Offshore multi-span distributed acoustic sensing with different sensing frequency ranges and sensing span lengths

Abstract

The invention discloses offshore multi-span distributed acoustic sensing with different sensing frequency ranges and sensing span lengths. A system may include a first DAS station including a first plurality of interrogator units to transmit a first plurality of DAS signals in a first direction, where the first interrogator unit is configured to transmit a first DAS signal at a first interrogation rate and a first wavelength, where the first interrogator unit is configured to transmit a second DAS signal at a second interrogation rate and a second wavelength. Wherein the second interrogator unit is positioned to transmit a second DAS signal at a second wavelength and a second interrogation rate less than the first interrogation rate. The system may include a loop array comprising a plurality of loops arranged over a plurality of span segments, where a first set of proximal loops closest to a first DAS station are configured to route a first DAS signal back to the first DAS station, and where a first set of intermediate loops farther from the first DAS station are configured to route a second DAS signal back to the first DAS station.

Description

Technical Field

[0001] Embodiments of the present disclosure relate to the field of optical communication systems. In particular, the present disclosure relates to techniques for extending and improving the sensitivity of distributed acoustic sensing (DAS) in submarine optical cables. Background Art

[0002] In a distributed acoustic sensing (DAS) system, optical cables may be used to provide continuous real-time or near real-time monitoring of disturbances or anomalies in the area surrounding a fiber optic cable (hereinafter referred to as the cable) and up to many kilometers from the cable. In other words, the cable itself may be used as a distributed sensing element to detect or monitor different types of disruptions, interferences, irregularities, natural or man-made activities, etc. occurring within or outside the subsea environment as acoustic vibrations in the DAS sensing environment (e.g., marine and terrestrial environments). To this end, optoelectronic devices / equipment coupled to the cable of the DAS system may detect and process reflected optical signals (e.g., Rayleigh backscatter signals or simply Rayleigh signals) over a distance (range) in the DAS sensing environment.

[0003] Typically, a DAS system may include a cable station equipped with a DAS Interrogator Unit (IU), which typically includes a DAS transmitter and receiver to probe a fiber optic cable using coherent laser pulses, wherein the phase change of the returned optical backscattered signal is measured. The optical phase shift between the received backscattered pulses may be proportional to the strain in the optical fiber, resulting in the ability to detect vibrations and the like, as measured by the effect of such disturbances on the phase. For example, a DAS system based on Rayleigh backscattering is also referred to in the art as a Rayleigh scattering-based DAS system.

[0004] To date, the capabilities of DAS IUs have been limited to cable lengths in the range of about 50 km for practical technology, and up to 150 km in experimental research units. Therefore, in a repeater system equipped with an erbium doped fiber amplifier (EDFA), only the first cable span (or spans hereinafter) adjacent to the DAS IU needs to be sensed. In order to sense the entire submarine cable link, where the link may cross a body of water to the opposite shore, in one approach, a DAS IU may be placed in each optical repeater of the submarine link, where the sensed data collected will need to be transmitted to either shore end via the same optical cable carrying the commercial optical signal.

[0005] Another approach for DAS sensing is to measure via multiple amplified spans from a single DAS IU located on shore. However, sensing long submarine links (e.g., up to 10,000 km) using only one DAS IU on either shore end of the system (similar to Coherent Optical Time Domain Reflectometry (C-OTDR) or Line Monitoring Equipment-Optical Time Domain Reflectometry (LME-OTDR) measurements deployed in some current commercial systems) will significantly reduce the sensitivity of DAS signal detection and therefore significantly reduce the maximum detectable frequency range due to the lower interrogation rate.

[0006] It is with reference to these and other considerations that the present disclosure is provided. Summary of the invention

[0007] In one embodiment, a system for a distributed acoustic sensing system is provided. The system may include a first distributed acoustic sensing (DAS) station including a first plurality of interrogator units for transmitting a first plurality of DAS signals in a first direction. Thus, the first interrogator unit of the first DAS station may be configured to transmit a first DAS signal at a first interrogation rate and a first wavelength, wherein a second interrogator unit of the first DAS station is configured to transmit a second DAS signal at a second wavelength and a second interrogation rate, the second interrogation rate being less than the first interrogation rate. The system may include a loop array including a plurality of loops arranged on a plurality of spans, wherein a first group of near-end loops located closest to the first DAS station is configured to route the first DAS signal back to the first DAS station, and wherein a first group of intermediate loops located further away from the first DAS station is configured to route the second DAS signal back to the first DAS station.

