reduce of crosstalk; rather, the addition of the phase-reversal element allows the digital signal processor to reduce or quantify the crosstalk in the final signal by isolation of the crosstalk component in a comparison of the received signal, and a phase reversed version of the received signal. 
     Referring now to FIG. 1, a logging tool  10  used to determine and acquire information in the exploration and production of hydrocarbons in an underground reservoir or formation is shown. FIG. 1 shows the logging tool  10  located in a wellbore  12  that passes through a formation  14 . The tool may include multiple transmitters  16   a  and  16   b , as well as multiple receivers  18   a  and  18   b . In the preferred embodiments, transmitters  16  and receivers  18  are toroidal or solenoid antennas for use in a tool for determining resistivity of downhole formations using electromagnetic radiation; however, transmitters  16  and receivers  18  may include other types of antennas and downhole sensors, such as acoustic sensors. The logging tool  10  is preferably a portion of a bottom hole assembly (BHA), but it is not so limited, and in fact the logging tool  10  also renders itself to wireline applications. 
     FIG. 2 shows an electrical system  50  of the preferred embodiment, which comprises a digital signal processor (DSP)  51  that executes algorithms used to determine formation properties. The DSP  51  uses these algorithms both in generating signals to be transmitted into formations and in processing signals returning from formations. The DSP  51  generally comprises a digital to analog converter (DAC)  52  which takes the digital signal and generates an analog equivalent. The analog signal generated by the DAC  52  may have properties that are undesirable (e.g. DC offset voltages, limited current and voltage, etc.), and therefore further analog signal processing using a signal conditioner  54  may be required. Signal conditioner  54  preferably performs, among other things, level shifting in order to prepare the signal to be amplified by power amp  56 . Amplification of the electromagnetic signal is often necessary before transmission into the formation so that sufficient signal strength is provided. Sufficient signal strength not only ensures adequate penetration of the electromagnetic signal, but also ensures the signal received from the formation has the desired signal to noise ratio in the receive circuitry. For example, it is common for the magnitude of the transmitter excitation to be in the ampere range. Finally, a transmitter  16  emits an electromagnetic representation of the signal into the formation, as is familiar to one of ordinary skill in the art. 
     A receiver  18  detects the returning electromagnetic signal, whose amplitude and phase are indicative of the formation&#39;s properties. FIG. 3A shows an example signal as received by receiver  18 . Typically, the amplitude of the received response signal is attenuated many orders of magnitude lower than that of the transmitted signal (e.g. in the microampere or milliampere range) and therefore requires amplification prior to analysis. As will be discussed in more detail below, a phase-reversal element  58  (controlled by a programmable phase control circuit  60 ), preferably couples the detected signal, or its phase-reversed version, to a pre-amplifier  53 . The pre-amplifier  53  is used to amplify the signal, which is then passed to a signal conditioner  55 . The signal conditioner  55  further prepares (e.g. level shift, filtering, etc.) the signal for the analog to digital converter (ADC)  56 . The ADC  56  preferably converts the received signal into digital form so that the DSP  51  can determine the formation properties. 
     However, in generating a high power electromagnetic signal for adequate formation penetration and signal-to-noise ratio at receiver  18 , the transmission path circuitry (i.e. signal conditioner  54 , power amp  56 , and transmitter  16 ) also generates electromagnetic interference (EMI). Additionally, due to confined space downhole, and particularly within the downhole tool  10 , the transmission and receive paths typically are in close proximity so that the EMI affects the received signal in the adjacent receive circuitry. This phenomena is known as crosstalk. It should be noted that the crosstalk may be introduced at any point along the receive path, and one element of the receive path may be more susceptible to crosstalk than other elements. FIG. 3B shows and exemplary incoming signal affected by crosstalk. So while the waveform of FIG. 3A exemplifies the received signal as it leaves receiver  18 , crosstalk distorts the signal as it propagates through the various receive path components. FIG. 3B exemplifies the received signal as it is sensed by the ADC  56 . It will be understood that the waveforms of FIGS. 3A and 3B (as well as FIGS. 3D and 3E) are simplified for purposes of explaining the preferred embodiments. Relative amplitudes, as well as the crosstalk components shown, are not necessarily those seen in an actual device, but are provided to simplify the description. 
