Marine Imaging Technology Good News for Oil and Gas Industry
April 18, 1997
By Lynn Yarris, LCYarris@LBL.gov
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.
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.