[0008] In another embodiment, a system for distributed acoustic sensing is provided. The system may include a first distributed acoustic sensing (DAS) station located at a first end of a multi-span link and configured to transmit a plurality of DAS signals in a first direction. The system may include a second distributed acoustic sensing (DAS) station located at a second end of the multi-span link and configured to transmit a plurality of DAS signals in a second direction. The system may include a loop array including: a first group of near-end loops located closest to the first DAS station; a first group of intermediate loops; a second group of near-end loops located closest to the second DAS station; and a second group of intermediate loops located closer to the second DAS station than the first group of intermediate loops. Thus, a first wavelength and a first interrogation rate of a first group of DAS signals routed through the first group of near-end loops may be different from a second wavelength and a second interrogation rate of a second group of DAS signals routed through the first group of intermediate loops, and wherein a third wavelength and a third interrogation rate of a third group of DAS signals routed through the second group of near-end loops are different from a fourth wavelength and a fourth interrogation rate routed through the second group of intermediate loops.

[0009] In another embodiment, a method for distributed acoustic sensing may include transmitting a first DAS signal from a first interrogator unit of a first DAS station at a first interrogation rate and a first wavelength and transmitting a second DAS signal from a second interrogator unit of the first DAS station at a second wavelength and a second interrogation rate, the second interrogation rate being less than the first interrogation rate. The method may include routing the first DAS signal back to the first DAS station via a loop in a first set of near-end loops that is closest to the first DAS station, and routing the second DAS signal back to the first DAS station via a loop in a first set of intermediate loops that is located farther from the first DAS station. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 An example of a DAS system according to an embodiment of the present disclosure is shown;

[0011] Figure 2 The method for operating according to an embodiment of the present disclosure is shown. Figure 1 An exemplary mode of a DAS system;

[0012] Figure 3 The method for operating according to an embodiment of the present disclosure is shown. Figure 1 Another exemplary mode of a DAS system;

[0013] Figure 4 The method for operating according to an embodiment of the present disclosure is shown. Figure 1 Another exemplary mode of a DAS system;

[0014] Figure 5An exemplary process flow for operating a DAS system is depicted. DETAILED DESCRIPTION

[0015] The present embodiment will now be described more fully below with reference to the accompanying drawings, in which exemplary embodiments are shown. The scope of the embodiments should not be construed as being limited to the embodiments set forth herein. On the contrary, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the embodiments to those skilled in the art. In the accompanying drawings, similar reference numerals refer to similar elements throughout.

[0016] Before describing specific embodiments in detail with respect to the drawings, general features regarding the embodiments will be reviewed.New DAS devices, systems, architectures, and techniques are provided to improve DAS sensing capabilities, particularly sensitivity and range across multiple spans of submarine systems including above-ground and underground fiber optic cables.

[0017] The present embodiments provide an architecture and method for DAS sensing that can customize the monitoring of different activities or events according to the location over a large range of up to thousands of kilometers in the seafloor environment. By way of example, in a seafloor sensing system, there are different activities that may occur near or along the length of a cable that can be detected and monitored, resulting in different measured acoustic frequencies or frequency signatures. Close to shore (e.g., <100km away), there are many activities related to humans, animals, ships, and other sources, where a suitable frequency response range may be up to several kHz. In the middle range (e.g., 100km-500km from shore), ship activities may be the most important event to be monitored. For monitoring activities within this distance range, detection of a 100Hz frequency response may be particularly suitable because most of the acoustic signals from commercial ships are generated at frequencies below 100Hz. For deep sea locations (e.g., >500km from shore), the main acoustic signals come from earthquakes, tsunamis, and underwater land movements, where a lower frequency range (down to a sub-Hz frequency response range) is sufficient to capture these natural events. Thus, the present embodiments customize DAS sensing along a multi-span submarine cable system based on span location.

[0018] Figure 1An example of a DAS system according to an embodiment of the present disclosure is shown, shown as system 100. System 100 is formed by two terminal DAS cable stations (cable stations equipped with DAS interrogator units or hereinafter referred to as DAS stations), which can be located at opposite ends of a submarine cable system, wherein the first DAS station is shown as DAS station 102A with DAS IU 103A and the second DAS station is shown as DAS station 102B with DAS IU 103B, wherein each DAS station can be located on shore. Each of the DAS stations can include multiple interrogator units (IUs) designed to independently transmit DAS signals at different wavelengths. For example, when two IUs are deployed from each end of the cable system, the IU wavelengths are designated as λ1, λ2 at one station and λ3, λ4 at the second station. When three IUs are deployed from each end of the cable system, the IU wavelengths are designated as λ1, λ2, λ5 at one station and λ3, λ4, λ6 at the second station. In certain embodiments, for certain span interrogations, the wavelength transmitted from the first DAS cable station may be the same as the wavelength transmitted from the second DAS cable station (as explained later); for example, Figure 1 In the specific example of 1, only 4 different wavelengths are needed to interrogate the entire span between the first DAS station and the second DAS station (λ1=λ3, λ2=λ4, λ5, λ6), where the 4 different wavelengths are allocated as λ1, λ2, λ3 at one station and as λ1, λ2, λ4 at the second station, as Figure 1 shown.

