Abstract:
Disclosed herein is a method of canceling noise in a wellbore telemetry system. The method includes, acquiring at least one signal in the system, predicting at least one deterministic component of the at least one signal based upon a change of at least one deterministic component from past signal values, and subtracting the at least one predicted deterministic component from the acquired at least one signal.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to U.S. provisional application, 60/949,595, filed Jul. 13, 2007, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    In mud pulse telemetry data from down hole is transmitted to surface using pressure pulses generated by a pulser. Pressure sensors at surface measure the pressure changes over time that represent the data. 
         [0003]    The pulser, however, is not the only source of pressure changes in a well. Other pressure varying sources, such as pumps, for example, which circulate mud within the well, also generate pressure changes. These pressure changes act as noise for the mud pulse transmission. In fact, pumps are generally also located at surface and are therefore closer to the pressure sensors and, as such, are a dominant source of noise. Signals received by the sensors that are generated by such pumps typically have higher energy than do the received telemetric data signals since the telemetric signals get highly attenuated while traveling from down hole to surface. As such, accurate detection of the telemetric data signals can be difficult. Methods to allow for accurate detection of the telemetric data signals in the presence of pump noise would be well received in the art. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    Disclosed herein is a method of canceling noise in a wellbore telemetry system. The method includes, acquiring at least one signal in the system, predicting at least one deterministic component of the at least one signal based upon a change of at least one deterministic component from past signal values, and subtracting the at least one predicted deterministic component from the acquired at least one signal. 
         [0005]    Further disclosed herein is a processor readable media having stored thereon a processor program product that is executed by the processor, the processor program product for canceling noise in a wellbore telemetry system in a processor environment, the processor program product comprising a storage medium readable by a processing circuit and storing instructions for execution by the processing circuit the processor readable media being readable by a processing circuit and the processor program product having instructions for execution by the processing circuit for facilitating a method. The method includes, acquiring at least one signal in the system, predicting at least one deterministic component of the at least one signal based upon a change of at least one deterministic component from past signal values, and subtracting the at least one predicted deterministic component from the acquired at least one signal. 
         [0006]    Further disclosed herein is a method of canceling noise in a downhole telemetry system. The method includes, acquiring at least one signal in the system, predicting at least one deterministic component of the at least one signal based upon one of at least one deterministic component from a past signal values and a change of at least one deterministic component from past signal values, and subtracting the at least one predicted deterministic component from the acquired at least one signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
           [0008]      FIG. 1  depicts a spectrogram of signals received by a sensor in a mud pulse telemetry system; 
           [0009]      FIG. 2  depicts a spectrogram that has had received pump signals canceled by methods disclosed herein; and 
           [0010]      FIG. 3  depicts a linear prediction algorithm disclosed herein. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    A detailed description of one or more embodiments of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures. 
         [0012]    Embodiments of the system disclosed herein are used during downhole measurement and telemetric communication such as the techniques of logging while drilling (LWD) and measurement while drilling (MWD), for example. It should be understood that although embodiments disclosed herein describe a system with telemetry from downhole to surface, alternate embodiments could include telemetry from surface to downhole as well as bidirectional telemetry. Additionally, embodiment may include telemetry between any desired positions within a well, such as, between a first downhole position and a second downhole position, for example. Referring to  FIG. 1 , a spectrogram  10  of a signal received from a sensor is illustrated. The spectrogram  10  is a plot of frequency  14  in hertz on the Y-axis versus time  18  in seconds on the X-axis. The spectrogram  10  is made such that the higher the energy of the received signal at a given frequency the darker the representation on the spectrogram  10 . The darkest areas, therefore, represent frequencies with high magnitudes of pressure pulsing. By contrast, the lightest areas represent frequencies with the lowest magnitude of pressure pulsing. 
         [0013]    Knowing some specifics about the frequency of the pulses being generated by the pulser and when the pulser is actually transmitting telemetric data helps to identify transmitted telemetric data signal  22  on the spectrogram  10 . In the example of  FIG. 1 , the pulser began transmitting telemetric data at the 40-second mark  26  and stopped transmitting telemetric data at the 250-second mark  30 . In this example the majority of the telemetric data is transmitted at a frequency of between 30 and 40 hertz. As such the telemetric data signal  22  can be identified by the dark area  34  positioned between the 40-second mark  26  and the 250-second mark  30  on the X-axis, and between the 30 hertz and the 40 hertz frequencies on the Y-axis. 
         [0014]    The spectrogram  10  also includes several dark horizontal lines  38 . These dark horizontal lines  38  represent noise from one or more pumps. The frequencies of the pumps&#39; noise correlate with frequencies of operation of the pumps themselves. As such the pumps&#39; noise is periodic and thus deterministic in nature with the periodicity being proportional to the frequency of operation of the pumps. It should be noted that although the periodic noise described in embodiments herein is pump noise, in alternate embodiments the periodic noise could be from other sources. Such sources include, mud motors, rotating bits, rotating drillstrings, reciprocating members and pulsing members, for example. In such alternate embodiment the periodic noise could be attributed to these alternate sources of periodic noise. 
