Abstract:
A system and method to synchronize distributed measurements in a borehole are described. The system includes a plurality of wired segments coupled together by couplers and a plurality of nodes configured to measure, process, or relay information obtained in the borehole to a surface processing system, each of the plurality of nodes comprising a local clock and being disposed at one of the couplers or between couplers. The system also includes a surface processing system coupled to a master clock and configured to determine a time offset between the master clock and the local clock of an nth node among the plurality of nodes based on a downhole generated synchronization signal.

Description:
BACKGROUND 
       [0001]    During downhole exploration and formation excavation, a number of sensors and measurement devices may be used to characterize the downhole environment. Each measurement, or record of measurements, may be time-stamped, or associated with a known time, so that the measurements from the various devices may be processed together at the surface. However, each of the downhole measurement platforms operates with a respective local clock that is typically not synchronized with the surface master clock. Thus, before the various distributed measurements may be processed together, they must be synchronized to a common time. In prior systems, the surface processing system has undertaken the synchronization. For example, the master clock at the surface generates a synchronization signal, and the local clocks downhole use the signal to set their time in agreement with the master clock&#39;s time so that all time stamps are referenced to the same (master clock) time. As another example, the master clock generates a synchronization signal, and based on a response to that signal from each downhole device, the surface processing system stores a measured offset from the master clock for each device. 
       SUMMARY 
       [0002]    According to one aspect of the invention, a system to synchronize distributed measurements in a borehole includes a plurality of wired segments coupled together by couplers; a plurality of nodes configured to measure, process, or relay information obtained in the borehole to a surface processing system, each of the plurality of nodes comprising a local clock and being disposed at one of the couplers or between couplers; and a surface processing system coupled to a master clock and configured to determine a time offset between the master clock and the local clock of an nth node among the plurality of nodes based on a downhole generated synchronization signal. 
         [0003]    According to another aspect of the invention, a method of synchronizing distributed measurements in a borehole includes disposing a known number of wired segments coupled together by couplers in the borehole; disposing nodes along the wired segments, the nodes being disposed at two or more of the couplers or between couplers and each node comprising a local clock and configured to measure or relay information obtained in the borehole to a surface processing system; generating a time-stamped signal at an nth node based on the local clock of the nth node; receiving the time-stamped signal at the surface processing system; and the surface processing system computing a time offset between a master clock associated with the surface processing system and the local clock of the nth node. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    Referring now to the drawings wherein like elements are numbered alike in the several Figures: 
           [0005]      FIG. 1  is a cross-sectional illustration of a borehole including nodes with local clocks according to an embodiment of the invention; and 
           [0006]      FIG. 2  is a flow diagram of a method of synchronizing distributed measurements in a borehole according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0007]    As noted above, prior systems have synchronized local clocks of downhole devices with a master clock at the surface by using a synchronization signal generated by the master clock or some other form of synchronization initiated by the master clock. Embodiments of the invention described herein use the fact that the time at a local clock need not be synchronized and reset as long as its offset from the master clock is determined and accounted for. Accordingly, embodiments described herein include downhole initiation of a synchronization process rather than synchronization by the master clock. 
         [0008]      FIG. 1  is a cross-sectional illustration of a borehole  1  including nodes  110  with local clocks  115  according to an embodiment of the invention. Nodes  110  are disposed in the borehole  1  penetrating the earth  3 , which may include a formation  4 . The formation  4  represents any subsurface material of interest that the nodes  110  may help to characterize. The nodes  110  may be conveyed through the borehole  1  by a carrier  2 . The carrier  2  may be a wireline used in wireline logging after drilling has ceased. In this case, the nodes  110  are disposed along the wireline. In alternate embodiments, the carrier  2  may be a drill string used in Logging While Drilling (LWD) with the nodes  110  disposed in a bottomhole assembly. In general, the nodes  110  with local clocks  115  may be part of any system for obtaining downhole measurements in a borehole  1  in which the nodes  110  are distributed along the borehole  1 , and in which the nodes  110  transfer data along the borehole  1 . That is, a given node  110  may obtain downhole data (act as a sensor measuring data) for transmission to the surface, relay downhole data sent from another node  110  without any additional processing, or may process measured data or data received from another node  110 . The data transfer by the nodes  110  may be via signals including, for example, mud pulse, acoustic, electro-magnetic, electrical, or optical. 