[0019] exist Figure 1 In the example, the individual IUs are represented by different wavelengths, in the case of DAS station 102A, by wavelengths λ1, λ2, and λ3, and in the case of DAS station 102B, by wavelengths λ1, λ2, and λ4. The two DAS stations may be connected to a single fiber pair of the cable, wherein one fiber of the fiber pair may be designed to transmit at Figure 1 In the embodiment, the DAS signal is transmitted on path 116 from "west" to "east", while the other fiber of the fiber pair can transmit the DAS signal along path 118 from east to west.

[0020] As discussed below, the different DAS stations of the system 100 may transmit on an optimized number of monitoring wavelengths so that the entire subsea link may be monitored together. In one aspect of this embodiment, the DAS pulses from the three DAS IUs may be synchronized and staggered in time in order to avoid or limit the optical signal to noise ratio (OSNR) consequences caused by fiber nonlinearities.

[0021] like Figure 1As depicted, link 101 between DAS station 102A and DAS station 102B features a series of cable spans, shown as span 120A, span 120B, span 120C, and span 120D, and repeaters, shown as repeater 130A, repeater 130C, remote repeater 130E, repeater 130D, and repeater 130B. By way of example, Figure 1 A single example of a repeater 130A and a repeater 130B is shown in FIG1 , but there may be multiple repeaters of type 130A and type 130B after the shore span. Repeaters 130A and 130B may be located, for example, at a distance of no more than about 100 km from the respective shores housing DAS IU 103A and DAS IU 103B. After the DAS IU 103A and DAS IU 103B type repeaters, a λ1 blocking filter will appear. Only the span after the repeater 130A furthest from the DAS IU 103A and the span after the repeater 130B furthest from the DAS IU 103B will be followed by a λ1 blocking filter. Link 101 also includes intermediate repeaters 130C and repeaters 130D and their associated optical spans, spans 120B and spans 120C, which according to various non-limiting embodiments may be located within a few hundred kilometers (e.g., 100 km to 500 km) offshore. At either end of the system, several repeaters and spans of type 130C and 130D may be employed. Only the span 120B furthest from DAS IU 103A and the span 120D furthest from DAS IU 103B are followed by a λ2 blocking filter. Link 101 may also include a set of remote repeaters 130E and associated spans covering a range of more than 500 km from each shore that accommodates DAS IU 103A and DAS IU 103B. In one example, remote repeaters 130E and associated spans may cover a range extending up to several thousand km (e.g., 900 km). This part of the 130E repeater and span has the following features: Figure 1 The λ3 and λ4 blocking filters are positioned as shown. Figure 1 In the particular embodiment shown, only one of the λ3 and λ4 blocking filters is provided, and these blocking filters are located at each end of the remote repeater 130E and the portion of the span.

[0022] Note that subsea repeaters may be located along the system 100, with the repeaters defining the boundaries of the spans, as indicated. Located at a given repeater may be a loop path, such as Figure 2130A, wherein the hardware associated with the repeaters and loop paths may be referred to herein as "loops". These loops include: a first set of near-end loops closest to DAS station 102A, wherein the loops may coincide with repeater 130A; a first set of intermediate loops located further from DAS station 102A, wherein the loops may coincide with repeater 130C; a set of far-end loops located further from DAS station 102A than the loops associated with repeater 130C, wherein the loops are shown as being coincident with far-end repeater 130E; a second set of near-end loops closest to DAS station 102B, which coincide with repeater 130B; and a second set of intermediate loops located between the loops associated with far-end repeater 130E and the loops associated with repeater 130B, which coincide with the loops associated with repeater 130D.

[0023] In various embodiments, these loops may be deployed with multiple designs. By way of example and not limitation, Figure 2 In the illustrated embodiment, the loop includes a circulator, an erbium-doped fiber amplifier (EDFA, for amplifying Rayleigh signals), and an optical add-drop multiplexer (OADM) filter. In alternative embodiments, the same functionality can be implemented using a combination of other hardware, such as passive couplers and optical filters. Compared to the traditional high loss loop (HLLB) path deployed in a conventional amplification path, we refer to this type of loop as a filtered and amplified loop (FALB). As described in detail below, this type of filtering component can act to pass only the sensing wavelength sent from the DAS cable station in the first direction associated with a given loop. The Rayleigh signal reflected at a given wavelength is then added to the return DAS signal in a given loop, which is sent back to the cable station for detection.

[0024] Therefore, a given loop needs to amplify and filter only the back-reflected Rayleigh signal corresponding to the wavelength of a given IU of the DAS station so that the return signal amplitude is suitable to maintain the output power of the amplifier at the designed power level in the return path.

