Patent Publication Number: US-6982925-B2

Title: Fiber-optic seismic array telemetry, system, and method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional application of, and claims priority from, U.S. Nonprovisional patent application Ser. No. 10/198,615, filed on Jul. 18, 2002 now U.S. Pat. No. 6,850,461, the entirety of which is incorporated herein by reference. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   BACKGROUND OF INVENTION 
   Field of the Invention 
   This invention relates to seismic cables that are used, for example, in marine and/or land-based seismic data acquisition. Specifically, the present invention relates to fiber-optic seismic cables utilizing dense wavelength division multiplexing (DWDM) and frequency division multiplexing (FDM). 
   Seismic sensor arrays extend over long distances—sometimes several miles. In such instances, optical fiber sensing of seismic arrays would become economical. However, the prior art optical systems and techniques have performance, reliability and maintenance problems. An example of such WDM/FDM prior art is seen in U.S. Pat. No. 4,648,083 and more recently in U.S. Pat. No. 5,696,857, both of which are incorporated herein by reference. Limitations of the prior art optical systems include: significant attenuation of optical signals passing through telemetry components over long distances and a poor signal-to-noise ratio. A time division multiplexed (TDM) system with input and return bus with optical amplifiers is described in U.S. Pat. No. 6,365,891. Such a system addresses some optical power issues but suffers from many other performance and assembly problems. Further, sensor failure or failure of optical telemetry components, in present fiber-optic seismic cable designs, results in very high repair and maintenance costs. Therefore, there is a need for increasing signal strength, and there is a further need to reduce problems of maintenance and repair. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the invention, a seismic cable is provided for optical sensing of seismic sensors, the cable comprising: at least one strength member; a plurality of optical fibers disposed in a plurality of fiber tubes and including at least one input bus telemetry fiber, at least one input distribution telemetry fiber, at least one return telemetry fiber, and at least one return bus telemetry fiber. 
   According to another aspect of the invention, a FDM/WDM seismic array telemetry system is provided for optical sensing of seismic sensors, the system comprising: an input distribution bus; a return telemetry bus with integral return optical amplifiers; and a telemetry module connected to the input distribution bus and to the return telemetry bus for connection, demultiplexing, remultiplexing and amplifying of signals from the optical sensing seismic sensors. 
   In still a further aspect of the invention, a method is provided for interrogating seismic sensors in a seismic cable, the seismic cable having a modular sensing stations spaced along the seismic cables and a connection module head end of the sensor section, the method comprising: dropping, at the connection modules, a wavelength of light from a input bus telemetry fiber that includes multiple wavelengths of light, distributing the dropped wavelength of light to the seismic sensors, returning the dropped wavelength to a return telemetry fiber, remultiplexing the dropped wavelength of light onto the return bus telemetry, and amplifying, in the seismic cable, the returned dropped wavelength. 
   According to still another aspect, a system is provided for interrogating seismic sensors in a seismic cable, the seismic cable having a modular sensing stations spaced along the seismic cables and connection modules at the head end of the sensor sections, the system comprising: means for dropping, at the connection modules, a wavelength of light from an input telemetry bus fiber that includes multiple wavelengths of light, means for distributing the dropped wavelength of light to seismic sensors, means for returning the dropped wavelength to a return telemetry fiber, and means for remultiplexing and amplifying, in the seismic cable, the returned dropped wavelength on a return bus. 
   In an even further aspect of the invention, a seismic cable is provided comprising: a sensing station, a seismic sensor positioned at the sensing station, a connection module connected to the sensor section, a wavelength drop from a multiple wavelength input telemetry bus fiber, a wavelength distributor from the wavelength drop to the seismic sensor, a wavelength return from the seismic sensor to a return telemetry fiber, and a multiplexer and amplifier on the return bus. 
   In yet another aspect of the invention, a method is provided for attaching seismic sensors in a seismic cable comprising a main strength member inside a cable jacket, at least one fiber tube wound around the strength member, and a sensor station base attached around the cable, the method comprising: removing the jacket, extracting the at least one fiber tube, and attaching a seismic sensor to the fiber tube. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a representational view of an example embodiment of the invention. 
