Multiplexing Architecture
System multiplexing is based on a two-level architecture. In the first level, up to 64 sensor channels (16 4-C packages) are combined using time division multiplexing (TDM). At the second level, up to eight sets of 64 TDM channels are multiplexed onto a single fibre pair using wavelength division multiplexing, giving a total of 512 channels on one fibre pair. The multiplexing architecture is based entirely on commercially available passive fibre optic components, developed over the last five years for commercial telecommunications applications.
Interrogator Unit
The interrogator unit is a key part of the system that generates the optical signals for interrogating the array and converts the returning TDM/wavelength division multiplexing optical signal stream into multiplexed digitised electronic data in the required output format. QinetiQ Ltd have developed an interrogator unit that is housed in two ruggedised 19- inch racks. One rack houses the transmit optics, including up to eight fibre lasers as optical sources. The other houses the receive and demodulation electronics, which are based on commercially available digital signal processors (DSPs). The receive electronics uses a flexible design in which most of the processing is carried out in firmware. Up to 64 sensor channels (16 4-C packages) can be driven from a single demodulator card. The current QinetiQ system, shown in Figure 2, has the capacity to drive 512 sensor channels (128 4-C packages). The system is modular in design and, as more powerful DSPs become available, has the potential to achieve significantly higher channel densities.
Figure 2:
Interrogator Unit (512 Channel Capacity)
Achieved Performance
All the elements of the system described above have been developed and tested. A summary of the most important achieved performance parameters is given in Table 2. Further improvements are expected in some of these figures as a result of recent development.
Table 2: Achieved System Performance
| Parameter |
Achieved performance |
| Accelerometer bandwidth |
> 500Hz |
| Accelerometer noise floor |
< -143dBg/(Hz @ 100Hz) |
| Hydrophone noise floor |
50dB re 1 microPa/(Hz @500 Hz) |
| Dynamic range |
> 120dB @ 100 Hz |
| Harmonic distortion |
< -60dB |
| Intra-sensor crosstalk |
< -80dB |
| Accelerometer cross-axis rejection |
< -30dB
|
A key test was carried out in February 2003 when a prototype four-component package was assembled and tested at sea. The package combined three QinetiQ accelerometers and a hydrophone into a proprietary Input/Output 4-C pressure housing (see Figure 3), which was deployed off the south coast of UK and interrogated using an air-gun. The package worked well, meeting all the performance goals that had been set. This is believed to be the first ever at sea demonstration of a fully fibre-optic 4-C package. A future goal is to carry out a similar test of the miniature 4-C package shown in Figure 1
Figure 3: Fully Fibre Optic 4-C
Package in Input/Output Body (Before Sea Test)
Future Plans
QinetiQ are currently working to have all the technology in place to be able to deploy a 4-C array with in excess of 100 stations in 2005. As part of the development programme a detailed comparison test is planned, which will compare the performance of a set of 10 fully fibre optic stations with a similar number of state-of-the art VectorSeis™ stations supplied by Input/Output. A consortium of operators is being assembled to participate in this test.
Conclusions
The E&P industry is now beginning to accept the use of fibre optic sensors for many downhole applications and to appreciate the benefits they bring. The next step is to apply fibre optic technology to the more difficult problem of seismic sensing and, in particular, to the application of very large permanent seabed arrays for LOFS. The benefits of fibre optic technology for this application may be at least as great as those for the existing E&P applications. A powerful fully fibre optic solution has been developed by QinetiQ, who are working with Input/Output to introduce this innovative new technology into service within the industry. Adoption of this technology will be another major step towards the vision of a fully optical oilfield.
Category:
Reservoir Engineering
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