[0025] Back to Figure 1, the loop associated with repeater 130A may be designed to filter and return Rayleigh signals corresponding to DAS signals transmitted from the IU of DAS station 102A at wavelength λ1. The same applies to the loop associated with repeater 130B regarding DAS signals transmitted from DAS station 102B. The loop associated with repeater 130C may be designed to filter and return Rayleigh signals corresponding to DAS signals transmitted from the IU of DAS station 102A at wavelength λ2, which is different from wavelength λ1. The same may apply to the loop associated with repeater 130D regarding DAS signals transmitted from DAS station 102B.

[0026] With respect to the loop associated with the remote repeater 130E, as particularly with respect to Figure 4 As detailed, these loops may be arranged as bidirectional loops to receive and loop back DAS signals transmitted from both DAS station 102A and DAS station 102B. According to various embodiments of the present disclosure, the wavelengths handled by the loop associated with repeater 130A and the loop associated with repeater 130B may (but need not) be the same as each other (e.g., Figure 1 1), and the wavelengths handled by the loop associated with repeater 130C and the loop associated with repeater 130D may (but need not) be identical to each other (eg, Figure 1 λ2 is shown).

[0027] Note that, according to an embodiment of the present disclosure, the outbound DAS IU forward sensing signal sent from the DAS station 102A or from the DAS station 102B is pulsed in time, while the returning back-reflected signal (such as a Rayleigh signal) is pulsed in time. Figure 1 The cable span lengths of the links shown are quasi-continuous. Therefore, the peak power of the backscattered signal directed into a given loop path is much lower than the peak power of the forward sensed signal.

[0028] According to various embodiments of the present disclosure, Figure 1 In a configuration, multiple spans (i.e., cable lengths between amplifiers) can be measured using a single outbound DAS signal transmitted from the IU at a selected wavelength (e.g., λ2) so that a back sensing channel can be added in the return (inbound) direction of the fiber pair based on the Rayleigh signals received from the multiple spans. Note that these different signals from different spans can be returned as pulses on the same channel (wavelength) of a given optical fiber used to carry the return DAS signal to a given DAS station. Therefore, in order to avoid pulse overlap of signals returned from different spans on the same channel, the pulse width of the DAS sense pulse can be set to be much smaller than the pulse travel time of the shortest span, and the pulse period is greater than the sum of the pulse width and pulse travel time of the longest span, so that the returned Rayleigh signal is quasi-continuous and DAS data can be correctly detected within any given span.

[0029] By way of example and not limitation, Table 1. summarizes exemplary sensing parameters for one non-limiting embodiment of the present disclosure, wherein three DAS IUs are provided at each of the DAS station 102A and the DAS station 102B to provide pulsed DAS signals at different wavelengths associated with a given sensing range to be detected. In this example, sensing capabilities are provided over a link extending a total of 10,000 km between the DAS station 102A and the DAS station 102B.

[0030]

[0031] Table I.

[0032] Figure 2 The method for operating according to an embodiment of the present disclosure is shown. Figure 1 1. In this case, a set of DAS signals are transmitted from DAS station 102A and DAS station 102B. Figure 2 Specifically, the system optical signal flow for outbound and inbound optical fibers is shown in bold to perform DAS sensing of shore spans, meaning span 120A and span 120D, where these spans may be located within 100 km of the opposite shore, represented by DAS station 102A and DAS station 102B.

[0033] As an example, the DAS station 102A transmits a first DAS signal 202 from a first IU at a first interrogation rate and wavelength λ1, for which an exemplary set of parameters may be specified from Table 1. For example, this wavelength may be used to detect acoustic interference within a range of 100 km from shore. Since the sensing range is 100 km, this segment may be divided into one (100-km-repeater spacing) or two (50-km repeater-spacing) sensing span segments, according to different embodiments. Therefore, the loop associated with the repeater 130A may generally represent one or two different loops in different embodiments. As shown, the loop associated with the repeater 130A (including the circulator, EDFA, and filter or OADM) only needs to be used in one direction (i.e., inbound to the nearest shore, represented by the DAS station 102A) to provide a return DAS signal 204, which provides DAS information within the sensing range of 100 km from shore. In this manner, the system cost of system 100 is reduced relative to systems that may require bidirectional loops in a given repeater, each requiring a circulator, EDFA, and filters or OADM.

