Abstract:
A system, method and device may be used to monitor acoustic energy in a borehole. Electromagnetic energy is used to energize a resonant circuit incorporating a sensor. The sensor modulates the electromagnetic energy in accordance with received acoustic energy and transmits the modulated energy so that it may be received and processed in order to obtain the desired measurements.

Description:
BACKGROUND 
   1. Field 
   The present invention relates generally to remote sensing and more particularly to passively communicating remote conditions by modulated reflectivity of an input electromagnetic signal. 
   2. Background 
   In resource recovery, it may be useful to monitor acoustic energy at locations remote from an observer. In particular, it may be useful to provide for monitoring conditions at or near to the bottom of a borehole that has been drilled either for exploratory or production purposes. Because such boreholes may extend several miles, it is not always practical to provide wired communications systems for such monitoring. 
   SUMMARY 
   An aspect of an embodiment of the present invention includes an apparatus for monitoring acoustic waves in a subsurface borehole, including a signal generator, constructed and arranged to produce an electromagnetic signal, a transmission line extendable along the borehole for transmitting the electromagnetic signal, a sensor including a resonant circuit and positionable in the borehole for receiving the transmitted electromagnetic signal, the resonant circuit having a resonant frequency that is variable in response to received acoustic energy, such that, in operation, the resonant circuit modulates the transmitted electromagnetic signal in accordance with the variation in the resonant frequency and returns the modulated signal along the transmission line, and a receiver, constructed and arranged to receive the modulated signal. 
   An aspect of an embodiment of the present invention includes a system for monitoring acoustic waves in a subsurface borehole, including generating an electromagnetic signal, transmitting the electromagnetic signal to a region of interest in the subsurface borehole, receiving acoustic energy with a sensor positioned in the region of interest, modulating, with the sensor, the electromagnetic signal in response to the received acoustic energy, and returning the modulated electromagnetic signal. 
   Aspects of embodiments of the invention may include a system incorporating the foregoing device and configured and arranged to provide control of the device in accordance with the foregoing method. Such a system may incorporate, for example, a computer programmed to allow a user to control the device in accordance with the method, or other methods. 
   These and other objects, features, and characteristics of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various Figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. As used in the specification and in the claims, the singular form of “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. 

   
     DESCRIPTION OF THE DRAWINGS 
     Other features described herein will be more readily apparent to those skilled in the art when reading the following detailed description in connection with the accompanying drawings, wherein: 
       FIG. 1  shows an embodiment of an apparatus for monitoring acoustic signals in a borehole; 
       FIG. 2  shows an embodiment of a variable capacitor for use in an embodiment of the apparatus illustrated in  FIG. 1 ; 
       FIG. 3  shows an embodiment of a crystal oscillator for use in an embodiment of the apparatus illustrated in  FIG. 1 ; 
       FIG. 4  schematically illustrates a circuit of which the sensors of  FIGS. 2 and 3  are elements; and 
       FIG. 5  is a cross-sectional illustration of a housing configured to hold a number of sensors in accordance with embodiments of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates an example of an apparatus  100  for monitoring acoustic waves in a subsurface borehole. The apparatus  100  includes an electromagnetically transmissive medium, such as a conductive line  102 , for conducting electromagnetic energy through the borehole. It will be appreciated by those having ordinary skill in that art that the conductive line  102  may take different forms or embodiments, depending on the state of the borehole. Thus, for example, the conductive line  102  may comprise a production tubing string in a completed borehole or a drillstring in a borehole under construction. Near the top of the conductive line  102 , a transformer  104  is provided to couple the conductive pipe to a source of electromagnetic energy. Alternate coupling methods to the transformer  104  may be employed. For example, the transmission line may directly couple to a coaxial cable or any other suitable cable. 
   In the example embodiment as shown, the transformer  104  includes a stack of ferrite rings  106 , and a wire  108  wound around the rings. The wire  108  includes leads  110  that may be coupled to a signal generator  112  which may be configured to produce a pulsed or a continuous wave signal, as necessary or desirable. The wire  108  may further be coupled to a receiver  114 . The receiver  114  may be embodied as a computer that includes a bus for receiving signals from the apparatus  100  for storage, processing and/or display. In this regard, the computer  114  may be provided with a display  118  which may include, for example, a graphical user interface. 
