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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 invention relates generally to the field of instrumentation used in wellbores drilled through Earth formations. More specifically, the invention relates to methods and apparatus for communicating signals to an instrument in a wellbore from the Earth&#39;s surface. 
     2. Background Art 
     Instruments used in wellbores drilled into the Earth&#39;s subsurface include a wide variety of sensing devices and mechanical operating devices. Examples of the former include pressure and temperature sensors, inclinometers and directional sensors, capacitance sensors, fluid density sensors, among others. In using such instruments, it is often necessary to send signals from the Earth&#39;s surface to the instrument to affect instrument operation or to provide information that may be used in the instrument. 
     For instruments deployed in a wellbore using armored electrical cable (“wireline” deployment) signals are transmitted along the cable to the instrument from the surface, typically from a surface recording system. For instruments deployed using a drilling rig or similar apparatus, where the instrument may be conveyed at the end of a drill pipe or tubing string, it is known in the art to send signals to the instrument by modulating the flow of drilling fluid through the drill pipe. Such modulation may be detected and decoded at the instrument by a flow sensor or a pressure sensor. It is also known in the art to send signals to the instrument by modulating the rate of rotation of the drill pipe. See, for example, U.S. Pat. No. 6,847,304 issued to McLoughlin and U.S. Pat. No. 5,113,379 issued to Scherbatskoy. It is also known in the art to communicated signals to an instrument in a wellbore by modulating fluid pressure from the Earth&#39;s surface. See, for example U.S. Pat. No. 4,856,595 issued to Upchurch and assigned to the assignee of the present invention. 
     In some cases, it is impractical to use any of the foregoing techniques for communicating signals to an instrument in a wellbore. For example, using “slickline” (a solid wire or wire rope conveyance having no insulated electrical conductors) there is no practical way to send electrical signals to the instrument from the Earth&#39;s surface. Further, it is not possible to rotate an instrument from the surface when conveyed by slickline or by coiled tubing. Finally, some wellbore instruments are materially complicated as to design by including a pressure or flow sensor. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention is a method for communicating a signal to an instrument in a wellbore. A method according to this aspect of the invention includes axially accelerating the instrument in a preselected pattern of acceleration. The predetermined pattern corresponds to the signal to be communicated. The axial acceleration of the instrument is detected, and the signal is decoded from the detected axial acceleration. 
     A signal detection system for an instrument in a wellbore according to another aspect of the invention includes an accelerometer oriented along a longitudinal axis of the instrument. The system also includes means for comparing measurements made by the accelerometer to at least one predetermined pattern. The predetermined pattern corresponds to a signal communicated from the Earth&#39;s surface to the instrument. 
     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an instrument deployed in a wellbore by a slickline unit. 
         FIG. 2  shows an instrument deployed in a wellbore by a coiled tubing unit. 
         FIG. 3  shows more detail of acceleration detection and sensor components of the instruments shown in  FIGS. 1 and 2 . 
         FIG. 4  shows one example of an automatic system for generating signals for communicating to an instrument in a wellbore. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an example of a wellbore instrument  10  having signal detection and decoding devices according to one aspect of the invention as it may be deployed in a wellbore using a conveyance device known as a “slickline unit”, shown generally at  20  in  FIG. 1 . “Slickline” is generally known in the art as a solid steel wire or wire rope deployed form a winch or similar spooling device to deploy or withdraw various instruments from a wellbore, and the term slicking unit includes such wire or wire rope, the winch and associated winch control devices. The present description of the invention is in terms of certain example conveyance devices, wherein the term conveyance device is intended to mean any device known in the art for inserting instruments into and removing instruments from a wellbore drilled through the Earth&#39;s subsurface. Such conveyance devices include slickline units and coiled tubing units as set forth in this description, because those conveyance devices represent particularly appropriate uses for a system and method according to the invention. It should be clearly understood, however, that any other conveyance device known in the art, including a drilling rig having a hoisting system, a workover rig having similar hoisting system that can convey devices into and out of a well using threadedly coupled segments of tubing or pipe, or a “wireline” unit having a winch that spools armored electrical cable having one or more insulated electrical conductors therein may also be used with the invention. Accordingly, the example conveyance devices shown herein are only meant to illustrate the general principle and are not intended to limit the scope of this invention. 