[0034] Note that DAS station 102B may function similarly to DAS station 102A to transmit second DAS signal 212 and monitor return signal 214 to detect DAS information from a sensing range of 100 km from shore of DAS station 102B. Note that the wavelength and interrogation rate of second DAS signal 212 may (but need not) be the same as the wavelength and interrogation rate of first DAS signal 202 to reduce system complexity. In operation, according to Figure 2 In the scenario of FIG. 1 , the sensing wavelength of a given outbound DAS signal (either the first DAS signal 202 or the second DAS signal 212) may be blocked at the end of the 100 km span length by the outbound optical fiber of the system 100 in order to boost the power of the remaining sensing wavelength and prevent interference at the opposite end. In other words, the first DAS signal 202 is dropped at the loop farthest from the DAS station 102A for the loop associated with the repeater 130A, and the second DAS signal 212 is blocked at the loop farthest from the DAS station 102B for the loop associated with the repeater 130B.

[0035] Figure 3 The operation according to the embodiment of the present disclosure is shown Figure 1 FIG. 1 is another scenario of a DAS system of FIG. 102A. Only the western portion of the system is shown. The eastern portion behaves in a similar manner. In this case, another set of DAS signals is transmitted from DAS station 102A. Figure 3 Specifically shown is the system optical signal flow through the outbound and inbound optical fibers for performing DAS sensing of the mid-spans, which means span 120C and span 120D (see also Figure 1 ), where the span may be located, for example, between 100 km and 500 km from shore of the DAS station 102A or the DAS station 102B, respectively.

[0036] As an example, the DAS station 102A transmits another DAS signal, shown as DAS signal 222, from the second IU at a second interrogation rate and wavelength λ2, for which an exemplary set of parameters may be specified from Table 1. This wavelength will be different from λ1 and may be used to detect acoustic interference within a range of 100 km to 500 km from shore. The interrogation rate of the DAS signal 222 will be less than the interrogation rate of the first DAS signal 202, as shown in Table 1., for example 200 Hz, with a maximum sensing frequency of 100 Hz. The individual loops within the loop array corresponding to repeater 130C (also referred to as a 130C-loop array) are shown as loops "130C 1" to "130C N" (each of which may coincide with a corresponding repeater and may be separated from each other by spans 120B 1 to 1210B N), and since the sensing range may be from 100 km to 500 km, the loops within the 130C-loop array may be divided into 4 (100-km-repeater spacing) to 8 (50-km-repeater spacing) sensing spans.

[0037] Again, individual ones of the loops within the loop array 130 may be unidirectional (including circulators, EDFAs and filters or OADMs) since these spans are meant to be covered by only the DAS station 102A and therefore, only one direction (e.g., the direction back to the shore of the DAS station 102A) again requires a loop, thereby reducing system cost.

[0038] Note that the return DAS signal 224 routed back to the DAS station 102A may be divided into a series of return signals that are routed through a series of loops of the 130C-loop array. These return signals are shown as DAS signal "224A" through DAS signal "224N" and are routed through loop "130C 1" through loop "130C N", respectively. Thus, in Figure 3 In the scenario of , the area associated with multiple spans covering distances of 100km to 500km from shore can be measured with a single outbound DAS IU wavelength λ2, so that a sensing channel corresponding to λ2 can be added back in the inbound direction (towards the DAS station 102a) of the fiber pair from Rayleigh reflections received at loops arranged across up to eight spans. Note again that the width of the sensing pulse, DAS signal 222, needs to be much smaller than the pulse travel time in the shortest span and the pulse period is greater than the sum of the pulse width and the pulse travel time in the longest span.

[0039] Figure 4 The method for operating according to an embodiment of the present disclosure is shown. Figure 1A further scenario of system 100 is shown. Only the western portion of the system is shown. The eastern portion behaves in a similar manner. In this scenario, the flow of optical signals for sensing in a deep sea environment is depicted. Thus, a set of loops is depicted that corresponds to the loops associated with remote repeater 130E discussed above. In particular, for illustrative purposes, loop 130E1 and loop 130EN are depicted, where the loops may represent loops arranged at opposite ends of a span associated with a first repeater 130E1 and a last repeater 130EN of remote repeater 130E. As noted, the sensing range covered by this set of loops may be up to 9,000 kilometers.

[0040] In this case, Figure 2 and Figure 3 Another set of DAS signals is transmitted from DAS station 102A and DAS station 102B at a different wavelength in the embodiment of the present invention. As an example, DAS station 102A transmits another DAS signal from a third IU at a third interrogation rate and wavelength λ3, shown as DAS signal 242, for which an exemplary set of parameters may be selected from Table 1. This wavelength will be different from λ1 and λ2 and may be used to detect acoustic interference within a range of 500 km from the shore of DAS station 102A to 500 km from DAS station 102B. In an example where the link between DAS station 102A and DAS station 102B spans exactly 10,000 km, for example, the range of the span associated with repeater 130E 1 to repeater 130E N may cover 9,000 km as an example. The interrogation rate of the return signal 244 derived from the DAS signal 242 will be less than the interrogation rates of the first DAS signal 202 and the DAS signal 222, as shown in Table I., e.g., 10 Hz, and the maximum sensing frequency < 5 Hz. In the case where the sensing range is from 500 km to 9500 km relative to the DAS station 102A (or the same relative distance as the DAS station 102B for that matter), the set of spans may be divided into 90 (100-km-repeater spacing) to 180 (50-km-repeater spacing) sensing spans.

