Advanced fibre-optic architecture
The Fosar system is designed to be a large-scale seabed seismic system suitable for seismic permanent reservoir monitoring (seismic PRM) applications. In addition to accommodating a very large number of multi-component seismic sensors, the system is designed to be robust, simple to install and reliable under harsh subsea conditions over the life of an oil or gas field.

Design Objectives

No underwater electronics Electrical systems typically require high voltage power to be delivered to the seabed. This raises issues of power generation, health and safety, and system maintenance at the surface. Also, and more critically, electrical systems are highly prone to failure due to water ingress and heat evolution. Fibre-optic systems feature no electrical parts installed subsea, making them inherently more robust and reliable than electrical systems. The power requirement of the topside interrogation and recording equipment is a fraction of that required for a complete electrical system.

Small size and weight
Installation of seabed equipment can represent up to 50% of the cost of a seismic PRM seabed system. A key design feature of the Fosar system is that it is compact and lightweight. This makes the system easier to install, with less impact on existing oilfield infrastructure.

Design Solution

A highly efficient optical architecture Many properties of light can be changed by a seismic signal, including its wavelength (colour), phase (time delay), amplitude and polarisation. Of these, phase is the property that can be measured with the greatest sensitivity.

To achieve high sensitivity, the optical fibre is wound into a small coil. Changes in pressure or acceleration squeeze the coil, resulting in changes in the length of the fibre. These changes are very small – of the order of the size of an atom – but result in changes to the phase of light that are measurable by sensitive optical systems.

The Fosar system utilises two types of sensor: accelerometers and hydrophones. The sensors are optically identical, but are distinguished by the way in which the fibre coil is mechanically packaged.

an approach that would enable the maximum number of channels per fibre. Optical input signals become slightly weaker at each sensor, so an architecture providing maximum optical efficiency will enable longer array lengths and less requirement for optical amplification. Another key design objective is to minimise the number of system components, including those within the sensors, connections to sensor units and lasers and electronic cards in the topside interrogator.

Stingray selected an architecture that combines time division multiplexing (TDM) and wavelength division multiplexing (WDM). TDM enables the recording on one fibre of signals generated by one laser returning from several sensors, the outputs of which are distinguished by their arrival time. WDM uses several lasers, each of which provides a unique input wavelength to a set of sensors. Thus a number of sensors are combined together using TDM at each wavelength.

Efficient measurements
The Fosar system uses a reflective coupler-based approach, in which very efficient coupler-based mirrors at each end of a sensor reflect some of the optical input signal. The reflections from before and after the sensor are compared to produce a measure of the phase shift within the sensor. This was selected in preference to a Fibre Bragg Grating (FBG) reflector approach, which has a lower multiplexing efficiency, is less cost-effective and suffers from crosstalk.

Stingray has developed a proprietary solution to address the impact of platform-generated noise on the seismic signals measured by the optical system. Stingray has also developed OASiS™, a unique solution to the challenges of overscale (sensor saturation).