       FIG. 2  is a schematic view of an example embodiment of the invention. 
       FIG. 3  is a side view of an example embodiment of the seismic cable having modular sensor sections having modular sensing stations spaced along the seismic cable. 
       FIG. 4  is a sectional view of an example embodiment of the optical cable. 
       FIG. 5  is a perspective view of an example embodiment of a modular sensor station breakout. 
       FIGS. 6   a  and  6   b  are a side view, in mated and unmated configurations, respectively, of an example embodiment of the telemetry module. 
       FIG. 7  is a schematic view of an example embodiment of the drop and distribution of a wavelength of light. 
       FIG. 8  is a schematic view of an example embodiment of the return multiplexing, coupling onto the return buss and amplifying the optical signals. 
       FIG. 9  is a top view of an example embodiment of a component storage tray in the telemetry module. 
       FIG. 10  is a schematic view of an example embodiment of the optical multiplexing in the telemetry module. 
   

   DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
   Referring now to  FIG. 1 , an example embodiment of a modular fiber-optic cable  1  is seen. In the illustrated example, sensor sections  12  and  16  include seismic sensors (not shown) that generate optical phase signals proportional to the seismic signals being measured. Sensor section  16  is coupled to a connection module  14  where the fiber-optic signals are demultiplexed, distributed, remultiplexed and amplified. Likewise, signals from sensor section  12  are connected through module  10 . An optical connector in the module  10  passes light from the front of one section to the aft end of another, carrying signals to and from the sensor sections  12 ,  16  on input and return busses. Mechanical load of the cable  1  is carried by the termination of a strength member (not shown), for example, a steel wire-rope, in an interconnection between connection modules and sensor sections, as more fully described, below. 
   Referring now to  FIG. 2 , a more specific embodiment of the cable of  FIG. 1  is seen. To obtain fiber-optic signals from sensor sections  12  and  16 , distribution and recombination telemetry  22  and  38  are provided through connection module  10  and  14 . Signals in section  16  pass through connection module  14 , section  12  and connection module  10 . In typical embodiments, many more than two connection modules and sensor sections will be used; and, in such examples, input bus  2  will continue at least to the last connection module  14  in the cable. Input telemetry bus  2 , in various embodiments, comprises multiple wavelengths of light modulated at multiple carrier frequencies. 
   Return telemetry bus  4  is also provided, again through module  10 , sensor section  12 , and at least to module  14 . Laser pump distribution telemetry  6  is provided, again through module  10 , sensor section  12 , and to module  14 , to provide power for amplification in modules  10  and  14  to signals on return telemetry bus  4 . 
   Referring still to  FIG. 2 , main input telemetry bus  2  includes a number of optical fibers with multiplexed wavelengths (λ), and main return telemetry bus  4  returns laser light that has been passed through the seismic sensors  8 . The system also includes a series of cable sections  12  and  16  and telemetry/amplifier modules  10  and  14  through which input telemetry fiber  2  and return telemetry fiber  4  run. In operation, wavelength drops  36  are optically coupled to main input telemetry bus  2  and to section distribution telemetry  22  and distribution fiber  24  for distribution of laser light to sensors  8 . 
   Sensors  8  comprise, in various embodiments, seismic sensors (for example, hydrophones, geophones, accelerometers, other interferometic sensors, Bragg-grating-based sensors, etc.) that are capable of interrogation of signal transmission via fiber optics. For example, see U.S. Pat. Nos. 5,363,342, 5,986,749, and 6,314,056 (all of which are incorporated herein by reference). The signals from the sensors  8  are passed through remultiplexing telemetry  38 , added to the return bus using tap coupler  40 , and amplified by amplifiers  18  in the modules  10  and  14 . According to various embodiments, the amplifiers  18  comprise optically pumped erbium-doped-fiber amplifiers. In a further embodiment, amplifiers  18  comprise waveguide optical amplifiers. The amplifiers  18  offset the loss associated with the combination onto the return bus and passing through connectors. 