[0041] like Figure 4 As further depicted in the exemplary embodiment of , the sensing wavelength 242 (λ3) of the DAS signal may be blocked from the outbound optical fiber of the system at the end of 9500 km (the span 120 farthest from the DAS station 102A, such as at loop "120C N") to boost the inbound (return) loop signal from the opposite direction. In other embodiments, the sensing wavelength of the DAS signal 242 may be retained in the system 100 within the last 500 km (the span closest to the DAS station 102B) for simplicity, as such retention may not unduly affect the inbound loop signal from the opposite direction.

[0042] Based on the architecture of the DAS system described above, since loops are provided for both directions of span 120C, a method of significantly improving the measurement sensing sensitivity is proposed. In particular, by measuring the DAS signals from DAS station 102A and DAS station 102B, and analyzing only the first 50 km of data for each span, the sensitivity can be improved by 20 dB (assuming a fiber loss of 0.2 dB / km). As an example, the digital signal processing (DSP) analysis algorithm used to analyze the returned DAS signals can be designed to process only the data from the first half of each span (from the EDFA output). For example, the DAS IU sensing capability for a particular span of span 120 may only cover the first 50 km for a 100 km span, depending on the DAS IU capability. Therefore, in the above example, for a distance of 0-500 km, the system span length may be 50 km, and the span length for the remainder of the system (from 500 km to 9500 km) may be doubled to 100 km. Stated differently, because the loop associated with remote repeater 130E is designed to be bidirectional, each 100 km long span in the range of 500 km-9500 km can be interrogated in two parts, one 50 km portion interrogated by the DAS IU of DAS station 102A and the other 50 km portion interrogated by the DAS IU of DAS station 102B. Thus, in this example, rather than providing, for example, 200 repeaters and loops to sense a 10,00 km link with 50 km sensitivity, one may interrogate the 10,00 km link with 50 km sensitivity. Figure 4 Only 110 repeaters (loops) are required in the system, with remote repeater 130E comprising 90 loops within 9,000 km, and repeaters 130A-130D together comprising 20 more loops, covering the nearest 500 km from each shore at 50 km span intervals.

[0043] Please note, Figure 1-Figure 4 The foregoing embodiments and examples of Table I. are provided for illustrative purposes. According to other embodiments, the DAS sensing system may be designed to have multiple numbers of sensing channels (DAS IU wavelengths) from each end. Each channel may cover a different distance in the DAS system with an optimized interrogation rate and sensing frequency range. In addition, the transmission of different DASIU signals may be time synchronized and wavelength staggered so that transmission and detection may be performed using synchronization techniques to facilitate correct detection of different signals.

[0044] Furthermore, in further embodiments, the link may be divided into only two types of spans. For example, the nearshore spans may be detected using a loop arranged to directly detect signals generated by a first IU at a first wavelength and an interrogation rate, while the mid-spans may be detected using a loop arranged to directly detect signals generated by a second IU at a second wavelength and a second interrogation rate. In the case where two DAS IUs in two different stations are arranged at opposite ends of a link, a first set of mid-spans associated with the first station may not overlap with a second set of mid-spans associated with the second station, such that each DAS IU in the two DAS stations detects only the first half of the link closest to that station.

[0045] Figure 5 An exemplary process flow 500 is presented. At block 502, an operation is performed to transmit a first DAS signal from a first DAS IU in a first DAS station at a first interrogation rate and a first wavelength. At block 504, an operation is performed to transmit a second DAS signal from a second DAS IU in the DAS station at a second wavelength different from the first wavelength and a second interrogation rate less than the first interrogation rate. At block 506, an operation is performed to transmit a third DAS signal from a third DAS IU in the DAS station at a third interrogation rate less than the second interrogation rate and at a third wavelength different from the first wavelength and the second wavelength. At block 508, an operation is performed to route the first DAS signal back to the DAS station via a particular loop or a group of near-end loops closest to the DAS station. At block 510, an operation is performed to route the second DAS signal back to the DAS station via a particular loop or a group of intermediate loops located further away from the DAS station. At block 512, an operation is performed to route the third DAS signal back to the DAS station via a particular loop or a group of far-end loops located further away from the DAS station than the group of intermediate loops.

