Whilst placing seismic sensors on the sea floor is actually the first way that marine seismic data were acquired, since the uptake of marine seismic methods by the oil industry in the mid-1960s ocean bottom seismic data acquisition has remained a minority activity in offshore exploration and production.
Historically there were a number of technical issues that were responsible for this and recent advances in instrumentation have addressed these limitations.
Cost has always been a constraining factor but the increasing awareness of the imaging limitations of conventional towed streamer acquisition with its inherently limited range of azimuths are redressing the perceived ‘price gap’ between towed streamer and seafloor seismic data.
The oil industry is about to witness a paradigm shift in the utilisation of both re-deployable and permanently installed ocean bottom data as the oil companies seek to maximise hydrocarbon recovery from existing fields and reduce drilling risk for newly discovered prospects.
Ocean bottom cable (OBC) data acquisition prices have historically been a factor of 5–10 times higher than multi-streamer towed streamer rates due to the inherently greater efficiency of the latter.
Recent improvements in OBC technology including buoy based recording and snaked cable operations have increased OBC productivity and reduced costs. Increased demand for towed streamer data driven by the oil and gas companies’ (no longer able to ‘find’ oil on Wall Street) need to improve reserve replacement ratios, which are close to their all time low at around 83 per cent (Enskilda) has resulted in a closing of the gap between OBC and towed streamer prices such that OBC rates are now approximately 1.5–3 times conventional – single azimuth – towed streamer rates. Increasing use of multi/wide azimuth towed streamer data to address imaging challenges in complex overburden areas has resulted in price inversion whereby OBC rates are arguably less expensive than the complex multi-vessel/multi-pass geometries required to widen the azimuth range of towed streamer data.
In addition to OBC techniques there have been two other methods that have begun to be employed commercially – Ocean Bottom Nodes (OBN) (Fig. 1) and Permanent Seismic Installations (PSI) (Fig, 2). OBN systems use autonomous individual sensor packages – nodes – which are typically deployed using an ROV as there is no inter-station cable, as in the case of OBC, between each node.
In PSI systems which are used in producing fields, cabled sensors are installed on or below the sea floor, using sea floor trenching tools, and are permanently connected to a recording system installed on a production facility.
OBC and Streamer Comparison
A recently published data comparison between towed streamer and OBC, clearly demonstratied the superior imaging capabilities of OBC.
One of the principle reasons for this quality uplift is the diversity of ray paths, especially at longer offsets, obtained when using orthogonal geometries, which can be readily acquired using ocean bottom seismic methods, compared to the inline geometries inherent with conventional towed streamer acquisition.
In a data comparison study presented by the author at the 2006 SEG Conference, the specific geophysical reasons for the improvement in OBC data quality were identified as the lower noise environment achieved by using stationary receivers on the sea floor rather than towing sensors through the water and the inherently wider bandwidth of modern MEMS sensors compared to traditional hydrophones.
Further analysis of the azimuth ranges acquired using the multi-/wide azimuth towed streamer geometries reveals that whilst there is more than one azimuth being acquired through the application of these complex and costly operations, there is nothing like the complete range of azimuths that can be achieved through the use of ocean bottom seismic acquisition.
The application of all three ocean bottom techniques – OBC, OBN and PSI – will increase as both the number of surveys and the size of each survey grows as oil companies seek to maximise hydrocarbon recovery from producing fields.
The application of ocean bottom seismic to 4D or timelapse seismic will be a specific part of this growth as longer offset/full azimuth data unable to be acquired with towed streamers in obstructed producing fields are recognised as being able to define reservoir structure with enhanced resolution. Specifically the application of fibre-optic sensors for PSI will also become the norm, since these systems allow their active components – the lasers which are used to interrogate the sea bed sensors – to remain topside.
Integration with Controlled Source Electro-Magnetic data (CSEM), which also utilises seabed sensors, will further drive the increased utilisation of ocean bottom seismic data as the combination of multi-component EM and seismic data allows reservoir geophysicists to quantify reservoir properties such as compartmentalisation, permeability, fluid saturation and net to gross ratios.
The future will truly be on the sea floor,
Dr Chris Walker is Vice President, Geophysics, Reservoir Exploration Technology ASA, Epsom, Surrey, KT18 5AD, UK. www.rxt.com