The transformation of seabed seismic
Seabed seismic surveys have been part of the hydrocarbon exploration industry for many decades. Initial implementations deployed cables which were populated with hydrophones and generally used for shallow water and transition zone projects. The cables were directly connected to a recording vessel to power the in-sea hardware, manage the spread and record the sensor measurements. Barr et al. in the 1980s implemented a technique, first postulated in 1954 by Haggerty and Backus, which uses a dual component measurement (pressure and vertical particle velocity) to eliminate the receiver side ghost, extending the reach of the technique into deeper waters. The potential of the seabed technique was further extended to make a measurement of the shear reflectivity through the addition of horizontal particle velocity components (four-component recording or 4-C). In the late 1990s the industry started experimenting with buried seafloor cables as part of permanent seismic installations to assist in production monitoring of the reservoir. As well as ocean bottom cable (OBC) systems, it is possible to make a seabed seismic measurement with an array of ocean bottom nodes (OBN). An ocean bottom node is an autonomous recording device with a self-contained recording system, clock and battery. As there is no connection with the surface, there is no limitation on length of the receiver line, no downtime due to telemetry/power line failures and no lost time associated with moving of the recording vessel. Ocean bottom nodes have been in use for many decades, but their use had primarily been limited to long-offset refraction surveys for studies of earth tectonics. The majority of the early node technologies were developed in academia and were neither industrially engineered nor designed with a focus on operational efficiency. After early development by Statoil in the 1990s, the first seismic reflection ocean bottom node survey was acquired in 2004 over Pemex’s Cantarell Field in the Gulf of Mexico. Ocean bottom nodes, when used for hydrocarbon exploration, were initially deployed by remotely operated vehicles (ROVs), but more recently ropes or wires have also been employed.