         [0015]    The spectrogram  10  also includes random dark speckles  42  that are from other undefined sources of noise. Such other noises can occur at various frequencies, various magnitudes and at various times. The magnitude of signals from these other noises may be low enough in the frequency range of the transmitted telemetric data signal  22  such that the data transmitted in the telemetric data signal  22  is legible above the other noises. 
         [0016]    The strength of the pump noise, being greater than the strength of the telemetric data signal  22  complicates deciphering the telemetric data signal  22  from the pump noise. It is, therefore, desirable to attenuate the magnitude of the pump noise to levels below that of the data. An embodiment of the invention uses the deterministic nature of the pump noise to attenuate the effect of the pump noise. This embodiment performs such attenuation even while the pump frequencies are changing over time due to changes in the operational frequency of the pump, for example. Such changes in pump noise may include changes in periodicity or changes in the shape of the periodic signal. 
         [0017]    Embodiments of the invention measure pressure in a well with at least one sensor. It should be noted, however, that alternate embodiments could use sensors that measure parameters other than pressure. Sensors that measure flow, magnetic forces or gravitational force, for example, could also be used with the disclosed system. The measured pressure signal is analyzed and some periodic signals are attributed to pump noise. The telemetric signal is assumed to be of non-periodic (stochastic) nature. If that is not the case for a significant time frame, data scrambling techniques may be applied. The periodic pump signals are analyzed for changes that are occurring over time. The changes occurring over time are used to predict values of the periodic signals at a future point in time. Then, at that future point in time, the predicted values of the periodic signals are subtracted from the pressure signals being measured. The remaining pressure signals are then analyzed to read the telemetric data transmitted. Algorithms to determine a periodic signal and then subtract the predicted signal from a signal in real time are called “Linear Prediction” and are described in several textbooks including “Adaptive Filter Theory” by Simon Haykin, published by Prentice Hall. 
         [0018]    Referring to  FIG. 2 , a second spectrogram  50  disclosed herein is illustrated. The second spectrogram  50  corresponds to a received signal that has been modified by subtraction of predicted periodic noise, including noise attributed to pumps, as disclosed herein. The dark horizontal lines  38 , of the periodic noise, as shown in  FIG. 1  are substantially eliminated. A dark area  54  representative of a telemetric data signal  58  is easily observable. 
         [0019]    Embodiments of the invention utilize mathematical algorithms in the analysis, prediction and subtraction of the periodic signals. For example, the received signal r(k) can be expressed as the superposition of the telemetric signal S T (k) with the periodic signal S PN (k) and other noise n(k). 
         [0000]        r ( k )= S   T ( k )+ S   PN ( k )+ n ( k ) 
         [0000]    The periodic noise signal has a periodicity and signal shape that may be changing slowly over time. Therefore it can be predicted from its previous values S PN (k−τ) except its variations. This can be done by a prediction filter h pred (k), which has to be adaptive to track variations of the periodic signal. 
         [0000]        {tilde over (S)}   PN ( k )= h   pred ( k )* S   PN ( k− −τ) 
         [0000]    Subtracting the predicted periodic signal {tilde over (S)} PN  (k) from the newest received signal, we get the telemetric signal distorted by noise and a pump noise prediction error e PN (k). 
         [0000]        r ( k )− {tilde over (S)}   PN ( k )= S   T ( k )+ e   PN ( k )+ n ( k )= ê ( k ) 
         [0000]    This signal is used to control the adaptation of the prediction filter in a way that the mean squared error E{|ê(k)| 2 } of ê(k) is minimal, which is only achievable by minimization of e PN (k) since the other signals are not predictable. If the mean squared error is minimal, ê(k) equals the telemetric signal plus minimum remaining noise. Since, however, the pump signal S PN (k−τ) is not directly available for prediction, the following may be used: 
         [0000]        r ( k —τ)= S   T ( k −τ)+ S   PN ( k −τ)+ n ( k −τ) 
         [0000]    which is the pump signal S PN (k−τ) distorted by the weak telemetric signal S T (k−τ) and noise n(k−τ). 
         [0020]    Referring to  FIG. 3 , a linear prediction algorithm  62  in accordance with an embodiment of the invention is illustrated. 
         [0021]    As described above, embodiments may be in the form of processor-implemented processes and apparatuses for practicing those processes. In exemplary embodiments, the invention is embodied in processor program code. Embodiments include processor program code containing instructions embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, or any other processor-readable storage medium, wherein, when the processor program code is loaded into and executed by a processor, the processor becomes an apparatus for practicing the invention. Embodiments include processor program code, for example, whether stored in a storage medium, loaded into and/or executed by a processor, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the processor program code is loaded into and executed by a processor, the processor becomes an apparatus for practicing the invention. The technical effect of the executable instructions is to cancel a pump signal received in a mud pulse telemetry system through analysis of data received by a single sensor. 
         [0022]    While the invention has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims.