NO20121451L - Procedure for collecting seismic data - Google Patents

Abstract

A method for gathering seismic data from a series of seismic sensor stations is provided. Each station houses a plurality of optical sensors that share a wave's wavelength. The common wavelength is different for each station in the same series. The sensors housed in each station are examined using hour division multiplexing (TDM) and sensors housed in different sensor stations in each series using wavelength division multiplexing (WDM).

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application has content related to content in the American

applications Serial No. 10/650,117 filed Aug. 27, 2003 (WEAT/0347), Serial No. 10/649,588 filed Aug. 27, 2003 (WEAT/0348), Serial No. 10/649,590 filed Aug. 27, 2003 (WEAT/0493), and Serial No. 11 /313,275 filed Dec. 20, 2005 (WEAT/0608). Each of these is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of the invention

[0002] Embodiments of the invention are generally related to seismic measurement, and specifically to components and techniques for deploying and examining arrays of seismic sensors, such as in ocean bottom seismic sensing (OBS) applications.

Description of related art

[0003] Investigations for marine seismic extraction for the extraction and monitoring of -hydrocarbon sonrproducer zones^^ sensor array, i.e. a plurality of sensor stations interconnected by cable sections. The cable arrays may include a large number of sensor stations (for example, several hundreds or thousands), and may be buried in a predetermined pattern on the seabed. Optical sensors may be particularly well suited for ocean bottom seismic (OBS) applications due to their robust nature, lack of sensitive electronics, and potential for light weight sensors and cable assemblies that are relatively inexpensive to install. An optical sensor station must include optical hydrophones, multi-axis accelerometers and/or geophones.

[0004] The individual sensors in a station, such as accelerometers oriented in orthogonal X, Y and Z axes can be interferometers. In such systems, a light source generates probing light pulse pairs (spaced according to a fiber length between reflectors in each interferometric sensor), resulting in interfering signals that are reflected back to the surface. These interfering signals can be analyzed by surface electronics, and stored and translated into seismic data.

[0005] As the total number of sensors in the arrays increases in high channel count (HCC) applications, it becomes a challenge to probe each sensor using a manageable number of optical fibers going to and from surface instrumentation. While multiplexing techniques such as wavelength division multiplexing (WDM) and time division multiplexing (TDM) are well known, there are typical limitations to Tiver. On the other hand, there is a practical limit to how many sensors can be probed by a single fiber due to a limited number of wavelengths and limitations on total transmitted power per fiber set by optical nonlinear interactions. On the other hand, TDM of multiple interferometric sensors using reflectors with a common wavelength is subject to unwanted reflections between sensor elements (causing crosstalk).

[0006] In some cases, in order to generate sufficient optical power to examine a large number of sensors in an OBS-array^anjelati^-animal-components-likeJcilder-which-are-pumped and optical amplifiers are used. Unfortunately, such remote components are relatively expensive and typically require special pressure-sealed housings to operate at the high pressures seen on the seabed. Replacing broken components that are remotely located underwater is an expensive and time-consuming process.

[0007] Packing and laying out OBS sensor arrays also creates challenges for

achieve efficient coupling of the seismic signals to the respective sensors. Station packing should ensure adequate protection of the sensors during installation and operation, and should also withstand hydrostatic pressures typical of the seabed (eg 50 - 200 bar). Packaging and the design of the drive should ensure high reliability over a relatively long expected life, and efficient assembly procedures to reduce overall production costs.

[0008] Cutting and splicing cables/fibres for data transmission in the cable array at each of the sensor stations increases time and cost at the same time as reliability decreases. The design of the sensor station and/or cable array can affect how many splices are required at each station. Consequently, any designs or techniques that reduce the number of splices in the cables/fibers for data transmission at each station reduce assembly time and cost while increasing the reliability of the cable array.

[0009] Therefore, there exists a need for a cheap and improved OBS sensor system with

a large number of sensors, as well as corresponding cable and

sensor station components, manufacturing and layout techniques.

SUMMARY OF THE INVENTION

[0010] Embodiments of the invention are generally related to methods,

devices and systems for seismic signal processing.

[0011] One embodiment provides a system for seismic measurement that generally includes one or more arrays of seismic sensor stations and instrumentation. Each station houses a plurality of optical sensors that share a common wavelength. The common JwavelengthiLÆrJcÆskjfiH^^ connected to the arrays of seismic sensor stations and configured to probe sensors housed in each station using time division multiplexing (TDM) and to probe sensors housed in different sensor stations in each array using wavelength division multiplexing (WDM ).

[0012] Another embodiment provides an array of seismic sensor stations that generally includes at least one array coupling module and a plurality of array cables extending from the array coupling module. Each array cable includes a series of array cable modules and a multi-fiber conductor cable. Each array cable module includes a series of seismic sensor stations. Each station houses a plurality of optical sensors that share a common wavelength. The common wavelength is different for each station in the same series. The array also includes, for each array cable module, a module connector node for connecting another fiber or fibers of the guide cable of a sensor fiber used to survey a series of corresponding seismic sensor stations.

[0013] Another embodiment provides an array cable module that generally includes a cable section extending a length of the array cable module and having a plurality of optical fibers and a plurality of seismic sensor stations. Each station houses a plurality of serially connected interferometric sensors sharing a common wavelength, wherein the common wavelength of each station is different. The system also includes at least one sensor fiber for probing the sensors in each of the stations and a module coupling node configured to optically couple at least one of the plurality of fibers of the cable section to the sensor fiber while a remaining one or more of the plurality of fibers of the cable section bypasses the sensor stations without optical coupling.

[0014] Another embodiment provides a method for collecting seismic data from a series of seismic sensor stations. Each station houses a plurality of optical sensors that share a common wavelength. The common wavelength is different for each station in the same series. The method generally includes examining, via a common optical path, sensors housed in each station using time division multiplexing (TDM) and examining, via the common optical path, sensors housed in different srøsorjstajoj^

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] A more detailed description of the invention, briefly summarized above, is obtained by reference to embodiments, some of which are illustrated in the attached figures so that the manner in which the above-mentioned characteristics of the present invention can be understood in detail. However, it must be noted that the attached drawings illustrate only typical embodiments of this invention, and should therefore not be considered as limiting its scope, for the invention may accommodate other equally effective

embodiments.

[0016] FIG. IA and IB illustrate an example of measurement system topologies for

ocean bottom seismic (OBS) according to embodiments of the present invention.

[0017] FIG. 2 illustrates an example of a sensor array cable module configuration,

according to an embodiment of the present invention.

[0018] FIG. 3 illustrates a schematic picture of an example of a sensor station i

according to an embodiment of the present invention.

[0019] FIG. 4 illustrates a basic configuration of sensors in a sensor station according to one embodiment of the present invention.

[0020] FIG. 5A and 5B illustrate a schematic view of the instrumentation of the system i

FIG. 1 according to an embodiment of the present invention.

[0021] FIG. 6A illustrates an example grating profile showing three adjacent wavelength channels, suitable for gratings in a seismic sensor station according to an embodiment of the present invention.

__[M22]JEJ^j6Bjllus^ reflectivities in a sensor station according to an embodiment of the present invention.

[0023] FIG. 7A and 7B illustrate an example of module connection nodes using

band wavelength division multiplexing (B-WDM) according to embodiments of the present invention.

[0024] FIG. 8 illustrates examples of groupings of wavelengths in several time slots for pulse pairs according to embodiments of the present invention.

[0025] FIG. 9 illustrates example reflected pulses from an array of reflectors separating sensors in a station crosstalk.

[0026] FIG. 10A - 10B illustrate an example OBS measurement system topology that allows probing from two ends according to an embodiment of the present invention.

[0027] FIG. 1 IA and 1 IB illustrate another example of OBS measurement system topology that allows probing from two ends according to an embodiment of the present invention.

"[0028TFIG. f27TogT2B lllustrereret" example of a configuration cable module for sensor array which is examined in first and second directions, respectively, according to an embodiment of the present invention.

[0029] FIG. 13A and 13B illustrate another example of a sensor array cable module configuration being examined in first and second directions, respectively, according to an embodiment of the present invention.

DETAILED DESCRIPTION

[0030] Embodiments of the invention generally relate to seismic sensor systems and sensor station topologies, as well as corresponding components, manufacturing, and cable and sensor station layout techniques. For some embodiments, networks of optical ocean bottom seismic (OBS) stations are provided, in which sensor stations are effectively laid out in a modular fashion as series of array cable modules laid out along a multi-fiber cable.

[0031] Interferometric sensors in each sensor station can share a common wavelength and be examined in a time division multiplex (TDM) manner. However, each sensor station may use a different wavelength for its sensors allowing multiple stations in series in an array cable module to be probed on a common "sensor" fiber by using wavelength division multiplexing (WDM).

[0032] For some embodiments, in each array cable module, only the sensor fiber is connected to the sensors in each station, while a multi-fiber pipe "bypasses" each sensor station without any connections and, thus, no cutting or splicing. At a transition point between array cable modules, a module coupling node cannot be used to connect another fiber from the multifiber pipe to the sensor fiber used to probe the next series of sensor stations. In such embodiments, only the sensor fiber needs to be spliced at the sensor station to connect to the sensors for that station.

[0033] Measuring systems for ocean bottom seismic (OBS) are described below, as a particular, but not limiting, example of an application in which embodiments of the present invention can be used to advantage. However, one skilled in the art will recognize that the concept described herein can be used to similar advantage in a wide range of other applications in which a large number of optical sensors have been investigated.

[0034] Other examples of other similar sensor arrangements that may be contained in a common housing and probed via the methods described herein (such as a tube element or core tube) include flowmeters using arrays of sensors that are linearly spaced. m in detail in TTS patent number 6,785,004 entitled "METHOD AND APPARATUS FOR EXAMINATION OF FIBER OPTIC SENSORS", herein incorporated by reference in its entirety.

[0035] Furthermore, while embodiments of the present invention will be described with reference to optical fibers, those skilled in the art will recognize that any type of suitable optical waveguide may also be used. Furthermore, while embodiments of the present invention will be described with reference to sensor elements that utilize in-line reflective elements such as FBGs to form interferometers, those skilled in the art will recognize that concepts described herein and set forth in the claims may, in in some cases, also be applied to interferometers that use transmissive elements (with analogies drawn between transmissive properties and reflective properties), and more generally to a wide range of optical sensors.

AN EXAMPLE OF AN ATTENTION SENSOR SYSTEM

[0036] FIG. 1A illustrates an example of an OBS system 100A according to an embodiment of the present invention. The system 100A includes an instrumentation unit 110 configured to survey an array of sensor stations 142 that may be laid out along a plurality of array cables 1060 extending from an array connector module 130A. As illustrated, each array cable 1060 may include a series of array cable modules 140 where each array cable module includes a module connection node 144 and a series of sensor stations 142 .

[0037] In some embodiments, the instrumentation unit 110 can be located on the sea surface ("topside"), for example on a boat or platform. For other embodiments, the instrumentation unit 110 may be located under water, for example in a watertight chamber in the sea (for example on the seabed). In such cases, either optical or electrical cables can be used to send processed data from the instrumentation unit to a platform, ship or to a data recording or processing center on land.

_[0038] F.n lead cable 1?,0 can connect the instrument irngen-UO-and the array coupling module— 13 OA. The guide cable 120 can be a separate or standard cable suitable for underwater laying, of varying length depending on the specific application, for example in the range from 1 - 50 km. The guide cable 120 may include one or more fibers to carry probing light pulses to the sensor stations, and to carry reflected light pulses from the sensor.

[0039] The total number of fibers in the guide cable 120 may depend on, among other things, the total number of array cable modules to be examined. As will be described in more detail below, for some embodiments, guide cable 120 may include at least two fibers for each array cable module, including one for carrying probing light pulses and a separate one for carrying return light pulses (eg, reflected). In such cases, array connector modules may include connectors to connect two fibers from the guide cable 120 to each array cable module in an array cable. Furthermore, the type of fibers contained therein can be selected based on a number of factors, such as non-linearity, polarization characteristics and total loss.

[0040] For some embodiments, the lead cable 120 may be laid out while separated

from the array cable modules 140 (for example on the seabed) and later be connected to the array cable modules. In such embodiments, the array connection module 130A may be referred to as a "wet connection" node because the connection is made underwater. The array connector module 130A may have a pressure-sealed housing containing a fiber distribution network with another fiber group 141 routed to probe sensor stations 142 along corresponding other cable arrays 1060. The array connector module 130A may also include optical connectors, such as "wet-mate" connectors. As will be described in more detail below, a module connector node 144 in each array cable module 140 may connect another fiber from a fiber group 141 to a sensor fiber 146 used to probe all sensor stations 142 in a single array cable module 140 .

[0041] As illustrated in FIG. IB, for some embodiments, multiple array link modules 13 OB may be utilized with one or more array cables 1060 (and corresponding series of array cable modules 140 ) extending from each. Jilsv^ende^ller-same-type^ — used in one of the topographies shown in FIG. 1A or FIG. IB. In practice, the specific choice of network topology (for example between that shown in FIG. IA, FIG. IB or some other topology types) will typically depend on the oilfield/reservoir topology to be mapped and existing infrastructure on the seabed.

[0042] In some embodiments, sensor stations 142 may be in a series of array cable modules

140 be examined by using a combination of both wavelength division multiplexing (WDM) and time division multiplexing (TDM). As an example, various sensors in a station 142 (eg, x, y, z accelerometers, a reference interferometer, and a hydrophone as shown in FIG. 3) may be interferometric sensors with reflective elements (eg, gratings) that share a common wavelength, and in this way can be examined via TDM.

[0043] However, as illustrated in FIG. 2, each of the (N) different stations 142 in an array cable module 140 (surveyed with a common sensor fiber 146) may use a different wavelength (eg XI-XN) for its sensors. Therefore, multiple stations 142 in an array cable module 140 can be surveyed, via WDM, using a common sensor fiber 146. The number of stations (N) can be limited by several parameters, such as the available spectral bandwidth, the amount of loss through each station, and the total fiber length with signals that propagate in both directions (Rayleigh scattering). In any case, a total length of several kilometers, with up to 10-100 stations per array cable module is achievable, for example with N=20 in one embodiment.

[UU44J Unfilæ^nsbrstations (ie examined on different wavelength channels) will experience

a different loss depending on their position in the array cable. Therefore, in some embodiments, the wavelength channel order in the sensor array can be controlled to reduce crosstalk. The sequence of wavelengths in an array cable module can be chosen to be any sequence of N different wavelengths.

[0045] Losses contributed by joints can be controlled by reducing the total number of joints required in the system. For one embodiment, only the splices required at the station 42 can be coupled to the sensor fiber 146 (shared with other stations 142 in the same array cable module 140) to the sensors in that station. A remaining set of fibers can "bypass" the sensors in the station in an uncut multi-fiber tube 148 (eg, a fiber in metal tube or FIMT).

[0046] As illustrated in FIG. 2, at a transition between array cable modules 140, a module connector node 144 may be used to connect another fiber from the multifiber tube 148 to the sensor fiber 146 of a following array cable module 140. The module connector node 144 may use any suitable components for such a transition, such as an optical circulator 149 , optical coupling, wavelength multiplexer and the like. Further, as will be described in more detail below, with reference to FIG. 12 and 13, the exact components may depend on the specific array topology used for a given embodiment.

[0047] In any case, another fiber pair of the multi-fiber tube 148 (where one fiber in the pair

is used for lowering and one for raising) is used to examine the N

the sensor stations 142 (for example with wavelengths XI-XN) of each different array cable module. Therefore, N sensor stations may be surveyed per fiber pair in the multifiber pipe 148 by cutting into the multifiber pipe 148 to connect a new fiber pair to the sensor fiber 146 by a following array cable module (via circulator(s), coupler(s), WDM(s) or similar components ) which occurs only at the module connection nodes 144. This reduced number of joints simplifies the overall array design and can significantly reduce manufacturing costs. This also reduces transmission loss to and from the array cable modules which reduces problems with optical nonlinearity and the need for expensive optical power amplifiers. The module connection nodes 144 may be separate components, or for some embodiments may be investigated in a seismic station 142 to provide a compact and efficient design. Specific designs, as well as procedures for such splicing and corresponding handling of a sensor fiber and multifiber tube are described in detail in US Patent Application Serial No. 11/313,275 filed Dec. 20, 2005 entitled “SEISMIC

SEA-BOTTOM STATION". —ET-EK£E-MBEtFÅ-E<J>^—

[0048] FIG. 3 illustrates a schematic image of an example of an arrangement of seismic sensors in a seismic sensor station housing 200 according to an embodiment of the present invention. As illustrated in FIG. 3, a section of sensor fiber 146 leading to or from a sensor station 144 can be spliced onto a sensor fiber section that passes through the sensor station housing 200. As illustrated, the housing 200 can include a variety of different sensors, to which the sensor fiber 146 can be connected with only two splices 201. At the last station (XN) in an array cable module 140, the sensor fiber 146 may be connected to a terminating connector 203.

[0049] As illustrated in FIG. 2, additional fibers can pass through the station 144 without connection to the sensors, thus avoiding cutting and splicing of those fibers. Each of these fibers can later be connected to a sensor fiber 146 in a following array cable module 140 in the series and used to probe the sensors therein. As illustrated by the small dashed lines, after a fiber has been connected to an array cable module 140, that fiber may then be left unconnected.

[0050] In the illustrated arrangement, the housing 200 contains the reference interferometer 210, orthogonal X, Y and Z accelerometers 220 (220X, 220Y and 200Z, respectively) and a hydrophone 230. As described in the above referenced application, the accelerometers 220 can be arranged in a type of liquid-filled container in the housing 201 to dampen mechanical resonances caused by mechanical disturbances and pressure fluctuations. The housing may also include a mechanism for transmitting pressure variations between the ambient environment and the interior of a second container containing the optical fiber coil of the hydrophone 230. The reference interferometer may be used to compensate for probing laser frequency fluctuations or phase shifts in a compensating interferometer 528 (described in more detail below) or in the guide cable as described in US Patent Application Serial No. 10/693,619 filed October 24, 2003 entitled "Downhole Optical Sensor System with Reference".

[0051] Each of the sensors 210, 220 and 230 may be formed from a length of fiber (for -example-a-coil^of-separating-a-p^— embodiments, the couplers 202 may be fiber-based Bragg gratings (FBGs). Furthermore, As will be described in more detail with reference to Figures 6A and 6B, the optical properties of the gratings, including the properties of the reflective spectrum, can be controlled to reduce cross talk between sensors in the same station as well as sensors from other stations.

[0052] While each sensor can be formed by two grids, grids can be shared between sensors,

so that only M+l gratings are required for M interferometric sensors. For example, in the illustrated arrangement, six gratings with overlapping channel (reflection) bands are used to form the five sensors shown.

[0053] As illustrated in FIG. 4, the reference interferometer 210 may be formed by a coil

which separate the gratings 202j and 2022. The x, y and z accelerometers 220 may be formed by coils separating gratings 2022 and 2023, 2023 and 2024, and 2024 and 2025, respectively, while the hydrophone 230 may be formed by a coil separating gratings 2025 and 2026. For

in some embodiments, the gratings may be formed in the fiber section, with suitable spacing before wrapping the coils resulting in spacing (optical path length), L, after wrapping. Shaping the grids in this way can eliminate the need for joints between sensors, reducing losses, production time and thus total cost.

