This article appeared in the SIO publication Explorations,
v4, No.2, Fall 1997.
All photographs and illustrations are property of Scripps
Institution of Oceanography, University of California, San Diego
EXPLORING NEW FIELDS
by Paige Jennings
Electromagnetic Technology Adapted to Detect Offshore Oil
Magnetotelluric Method (MT)
Steven Constable
Cecil H. and Ida M. Green Institute of Geophysics and Planetary
Physics
Scripps Institution of Oceanography
For decades, scientists have studied Earth's structure, on land and deep
in the ocean, aided by the power of the sun and the force of lightning.
Using the magnetotelluric (MT) method, geophysicists trace variations in
the planet's magnetic field created by solar winds and lightning storms.
These variations cause electrical currents to course deep into the planet's
crust.
Marine geophysicists at Scripps are revising existing electromagnetic technology
to explore the resource-rich sediments of the continental shelf. These technological
advances give scientists another tool for understanding the formation and
structure of the planet, and the oil industry acquires access to potential
new reserves of oil and natural gas.
An industrial collaboration began in 1994 when Scripps geophysicist Steven
Constable met with Arnold Orange, a scientist and oil-industry consultant.
Orange knew that Constable and his colleagues in the Cecil H. and Ida M.
Green Institute of Geophysics and Planetary Physics had successfully mapped
deep-ocean rocks by measuring electrical conductivity, and that Constable
was familiar with the MT method, which was frequently used on land for oil
exploration. The consultant wanted to know if it would be possible to survey
geologic structures less than one mile deep, as well as in depths up to
three miles, using the MT method. Constable immediately took up the challenge.
Eventually his Scripps group joined a team that included researchers from
Ernest Orlando Lawrence Berkeley National Laboratory and scientists and
industrialists with several of the world's leading oil companies.
"I said yes without thinking about it," recalled Constable, "but
as we started to discuss it more, I realized that the traditional method,
which is to measure very low frequency electromagnetic fields, wasn't going
to work in the areas of the ocean where oil might be found."
Seafloor instrument being recovered from the water.
First developed in 1957, the MT method exploits a basic physical phenomenon:
as the planet's relatively constant magnetic field is altered by variations
from solar winds and electrical storms, electric currents form within the
earth that work to counteract the magnetic field variations. These currents
are stronger in good electrical conductors, such as sandstone and shale,
and weaker in resistive materials, including limestones, ancient lava flows,
and salt structures.
To monitor these currents, a magnetometer records the magnetic field along
the surface, and a voltmeter gauges the flow of electric currents through
the ground. Traditional seafloor methods rely on low-frequency waves, usually
with periods of 1,000 seconds or more, to study Earth's mantle (or deep
structure). But, to study the relatively shallow sediment layers of the
continental shelf, scientists need to focus on periods between 1 and 1,000
seconds. Designing equipment to read very small, high-frequency electric
fields in the magnetically noisy, highly conductive ocean environment atop
the continental shelf was a bit tricky. Fortunately, Scripps has a long
history of developing magnetic and electric sensors for marine exploration.
R/V Pelican awaits the loading of Steve Constable's stock of MT
instrument packages.
Constable (above right) and members of his team (below) assemble and
test each instrument package before deployment.
With the resources and knowledge available at the institution, Constable
modified existing technology and created a spidery looking instrument package
sturdy enough to survive ocean conditions, compact enough to easily transport,
and sensitive enough to detect faint, small-scale variations through the
earth's upper layers. His alterations included adding a special amplifier
to bolster weaker electric signals and an autonomous seafloor data logger
capable of gathering information rapidly at each deployment. This allowed
for the shorter and more frequent instrument deployments necessary for compiling
a comprehensive survey. Each package was linked to a detachable cement anchor
and affixed with a compass to record its orientation on the ocean floor.
Magnetic readings from land-based magnetometers set up near the offshore
survey site provided a remote reference to help process noisy underwater
measurements.
To test his initial modifications, trials were conducted off the San Diego
coast. The results were advertised to the oil industry. One company, AGIP,
was aggressively exploring the Mediterranean Sea and in 1995 asked Constable
and Orange to conduct surveys on their behalf off the coast of Italy. They
surveyed 20 sites with limited success.