     The slickline unit  20  includes a winch  20 A or similar device of any type known in the art. As will be further explained with reference to  FIG. 4 , the winch  20 A may be rotated by a motor (not shown in  FIG. 1 ) or similar source of rotary motive power Slickline  18  is shown deployed from the slickline unit  20  into a wellbore  16  drilled through the Earth&#39;s subsurface. In  FIG. 1 , the slickline  18  is routed through an upper sheave  24  and lower sheave  26  of types well known in the art. The sheaves  26 ,  24  redirect the slickline  18  so that it extends vertically over the wellbore  16  for extension therein and withdrawal therefrom. The sheaves  24 ,  26  may be supported by a portable mast unit  22  of any type well known in the art. 
     The instrument  10  is shown deployed in the wellbore  16  at the lower end of the slickline  18 . The instrument  10  may include sensors or other devices and a data acquisition processor, shown generally at  14 , and an accelerometer and associated signal processing circuit devices, shown generally at  12 . The accelerometer  12  is oriented in the instrument  10  to be sensitive primarily to acceleration along the longitudinal axis of the instrument, as shown generally by line  16 A. 
     A tensile stress sensing element, or “load cell”  60  may be coupled between the upper sheave  24  and the derrick portion of the mast unit  22  to enable estimating the tensile stress (“weight”) on the slickline  18 . In addition to providing the slickline unit  20  operator with indication of the condition of the instrument  10  as it is moved along the wellbore  16 , tensile stress measurements may be used, as will be explained below with reference to  FIG. 4 , to assist in operating the winch  20 A so as to generate a signal for communication from the Earth&#39;s surface to the instrument  10  in the wellbore  16 . 
     Another example of deployment device for a wellbore instrument is shown in  FIG. 2 . The deployment device shown in  FIG. 2  is a coiled tubing unit  30 . Coiled tubing  18 A is stored on a reel  36 . The coiled tubing  18 A can be extended into the wellbore  16  and withdrawn from the wellbore  16  to move the instrument  10 . The coiled tubing unit  30  typically includes a tractor device called an “injector head”, shown generally at  34 . The injector head  34  includes tractor belts or similar devices that move the coiled tubing  18 A upwardly and downwardly. The coiled tubing  18 A is redirected from the reel  36  to a generally vertical orientation over the injector head  34  using a device called a “gooseneck”, shown generally at  32  and which typically includes a plurality of rollers disposed along an arcuate path in a support structure. Although not shown separately in  FIG. 2 , typically the coiled tubing unit  30  will include a weight indicator or load cell similar in purpose to the load cell  60  shown in  FIG. 1  as used with the slickline unit ( 20  in  FIG. 1 ). 
     The deployment devices shown in  FIG. 1  and  FIG. 2  are only examples of deployment devices that may be used with a method and apparatus according to the invention, as explained above. The devices shown in  FIG. 1  and  FIG. 2  are those for which the invention is intended because both do not necessarily include an electrical signal channel, optical signal channel or pressure signal channel to communicate a signal from the Earth&#39;s surface to the instrument in the wellbore, neither can they readily cause the instrument to rotate in the wellbore. 
     Having shown generally conveyance devices for deploying the instrument in the wellbore, an example of a signal detection and decoding apparatus according to one aspect of the invention will be explained with reference to  FIG. 3 . The instrument  10  may include an elongated housing  11  configured to move along the interior of the wellbore. The housing  11  typically defines a sealed interior chamber therein. The housing  11  maybe coupled to the end of the slickline  18  (or coiled tubing  18 A) by means of a cable head  40  of any type known in the art for coupling a slickline instrument thereto. Devices for signal acquisition and processing are typically disposed in such sealed chamber. 