[0054] Any change in the optical path lengths between the fiber-based Bragg gratings, which would typically result from external influences on the accelerometer or hydrophone fiber coils, would change the superimposed resulting reflected signal from a seismic station. U.S. Patent No. 205/009/<y>!)S describes examples of interferometric accelerometers for determining acceleration, and methods of fabricating such accelerometers. The accelerometers are based on a rigid frame, a mass movably suspended in the rigid frame, and a measuring coil partially wrapped around surfaces of first and second elements to detect movement of the mass in response to an acceleration based on a change in the length of the measuring coil.

[ 0055] A general problem with arranging a hydrophone along with x, y and_z accelerometers (or geophones) in a four-component (4-C) seismic station is the cross-sensitivity between the hydrophone and the accelerometers. While it is generally desirable to optimize the exposure of the hydrophones to pressure variations, it is generally undesirable to allow pressure variations to affect the geophones/accelerometers. This influence of the pressure signal on the geophones/accelerometers forms an unwanted cross-sensitivity. For some embodiments, the sensor station and housing may be designed to reduce such effects, and may be designed according to one of the seismic sensor station housings described in detail in the US patent application entitled "Seismic Sensor Station Housing" and filed herewith (document number WEAT/ 0740).

[0056] Referring back to FIG. 3. To examine the sensors is an optical "double

pulse" signal 310 with a wavelength (XI in the illustrated example) in the channel (reflection) bandwidth of the gratings 202i-202é sent up the optical fiber section inside the housing 200. The time delay between the two pulses is chosen to fit the the optical propagation (round trip) delay between each pair of successive gratings

202. Therefore, in the optical signal reflected from the gratings, a reflection of the second pulse from the first fiber grating will superimpose the reflection of the first pulse from the second fiber-based Bragg grating.

[0057] For the illustrated arrangement of sensors, the use of a total of six gratings 202 will result in a total of five interfering (superimposed reflected) pulses 320 being produced and containing the sensor signals followed by back pulses 322. In some cases, sensor crosstalk can be caused by multiple reflections (ie .reflected pulses that are -reflected and interfere TermeTim"aTmmpulse)r1 As described in more detail below with reference to FIG. 9, measurements can be taken to reduce the influence of such crosstalk, for example by applying an inverse scattering algorithm, such as layer peeling (layer-peeling), in the instrumentation unit.

[0058] As previously described, multiple seismic stations can be surveyed via a common sensor fiber 146 using wavelength multiplexing, by selecting different wavelengths for the gratings of each seismic sensor station. Illustratively, the gratings of the shown seismic sensor station are set to a first optical wavelength Al. while the probing optical signal may include light at other wavelengths A2, A3, A4,... AN, intended for other seismic sensor stations and will pass virtually unreflected through the shown seismic station.

[0059] The embodiment described above uses TDM in each station and WDM between stations. Alternatively, some embodiments may employ a multiplex configuration using WDM in each station and TDM between each station. General concepts for such a multiplex scheme are described in U.S. Patent No. 5,987,197, herein incorporated by reference. When compared to the multiplex scheme shown in the figures and described above, a scheme using WDM in a sensor station will require additional gratings (and possibly additional splices), since gratings of different wavelengths will not be shared between series-connected interferometers. Further, for some embodiments in the same station, a plurality of sensors sharing a common wavelength may be interrogated via TDM, while one or more sensors having different wavelengths may be interrogated via WDM.

EXAMPLE OF INSTRUMENTATION

[0060] FIG. 5A illustrates a schematic view of the instrumentation unit 110 in the system of FIG. 1, according to an embodiment of the present invention. In general, the common components of the instrumentation are designed to generate probing light pulse pairs of wavelengths (Al-AN) corresponding to the sensor stations, as described above, and process the resulting interfering pulses. both of these functions, separate components performing pulse generation and signal processing functions may be provided in separate units for some embodiments.

[0061] As illustrated, the instrumentation may include a light source 510 capable of producing light signals with multiple wavelengths (XI-AN). The light source 510 may include any suitable components, such as multiple fiber lasers, to generate suitable light signals. Suitable light signals may include, for example, continuous wave light signals of low intensity and frequency fluctuations, unless coherence modulation can be applied directly to each wavelength inside the light source (which will be discussed in more detail below). For some embodiments, the light source 510 may be configured with a high birefringence polarization that maintains the output fiber with the polarization of the fiber's output light aligned with one of the birefringence axes.

[0062] As illustrated, separate light signals at different wavelengths can be sent out to one or more modulator channels 520. If several modulator channels are used, different wavelengths can provide input to each modulator channel. As illustrated in FIG. 5B, multiple wavelength inputs to a modulator channel may be combined by a wavelength division multiplexing (WDM) unit 522 to combine the signals of multiple wavelengths onto a common fiber. Each modulator channel 520 may include any suitable components to form the amplitude, coherence characteristics, phase and polarization state of the light signals generated by the source 510. For some embodiments, a modulator channel 520 may include an intensity modulator unit 523 and a coherence modulator unit 524 to shape optical pulses and controlling their coherence characteristics, amplifier(s) 526 to compensate for losses in the modulators. A modulator channel may also include a compensating interferometer (CIF) 528 to split single pulses into double pulses, phase modulator(s) 532 to control the phase of the pulses, and polarization modulator(s) 534 to control the polarization state of the output light . The exact type of modulators to be used may depend on, for example, the output of the light source. For example, assuming that light produces polarization, sustained light signals, particularly modulators, such as lithium niobate modulators, may be used for one or more elements 523, 524, 532 and 534.

[0063] As described in US Patent Application 10/961,326 entitled

"Active Coherence Reduction for Interferometer Survey", herein incorporated by reference in its entirety, the complex field amplitude of the signal probing an optical interferometer can be modulated (coherence modulation) in such a way that the temporal coherence is reduced, thereby reducing the sensitivity to unwanted _ reflections by time delays different from the sensor reflector. For some embodiments, the optical field indicator of the light source 510 may be modulated in a controlled manner to produce a wider optical source power spectrum. This can be achieved through some direct source modulation, for example through modulation of laser cavity parameters that change the laser frequency or phase, or through modulation of laser pump signals. It can also be achieved through modulation of the light inside the coherence modulation unit 523 which can be included in the modulation channel(s).