   In the illustrated embodiment, the section connection modules  10  and  14  include main input distribution drops  36  which are optically-coupled in the module to section input distribution telemetry coupler  22  and telemetry fiber  24 . Telemetry fiber  24  is passed down the cable inside the fiber tubes and spliced in at the sensor station for input of laser light to optical sensors  8 . Also included are section return couplers  38  and return bus couplers  40 , optically-coupled to section return telemetry fiber  28 . Return telemetry fiber  28  passes down the cable inside a fiber tube and is spliced in at a sensor station for receipt of return laser light. Optical amplifiers  18  are optically coupled in return telemetry  4  and activated by laser pump distribution telemetry  6  to amplify the optical signals from sensors  8 . 
   In typical embodiments, the section connection modules  10  and  14  and the cable sections  12  and  16  are optically-coupled through optical connectors and physically-coupled through strength members (not shown). 
   In various embodiments, the distribution laser light borne by the main input telemetry is wavelength division multiplexed (WDM). In many embodiments, the distribution laser light borne by the main input distribution telemetry is both wavelength division multiplexed (WDM) and frequency division multiplexed (FDM) (for example, one carrier frequency and a multiplicity of laser wavelengths on each distribution optic fiber). Also in various embodiments, the return laser light borne by the main return telemetry  4  is both wavelength division multiplexed and frequency division multiplexed (WDM/FDM). 
   In a specific example, the section wavelength drops  36  demultiplex, from the main input telemetry  2 , a unique wavelength of distribution laser light for each cable section  12  and  16 . The sensors  8  in the particular cable section are all illuminated by the unique wavelength. For example, all of sensors  8  of cable section  12  are illuminated by wavelength λ 1 , and all of sensors  8  of section  16  are illuminated by wavelength λ 2 . The sensors within a particular sensor group  34  in a particular section (e.g., section ( 16 )) are illuminated by a particular carrier frequency. Accordingly, any particular group  34  in section  12  and  16  is illuminated by a unique combination of wavelength and carrier frequency. Group size varies depending on a variety of array design principles known to those of skill in the art. 
   The section return couplers  38  and the return bus couplers  40  multiplex, onto return optical fibers in the main return telemetry  4 , a multiplicity of wavelengths (λ) and carrier frequencies (ω) containing the signals from sensors  8 . In the specific embodiment illustrated, the return couplers  38  multiplex, onto each return optical fiber in the return telemetry  4 , return laser light output from only one sensor in each sensor group  34 . 
   In some specific embodiments, the passband of a particular wavelength drop is based on the ITU grid of 100 GHz or about 0.8 nanometer; in other embodiments, the passband is narrower or broader. One specific embodiment for the section wavelength drop comprises a 3-port, thin-film filter of the kind sometimes known in the industry as a “drop filter,” (for example, those manufactured by Excelight Communication, Inc., of 4021 Stirrup Creek Dr., Durham, N.C. 27703, model number DWDM10C270BCCZ-01, the particular model being a “100 GHz High Isolation WDM filter”). In various specific examples, the filter comprises a dual-stage, single-stage, or any number of filter stages. Isolation of the filter directly affects the crosstalk of the system; dual stage filters typically provide isolation of greater than 40 dB. The high isolation and low loss associated with these types of devices makes them preferred. 
   According to some examples, the section wavelength drop comprises a fused optical coupler and a Bragg-grating. In other examples, the section wavelength drop comprises an optical circulator and a Bragg-grating. Optical amplifiers are included, in various examples, in an input bus if the array length is such that attenuation over distance becomes higher than can be tolerated. 
   Typical embodiments of the kind illustrated include a laser source of distribution, multiplexed laser light. In many such embodiments, the laser source comprises a distributed feedback laser. Also, in some such embodiments, the laser source comprises a tunable laser, a fiber laser, or any other narrow linewidth laser source. A carrier frequency is added to the light using an optical phase modulator driven by a frequency synthesizer. 
   In various embodiments of the kind illustrated in  FIG. 2 , the return couplers  38  and  40  comprise wavelength-independent fused biconic taper (FBT) coupler; and in some examples, the return coupler  40  comprises an optical circulator and a fiber Bragg-grating. 
