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Seeing clearly beneath the seabed
Thick layers of salt deep in the seabed in places like the Gulf of Mexico make pinpointing oil and gas deposits a big challenge. But engineers are discovering new resources by using advanced technology to create sharper images of the rock formations under the salt.
In their quest to find more energy deep below the surface of the sea, engineers are using new techniques to create increasingly detailed maps of the earth’s crust.
When engineers carry out seismic surveys to map reservoirs, they use an air gun to shoot sound waves from a boat towing receivers. The waves bounce off the rock below the sea bed and back to the receivers. The speed and direction of the sound waves as they bounce back helps engineers to build a three-dimensional model of the rock and assess if it might contain oil and gas reservoirs.
But in some locations layers of salt distort the sound waves the way static on a telephone connection breaks up a conversation, making it hard to build an accurate picture.
To overcome the challenge of salt, companies have started to place sound wave receivers on a second boat or on the seabed. This creates a wide angle between the source of the sound and the receivers.
The technique is known as wide azimuth seismic, a mathematical term that describes the precise measurement of the angle in relation to the sea’s surface. Sound waves travelling at this angle pass below the salt layer and provide information on rock that was previously invisible.
Until recently the high cost of using a second boat and the computing power needed to process the results restricted the use of wide-angled surveys. But advances in technology over recent years have made the process more affordable and it is now more widespread.
In 2009-2010, Shell’s use of wide-angle seismic surveying led to the discovery of four major fields in the Gulf of Mexico, adding hundreds of millions barrels of oil and gas to the region’s resources.
There are plans to use this approach to overcome the challenges of salt layers in rock beneath the deep waters off Brazil and West Africa and in parts of the Mediterranean.
Shell and a partner are testing a way to place seismic receivers using self-propelled robots equipped with satellite navigation systems. This would allow the use of more receivers and improve the accuracy of surveys. The new robots would navigate under water to preset locations to record sound waves, then float to the surface for reassignment.