     The signal detection and processing device  12  may include an accelerometer  42 , such as a quartz flexure accelerometer, as previously explained, oriented so that its sensitive axis is generally along the longitudinal axis ( 16 A in  FIG. 1 ) of the instrument  10 . One such accelerometer is sold under model designation QAT160 by Honeywell International, 101 Columbia Rd., Morristown, NJ 07960. So arranged, the accelerometer  42  will generate a signal related to the axial acceleration on the instrument  10 . Output of the accelerometer  42  may be coupled to an operational amplifier, single pole bandpass filter combination  44  (“filter”), which may condition the accelerometer  42  output and filter acceleration components above and/or below a selected frequency. In one example, the filter  44  has a high cut frequency of about 50 Hz. Output of the filter  44  may be conducted to a digital signal processor (“DSP”)  46 . The DSP  46  may include an internal analog to digital converter (“ADC”) or may use a separate ADC (not shown) coupled between the output of the filter  44  and the input of the DSP  46 . One suitable DSP is sold under model designation TMS320C33 by Texas Instruments Inc., 12500 TI Boulevard, Dallas, TX 75243-4136. 
     The other signal acquisition and processing devices  14  may include a central processor  50  to process and/or record signals output from the DSP  46  as well as signals generated by one or more other sensors  52  or other devices in the instrument  10 . Non-limiting examples of such other sensors  52  may include pressure and/or temperature sensors and calipers (wellbore internal diameter measuring devices). Any other device ordinarily operated by a slickline or coiled tubing conveyed instrument may also be disposed in or associated with the housing  11 . Accordingly, the structure shown in  FIG. 3  is not intended to limit the scope of the types of other sensors or devices that may be used in the instrument  10 . 
     Electrical power to operate all the foregoing devices may be supplied by a battery  48  or other energy storage device. The source of electrical power to operate the various devices in the instrument, however, is not intended to limit the scope of this invention. 
     In one example, the DSP  46  may be configured to measure the filtered output of the accelerometer  42  for a selected period of time, for example, by buffering a selected number of accelerometer measurement samples, and calculating certain attributes of the measured acceleration. Such attributes may include maximum acceleration, minimum acceleration, means acceleration and variance (or standard deviation). The statistical information may be used in some examples to discriminate between true signals communicated from the Earth&#39;s surface and noise that is unlikely to represent a signal from the Earth&#39;s surface. For example, if the maximum and minimum acceleration values within a selected time interval are not outside selected threshold criteria, the measured acceleration may be attributed to ordinary operation of the conveyance device rather than signal elements. 
     The DSP  46  may be configured to compare the measured acceleration to one or more predetermined acceleration patterns. If a predetermined acceleration pattern is matched, the DSP  46  may communicate a signal to the processor  50  corresponding to the detected pattern indicating that a signal has been detected. The processor  50  may operate one or more devices in the instrument  10  according to instructions corresponding to the detected signal. For example, a sensor may be switched on or off. A recording device in the processor  50  may be switched to record a particular type of sensor output or change a sample rate of sensor signal recording. It is not a limit on the scope of this invention as to the type of operation initiated (or stopped) by the instrument  10  in response to a detected pattern. In addition, while the foregoing examples of signals from the Earth&#39;s surface have been explained in terms of commands or instructions, it is also within the scope of this invention that data may also be communicated to the instrument. Accordingly, the term “signal” as used herein with reference to information being transmitted from the Earth&#39;s surface to the instrument is intended to mean any information that can be encoded into a particular acceleration pattern and detected by suitable processing of acceleration signals in the DSP  46  and/or processor  50 , or any similar signal detection and decoding device. 
     Acceleration as that term is used in the present description is intended to mean a force applied for a sufficient duration of time so as to change the velocity of the instrument  10 . Such definition is intended to distinguish from acoustic signal transmission (which may be detected by an accelerometer), in which elastic or shear waves are moved through the instrument  10  but do not change its velocity. 