During this time, researchers with the Earth Sciences Division at Berkeley
Lab were working on developing computer-generated models of salt structures
in the Gulf of Mexico. They were interested in utilizing Constable's developing
technology to gather data for their project, thus an academic collaboration
between the two research centers was struck.
With support from Berkeley Lab's model studies of the Gulf of Mexico, Constable
and Orange began forming a consortium of oil companies to sponsor the first
field trials off the coast of Louisiana.
The large concrete anchor prevents the instrument from moving while on
the seafloor.
Instrument orientation on the seafloor is recorded using a magnetic
compass with a timed locking mechanism.
Electromagnetic energy (descending arrows), generated by Earth's ionosphere
(horizontal wave, top), propagates with almost no attenuation in the non-conductive
atmosphere, but quickly dissipates in the electrically conductive ocean
and sediments. The rate at which the energy decays depends on seafloor electrical
structure and on an electrical charge that builds up on the edges of conductive
boundaries, such as the salt structures pictured in gray. An array of seafloor
instruments, with a land instrument for reference, maps the influence of
the deeply buried conductors.
"Now began a three-legged operation-Arnold Orange representing industry
applications, Scripps providing the methodology, and Berkeley Lab working
on their modeling theories," described Constable.
Six oil companies joined the consortium and operations were quickly organized
to begin field trials in the gulf. During August of 1996, Constable made
approximately 40 gulf deployments for academic purposes using consortium
funding. Two months later he was back in the Mediterranean assisting AGIP
with an extensive 100-site commercial survey.
In 1997 Constable spent nearly the entire humid month of June in the small
fishing and recreational community of Cocodrie, Louisiana, and amidst the
cobalt blue waters of the Gulf of Mexico conducting more academic research
for Scripps and Berkeley Lab.
From the stern of the 105 foot (32 m) research vessel Pelican, Constable
and a team representing Scripps, Berkeley Lab, and Arnold Orange Associates
made 46 instrument deployments. On land and at sea, their efforts were supported
by the Louisiana Universities Marine Consortium, which operates two research
vessels, including R/V Pelican, from a marine research center nestled in
the heart of estuarine wetlands between the Mississippi and Atchafalaya
Rivers. Prior to the first shakedown cruise, Constable's team headed for
the town of Thibodaux to establish a land-based station. In a muddy field
of knee-high grass sandwiched between an early season sugarcane field and
the overgrown tangles of a bayou, they aligned and buried several magnetometers
and hooked up the monitoring equipment in a camping tent purchased from
the local Wal-Mart. While in the field, Constable and his team were warned
to watch out for poisonous snakes, chiggers, mosquitoes, fire ants, and
swamp rats, making their upcoming work at sea sound almost hazard-free by
comparison.
Constable tests the magnetometers before they are delivered to the land
station.
Oil platforms are a common sight for Constable's team while working in
the Gulf of Mexico.
During the establishment and maintenance of the land station, the
equipment must be checked periodically, and the data downloaded for analysis.
Testing battery voltages.
After the first land station was up and running, one person was left
on land to monitor it, and the rest of the team boarded the ship in search
of salt structures.
The salt structures found in the Gulf of Mexico developed during the Jurassic
period when the continents were separating. It is believed that as a rift
formed between North and South America, seawater spilled over from the Pacific.
As the water evaporated, salt deposits were laid down in what is now the
Gulf of Mexico and on land throughout Texas, Louisiana, and Mexico. Over
time some of the salt sheets protruded up through heavier marine sediments
as domes and formed pockets that now harbor rich oil reservoirs. Geologists
are interested in how these structures formed, and oil explorers are interested
in the resources that might lie in the sediments below.
In most industrial surveys, the seismic method is the preferred mode of
exploration. This technique measures the sounds from small explosions as
they bounce off the differing sediment layers. But the reliability of this
method decreases when used with certain geologic structures, such as salt.
"Salt is a problem for the seismic method," explained Constable.
"The oil industry wouldn't be interested in MT if the other method
worked all of the time. In materials such as salt, basalt, and carbonates,
there is too much reverberation within the structure to get an accurate
seismic reading."
These formations reflect so much of the sound that lower-lying sediment
layers can't be detected with explosions. This problem is averted using
the marine MT method.