     The phase-reversal element  58  and its operation in correcting crosstalk will now be discussed in detail. The programmable phase control element  60  is preferably included in DSP  51 , and provides control for the phase-reversal element  58 . This control is robust in that it may power down the phase-reversal element  58  and/or it may cause the phase-reversal element to cycle between phase states. For example, the programmable phase control element  60  may produce a pulse width modulated control signal as shown in FIG. 3C, which when applied to the phase reversal element  58  either couples the signal received from the receiver  18 , or a phase-reversed version, as shown in FIG.  3 D. The coupled signal from the phase-reversal element would then be subject to the typical noise and crosstalk as would be introduced in the receive path between the output of the phase-reversal element  58  and the input of DSP  51 , as shown in FIG.  3 E. Moreover, the phase-reversal element  58  provides subsequent phase-cycles to DSP  51  where the desired information has an opposite phase from cycle to cycle, yet the noise induced because of crosstalk between the transmit and receive circuitry has the same phase from cycle to cycle. Upon arrival at the DSP  51  the signal is then digitized using the ADC  56 , so that the DSP  51  may then utilize the signal to quantify and/or nullify any noise or crosstalk introduced into the line after the phase-reversal element  58 . Referring to FIG. 3E, cycle B represents the phase cycle including any induced noise and crosstalk introduced in the receive path, and cycle A represents a phase-reversed version. The received signal has an opposite phase in phase cycle A than it does in phase cycle B, while the crosstalk has the same phase in both phase cycles. Accordingly, the DSP may compare the signals of the two phases cycles A and B to determine the crosstalk component. While the comparison of the two signals (and the embedded crosstalk) may take many forms, in the preferred embodiments the DSP  51  subtracts the signals, on a point by point basis. The subtraction of the signals from phase cycles A and B has a resultant a scaled received signal free from the crosstalk component. Equation (1) through (3) below describe the process more mathematically:                A   1     =     S   +   C             (   1   )                     (   -   )          A   2       =       -   S     +   C                          (   2   )                            R   =     2      S                          (   3   )                                
     where A 1  is the composite received signal comprising the received signal S and the crosstalk component C, A 2  is the composite signal received comprising a phase-reversed version of the received signal S (indicated by a leading negative sign) and the crosstalk component C, and R is the resultant signal (equal to 2S) created by the addition of signals A 1  and A 2 . Thus, the DSP  51  may perform further calculations necessary to ascertain formation properties with a resultant substantially free from crosstalk. Indeed, it is believed that using the phase-reversal element  58  to compensate for crosstalk provides at least a 40-dB improvement over the non-compensated case. 
     Alternatively, the DSP may add the signals of the phase cycles A and B to reveal the crosstalk. This situation is shown mathematically in equations (4) through (6) below.                A   1     =     S   +   C             (   4   )                     (   +   )          A   2       =       -   S     +   C                          (   5   )                            R   =     2      C                          (   6   )                                
     It is seen then that in the case of addition of the two signals from the phase cycles, the resultant signal is a scaled version of the crosstalk experienced by the receive circuits. In another embodiment, the DSP  51  may periodically monitor the overall noise floor of the tool using the resultant signal produced by addition of the signals from the two phase cycles, thereby providing insight into developing problems and enabling a user to predict costly downtime. It should be noted that in the preferred embodiment of FIG. 2 the phase-reversal element  58  is shown directly after the sensor  18 ; however, alternate embodiments include the phase-reversal element  58  placed at any of the various points in the receive path. In this manner, crosstalk introduced after the phase-reversal element  58  may be quantified and/or nullified by the DSP  51 . Also, the phase-reversal element  58  may be implemented using various methods such as mechanical relays and/or solid state devices. 
     FIG. 4 shows an exemplary embodiment of phase-reversal element  58  connected to an exemplary receiver  18  embodied as two coils with opposite polarity, as is familiar to one of ordinary skill in the art. A pulse width modulated (PWM) control signal from the programmable phase control element  60  is tied to the clock input of a D type flip-flop  70 , which also has its output Q′ connected to the D input so as to toggle states on each rising clock edge of the PWM control signal. The Q output of flip-flop  70  is coupled to the power down pin of a differential amplifier  75 , and the Q′ output of flip-flop  70  is coupled to the power down pin of another differential amplifier  80 . In this manner, the toggling signal from flip-flop  70  serves to toggle powering up either differential amplifier  75  or differential amplifier  80  for half of the period of the PWM control signal. The inputs of differential amplifiers  75  and  80  are connected to the outputs of receiver  18  similarly. That is, the positive output of the receiver  18  is connected to the positive inputs of differential amplifiers  75  and  80 , and the negative output of the receiver  18  is connected to the negative inputs of differential amplifiers  75  and  80 . However, the differential outputs of differential amplifiers  75  and  80  are oppositely connected to the input of a third differential amplifier  85 . Thus, as operation of differential amplifiers  75  and  80  are toggled as described above, differential amplifier  85  receives either the original signal from receiver  18  or a phase-reversed version. The output of differential amplifier  85  produces a single ended signal out to be sent to the rest of the receive path. Differential amplifier  85  also has its power down circuitry connected to the PWM control signal such that differential amplifier  85  may be powered down completely while coupling the output to ground. 
     FIG. 5 shows another exemplary embodiment of phase-reversal element  58  connected to an exemplary receiver  18 . A control signal from the programmable phase control element  60  is tied to switches S 1  and S 2 . Switches S 1  and S 2  may be any type that allows the control signal to alternate between polarity configurations for coupling receiver  18  to the phase-reversal element  58 . For example, S 1  and S 2  may be relays. In any case, switches S 1  and S 2  serve to provide either the received signal or a phase-reversed version to amplifier  71 . 
     Therefore, the embodiments of the present invention provide a method and apparatus that employs phase cycling to reduce crosstalk and other sources of noise in downhole tools. Also, the ADC  56 , DAC  52 , and programmable phase control element  60  are shown included in DSP  51 , however any or all of these elements may be separate from DSP  51 . Moreover, while the specification has discussed quantifying crosstalk so as to remove the crosstalk from the signal reaching the digital signal processor  51 , the amount of crosstalk may also be indicative of the logging tool&#39;s condition. That is, the resultant signal created by combining the receive signal and a phased reversed version of the receive signal may be indicative of the logging tool&#39;s condition. 
     The embodiments set forth herein are merely illustrative and do not limit the scope of the invention or the details therein. It will be appreciated that many other modifications and improvements to the disclosure herein may be made without departing from the scope of the invention or the inventive concepts herein disclosed. Because many varying and different embodiments may be made within the scope of the inventive concept herein taught, including equivalent methods of implementing the phase-reversal element  58 , many modifications may be made in the embodiments herein detailed in accordance with the descriptive requirements of the law, and it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.