     To generate a selected acceleration pattern at the Earth&#39;s surface to represent a signal to be communicated to the instrument  10 , the winch ( 20 A in  FIG. 1 ) or the coiled tubing unit ( 30  in  FIG. 2 ) may be operated to accelerate the instrument in a predetermined manner. For example, the winch or coiled tubing unit may be operated to momentarily apply upward motion to the slickline ( 18  in  FIG. 1 ) or coiled tubing ( 18 A in  FIG. 2 ), momentarily stop the slickline or tubing, and repeat the foregoing for a selected number of accelerate/stop operations. As another example, the foregoing upward acceleration/stopping sequences may be followed by a selected duration waiting period, followed by another selected number of upward acceleration/stop sequences. Downward acceleration and/or acceleration and stopping sequences may also be used. 
     In one example, the slickline unit or coiled tubing unit operator may cause the upward (or downward) motion to generate a selected increase (or decrease) in measured tensile stress (as measured by the load cell  60  in  FIG. 1 ) over the tensile stress measured while the instrument is stationary in the wellbore. Such increase in tensile stress will be related to acceleration of the slickline or coiled tubing, and consequently, will be related to the acceleration applied to the instrument  10 . By selecting a predetermined tensile stress increase (“overpull”), the acceleration applied to the instrument  10  is more likely to be detected as part of a signal sequence, rather than ordinary operation of the slickline or coiled tubing unit for moving the instrument. 
     In another example, automatic operation of the slickline or coiled tubing unit for signal generation may be provided by an apparatus such as the one shown in  FIG. 4 . The components shown in  FIG. 4 , other than the load cell  60  may be associated with or disposed in the coiled tubing unit ( 30  in  FIG. 2 ) or the slickline unit  20  as shown. A central processor  64  such as a microprocessor based controller or programmable logic controller (PLC) may include program code intended to operate the slickline winch (or coiled tubing winch) in a predetermined sequence of start/stop operations in order to communicate a signal from the Earth&#39;s surface to an instrument in the wellbore. When an appropriate input signal is provided to the central processor  64  by the system operator, the central processor  64  can apply electrical power to actuate a solenoid-operated hydraulic valve  66 . The valve  66  may be included in an hydraulic system  68  functionally associated with an hydraulic motor  70 . The motor  70  provides the motive power to drive the winch ( 20 A in  FIG. 2 ). When oriented, the solenoid valve  66  will cause the motor  70  to start and stop. The central processor  64  may accept input signals from the load cell  60 , suitably digitized in an analog to digital converter  62 . The central processor  64  may be programmed to operate the valve  66  start the motor  70  until a preselected increase in detected stress is measured by the load cell  60 , and then operate the valve  66  to stop the motor  70 . Such process may continue for a preselected number of cycles until the selected signal is communicated to the instrument ( 10  in  FIG. 1 ). 
     The example system shown in  FIG. 4  may be applied to the coiled tubing unit as well. Although the example shown in  FIG. 4  provides electrical control of an hydraulic motor, those skilled in the art will appreciate that an electric motor or a prime mover may also be controlled by a similar system. 
     Alternatively, an as explained above, the winch or coiled tubing unit may be operated to momentarily move the instrument downward at full speed and then stop motion of the instrument. The winch or coiled tubing unit may also be operated to move the instrument downward and then reverse motion, either prior to stopping motion of subsequent reversing the direction of motion of the instrument. 
     By operations such as suggested above, a signal may be transmitted from the Earth&#39;s surface to the instrument in the wellbore without the need for a directly coupled signal communication channel (e.g., electrical power, optical signal or pressure modulation). 
     While the invention has been described with respect to a limited number of embodiments, these skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

Summary:
A method for communicating a signal to an instrument in a wellbore includes axially accelerating the instrument in a preselected pattern of acceleration. The predetermined pattern corresponds to the signal to be communicated. The axial acceleration of the instrument is detected, and the signal is decoded from the detected axial acceleration. A signal detection system for an instrument in a wellbore includes an accelerometer oriented along a longitudinal axis of the instrument and means for comparing measurements made by the accelerometer to at least one predetermined pattern.