   The computer  114  may be programmed to process the modulated frequency to provide a measure of the sensed characteristic. The computer  114  may perform any desired processing of the detected signal including, but not limited to, a statistical (e.g., Fourier) analysis of the modulated vibration frequency, a deconvolution of the signal, a correlation with another signal or the like. Commercial products are readily available and known to those skilled in the art can be to perform any suitable frequency detection. Alternately, the computer may be provided with a look-up table in memory or in accessible storage, that correlates received modulated frequencies to sensed acoustic energy. 
   In a typical drilling application, the borehole will be lined with a borehole casing  120  which is used to provide structural support to the borehole. This casing  120  is frequently made from a conductive material such as steel, in which case it will cooperate with the line  102  in order to form a coaxial transmission line, and it is not necessary to provide any additional conductive medium. Where the casing is not conductive, a conductive sleeve may be provided within the casing in order to form the coaxial structure. In order to maintain a spacing between the line  102  and the casing  120 , the apparatus  100  may include dielectric rings  122  disposed periodically along the conductive line  102 . 
   The spacers can, for example, be configured as insulated centralizers which can be disks formed from any suitable material including, but not limited to, nylon or polytetrafluoroethylene (PTFE). Though the illustrated embodiment makes use of a coaxial transmission line, it is contemplated that alternate embodiments of a transmission line may be employed, such as a single conductive line, paired conductive lines, or a waveguide. For example, the casing alone may act as a waveguide for certain frequencies of electromagnetic waves. Furthermore, lengths of coaxial cable may be used in all or part of the line. Such coaxial cable may be particularly useful when dielectric fluid cannot be used within the casing  120  (e.g., when saline water or other conductive fluid is present in the casing  120 ). 
   A probe portion  124  is located near the distal end of the apparatus  100 . In principle, the probe portion may be located at any point along the length of the transmission line. Indeed, multiple such probe portions may be placed at intervals along the length, though this would tend to create additional signal processing burdens in order to differentiate signals from the several probes. Setting a natural resonance frequency of each probe at a different frequency would, in principle, allow for a type of wavelength multiplexing on the coaxial line that could simplify the processing. 
   The probe portion includes a port  126  that is configured to communicate acoustic energy in the ambient fluid of the borehole into the probe where it may be sensed by the sensor (not shown in  FIG. 1 ). Below the probe is illustrated a packer  128  and packer teeth  130 . 
   In use, the signal generator  112  generates an electromagnetic pulse that is transmitted through the transmission line to the probe  124 . In an alternate arrangement, the pulse may be generated locally as described in U.S. patent application Ser. No. 11/898,066, herein incorporated by reference. 
   The probe includes a sensor that includes a resonant circuit portion that, upon receiving the pulse, modulates and re-emits or reflects the pulse back up the transmission line. The resonant circuit may be, for example, a tank circuit that includes inductive and capacitive components. In a tank circuit arrangement, either the inductive or the capacitive component may be configured to be sensitive to acoustic energy such that as acoustic energy impinges on the component, the resonant frequency of the tank circuit changes, thereby modulating the returned electromagnetic signal in accordance with the received energy. 
   In an embodiment, the sensor module is based on a capacitive sensing element  200  as illustrated in  FIG. 2 . A housing  202  has at one end a pressure feed-in tube  204  that allows acoustic energy from the borehole environment, for example via the port  126 , to pass to an interior space  206  of the housing, such that it impinges on a flexible membrane  208 . 
   Motions of the flexible membrane  208  are transmitted to a male conical portion  210  that engages a female conical portion  212  to form a variable capacitor. As shown in  FIG. 2 , the female conical portion  212  may simply comprise a cavity in the housing  202 . Alternately, the female conical portion may be a separate structure held within the housing. One or both of the conical portions  210 ,  212  may include a layer of dielectric material to ensure that even when the portions are in mutual contact, they have some degree of capacitance rather than acting as a short. 
   The male cone  210  is spring biased towards the female cone by a spring mechanism  214 , resulting in a minimum capacitance in the absence of any deflection of the membrane  208 . As acoustic energy vibrates the flexible membrane  208 , the male cone  210  moves relative to the female cone  212 , changing a distance therebetween and altering a capacitance of the device. 
   A spring constant of the spring mechanism, a flexibility of the flexible membrane and a mass of the male cone cooperate to define a physical element of a frequency response of the sensor. As will be appreciated, to monitor high frequency vibrations, the inertial mass of the moveable parts should be minimized, the spring should be relatively soft and the membrane should be highly flexible. For lower frequency monitoring, these factors become less important, and sensitivity may be sacrificed in favor of more durable construction. 