[0064] A compensating interferometer (CIF) 528 which

having a delay difference corresponding to the delay difference of the sensors in the sensor stations 142 can be arranged in a series-connected manner at the output of the signal conditioning logic 520 to produce pulse pairs suitable for producing interfering reflected pulses from the grids in the sensor stations 142.

[0065] Output from the compensating interferometer 528 may be sent to additional modulators which may include appropriate components such as phase modulator 532 and polarization modulator 534 to modulate the phase difference between pulses in each pulse pair which will result in subcarrier modulation of the interference signals reflected from the sensors allowing sensor phase demodulation without ambiguity, and to perform polarization processing for polarization insensitive sensor examination, for example, according to US Patent Application Nos. 10/6493,590 and 11/056,970 entitled “METHOD AND APPARATUS FOR

T1EVEIEBRTNGFL>EP0LAR1SERT LIGHT", 'TRAM METHOD AND APPARATUS' FOR PROVIDING POLARIZATION-INSENSITIVE SIGNAL PROCESSING

FOR INTERFEROMETRIC SENSORS". In addition, the common phase or frequency of probing pulse pairs can be modulated to reduce crosstalk and noise caused by unwanted reflections in the system, according to US patent application number 11/056,970 entitled "METHOD AND

APPARATUS FOR SUPPRESSING CROSSTALK AND NOISE IN TIME DIVISION MULTIPLEXED INTERFEROMETRIC SENSOR SYSTEMS", all of

which are hereby incorporated by reference in their entirety

[0066] As an option for some embodiments, the instrumentation may include means to spread the different wavelength channels out in time (eg, distribute them into different pulse pairs). Spreading the wavelengths out in this way can reduce peak levels of optical power, thereby reducing non-linear effects such as stimulated Raman scattering (SRS), four wave mixing (FWM), self-phase modulation and cross-phase modulation (self-phase modulation and cross-phase modulation) (SPM/XPM) in the fibers which can degrade system performance.

[0067] The wavelengths can be spread out in time by grouping the wavelengths, for example with XI - X4 in group 1, A.5 - X8 in group 2 etc. Each group can then allocate a different time segment. As an example, a TDM rate of 2000ns and 300ns duration for each pulse pair allows 6 time segments, as illustrated in FIG. 8.

[0068] Allocation of different wavelengths for different time segments can be achieved by sending different groups of wavelengths through different modulator channels 520, as suggested in FIG. 5, and activate the modulators in different channels to generate pulses at different times.

In some cases this can be advantageous by allowing each modulator to be optimized for a limited wavelength range. Alternatively, dispersion of wavelengths in time can be achieved by having different optical delays (fiber coils of different lengths) for different groups of wavelengths.

~[0069] Refers backTlfTIG. 5A. In either case, the modulated signals from groups of wavelengths in different wavelength bands can then be fed into a wavelength division multiplexing (WDM) unit 538 to combine all wavelengths into one fiber. Output from 538 may pass through a branching module 540 containing a splitter 544 which splits the multipulse multiwavelength signal into multiple fibers. Multiple branching modules 540 can be cascaded in a tree topology to split the survey signal into a required number of downlink fibers 122 which can be combined into one or more downlink cables 120. Note that a downlink cable can in many cases also serve as an access cable containing access cables in addition - for lowering cables. Some branching units may include a broadband optical power amplifier 542 to compensate for the splitting loss and generate required output power. It may also be possible to amplify signals and perhaps also compensate for splitting losses at a later point (for example by locally or remotely pumped underwater amplifiers and/or sources). However, it is typically less expensive to use an amplifier to amplify the signal at the surface, rather than placing amplifiers underwater.

[0070] The descending fibers 122 can propagate the probing pulse pairs to the sensor interferometers of the seismic sensor stations 142 (for example, located on the seabed). As previously described, use of wavelength-selective FBG reflectors in the interferometric sensors in the stations 142 allows wavelength division multiplexing (WDM) of several stations in series on a single downlink fiber 122 (for example, N stations with corresponding wavelengths XI - AN).

[0071] In response to the probing pulse pair 310, the gratings in each sensor station will reflect light in a corresponding wavelength channel and form interfering pulses 320. The pulses 320 can be directed back up to the detecting part of the instrumentation unit 110 (for example by a circulator 123 contained in a module switch node 144), via uplink fibers 124. As illustrated, an array switch module 130 may be included to route uplink and downlink fibers from the conductor cable 120 to/from appropriate series of array cable modules 140. As illustrated, the detecting components may include WDM demultiplexers 550 (e.g. one for each inventive cell 124) which splits the different sensor wavelengths into different detector circuits 562. Electrical signals generated at each detector circuit 562 can be fed to a demodulation processing unit 563 to be processed, for example by any technique known in the art to extract the sensor phases of that wavelength channel and corresp transmitting seismic data from each sensor station 142. Demodulated sensor data from the processing unit may be forwarded (via a host interface 564) to a host computer for storage and quality control.

[ 0072] For some implementations, cameras can monitor the outgoing light signals and accordingly adjust one or more parameters of the modulator channels. As will be described in more detail below with reference to FIG. 9, the demodulation processing unit 563 or the host computer 570 may be configured to perform a layer descaling algorithm to reduce the effects of crosstalk between the sensors in a station for some embodiments. Furthermore, for some embodiments, the host computer 570 may be configured to command the source device 510 to adjust the wavelength of the optical signals generated, for example, to account for detected changes in wavelengths of the sensor station gratings over time, for example through changes in temperature.

COMBINE WDM WITH IN-LINE TDM AND AN ALGORITHM FOR

LAW PEELING

[0073] In order to limit the number of leading fibers, it is necessary to maximize the number of sensors that can be multiplexed on a pair of downlink and uplink fibers. As described in previous sections, this has been achieved by combining time division multiplexing (TDM) in each station with wavelength division multiplexing (WDM) between stations. For example, with 5 sensors per station (reference, 3 accelerometers and hydrophone) and N=20 wavelength channels, the total number of sensors that can be examined through a pair of guide fibers becomes 5x20=100. However, the fact that the sensors in a sensor are aligned in-line with multiple reflectors on the same fiber causes distortions in the detected TDM multiplexed interference signals. The effect of these distortions can be reduced by using an inverse scattering algorithm, such as denoising.

[0074] Multiple reflections can result in crosstalk between sensors in a station. FIG. 9 illustrates how the detected pulses include a composition of reflections from all the gratings along the sensor fiber in a sensor station. However, embodiments of the present invention can reduce crosstalk interference between sensors in an OBS sensor station by using algorithms, for example, in the demodulation processing unit 546 located in the instrumentation unit 110 shown in FIG. 5.

