Conventional towed streamer 3D seismic configurations have been widely used to acquire repeat surveys for 4D analysis. The cost of deploying vessels and equipment for a towed-streamer 3D survey has, to date, been considerably less than the cost of installing seabed equipment and acquiring seabed data over an equivalent area.

However, towed streamer surveys are likely to exhibit differences between surveys due to several factors, including different weather conditions, tides, sea swell, currents and receiver positions. These differences cause noise in the subsequent 4D analysis that can obscure differences relating to changes in the reservoir.

Seabed receivers
Deploying receivers on the seabed avoids many of the causes of 4D noise inherent to towed-streamer methods, and improves the repeatability of sequential surveys. Seabed receivers are immune from swell, tides, currents and other near-surface noise. If permanently installed, seabed receivers are also guaranteed to be in the same location from one survey to the next.

A further major advantage of seabed receiver systems is that they can record both P- and S-wave seismic data (multi-component), which can reveal considerably more about a reservoir than conventional towed streamer systems, which record only P-waves. The system sits on the seabed and a source vessel periodically shoots over the network to provide an updated image of the reservoir. Comparison with images from previous surveys enables some key reservoir performance characteristics to be deduced.

Permanent seabed systems
While it is likely that towed-streamer 3D surveys will continue to be cheaper than an equivalent single seabed receiver 3D survey, the balance can quickly swing in favour of seabed solutions when an operator invests in a long-term reservoir monitoring strategy. Once a seabed receiver system is permanently installed and operational, the cost of acquiring a new 3D dataset is much less than that of mobilising and deploying another 3D towed-streamer vessel.