Fibre optic sensors employ a length of optic fibre arranged in such a way that a sensed parameter causes a strain to be imposed on the fibre. Typically the fibre is arranged in a coil, although other arrangements are possible. Such strain causes a change in phase of the optical signal propagating in that fibre, which change can be detected by interferometric techniques. A variety of different arrangements for this type of transducer have previously been proposed, many of which have the coil of optic fibre wound on a deformable core or mandrel, which undergoes radial expansion or contraction in response to the sensed parameter, such as sensed vibration.
Such fibre optic sensors can exhibit extremely high sensitivities, and have the advantage of being completely passive, employing no power at the sensing transducer. Such sensors have also proved popular in applications where large arrays of sensors are required, on account of the relative ease with which they can be multiplexed.
An example of such an application is seismic surveying in the oil and gas exploration industry, where large time multiplexed arrays comprising hundreds or even thousands of vibration sensors and/or hydrophones can be used to sense reflections of an incident pulse from geological formations beneath the sea bed. Sampling such an array at regular periods provides 3D time lapsed data on existing or potential new reserves.
In greater detail, a high amplitude seismic source (usually an airgun) is towed across the top of a (known or potential) oilfield, firing the source at regular intervals, and the reflected returns form the source are monitored using sensors which are either towed together with the source or are positioned on the seabed. It is desired to be able to measure directly both the direct signal from the airgun when it first hits the sensors (which is a very high amplitude), and the seismic returns reflected from the underground features within the field (which are much lower amplitudes). Two examples of the relationship between the sensor output and time are shown in FIG. 1. In the top plot, the airgun is closer to the sensor than in the lower plot, and the amplitude of the signal is correspondingly greater. The large variations of sensor output at the left-hand side of the graph represent the direct signal from the seismic source impinging on the sensor. After a short interval of little activity, the smaller variation sensor outputs at the right hand side represent seismic returns from underground formations detected by the sensor
A problem experienced with this approach to sensing is that, for a given sampling rate, signals above a certain amplitude threshold cause the phase based sensed information to become distorted, and can cause failure of the demodulation process. This effect, commonly referred to as overloading or overscaling is dependent on the frequency of the measured signal. In seismic systems this can cause a particular problem with the direct arrival of the incident pulse, which is used to determine the orientation of the sensors with respect to the source. This is especially true when the pulse has been generated close to the sensors, however at greater ranges even the direct arrival may not be overscaled. It is desirable to be able to record this incident pulse without the distortion that overscaling can produce.