The system consists of a passive, fully fibre-optic array of Optical Sensor Units (OSU) connected by armoured fibre optic cable. Fibre-optic systems are inherently more reliable than electrical systems under harsh subsea conditions. The array is installed subsea and connected through a riser to an interrogation device at the surface that communicates with the sensors by pulsing light into the array and receiving and decoding the returning signals.

The system is designed to enable large multi-component seismic sensor arrays, permanently installed on the seabed, that are highly reliable and robust.

A highly efficient proprietary optical architecture ensures maximum channel count per fibre and a minimum number of optical components. The proprietary OASiS™ approach offers a unique solution to the potential challenges of overscale (sensor saturation) by extending the system's effective dynamic range to 180dB. This large dynamic range enables recording of data even at very close ranges to full size airgun arrays. The Fosar architecture facilitates flexible array geometries using reliable industry standard subsea wet-mate connectors and small diameter risers to connect with the surface recording system.

The architecture also employs a high channel count per laser, so the interrogator installed topside on the production facility can be very compact, with low power and maintenance requirements. Lightweight cable and sensor packaging help to enable safe, efficient, installation.

Scalability
The Fosar system architecture is modular and scalable. The basic unit of an array is a 6.4 km length of sensing cable containing 128 4C optical sensor units (OSUs). This requires only 2 pairs of optical fibres to be connected to the surface interrogation and recording equipment. Keeping the number of fibres to a minimum ensures that the seabed-surface connector is as light and compact as possible. This is important in terms of reliability of the design and ease of installation, particularly in deep water or when there are long distances from the array to the surface infrastructure where the interrogation and recording equipment are situated.

Future-proof
Stingray has adopted a systems approach to the development of Fosar. This is essential in optical sensing systems as the surface opto-electronics and the optical fibres connecting it to the sensors all form part of the sensing system, as does the multiplexing architecture used to maximise throughput on the system. It also impacts characteristics such as dynamic range, noise and crosstalk. The system is also designed to simplify the updating of components if and when future developments in technology or manufacturing provide better alternatives.