         [0009]    In the embodiment shown in  FIG. 1 , the carrier  2  is a wired pipe system composed of multiple wired segments  120  with interspersed couplers  125 . These couplers  125  are used to transfer data between sections of tubular elements that make up the carrier  2  (e.g. drill string). The wired segments  120  may, therefore, all be of approximately the same length and, therefore, periodic. The couplers  125  may be nodes  110 , though not all couplers  125  may be nodes  110 , and. a node may lie between couplers  125  (see e.g.,  110   m ). For example, coupler  125   x  is not a node  110  while coupler  125   y  is a node  110  that measures or senses some information downhole. Each of the nodes  110  relays information from another node  110  farther from the surface. Each node  110  includes a local clock  115  so that information provided by each node  110  is time-stamped with the local time at which the information was obtained. Information relayed to the surface may be processed by a surface processing system  130  that includes one or more processors and memory devices. The surface processing system  130  also includes or is coupled to the master clock  135 . 
         [0010]    The local clocks  115  of the nodes  110  may be relatively simple crystal oscillators that are intended to work in an asynchronous manner (i.e., independent of any other timing device in the borehole  1 ). The local clocks  115  are intended to ensure that inter-sample time periods are accurate to a specified amount, but a local clock  115  at a node  110   m  may experience time drift with respect to a local clock  115  at another node  110   n  over the long term. Local clocks  115  are used to time-stamp individual measurements, or records of measurements, with a local time. In general, the master clock  135  may be a very accurate time keeping system, possibly synchronized to a remote timing system, such as that supplied by a Global Positioning System (GPS). 
         [0011]    The couplers  125 , wired segments  120 , and nodes  110  operate in a time-varying thermal environment and are also subject to vibration. As a result, the local clocks  115  of the nodes  110  drift with respect to one another. In addition, signal propagation speeds vary with temperature because, for example, material properties of the wired segments  120  change with temperature. The signal propagation delay includes time delay through the multiple wired segments  120 , time delay through the multiple couplers  125 , and time delay at each of the nodes  110  encountered by the signal prior to reaching the surface processing system  130 . Each of these is discussed in turn. 
         [0012]    With regard to the wired segments  120 , when these are coaxial segments, then signal propagation velocity is approximately 180 m/μs, assuming a velocity factor of 60% for the dielectric. If each coaxial segment is 10 m in length, then the delay over each segment is 1/18 μs. Over an exemplary borehole  1  of length 10 km, the resulting delay over the combined coaxial segments is 0.056 ms. Because seismic recording rates are 1000 s/s, a clock accuracy (offset accuracy) to 0.5 ms is needed. As illustrated by the exemplary case, this accuracy is apparently achievable by at least a factor of 10, even with this conservative estimate of the coaxial velocity factor. Thus, delays through the wired segments  120  may be ignored, although they may be compensated for if needed. The delay through the wired segments  120  may be modeled to account for temperature effects, for example. With regard to the couplers  125 , their cumulative effect may be significant. However, because there are a relatively large number of couplers  125  within a carrier  2  (e.g., wired pipe), the delay introduced by the couplers  125  may be measured experimentally and handled statistically. The surface processing system  130  has knowledge of the number of couplers  125  between itself and a given node  110  and can, therefore, estimate the delay resulting from the couplers  125 . If the coupler  125  delay is found to be affected by temperature, then periodic temperature measurements along the carrier  2  may be used to correct the delay values. With regard to the nodes  110 , the delay associated with each may be more problematic because nodes  110  are computation points and may read and write messages, as well. As such, the delay at each node  110  is unlikely to be the same from one transmission to the next. Further, the delay at a given node  110  is likely to be different from the delay at another node  110 . However, the delay at a node  110  may be made deterministic by delaying a time synchronization signal a specified amount of time at each node  110 . That is, as long as the specified amount of time that is assumed as the delay is greater than the actual computation, read/write time taken by any node  110 , the actual delay at the node  110  need not be known. For example, if the longest delay at any node  110  is x, a forced delay of x+some margin may be imposed on all the nodes  110  in order to make the delay related to the nodes  110  deterministic. In alternate embodiments, a processing delay (x+some margin) may be imposed on those nodes  110  that process data but not on nodes  110  that merely relay data. In alternate embodiments, the delay at each node  110  may also be determined statistically. 
         [0013]    The offset associated with a particular node  110   n  may then be calculated as: 
         [0000]        B[n]=C[n]+da[n]+dc[n]+dr[n]−C   M   [EQ. 1]
 