"Rocks such as basalt, limestone, and salt domes, are usually less
electrically conductive than surrounding sediments of sandstone and shale,
because they have less water trapped in pore spaces, and it is the water
that usually conducts electricity in the ground," said Constable. Using
the new modified MT instrumentation, the electrical contrast between sediments
buried beneath the reflective layers of rock can be measured.
The results from Constable's Louisiana expedition were successful. The data,
when processed, allowed them to accurately image the salt structure with
enough detail to be useful to the oil industry. The positive outcome of
the latest expedition has heightened Constable's confidence in the effectiveness
of the marine MT method as a tool for natural resource exploration. He also
is encouraged by the strengthening ties, both academic and industrial, that
his improving technology is helping to form.
Multifaceted collaborations involving the oil industry are rare at Scripps;
but with a growing need for alternative academic funding sources and the
decreasing availability of petroleum products from existing sites, relationships
such as these may become more common.
Of the 46 instrument deployments, all were recovered, and nearly all
supplied useful data for Constable's survey.
An instrument is released into 900 m of water.
A successful recovery.
Constable explained further, "What we are doing is a mixture of
academically funded field trials and commercial surveys. The funding we
are receiving is important. It generates money to purchase equipment, to
hire engineers to develop new instruments, and to support the education
of graduate students and postdocs. Plus, it's given me experience very quickly
that is invaluable to my academic work."
Constable envisions that the work that has gone into developing the new
MT method for research in shallow waters may improve existing techniques
for mapping crustal structures, such as mid-ocean ridges, in the deepest
parts of the ocean. One of his new instruments was tested to depths of approximately
2.5 miles off Hawaii as part of a traditional marine MT survey last April.
"From an industrial point of view, the new MT will probably never be
as widely used as the seismic method," concluded Constable. "But
I think it has the potential to change the way scientists conduct offshore
exploration."
EPILOGUE
Innovative Technology Makes Data Affordable, L-CHEAPO
Tucked neatly inside Steve Constable's magnetotelluric (MT) instrument package
is an autonomous data logger appropriately named L-CHEAPO by its developers
at Scripps. It is described as Low-Cost Hardware for Earth Applications
and Physical Oceanography, and it provides geophysicists with an affordable
and reliable method for gathering digital information from many types of
Earth monitoring instruments, including undersea hydrophones and Constable's
MT system. Although based on decades of technological development at Scripps,
L-CHEAPO was designed, built, tested, and deployed in just four months.
The process began in 1993 when Scripps geophysicist John Orcutt and his
engineers, David Willoughby, Paul Zimmer, and Crispin Hollinshead, found
themselves in urgent need of an affordable system with which to compile
seismic data from ocean-bottom hydrophones. Commercially available systems
were not considered suitable because they required massive power to operate,
were too bulky to transport easily, and did not always keep accurate time.
With the tools, knowledge, and experience all available at Scripps, Orcutt's
team decided to create smaller, more efficient and reliable loggers. Constable
collaborated with Orcutt's team by providing the acoustic navigation and
release systems that allow scientists to remotely locate, communicate with,
and retrieve seafloor instruments. The acoustic system involved-the result
of years of work by Constable and two other Scripps scientists, Chip Cox
and Spahr Webb-also was more affordable than similar commercial products.
In an impressive development cycle, design of L-CHEAPO began in December
1993 and six new instruments were assembled by March 1994. In that same
month, as five of the loggers were in transit to Orcutt's research team
in Barbados, Constable was testing the sixth instrument in the waters off
San Diego. "The instrument worked," said Constable. "We actually
recorded some aftershocks of the Northridge, California, earthquake, and
were able to e-mail corrections of minor bugs to Orcutt's group in Barbados.
His experiment was a great success." The next year the U.S. Navy sponsored
further improvements to L-CHEAPO with a specific goal of extending its endurance
to one year deployments collecting up to 10 giga bytes of data. "L-CHEAPOs
have not pioneered the use of any new or exotic technologies," explained
Willoughby of the new research tool. "They simply combine state-of-the-art
components from the computer and oceanographic industries to make a small,
simple, and cheap package that can be used to record any kind of data on
the seafloor." To date, nearly 50 L-CHEAPOs have been built at Scripps
and used for numerous projects including passive and active marine seismology,
marine MT, ocean acoustic studies, and monitoring of whale vocalizations.
David Willoughby and Constable ready a team of L-CHEAPOs, prior to
embarking on the latest research cruise in the Gulf of Mexico.