[0075] For example, the host computer may be configured to use an inverse scattering algorithm to detect an accurate transmission phase delay response between each pair of following reflectors while reducing crosstalk from other reflectors in the array. One form of inverse scattering algorithm is the layer peeling algorithm. This algorithm allows the use of right-reflectivity gratings in a TDM system without creating unacceptable crosstalk, thus improving the power budget and in many cases allowing a system without the use of remote/remote-pumped amplifiers.

[0076] Signal processing software, for example, running in the host demodulation processing unit 564 may be used to process the light detection output from the detection circuit 562 to eliminate crosstalk from higher order reflections according to an embodiment of the present invention. Appropriate layer peeling algorithms are described in detail in U.S. Patent Application Serial No. 10/649,588 entitled “TRANSMISSION METHOD AND APPARATUS FOR REDUCING CROSSTALK INTERFERENCE IN AN IN-LINE-BASED FABRY-PEROT SENSOR ARRAY”, herein incorporated by reference.

[0077] When the light pulses propagate through a sensor station, they can experience coupling between the modes of polarization propagation in the fiber. The influence of multiple reflections on the detected interference signals will generally depend on this polarization mode coupling. To ensure accurate results from an inverse scattering algorithm, a polarization-resolved measurement of interference responses may therefore be required. Appropriate procedures for polarization-resolved examination of the interference responses are described in detail in the previously mentioned US patent application serial numbers US 10/649,588 and US 11/056,970.

GRID ARRAY DESIGN

[0078] As previously described, the sensors in each station 142 can be formed by a series of gratings with overlapping reflection (channel) bands. By using multiple wavelength channels Jkan flej£jl^jonej bJ^ division multiplexing, as illustrated in FIG. 6A, showing three adjacent wavelength channels. As illustrated, the probing laser frequency (wavelength) of channel N, vn(hi) can be controlled to be in the grating channel bandwidth BChav grating N at all grating operating temperatures and all times, taking into account possible wavelength shifts in the grating spectrum over time. For some embodiments, the gratings may be designed for wavelength channels selected such that at a nominal temperature (eg 4°C) the wavelengths may vary from approximately 1530nm (XI) to approximately 1560nm (X20) with a relatively constant frequency interval, AvCh (see FIG. 6A ) between the wavelength channels, for example, 200 GHz.

[0079] For some embodiments, the channel bandwidth (BCh) may be approximately 25% of

the channel interval (for example 50GHz assuming an interval of 200 GHz). The reflectivity in the channel bandwidth may be equal or close to R (eg between .9R and R as shown in FIG. 6A), where R may vary from eg 1 to 10%.

[0080] The grating reflectivity of a specific grating in the other wavelength channel bands should be kept below a level R-x, for example, but x typically > 40dB to suppress demodulation errors and inter-station crosstalk i due to multiple reflections between sensor stations along the same fiber.

[0081] As alluded to above and as shown in FIG. 6B, the reflectivity of each grating (R1-R6) in a sensor station can be varied to optimize the signal-to-noise ratio for all sensors, and to reduce errors, including crosstalk, introduced by multiple reflections between gratings. As mentioned above, the unwanted effects caused by multiple reflections can generally be reduced using layer peeling, or other inverse scattering processing techniques. However, the accuracy of the output from the inverse scattering processing will generally be more accurate if the magnitudes of the errors to be removed through such processing techniques are small. Thus, for some embodiments, reduction in error/cross talk due to multiple reflections can be achieved by allowing the reflectivities of later gratings to be stronger than earlier gratings. For example, for some embodiments, the reflectivities of the six sensors may be as Jølger.:JL]^Q%,-R2^ —

[0082] Furthermore, the (optical) distance, L, of fiber between the center position of (any) two gratings forming a sensor can for example be in the range from 4 to 20 meters. At nominal operating conditions, L should be equal to half the distance between the two optical pulses in a probe pulse pair, corresponding to the delay in the previously mentioned compensating interferometer (compensating interferometer), CIF.

BAND WAVELENGTH DIVISION MULTIPLEXING (B-WDM)

[0083] While the array cable module 140 shown in FIG. 2 uses a single sensor fiber 146 to examine all sensor stations 142 (with corresponding wavelengths Al - AN), for some embodiments, band wavelength division multiplexing (b-WDM) can be used to divide wavelengths of an array cable module into two (or more) sensor fibers. In other words, each sensor fiber can carry a band of wavelengths, and each sensor fiber can then be connected to the sensor stations in the array cable module that have corresponding

wavelengths in the corresponding wavelength band.

[0084] Using B-WDM in a switching node can minimize the variation in reflected power levels from the stations all the way through an array cable module. In contrast, if all sensors are along a single fiber there may be a major difference in loss seen by the first sensor in the series and the last sensor due to the distributed loss all the way through the sensor array (each sensor station has some transmission loss).

[0085] As illustrated in FIG. 7A and 7B, for some embodiments, a module coupling node 744 may include a ToF-band WDM unit 745 downstream from a circulator 723 configured to divide wavelengths in an optical signal receiver from a circulator 723 into groups of wavelengths of different bandwidths. The groups of bandwidths may be carried by multiple sensor fibers, such as sensor fibers 746 and 747 shown in the figures. Band-WDM can be any suitable type of component or components, such as a C-band red/blue splitter or a C/L-band splitter. For some embodiments, a C-band (~1525-1565nm) and L-band (~1570-1610nm) may be used to approximately double the number of usable wavelengths compared to the use of a single band (for example).

[0086] For some embodiments, the arrays of sensor stations 742 may be evenly spaced across multiple fibers. For example, assuming an array cable module with N stations, N/2 sensor stations (for example, 1 to N/2) can be probed with sensor fiber 746 and N/2 sensor stations (for example, N/2+1 to N) can be probed with sensor fiber 747.

[0087] As illustrated, a multi-fiber tube, such as fiber in metal tube 748 can bypass each

station and eliminate the need for corresponding cutting and splicing at each station, while only one sensor fiber needs to be cut to splice into the sensor housing. As previously described, at a link between array cable modules, the circulator 723 may connect another fiber from the multi-fiber pipe 748 to a sensor fiber that is routed into the B-WDM unit 745 .