         [0000]    where B[n] is the offset for the nth node  110 ;
 
C[n] is the local clock time of the nth node  110 ;
 
da[n] is an aggregate of the delay associated with every acquisition platform or node  110  from the surface to the nth node  110 ;
 
dc[n] is an aggregate of the delay associated with every wired segment  120  from the surface to the nth node  110 ;
 
dr[n] is an aggregate of the delay associated with every coupler  125  from the surface to the nth node  110 ; and
 
C M  is the master clock  135  time.
 
As noted above, the delay associated with every node  110  (da[n]) may be an artificial delay that is introduced at each node  110  (or each processing node  110 ) and ensured to be greater than the actual processing time at every node  110 . Also, a node  110  may not be disposed at every coupler  125  but may also be disposed between couplers  125 . Thus, the number of couplers  125  between the nth node  110  and the surface may be greater or less than the number of nodes  110  between the nth node  110  and the surface. Once the offset (B[n]) is determined, the timestamp of a data point received from the nth node  110  may be standardized to the time of the master clock  135  as follows:
 
         [0000]      master timestamp=timestamp from nth node− B[n]   [EQ. 2]
 
         [0000]    Once all the data points from all the nodes  110  are adjusted to have timestamps standardized to the master clock  135  according to EQ. 2, measurements taken at different nodes  110  at the same time may be matched up and used in the analysis of the downhole environment. 
         [0014]    In one embodiment, node  110   n  is the farthest node  110  in the borehole  1  from the surface processing system  130 . A time-stamped signal originates at node  110   n  and is relayed to the surface processing system  130  by each node  110  between node  110   n  and the surface processing system  130 . By having each node  110  add its local timestamp to the relayed signal, the offset associated with each node  110  from the farthest (node  110   n ) to the closest to the surface processing system  130  may be determined. As such, the node  110   n  would have generated the synchronization signal. In alternate embodiments, any node  110  may generate the synchronization signal for the surface processing system  130  to determine its offset and the offset of nodes  110  between the node  110  generating the synchronization signal and the surface processing system  130 . 
         [0015]      FIG. 2  is a flow diagram of a method  200  of synchronizing distributed measurements in a borehole  1  according to an embodiment of the invention. Disposing a known number of couplers  125 , nodes  110 , and wired segments  120  in the borehole  1  (block  210 ) may be as shown in  FIG. 1 , for example. Generating a time-stamped signal at a node  110  (block  220 ) includes generating the synchronization signal at the node  110   n  that is farthest from the surface processing system  130 . The method  200  also includes receiving the signal at the surface processing system  130  at block  230 . Computing the offset at block  240  may be for one or more nodes  110 . For example, as discussed above, in one embodiment, a synchronization signal generated at the farthest node  110   n  is used to determine offsets for every node  110  from the farthest node  110  to the closest node  110  to the surface processing system  130 . 
         [0016]    While one or more embodiments have been shown and described, modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.