Mark B. Amelang - OCTIO Geophysical
While fiber optics certainly have a place in seismic monitoring, even on the seafloor, it is worthwhile to consider the inherent limitations of this technology for certain cases.
Passive fiber optic sensors have been used in various applications for quite some time. More recently they have been fashioned into configurations for sensing seismic, and are also used in permanent seismic sensor cables.
The deployment of fiber optic sensors to record seismic data for exploration poses specific challenges. Several factors contribute to data quality and reliability challenges for fiber optic sensors
Fiber optic cables are widely held to be the standard solution for telecommunications. This telecommunication use has led some to believe that the same technology is suitable for seismic acquisition. Fiber optic cables are small, lightweight, and can conduct huge amounts of data. This broadband digital data-transmission medium has been in use underground and even under the ocean for years now. Transocean fiber optic cables are responsible for much of our ability to rapidly communicate on a global basis.
Transmission of digital signals across these expanses of fiber optic cables can be monitored easily and adjusted to ensure data quality and to detect data deterioration or even failure. These cables are contiguous across long distances, with few breaks. This means that the fibers can be installed in steel tubes or coated to protect them from water and H2 intrusion under the ocean. Long stretches of cable with few connections are inherently more reliable than when many connection points are used (such as for passive fiber optic sensors).
Another reason that fiber optic cables conducting digital communications are wide spread is because of reliability and predictability available with optical fibers combined with proven failure detection and correction algorithms. Even though the technology is recent, accepted standards are available to use in reliability models such as Telecordia. This is much like the reliability standards used for electronic components, where the mean time between failure calculations are well known and accepted.
Fiber optic seismic sensors
Dynamic range is one challenge for fiber optic sensors. Fiber optic sensors typically require a high dynamic range that can exceed 180 db. Standard bandwidth acquisition parameters can lead to data clipping and the loss of data fidelity, or can limit the choice in size of seismic source that can be used. Fiber optic sensing is based on measurement of time delays in light traveling through the fiber. Longer delays correspond to larger sensor signal amplitude. These time delays are influenced by all signals hitting the sensor, also the high frequency signals, which in an electric system are filtered out by the anti-aliasing filter. In a fiber optic system (FOS) there is no anti-aliasing filtering of the analog signal. High-frequency noise can show up as noise spikes in the seismic band.
Signal integrity can be an obstacle to recording and processing quality seismic data collected from fiber optic sensors. The typical deployment of fiber optic sensors includes many fiber optic cables deployed together. To reduce the number of optical fibers, the systems include both time and wavelength multiplexing on the same fiber.
While the transmission of digital signals will not be affected much with this kind of dense signaling, the propagation of analog signals along an optical fiber using time division multiplexing and wavelength division multiplexing can lead to cross talk and noise.
A fiber optic sensor line can be compared with the analog electrical seismic streamers in use years ago. A single sensor on a single fiber may have decent performance. But when many sensors are multiplexed on the same optical fiber, the noise and distortion values can become unacceptable.
In addition, sensor orientation can be a problem. Fiber optic sensors typically do not have built-in gravity meters and do not sense orientation. They use first arrival analysis of the seismic signal to calculate it. This can cause an issue with accuracy of the sensors orientation which shows when it comes time to process the seismic data.
Fiber optics in a marine environment
Several design and operational factors can limit the effectiveness of fiber optic sensors on the seabed. While some of these shortcomings are little more than a nuisance on the surface, they can jeopardize system integrity and use in deepwater.
Achieving the required lifetime is a challenge for fiber optic systems, especially due to hydrogen permeation into the fiber core. When the optical fibers all are contained in a contiguous, hermetically sealed (typically steel) tube, such as is for digital communications, this is not a problem. However, for fiber optic sensors with breaks in the seal every 25 or 50 m (roughly 80 to 160 ft) this is a serious concern, even though various methods are used to slow down the hydrogen permeation.
Fiber optic accelerometers are a sort of spring-mass system without a feedback system to avoid movements of the mass. The mass “squeezes” the fiber-optic coil to measure the acceleration and that causes a change in characteristics of the sensor over its life. To avoid that, it is possible to use a hybrid system (e.g. H. Asanuma et al.) with optical feedback. However, the electronics need to connect back from the seafloor.
The design of an FOS typically calls for all electronic components to be installed on the production platform. One consequence is that the electronics need powerful air conditioning to cool the high temperature laser source and electronics. It also becomes difficult to stabilize the laser in a high vibration area such as an offshore platform. This creates noise in the seismic signals.
The laser source is a vulnerable part of the FOS system. Accuracy of results is related directly to accuracy of the sensors. The ambient conditions (temperature, alteration, vibration, instability in the laser power, etc.) of the laser source change the light frequencies. To avoid this, the laser source has to be recalibrated frequently, thus making the FOS array unusable for continuous recording.
It is also difficult and costly to increase the usable lifespan of an FOS with bypass techniques and redundant components. In a standard array, if one component fails, the entire sensor chain fails. This makes putting many seismic sensors on one fiber risky. The alternative is to run multiple fibers to each sensor, but that dramatically increases the number of fibers to be connected, repaired, etc. In any case, adding multiple fibers provides for many underwater connections in one cable, increasing the chance for water intrusion.
Troubleshooting a fiber system with a mass of cables in a bundle is compared to looking for a needle in a haystack. This makes repair somewhat problematic. Deploying fiber optic systems also can be a problem when wet mate connectors are used. These connectors are expensive and are not proven to be reliable, even for a limited number of fibers (qualified long-life wet-mate connectors are, however, available for eight fibers or less).
To install a large sensor grid at the seabed normally requires the sensor cables to be connected to a backbone, or central connection points to minimize the number of cables going to the surface. This can be done by splicing the cables at surface during the installation process, or by use of wet-mate connectors. Splicing is time consuming, and in deepwater (more than a few hundred meters) splicing is not viable. Connecting hundreds of fibers using wet-mate connectors is risky and unreliable, in addition to being expensive.
So, while fiber optics has its place in marine seismic acquisition, fiber optic seismic sensors could prove to be a problem. Alternatives exist in the market today including electrical systems that use fiber optic for digital communication (the design purpose of fiber optic technology) and MEMS-based (micro-electric mechanical system-based) seismic sensors to ensure 25-year system longevity and high-fidelity seismic acquisition.
The key to success is using best in class technology for each case in permanent reservoir monitoring system, especially in deepwater.
