Optimum Optical Architectures for Seismic Reservoir Monitoring
Authors: Strudley alan , Nash Philip 
Published: October 2008

Abstract
Fibre-optic seismic sensing systems for permanent seismic reservoir monitoring have received significant recent interest. Such technology offers the benefits of increased reliability and lower through life cost compared to existing systems based on electrical sensors. However, to fully realise these benefits, an efficient optical system architecture is required which can combine large numbers of sensors onto a small number of optical fibres, whilst minimising the size and complexity of the array and associated platform-mounted opto-electronic interrogation systems. A comparative study of alternative optical system architectures is presented, and a preferred approach based on the combination of time and wavelength division multiplexing is presented. This approach allows up to 480 fibre-optic sensor channels to be combined onto a single optical fibre pair, significantly greater than other approaches considered. It also permits a simple optical design for the sensor and the array architecture, while minimising requirements for lasers and optical receivers, compared to other architecture approaches.
Results from the implementation of this preferred architecture in a field test of a 4C seismic optical sensor array are presented. A system noise floor below -148dB g/rt Hz, maximum signal level of greater than 0.9g in the seismic band and sensor to sensor cross talk better than -70dB were achieved, confirming the viability of this approach for optical seismic sensing.

Introduction
The use of permanent seabed seismic arrays for monitoring of offshore fields is an area which has been receiving considerable recent attention. Such systems offer significant benefits in facilitating enhanced reservoir recovery, but make exacting demands in terms of system fidelity, repeatability and reliability over the typical 15-20 year lifetime of a field. The high initial, and through life cost of existing Ocean Bottom Cable designs, based on traditional, electrical accelerometers and/or geophones, has been a barrier to the adaptation of this technology.
Systems based on the use of fully fibre-optic sensors and telemetry offer significant potential benefits, by eliminating the requirement for costly in-sea electronics and associated power cabling, and by increasing reliability through the use of robust, electrically passive optical fiber components. However, to fully realise these benefits, such systems require highly efficient multiplexing and telemetry architectures which combine large numbers of sensors onto a small number of optical fibres. In addition since the topsides facilities may be some distance from the array, the system architecture must allow interrogation distances of tens of km. This paper considers a range of alternative architectures for such system, discusses the various trade-offs involved, and reports initial seismic results from one optimized architecture.
The architectures described here have been made possible by the convergence of 2 parallel technology development streams. The use of fibre-optic sensors for underwater sensor applications (principally in the context of military sonar) was first mooted in the late 1970s (Bucaro et al, 1977). Throughout the 1980s optical hydrophone systems were developed for sonar applications, both in the UK and US (Giallorenzi et al, 1982). Specific applications included towed array systems, submarine hull arrays, and seabed arrays.
At the same time, the massive explosion in demand for fibre-optic telecoms based systems led to the development of various key enabling technologies which allowed remote transmission of high data rate optical signals. These technologies included the optical fibre itself, the optical amplifier, low noise fibre lasers, and wavelength division multiplexing systems.
During the late 1990s, these technology streams converged and led to the development of efficient architectures for large scale hydrophone arrays (Nash, Cranch and Hill, 2000)