Seismic survey is a vital first step taken in the process of finding hydrocarbons within the oil and gas industry- both offshore and onshore. It is an operation that involves transmitting sound pulses through the earth’s surface and capturing the returning signals.
Geophysicists will in turn interpret the data from the signals received and process it into either a 2D or 3D model, allowing geologists to assess whether it is worth proceeding to the next step of drilling an exploratory well.
In the offshore oil and gas industry, the operation is carried out by towing the seismic equipment behind a specially commissioned ship.
In today’s modern offshore seismic survey industry, seismic survey ship design is not purely dictated by the large amount of seismic hardware needed, but also by the vast amount of data processing equipment needed to evaluate, process and produce the ever increasing complex models demanded by the oil and gas companies.
A sound wave source is towed behind the seismic survey vessel. This is most commonly an air gun or series of air guns where a faster rate of shooting is required.
Typically the sound wave source or air guns are fired in intervals every 15 to 20 seconds. The sound waves from the source are directed towards the earth’s surface, where they meet varying degrees of resistance due to the differing density of rock and hopefully hydrocarbon beit gas, oil or condensate.
Behind the sound wave source, a series of cables known as streamers are towed by the same seismic survey vessel. The streamers contain equally spaced hydrophones or acoustic receivers that capture the sound wave that has bounced back from within the earth’s surface. Streamers can vary in size of anything up to 12km and number up to the current record of 17 by the PGS Ramform Tethys.
Seismic Survey Types NATS
A single offshore seismic survey vessel that makes only one pass, in a single direction over the same area, is known as a Narrow-Azimuth Towed Streamer or NATS.
This is used for preliminary surveys, as it is quick and therefore cheaper than other methods of offshore seismic survey. It is however not accurate enough to enable modern day complex drilling methods to pinpoint isolated reservoirs.
A single offshore seismic survey vessel that makes multiple passes, in multiple directions over the same area, is known as a Multi-Azimuth Towed Streamer or MAZ.
As the seismic survey vessel makes multiple passes, it is slower and more expensive. It does however mean a much greater accuracy when producing seismic survey results to where drilling crews can utilise directional drilling methods to pinpoint isolated reservoirs. Using this method can also produce highly accurate 3D seismic models.
A method using a seismic survey vessel purely for towing streamers and two separate vessels spaced at either end of the streamers to operate sound sources is known as wide-azimuth towed streamer or WATS.
This method is used to cover greater area and produce much larger datasets.