[0046] The present disclosure is not limited in scope by the specific embodiments described herein. In fact, in addition to those described herein, various other embodiments and modifications of the present disclosure will be apparent to those of ordinary skill in the art from the foregoing description and the accompanying drawings. Therefore, such other embodiments and modifications are intended to fall within the scope of the present disclosure. In addition, although the present disclosure has been described herein in the context of a specific embodiment, for a specific purpose in a specific environment, it will be recognized by those of ordinary skill in the art that its usefulness is not limited thereto, and the present disclosure can be beneficially used for any number of purposes in any number of environments. Therefore, the claims set forth below should be interpreted in view of the full scope and breadth and spirit of the present disclosure as described herein.

Claims

1. A system for distributed acoustic sensing, comprising: a first distributed acoustic sensing (DAS) station comprising a first plurality of interrogator units for transmitting a first plurality of DAS signals in a first direction, wherein the first interrogator unit of the first DAS station is configured to transmit a first DAS signal at a first interrogation rate and a first wavelength, wherein the second interrogator unit of the first DAS station is configured to transmit a second DAS signal at a second wavelength and a second interrogation rate, the second interrogation rate being less than the first interrogation rate; and a loop array, the loop array comprising a plurality of loops arranged on a plurality of spans, wherein a first set of near-end loops located closest to the first DAS station is configured to route the first DAS signal back to the first DAS station, and Wherein, a first set of intermediate loops located farther from the first DAS station is configured to route the second DAS signal back to the first DAS station.

2. The system according to claim 1, wherein: The first DAS station is located at a first end of the plurality of spans, the system further comprising: a second distributed acoustic sensing (DAS) station located at a second end of the plurality of spans, further comprising a second plurality of interrogator units transmitting a second plurality of DAS signals in a second direction, the second direction being opposite to the first direction, wherein the first interrogator unit of the second DAS station is configured to transmit a third DAS signal at a third interrogation rate and a third wavelength, wherein the second interrogator unit of the second DAS station is configured to transmit a fourth DAS signal at a fourth wavelength and a fourth interrogation rate, the fourth interrogation rate being less than the third interrogation rate; wherein a second set of near-end loops located closest to the second DAS station is configured to route the third DAS signal back to the second DAS station, and Wherein, a second set of intermediate loops located farther from the second DAS station is configured to route the fourth DAS signal back to the second DAS station.

3. The system according to claim 1, in, the first plurality of interrogator units comprises three interrogator units arranged to transmit a set of three DAS signals at three different wavelengths, wherein the third interrogator unit is configured to transmit a third DAS signal at a third wavelength and a third interrogation rate, the third interrogation rate being less than the first interrogation rate and greater than the second interrogation rate, and Wherein a set of remote loops located further away from the first DAS station than the first set of intermediate loops is arranged to route the third DAS signal back to the first DAS station.

4. The system according to claim 3, wherein: The first DAS station is located at a first end of the plurality of spans, the system further comprising: a second distributed acoustic sensing (DAS) station located at a second end of the plurality of spans and comprising an additional set of three interrogator units, wherein the first interrogator unit of the second DAS station is configured to transmit a fourth DAS signal in a second direction at a fourth interrogation rate and a fourth wavelength, wherein the second interrogator unit of the second DAS station is configured to transmit a fifth DAS signal at a fifth wavelength and a fifth interrogation rate, the fifth interrogation rate being less than the fourth interrogation rate; wherein the third interrogator unit of the second DAS station is configured to transmit a sixth DAS signal at a sixth wavelength and a sixth interrogation rate, the sixth interrogation rate being less than the fifth interrogation rate, the sixth wavelength being different from the third wavelength, wherein a second set of near-end loops located closest to the second DAS station is configured to route the fourth DAS signal back to the second DAS station, wherein a second set of intermediate loops located further away from the second DAS station is configured to route the fifth DAS signal back to the second DAS station, and Wherein, the set of remote loops is configured to route the sixth DAS signal back to the second DAS station.

5. The system according to claim 4, wherein: The first wavelength is equal to the fourth wavelength, wherein the second wavelength is equal to the fifth wavelength.

6. The system according to claim 4, wherein: The first interrogation rate is equal to the fourth interrogation rate, wherein the second interrogation rate is equal to the fifth interrogation rate, and wherein the third interrogation rate is equal to the sixth interrogation rate.

7. The system according to claim 3, wherein: The first interrogation rate is 500 Hz to 2000 Hz, wherein the second interrogation rate is 100 Hz to 400 Hz, and wherein the third interrogation rate is 5 Hz to 20 Hz.

8. The system according to claim 1, wherein: The first DAS signal and the second DAS signal are each transmitted as a group of pulses, and the system further includes a controller for synchronizing the first DAS signal and the second DAS signal, wherein the first DAS signal is staggered in time from the second DAS signal.