   Referring now to  FIG. 3 , an example of a section connection module  10  and sensor section  12  is seen. Sensor section  12  comprises a plurality of sensor station assemblies  51   a – 51   c . Although three assemblies  51   a – 51   c  are shown, those of skill in the art will understand that many more or less are used in various alternative embodiments. 
   Referring now to  FIG. 4 , a cross-section through line A of  FIG. 3  is seen of sensor section  12 , between sensor station assemblies  51   a  and  51   b . Cable sheath  62  surrounds optical fiber tubes  64 , each of which holds a plurality of optic fibers  66  (used, for example, for the various functions described with reference to the earlier figures). Fiber tubes  64  are disposed between the interior  69  of cable sheath  62  and strength members  68  (which take mechanical loads of dragging or towing of the cable off fibers  66  and other non-load-bearing components). According to various examples, a first fiber tube  64  may house the input telemetry fiber  2 , while another tube houses return telemetry  4 , and still another houses amplifier pump fiber  6 , leaving the remaining tubes to connect the sensor stations to throughout the sensor section  12 . 
   Placement of fiber tubes  64  near the interior surface  69  of the cable jacket  62  facilitates installation of sensor-station assemblies  51  ( FIG. 3 ). Fiber tubes  64  are easy to extract; and, therefore, individual optical fibers  66  are easily extracted from particular tubes  64 . Cutting of strength members  68  when sensor-station assemblies  51  are connected is avoided by placement of fiber tubes  64  between jacket  62  and strength members  68 . Cable termination hardware at each sensor station is thus avoided, representing a substantial savings in hardware and labor cost in seismic cables. 
   Referring now to  FIGS. 5   a  and  5   b , an example embodiment of sensor station  51  ( FIG. 3 ) is seen. Cable jacket  62  ( FIG. 4 ) is removed and the appropriate fiber tube  64  ( FIG. 4 ) is extracted. The pad base  70  is attached and epoxied into place  72 . A splice housing  71  is opened through cover  71 ′ to access the fibers from the tube  74  which make up system distribution telemetry fiber  24  and return telemetry fiber  28 . Fusion splices  76  are used in various examples connecting sensors  8  to distribution telemetry fiber  24  and return telemetry fiber  28  and to the module components distribution telemetry  26  and return telemetry  30 . 
   In the specific embodiment shown in  FIGS. 5   a  and  5   b  an additional telemetry distribution coupler  22 ′ is used to further distribute laser light to sensor  8   b – 8   d . Sensors  8  are held in a sensor housing  82 , which is then held to the splice housing  71 , for example, by straps  73 . While two straps are shown, of course other straps and alternative means of attachment of sensor housing  82  are used in various embodiments. This assembly technique greatly maximizes the reliability and simplifies any rework, because only optical splices are included in the section that can be damaged; the telemetry components are collocated in a module and not distributed throughout the array sections. Should a channel go down, the splice tray is easily opened and a new sensor housing is spliced in. The manufacture of sensor housing  72  and sensors  8  is performed according to various methods that will occur to those of skill in the art, depending on the particular environment of use for which the sensor are intended. Potting material (not shown) and seals, for example, are used in some water-tight, high-pressure embodiments. 
   It should also be noted that  FIGS. 5   a  and  5   b  illustrates an example embodiment in which there are multiple sensor taps per connection modules. In other words, the modular section served by connection module  10  ( FIGS. 1 ,  3 ) comprises multiple sensor stations, and the fiber tubes  64  ( FIGS. 4 ,  5   a , and  5   b ). Once a group of fibers are terminated to a group of sensors they are not used again in that section. As shown in  FIG. 5   b  six optical fibers are connected to the sensor station. However, in some cases, a tube contains extra fibers that are connected to another sensor station, (e.g., additional fibers that need to be passed through one sensor station to get to another. Therefore, multiple stations are run through the same fiber tube, in some example embodiments of the invention, and at a further sensor station (e.g.,  51   b  of  FIG. 3 ), another fiber tube ( FIG. 4 ) is used. 
