Marine Imaging Technology Good News for Oil and Gas Industry

April 18, 1997

By Lynn Yarris, [email protected]

A new technique being developed by Berkeley Lab scientists has the oil industry keenly anticipating its potential use in finding petroleum and natural gas reservoirs hidden beneath underwater bodies of salt. Called "marine magnetotellurics," or marine MT, the new technique is designed to augment seismic imaging in geological surveys by revealing the size and thickness of underwater salt structures. This information can help researchers gauge the prospects for the sediment underlying the salt to be rich in oil or gas.

Marine MT is the work of Michael Hoversten and Frank Morrison, geophysicists with the Earth Sciences Division, and Steven Constable, a marine geophysicist with the Scripps Institute of Oceanography at UC San Diego. It is based on measurements of salt's electrical resistance to low-frequency electromagnetic radiation from the earth's ionosphere.

"We've found that these low-frequency electromagnetic fields are still recordable and capable of being used for sub-bottom imaging even at ocean depths of up to two kilometers," says Morrison, who is also a professor on the UC Berkeley campus.

With conventional seismic imaging, soundwaves are bounced off underground rock layers and reflected back to the surface, where instruments record their travel time. This yields valuable information about rock formations and structures that can be used, among other purposes, to predict the presence and approximate size of petroleum and gas reservoirs. Seismic imaging, however, runs into problems in marine surveys, especially in deep sedimentary basins, because of interference from salt bodies. Sometimes covering hundreds of square miles in area, these salt bodies are highly reflective of soundwaves and prevent surveyors from getting an accurate reading of the geological formations underneath it.

However, in addition to being highly reflective of soundwaves, salt is also highly resistant to the flow of an electrical current, a fact that marine MT exploits through the utilization of atmospheric electromagnetic radiation. This low-frequency radiation occurs naturally as a result of solar wind striking the ionosphere, that region of the atmosphere extending from about 55 to 306 kilometers (34 to 190 miles) above sea level, which is electrically charged. The solar wind causes the ionosphere to vibrate, generating electromagnetic waves that can penetrate deep into the earth's crust. These waves create electrical currents through rock strata and sea floor sediment but not through salt and other substances that have a high degree of electrical resistance.

"The electrical resistivity of salt is often more than 10 times greater than that of the surrounding sediments," says Hoversten. "By measuring the distortion in the flow of electrical currents through seawater and sediment produced by the presence of salt, we can easily map major (salt) structures and resolve questions not answered by seismic imaging. In this manner, marine MT provides us with complementary as well as independent information."

The basic premise of marine MT is an extension of a land-based technology with a major caveat: essential to the success of marine MT is the ability to deploy and retrieve instrumentation from the bottom of the sea. To this end, Hoversten, Morrison, and Constable conducted tests in the Gulf of Mexico, where huge oil and gas reservoirs are believed to be hidden under vast expanses of salt.

Marine MT surveys were conducted over two sites, known as "Mahogany" and "Gemini," where the prospects for finding oil and gas are rated good. Mahogany is a relatively shallow water site, about 100 meters in depth, off the Louisiana coast. Gemini is further off-shore in water as deep as 1.5 kilometers (nearly 5,000 feet). The device used to measure underwater electrical resistivity consists of an X-shaped frame packed with electrodes and special magnetic field sensors that were developed at Berkeley Lab and are among the most sensitive ever made.

To this assembly is added a buoyancy chamber and a concrete anchor. The complete package, which looks somewhat like a four-legged spider the size of a small raft, gets dropped overboard off a ship, sinks to the sea floor, and remains in the sediment for a couple of days. A remote signal is then used to detach the anchor from the frame and the floatation chamber brings it to the surface.

"This is the first time where MT instrumentation has been successfully deployed and retrieved from deep water," says Hoversten, who credits Constable and his colleagues at Scripps for the design of the marine equipment. The Scripps researchers believe their assembly will operate in water depths up to five kilometers (16,500 feet).

Data from the Mahogany and Gemini surveys are still being processed, but based on numerical modeling, Hoversten says he and Morrison are confident that marine MT can map the extent and thickness of salt structures with sufficient resolution to determine the prospects of finding new oil or gas deposits.

"Most of the undiscovered oil and gas in the Gulf and other bodies of water throughout the world are hidden under salt, where the companies couldn't see it using seismic imaging," says Hoversten. "By showing where and how deep the possible pay zones are, marine MT can go a long way toward helping a company pick its drilling targets."

The cost of marine MT pales before the cost of drilling or even the cost of seismic imaging. Marine surveys are divided into "blocks," each of which constitutes an area of three square miles. It costs about $500,000 to survey one block with seismic imaging and about $50,000 to survey it with marine MT.

Currently, Hoversten and Morrison are developing computer programs that will allow the data from marine MT to be integrated with data from seismic imaging. This should improve the accuracy of predicting underwater petroleum and gas reservoirs, and also enable the technique to be applied to scientific studies of geologic structures that are under lava flows, and other formations that pose difficulties for seismic methods alone.

The marine MT surveys of the Mahogany and Gemini sites in the Gulf of Mexico were funded by a consortium of oil companies including AGIP, Chevron, BP, BHP, and Texaco. This project is administered at Berkeley Lab through ESD's Norm Goldstein.

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