[0088] The several sensor fibers can also be housed in one or more tubes. As illustrated in FIG. 7A, multiple sensor fibers carrying different wavelength bands can also be housed in a single protective tube, such as a fiber in metal tube (FIMT) 749. If the FIMT 749 and both sensor fibers are cut to connect the appropriate sensor fiber to the sensors in the station, a splice 751 may be required at each station, even for the sensor fiber that is not connected to the sensors in the station. As illustrated in FIG. 7B, however, for some embodiments, multiple FIMTs may be provided for multiple sensor fibers. For example, as illustrated, each sensor fiber 746 and 747 may have its own FIMT 749 and 747, respectively.

OTRRnXSION EXAMINATION FROM TWO ENDS

[0089] In applications, such as OBS, involving significant material, manufacturing and installation costs, it is often desirable to design in some degree of redundancy to allow continued examination of at least some sensors in the event of a failure. Examples of such failures include, but are not limited to, breakage (or other types of damage) in a conductor cable, damage to one or more fibers contained in a cable, or cable failure or breakage. In any case, some embodiments of the present invention provide sensor topologies with inherent redundancy that allow

■^sensjarslasjjans to be examined ftailers&jettings^

[0090] In the present description, the term direction refers to the direction in which probing (and reflected) pulses move, relative to probed sensor stations in various operating modes. In other words, in a first (e.g. normal) mode of operation, probing pulses may travel to a sensor station from one direction, while in a second (e.g., redundancy-connected) mode of operation, probing pulses may travel to the sensor station from another direction .

[0091] FIG. 10A and 10B illustrate an example of a topology for an OBS measurement system

which allows examination from different directions before and after a break 941 in a conductor cable (for example 141 j) or an array cable module 140, according to an embodiment of the present invention. The redundancy provided by the illustrated topology allows sensor stations 142 located both before and after the breach 941 to be examined. The illustrated topology uses a link 950 of cables 141 to

examine various series 960 of array cable modules 140 extending from an array connector module 930 .

[0092] In normal operation, all stations along series 960] and 9602 (together these form an array cable) are examined via lead cable 141 ]. In this arrangement, the conductor cable 120 may contain twice as many fibers as the conductor cable in a system without redundancy (for example, as shown in FIG. IA). All fibers will normally be connected to the corresponding fibers in all guide cables (ie 141 i, 1412, etc.). To be able to interrogate sensors on both sides of an array 94T, the optical fiber can be connected to an additional downlink fiber 122 from the instrumentation (for example, by adding an additional lx2 splitter), and an additional uplink fiber 124 can be interrogated at the receiving end. by connecting this additional fiber to an additional WDM demultiplexer 550. The number of detector circuits need not have increased since the number of stations examined will be the same, but the connections from the WDM demultiplexers 550 to the detector circuits may need to be regrouped to couple the reflected light from the various stations to the corresponding detector circuits.

[0093] As illustrated in FIG. 10A, in a first (e.g., normal) mode of operation, a first series 960j of array cable modules 140 may be probed via a first guide cable 141j extending from an array connector module 930. As described above, the guide cable 141] may include multiple fibers to allow for N sensor stations 142 (for example with wavelengths Al-AN) in a common array cable module 140 to be examined via a common sensor fiber 146. Connection nodes 144 can be used to connect another fiber from the guide cable 141] to the sensor fiber 146 in a following array cable module 140. Which will be described in more detail below, with reference to FIG. 12 and 13, different designs of connector nodes 144 may facilitate examination from multiple ends, with the exact components used depending on the specific embodiment.

[0094] Therefore, as illustrated in FIG. 10B, in a second mode of operation (for example, upon the occurrence of an event, such as a break 941 in the guide cable 141 ] or some other type of failure that prevents probe pulses 910 or reflected interference pulses 920 from being sent to or from sensor stations 142 in the same or following array cable modules 140) at least some of the sensor stations 142 in series 960] can be probed via a guide cable 1412 from the opposite direction. As illustrated, the coupler 950 may allow probing pulses 930 carried in one or more fibers of a guide cable 1412 to reach sensor stations 142 in the array 9602 that occur after the fracture 941. The coupler 950 may also allow reflected/interfering pulses 940 to be carried back to the surface instrumentation via the guide cable 1412 .In the event of breakage in the lead cable 120, the array connector module 930 or the lead cables 141 (141]1412,...,t, etc.) these components may need to be replaced.

[0095] FIG. 1 IA and 1 IB illustrate another example of a topology for an OBS measurement system that allows examination from two ends, according to an embodiment of the present invention. Rather than using a link 950, as shown in the topology of FIG. 10A - 10B use the topology as shown in FIG. 1 IA and 1 IB separate control cables 120] and 1202, and separate (or common) array coupling modules 930] and 9302. The control cables will normally have the same number of fibers as the control cable in a system without redundancy (cf. FIG. IA). Otherwise, examination of sensor stations 142 in both directions can be carried out in a relatively similar manner. In normal operation, light will only be sent through the fibers in one of the guide cables (eg 120]).

[0096] As illustrated in FIG. 1 IA, prior to the occurrence of a fault, sensor stations 142 in a first array cable 1060]in array cable modules 140 may be probed as described above with the first guide cable 120]used to carry probing pulse pairs 1010 and reflected pulses 1020. After a break 1041, as shown in FIG. 1 IB, the second guide cable 1202 can be used to probe sensor stations in the first array cable 1060] in the other direction with pulse pairs 1030 and to carry reflected pulses 1040 back to the instrumentation.

[0097] A potential advantage of the topology shown in FIG. 1 IA - 1 IB in relation to the topology shown in FIG. 1 OA -1 OB is that the array cable covers a longer distance for the same number of array cable modules, since the array cables in FIG. 1 OA - 1 OB is folded.

[0098] As illustrated in FIG. 1 IA and 1 IB, for some embodiments, the lead cables 120] and 1202 may have separate corresponding array connector modules 930] and 9302, respectively. For some embodiments, the array connector module 9302 may be installed during installation of the array cables 1060 (1060], IO6O2, etc.), but without initially running the corresponding "redundant" lead cable I2O2. By installing the array connector module 9302 in this way, the lead cable 1202 can be laid out only if a cable or station error is detected, thereby at least deferring costs, while still accommodating the laying out of the lead cable I2O2 and thus enabling

-redurrian inner rø^

EXAMPLE OF CONNECTOR NODES

[0099] While Fig. 1 OA - 1 OB and FIG. 1 IA - 1 IB illustrate various sensor topologies that allow examination from two sides, illustrate FIG. 12A-12B and FIG. 13A - 13B examples of various module connection nodes that can be used with one of the sensor topologies shown in FIG. 10A-10BookFIGS. 11A-11B.