   According to still another alternative embodiment (not shown), there is a single connection module  10  for all sensor stations  51   a  in a section  12 , thus reducing the fibers required for passing the laser light to and from the section to only those needed to hold the main distribution telemetry  2 , the main return telemetry  4 , and in the case of remote amplifier pumping the laser pump drive  6 . This greatly simplifies the optical connector requirement in the system. 
   Referring now to  FIGS. 6   a  and  6   b , a specific example embodiment of a connection module  10  ( FIGS. 1 ,  3 ) is shown in mated and unmated configurations, respectively. Optical cable  7  connects to strength termination member  81  on each end of connection module housing  82  through locking rings  83 . Module pressure barriers  84  isolate the interior of housing  82  and are penetrated by fiber pressure feed-throughs  85 . On one end of the interior housing  82 , optical connection inserts are attached via standoffs from pressure barriers  84  and connect optical cable  8  to optical storage trays  88  which are mounted in tray support brackets  89 . Storage trays  88  connect on the other end via fusion splicing, or other means, to filter and telemetry components. The mounting brackets  89  are attached to the pressure bulkhead  84 . 
   A schematic of example distribution telemetry held on a tray  88  is seen in  FIG. 7 , where a multiple wavelength signal is provided on fiber  2 . A thin-film filter-drop  36  takes a wavelength that is then split by 50/50 distribution couplers  22  to supply interrogation signals for sensors  8  ( FIGS. 2 ,  5 ) via distribution fibers  24 . The number of splits is dependent on array configurations and performance known to those skilled in the art. 
     FIG. 8  shows a schematic of an example return telemetry held on a tray  88  ( FIG. 6 ). In the illustrated example, 50/50 return couplers  38  receive signals from sensors  8  ( FIGS. 2 ,  5 ) via return fibers  28 . A return tap coupler  40  then couples the signals to return fiber  4 . An erbium-doped fiber  94 , pumped via a WDM  95  and using a 1480 laser pump signal on pump fiber  6  amplify the return, multiplexed signal. An optical isolator  92  is provided to keep unwanted light from getting into the sensors. 
   As seen in  FIG. 9 , according to a specific example, a storage tray  88  stores the optical components and fiber from the devices shown in  FIGS. 7 and 8  using, for example, an Europlus EFA0404D1 fiber-storage reel. In the case of  FIG. 8 , Erbium-doped optical fiber  94 , return multiplex couplers  38 , main return bus couplers  40 , optical isolator  92 , and 1480/1500 WDM coupler  95 , are all mounted to tray  88 . Such trays  88  hold four amplifiers per tray (two per side). Similarly the all components of  FIG. 7  are stored on a tray. 
   Referring now to  FIG. 10 , an example of the configuration of the connections used in an entire connection module  10  to house the above optics, where drop distribution  100  resides on a storage tray (or trays) in the same connection module  10  as return/amplification components  102  and pump optics  104 , on different trays. This greatly simplifies the construction and assembly steps used in the optical array. 
   In a specific assembly process, return telemetry couplers are mounted to a tray, followed by a main return bus coupler, an optical isolator, erbium-doped fiber, and WDM coupler. Fiber length between components is maintained to avoid excess fiber loops and to keep the various components collocated. Optical power is monitored during assembly to insure splices have acceptable losses. 
   In a specific assembly embodiment of 1×4 return telemetry couplers, data on each of the four couplers is monitored, and the coupler showing the best uniformity from the monitoring is used for the main return bus coupler. Of the three remaining couplers, the one showing the next best uniformity is used for the tray base, which is then spliced into a laser source for measurement of outputs. A splice of the outputs is then made to the input of the two remaining couplers, and their outputs are also monitored to ensure splice quality. 
   In some embodiments, heat-shrink splice protection is used with a micro-protection sleeve. Components are taped into the tray and cutback measurements are made to verify losses on the leads. A 1550 nm source is spliced into a coupler lead in the direction of travel for the amplifier chain; optical power is measured exiting the base coupler for quality control. Next, a main return bus 50/50 coupler is spliced on, and optical power is again measured. Then, an optical isolator is spliced in and power is again measured. A WDM coupler is then spliced in, and output of the coupler is measured for quality control. 
