Almost all physical measurements are based on a simple principle: measurement of a change in length. When we measure temperature, we measure how much a column of a fluid changes due the change in temperature. When we measure blood pressure we also a measure the change in length of a fluid column. If we want to measure the mass of an object we can measure the displacement of a spring.
For seismic measurements we recognize the same principle: conventional hydrophones, responding to variations in acoustic pressure, convert a small length change into a piezoelectric signal that is proportional to the amplitude of the pressure wave. The output voltage is digitized and transmitted via a cable to the data recording system.
However, such measurements require electrical instrumentation that may become unreliable due to corrosion, electrical leakage, or sensor degradation when stationed on the sea bed for “life of field” permanent monitoring.
A bundle of optical fibres (Frontiers, April 2002).
Fibre optics for seismic
However, there is an alternative to electronics for measuring seismic waves: fibre optic technology. The fibre optic system is entirely passive, with no electronic components at the wet end, and is therefore expected to be more reliable than in-sea electrical instrumentation. A strong interest in the use of fibre optics in permanently installed reservoir monitoring systems has therefore recently developed.
The present article focuses on how is it possible to “listen with light” to measure seismic signals.
Reading on, you will see that the principle behind fibre optic seismic sensors is again the measurement of changes in length.
To help us present a simple explanation, we visited the leading company in the fibre optic 4D seismic market, Optoplan AS. They have developed a unique seismic fibre optic sensor system – Optowave – by innovatively using fibre Bragg gratings (see box) as reflectors in interferometers. Optoplan, today part of CGGVeritas’ Sercel subsidiary, installed a pilot seismic fibre optic cable system for StatoilHydro at the North Sea Snorre field in the summer of 2008, and in October 2008 won the ConocoPhillips contract to supply the Ekofisk life-of-field seismic system for reservoir monitoring.
The optical fibre
The optical fibre.The optical fibre is composed of several elements: core, cladding and buffer. The core at the centre is the light carrying element. It is usually made up of silica doped with another material to achieve the proper index of refraction. The core is surrounded by the cladding layer which is still based on silica, but with another dopant* or different concentration of the dopant, again to achieve the proper index of refraction. The fibre is then coated with a protective plastic covering called the buffer.
To confine the light in the core, its refractive index must be greater than that of the cladding. Typically, the core and cladding values are around 1.5, with a difference of approximately 0.5 percent.
The core of the fibre is only about 8-10 microns. This is the standard single-mode fibre used for long-range communication like telephony and cable TV.
The optical system also includes a transmitter that converts an electric signal into light and sends it down the fibre. The transmitter is a laser diode designed to emit light pulses at wavelengths around 1550 nanometers (nm).
Attenuation of the optic signal
The attenuation in the fibre is minimum around 1550 nm.The attenuation of light in an optical fibre is mainly caused by scattering due to minor imperfections in the glass. This attenuation is strongly dependent on the wavelength of the light. By choosing wavelengths corresponding to low attenuation (1550 nm), it is possible to transmit optical signals over tens of kilometres with little loss. Typical losses for single-mode fibres are of the order of 0.2 decibels (dB) per kilometre (dB is measured as 10 times the logarithm of the output power divided by the input power).
Fibre optic hydrophones and geophones
Optoplan’s fibre optic cableThe sensors are hydrophones and three-component accelerometers.
A fibre optic hydrophone is composed of a cylinder of a plastic-like material, where the fibre is coiled around the cylinder. When a pressure wave is passing the cylinder, the length of the coil will be slightly changed, and this displacement can be measured with high resolution as a shift in reflection time or phase delay between two identical Bragg gratings situated on opposite sides of the hydrophone. The wavelength of the incident light must be equal to the Bragg wavelength. In this way the hydrophone converts pressure waves into a change in the fibre length. The change in the length is proportional to the amplitude of the seismic wave.
The fibre optic accelerometer similarly modulates the fibre length as the sensor is exposed to acceleration. To achieve a directional measurement, a special design utilizing two half-spheres at each side of a rod ensures that the coil length is changed only for a directional signal. If the seismic wave hits the optic accelerometer perpendicular to the rod, the coil length will not change. When the wave hits along the rod, the coil length will change, and a non-zero seismic signal is measured.
One advantage of fibres is that a number of sensors can be multiplexed on one fibre. In the Optowave system this is implemented through a combination of time division multiplexing and wavelength division multiplexing. All sensors within one 4C seismic station are multiplexed using time division multiplexing, i.e., separated in time at the detector. Several 4C stations are interrogated through the same fibre using wavelength division multiplexing, meaning that every station is interrogated at a different wavelength. This requires several lasers emitting light with distinctive wavelengths to matching Bragg gratings.
Seismic stations and cables
A fibre Bragg grating (FBG) acts as a light reflector with maximum reflection at a very specific wavelength. Other wavelengths are transmitted. The Bragg grating is “written” into a short segment (about 1 cm) of the fibre by using an intensive ultraviolet laser that alters the refractive index of the fibre core. The core is photosensitive with exposure to UV light, and its change in refractive index is a function of the intensity and duration of the exposure. The grating will typically have a sinusoidal refractive index variation. The reflected wavelength λ, called the Bragg wavelength, is defined by the relationship λ=2nΛ, where n=(n3+n2)/2 is the average refractive index of the grating in the fibre core and Λ is the grating period. Using an average refractive index of 1.55 and grating period of 500 nm, the grating reflects at λ=1550 nm. The reflection strength Rλ(peak reflectivity) is determined by the grating length L=NΛ, where N is the number of periodic variations, and the grating strength n3-n2. The refractive index is a measure for how much the speed of light is reduced inside the fibre relative to a vacuum. For example, the glass core has refractive index of 1.5, which means that in the core, light travels at 1 / 1.5 = 0.67 times the speed of light in a vacuum. This is fast, 200,000,000 metres per second. The Bragg grating acts as light reflector for a specific wavelength while other wavelengths are transmitted.
Left: Basic principle of a fibre optic hydrophone. B1 and B2 are two identical Bragg gratings which reflect light with the same wavelength as that emitted by the laser. When an acoustic pressure wave passes the fibre coil, the length of the fibre between the gratings will change by a small amount (dL). This length change is precisely measured as a change in travel time (phase delay) between the Bragg reflections. The relationship between dL and the amplitude of the seismic pressure wave is linear. Right: Optoplan’s fibre optic hydrophone (top). The fibre is wrapped around a cylinder of a material that is sensitive to the pressure wave. The fibre optic accelerometer (bottom).The hydrophone and three-component accelerometer are mounted into a fibre optic four-component seismic station or sensor package which is spliced into the fibre optic cable. The cable protects the fibres in their hostile environment. In a full-scale ocean bottom seismic system covering an area of about 50-60 square kilometres., several thousands of these stations are connected in cables. The cables are trenched in the seafloor and connected back to topside facilities on a platform. An advanced optical interrogation system is used to read out the optical signals from the sensors as they measure seismic reflections. Finally, the signals are converted to seismic formats and transferred to shore for data processing and interpretation.