[ 00100 ] Referring first to FIG. 12A and 12B. Array cable modules 140 using module connection nodes 1144 with a pair of circulators 1149 and a coupler 1147 are shown. As shown in FIG. 12A, "forward" probing may take place as described above, with pulse pairs 1110 used to probe sensor stations 142 in a first array cable module 140 carried in a first conducting fiber 1150. The pulse pair 1110 is directed to a sensor fiber 1146 connected to the sensor stations 142 through the bottom circulator 1149 and the coupler 1147. Reflected pulses 1120 are then passed from the sensor stations 142 through the coupler 1147 and the upper circulator 1149 to a return fiber 1151. The second array cable module 140 is examined in a similar way, through another guide fiber 1152 and return fiber 1153.

[00101] Referring to FIG. 12B. However, after a cable break 941 occurs between

stations in the first array cable module 140, "backside" probing from the other end of the series of array cable modules 140 may be engaged. Rear examination can be automatically switched on, for example after automatic detection of a cable break 941 off

the instrumentation, as indicated by a lack of reflected pulses from stations located after the breach.

[00102] In any case, as illustrated in FIG. 12B, while sensor stations 142 located before the break (eg, XI) are reachable in a conventional manner, sensor stations (eg, A2-AN) located after the break 941 are not reachable in this arrangement. However, sensor stations in the following array cable module 140, although located after the break 941 are available. For example, as illustrated, probe pulse paTlTTOpTeirleclel^ sensor fiber 1146 of the sensor stations 142 via top coupler 1149 (in module connector node 1144 of the array cable module). Resulting reflected pulses 1120 are then passed from sensor stations 142 through coupler 1147 and bottom circulator 1149 to a return fiber 1155.

[00103] An advantage of the arrangement is that, even if examination occurs from the "back" direction, as shown in FIG. 12B, the sensors in station 142 are still examined in the normal manner. For example, as shown by the reflected pulses 1120 carried on the guide fiber 1155, the reference interferometer (R) may still be probed first, followed by the X, Y and Z accelerometers and finally the hydrophone (H). Examining the reference first can be beneficial because it will not be subject to crosstalk from the other sensors in the station. However, it should be noted that an amount of optical loss occurs with each path taken through the couplers 1147.

[00104] FIG. 13A and 13B illustrate another example of a configuration of array cable modules that allow bidirectional surveying in accordance with another embodiment of the present invention. In the illustrated arrangement, array cable modules 140 are shown using module connection nodes 1144 with only circulators 1149, without couplers. As shown in FIG. 13A, "forward" probing may occur as described above, with pulse pairs 1110 used to probe sensor stations 142 in a first array cable module 140 carried in a first guide fiber 1150 to a sensor fiber 1146 via a circulator 1149, which also directs resulting reflected pulses 1120 to a return fiber 1151.

[00105] Referring to FIG. 13B. However, after a cable break 941 occurs between stations 142 in the first array cable module 140, backside probing from the other end may be engaged. As in the previous arrangement shown in FIG. 12A and 12B, the first station (A1) located before the break 941 is accessible in the forward direction, while the remaining sensor stations (A2-AN) in the same array cable module 140 are not. However, in the arrangement shown in FIG. 13A and 13B, these remaining sensor stations, although located after the breach 941, may become available in the ijaksideTelningen.

[00106] As illustrated, for some embodiments, when examined in the "opposite" direction, a connector node 1144 in an array cable module 140 may be used to connect lead and/or return fibers (1156 and 1157) to a sensor fiber 1246, via a circulator. 1149. In this way, probe pulses 1110 from the back can be used to probe these sensor stations (A2-AN) that occur after the break 941 in another array cable module 140 by passing light from the backside guide fiber 1156 to a sensor fiber 1246 via a circulator 1247. pulses 1120 can be stvrt back-on-tetijrfib&T&n-l-I^T^

[00107] This technique has an advantage in that the sensor stations 140 located after a break 941 can still be examined. However, the accuracy of measurements obtained from the reflected/interfering pulses 1120 may be reduced to some extent (relative to measurements obtained via survey in the first direction), due in part to the fact that the sensor stations have been surveyed in the wrong direction. For example, as shown by the reflected pulses 1340, the sensors, in order, as shown under backside examination: hydrophone (H), Z, Y and X accelerometers, and finally the reference interferometer (R).

[00108] By examining the reference interferometer finally, the reference reading can be affected by crosstalk from the hydrophone, which can reduce resolution. However, depending on the application, the reduced resolution may be acceptable, especially given the possibility of examining sensors located after a cable break. Further, for some embodiments, a second reference interferometer (not shown) may be included and positioned so that it occurs earlier (first) in the optical path when a sensor station is probed from the rear.

CONCLUSION

[00109] Features and aspects of any of the embodiments described herein may be combined or otherwise modified with any features or aspects of other embodiments described herein. While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may depart from the basic scope, etc., and the scope thereof is determined by the claims that follow.

Claims (7)

1. A method for collecting seismic data from a series of seismic sensor stations (142) characterized in that each station (142) houses a plurality of optical sensors that share a common wavelength, the common wavelength being different for each station (142) in a same series, the procedure includes: interrogating, via a common optical path, sensors housing in each station (142) using time division multiplexing (TDM); and to examine, via the common optical path, sensors housed in different sensor stations (142) in each series using wavelength division multiplexing (WDM). 2. The method of claim 1, wherein examining, via a common optical path, sensors housed in each station (142) using time division multiplexing (TDM) includes examining a reference interferometer (210) housed in a sensor station (142) and positioned previously in the common optical path than other sensors in the same sensor station (142). 3. The method of claim 1, wherein the sensors in each station (142) are interferometric sensors and examining the sensors in each station (142) using TDM includes generating a pulse pair (310) with a distance in between the pulses corresponding to a common sensor delay . 4. The method in claim 3, wherein the interferometric sensors include interferometers of the Fabry-Perot type that use wavelength-dependent reflectors. 5. The method in claim 4, wherein the wavelength-dependent reflectors include Bragg gratings (202). 6. The method of claim 2, wherein examining sensors housed in different sensor stations (142) in each series using wavelength division multiplexing (WDM) includes generating the pulse pairs (310) with different wavelength components corresponding to wavelengths of different sensor stations (142) in the series . 7. The method of claim 6, wherein the series of sensor stations (142) are located along array cables extending from an array connector module (130A); and the array cables (140) are connected to the array connector module (130A) via wet connectors after laying out the array cables (140) and the array connector module (130A).