   The assembly of the wavelength drop of a tray is performed in a similar process, using, for example, a thin-film filter or other drop components replacing the amplifier and return bus coupler. A particular benefit of such process is the ease of connection of an assembled tray to an optical cable. Various embodiments of narrow-band wavelength drops (for example, thin-film filters with three ports) provide several technical benefits, including improved isolation between channels. 
   A comparison of main distribution fibers to section fibers is performed, in some embodiments; the section fibers contain not only the drop wavelength but also low levels of the other wavelengths distributed in a system, this makes up the system crosstalk. That crosstalk level is much smaller in the 3-port filter configuration because of the higher isolation than it is for other embodiments, comparable performance has been achieved with other embodiments but with a significant price penalty. A specific type of noise dealt with by this embodiment includes a kind of crosstalk from other wavelengths, as opposed to thermal, ASE, polarization-induced, or other noise types. 
   Typical drop filter components be it thin film filters or single circulator and grating provide about 30 dB isolation, while a single-fused coupler gives only about 15 dB. A new generation of relatively inexpensive dual-stage, thin-film filters, such as those mentioned above from Excelight communications, gives some embodiments more than 40 dB of isolation. In some embodiments, the standard components are ganged or cascaded to achieve better isolation results or noise performance; however, ganging couplers incurs additional hardware expense. 
   Narrower bandwidth in various embodiments allows for more channels, or more wavelengths of distribution light, in a passband (such as, for example, the passband of Erbium-doped fiber-type optical amplifiers). The Erbium bandwidth is about 30–70 nanometers, centered on about 1550 nanometers. More channels in the band means more wavelengths per fiber, more optical sensors (more channels per array), fewer distribution and return fibers per optical sensor or channel, fewer contacts on each connector, less fiber in the cable, much less hardware overall, and much less expense for the same sensing capacity. For example, traditional FDM sensing is achieved through a 12×12 array with one wavelength of distribution light that gives 144 channels. Example embodiments of the present invention, on a 12×12 array, yield 12×12×N wavelengths or 144×N channels. 
   Further technical benefits of various embodiments include improved filtering and improved exclusion of all “other” optical noise. Less crosstalk means less noise and narrow bandwidth mean more channels. Thin-film filters or Bragg-gratings typically can be made to yield a single transmission/reflection bandwidth of less than a nanometer (0.8 nm is a telecommunications standard) somewhere in the Erbium spectrum. Fused couplers have narrow bandwidths at a wavelength of interest, but fused couplers also pass so many other wavelengths in the Erbium spectrum as to result in much poorer overall noise performance compared to thin-film filters or circulators with Bragg-gratings. 
   Embodiments in which drops are modular and wavelength-specific results in optical sensor sections modular and non-wavelength-specific. Seismic cable sections and module in typical embodiments are installed literally anywhere in an array having thousands of sensors, completely plug-compatible at any location in the entire array. Adding the wavelength drops to the module make a section wavelength specific. In some example embodiments, the drops are used as a type of program plug for the section in the array. Switching the program plug allows for a section to be used anywhere. 
   In the illustrated embodiments, pump laser light for amplification is provided by remote pumping (for example, in a cable truck or marine vessel). However, in alternative embodiments, each connection module includes a separate laser source for the amplifier. In various such embodiments, power for the pump is supplied through the cable by a power line or batteries in the connection modules, according to two power supply examples. In this case pump wavelengths of 980 nanometers could also be used. 
   Modularity greatly reduces the expense and difficulty of field repairs, since optical sensor cables are typically many kilometers in length. It is very difficult to treat an entire cable as a single unit for repair. It is much more efficient to identify a section of the cable as defective, than simply replace the defective section by unplugging it in its entirety and plugging in another identical wavelength-independent cable section and adding the drops for a single wavelength into a single section connection module makes for simple, convenient, and inexpensive troubleshooting and field repair of large optical sensor arrays. 
   The above description has been given by way of example only; other embodiments and further benefits will occur to those of skill in the art upon review of the present specification without departing from the spirit or scope of the invention as defined herein.