9. The system according to claim 1, wherein: A given loop in the plurality of loops comprises: Circulator; Erbium-doped fiber amplifier (EDFA); and An optical filter arranged to filter out only a given wavelength to be routed back to the DAS station, the given wavelength corresponding to the first wavelength or the second wavelength.

10. A distributed acoustic sensing system, comprising: a first distributed acoustic sensing (DAS) station located at a first end of the multi-span link and configured to transmit a plurality of DAS signals in a first direction; a second distributed acoustic sensing (DAS) station located at a second end of the multi-span link and configured to transmit a plurality of DAS signals in a second direction; as well as Loop array, including: a first group of near-end loops located closest to the first DAS station; The first set of intermediate circuits; a second set of near-end loops located closest to the second DAS station; and a second set of intermediate loops located closer to the second DAS station than the first set of intermediate loops, wherein a first wavelength and a first interrogation rate of a first set of DAS signals routed through the first set of near-end loops are different from a second wavelength and a second interrogation rate of a second set of DAS signals routed through the first set of intermediate loops, and The third wavelength and the third interrogation rate of the third group of DAS signals routed through the second group of near-end loops are different from the fourth wavelength and the fourth interrogation rate of the signals routed through the second group of intermediate loops.

11. The system according to claim 10, wherein: The first and second sets of DAS signals are transmitted from first and second interrogator units of the first DAS station, respectively, and wherein the third and fourth sets of DAS signals are transmitted from first and second interrogator units of the second DAS station, respectively.

12. The system according to claim 10, further comprising: a group of remote loops located further from the first DAS station than the first group of intermediate loops and located further from the second DAS station than the second group of intermediate loops, wherein a fifth wavelength and a fifth interrogation rate of a fifth set of DAS signals routed from the first DAS station through the set of remote loops are different from the first wavelength and the first interrogation rate and different from the second wavelength and the second interrogation rate, wherein a sixth wavelength and a sixth interrogation rate of a sixth set of DAS signals routed from the second DAS station through the set of remote loops are different from the third wavelength and the third interrogation rate and different from the fourth wavelength and the fourth interrogation rate, and Wherein, the sixth wavelength is different from the fifth wavelength.

13. The system according to claim 12, wherein: The first wavelength is equal to the third wavelength, wherein the second wavelength is equal to the fourth wavelength.

14. The system of claim 12, wherein: The first interrogation rate is equal to the third interrogation rate, wherein the second interrogation rate is equal to the fourth interrogation rate, and wherein the fifth interrogation rate is equal to the sixth interrogation rate.

15. The system of claim 12, wherein: The first interrogation rate and the third interrogation rate are between 500 Hz and 2000 Hz, wherein the second interrogation rate and the fourth interrogation rate are between 100 Hz and 400 Hz, and wherein the fifth interrogation rate and the sixth interrogation rate are between 5 Hz and 20 Hz.

16. The system of claim 10, wherein: The first set of DAS signals, the second set of DAS signals, the third set of DAS signals and the fourth set of DAS signals are each transmitted as a set of pulses, the system further comprising a controller arranged to: synchronizing the first set of DAS signals and the second set of DAS signals, wherein the first set of DAS signals from one station are staggered in time from the second set of DAS signals; and The third set of DAS signals and the fourth set of DAS signals are synchronized, wherein the third set of DAS signals from another station are staggered in time from the fourth set of DAS signals.

17. A method for distributed acoustic sensing, comprising: transmitting a first DAS signal from a first interrogator unit of a first DAS station at a first interrogation rate and a first wavelength; transmitting a second DAS signal from a second interrogator unit of the first DAS station at a second wavelength and a second interrogation rate less than the first interrogation rate; routing the first DAS signal back to the first DAS station via a loop in a first set of near-end loops closest to the first DAS station; as well as The second DAS signal is routed back to the first DAS station via a loop in a first set of intermediate loops located further away from the first DAS station.

18. The method according to claim 17, further comprising: transmitting a third DAS signal from a third interrogator unit of the first DAS station at a third interrogation rate less than the second interrogation rate and at a third wavelength; as well as The third DAS signal is routed back to the first DAS station via a loop in a set of remote loops, the set of remote loops being located further from the first DAS station than the first set of intermediate loops.

19. The method according to claim 18, in, The first DAS signal, the second DAS signal and the third DAS signal are formed by a first group of pulses, a second group of pulses and a third group of pulses, respectively. wherein the first group of pulses is staggered in time relative to the second group of pulses and the third group of pulses, and The second group of pulses is staggered in time relative to the third group of pulses.

20. The method according to claim 17, wherein: The first DAS signal is transmitted over a system comprising a plurality of spans, the method further comprising: An analysis algorithm is employed to sense only a portion of the first DAS signal returned from only a first half of a given span of the plurality of spans.

Images