   An electrical lead  216  is included for connecting the sensor  200  to other electrical components of the probe, not shown in this view, but illustrated in  FIG. 4  and described in greater detail below. 
   An embodiment makes use of a crystal-based oscillator  300  as illustrated in  FIG. 3 . The structure of the housing  302  may be similar to that of the housing  202 . For example, the feed-in tube  304  may be similar to the feed-in tube  204  illustrated in  FIG. 2 . The feed-in tube  304  allows acoustic energy to pass into an interior space  206  of the sensor  300 , where it is allowed to impinge on a flexible diaphragm  308 . 
   Motion of the diaphragm  308  is transmitted to a quartz crystal  310 . As pressure is transmitted to an edge of the quartz crystal, its resonant frequency changes. By correct selection of a direction of cut of the crystal, the sensor may be made to be more sensitive to pressure or to temperature. For acoustic monitoring, the crystal should be preferentially sensitive to pressure and relatively less sensitive to temperature. Furthermore, for acoustic monitoring, it is useful for the crystal to be generally relatively thin (e.g., 0.2-2.0 mm) and a typical size is on the order of 1 cm in diameter. 
   A return spring mechanism  314  is provided to bias the crystal  310  and its holders towards the feed-in tube  304  and thereby to tend to cause the diaphragm to return to a neutral position. As with the capacitive sensor, an electrical feed through  316  is provided to couple the sensor  300  to the sensor circuit (not shown). 
   In place of either a variable capacitor or a variable oscillator, a variable inductor (not illustrated) may be used as the sensor component. In such an arrangement, a voice coil or other type of variable inductor may be used to change a resonant frequency of the sensor circuit in response to the received acoustic energy. 
   It should be noted that for any of the embodiments, the sensor pressure feed-in tube may be configured such that it acts as an acoustic filter. In this regard, it may include an opening that acts as a high pass filter and/or an expansion chamber that acts as a low pass filter. By placing multiple such structures in series along the feed-in tube, a band pass filter may be implemented. Rather than filtering in the acoustic domain, filtering may be performed on the electronic signals, either in circuitry or at the computer as desired. 
   Whichever sensor  200  or  300  is used, it may be coupled to the transmission line via an inductive ferrite ring  400  as illustrated in  FIG. 4 . Electrical leads  402  are provided through the electrical feed through  216  or  316  of the sensor module. The leads  402  couple wire loops around the ferrite ring  400  and comprise an inductor. When used with a capacitive sensor  200 , the inductor comprises a portion of the tank circuit along with the sensor. When used with an oscillator based sensor  300 , the oscillator itself has the characteristics of an L-C circuit and the ferrite ring merely acts as a transformer to couple the oscillator to the transmission line. When used with an inductor based sensor (not illustrated), an additional capacitor should be provided in the circuit so that a complete tank is present. In this arrangement, the ferrite ring may be considered as merely a transformer (as in the oscillator configuration) or may be considered to constitute a portion of the inductance of the L-C circuit. 
     FIG. 5  illustrates a package for sensors in accordance with embodiments of the present invention. A number of sensors  500  are disposed within a common housing  502 . For each sensor  500 , there is a corresponding ferrite ring  400 , which is disposed in a portion  504  of the housing  502  that is made from a dielectric material, for example PTFE. As described above, the rings  400  couple the sensors to the transmission line  102 . The sensors, in turn, are held in a metal block portion  506  of the sensor module. Tubing  508  is threaded into the metal block in order to positively locate the sensor package. In a typical application, this tubing may constitute either the production tubing itself, or an extension of the production string. 
   As will be appreciated, it is possible to combine pressure and temperature sensors in a single package, such that the temperature measurements may be used to help account for temperature related drift of the pressure sensor. 
   Depending on the particular use of the sensor, it may be useful to filter the signal so as to emphasize a particular frequency spectrum. For example, in acoustic emission monitoring of rock fracture or structural failures, relatively higher frequency acoustic energy may be of interest. On the other hand, for monitoring fluid movement, lower frequency information is likely to be relevant. In this regard, a number of sensors may be used in a given environment with each optimized for a particular range of frequencies. Likewise, when a number of sensors are used in a single region, information derived from them may be combined to provide directional information about the source of the acoustic energy. 
   Those skilled in the art will appreciate that the disclosed embodiments described herein are by way of example only, and that numerous variations will exist. For example, other arrangements of capacitors, inductors and oscillators may be employed. The invention is limited only by the claims, which encompass the embodiments described herein as well as variants apparent to those skilled in the art.