‘A better look’ below

Array Unlocks Sub-Secrets

American Association of Petroleum Geologists (AAPG)

You could think of USArray as being a continental-scale cosmic observatory, except rather than looking up at the sky it looks down into the earth – a CAT scan of the subsurface.

That’s because USArray delves, both literally and figuratively, into the most complex and diverse parts of the planet – those parts that contain the oldest rocks and, therefore, record the planet’s early formation and evolution.

And in doing so it gives us, according to Bob Woodward, director of instrumentation services for USArray, “A better look at what we think is there.”

The key word is “better,” because for geologists, Woodward said, the program will continue to “provide a consistent, synoptic image of the crust and upper mantle beneath the United States.”

The USArray (www.usarray.org) is a component of EarthScope (www.earthscope.org), which is funded by the National Science Foundation. Since its first steps a decade ago, it was and is designed to provide a foundation for integrated studies of the continental lithosphere and deep Earth structure.

It comprises four components:

  • The Flexible Array – a pool of portable seismic instruments (2,146 in total), which are available to the research community for customized targeted studies.
  • The Reference Network – a dispersed, permanent network of seismic instruments that provides a long-term reference frame for comparison of observations.
  • The Magnetotelluric Array – a collection comprising both permanent and portable elements that measure naturally occurring electric and magnetic fields. And even though there’s not a lot to see, Woodward says, “People take care of them.”
  • The Transportable Array – a network of 400 high-quality broadband seismographs that have been placed in temporary sites (spaced approximately 70 kilometers apart) across the conterminous United States.

It is this fourth element that is yielding the most striking results. 

Linking the Old With the ‘New’

The Transportable Array seismographs were first placed in the western states – California, primarily – and then, following a two-year period of operation, each instrument gets picked up and moved to the next carefully selected location on the array’s eastern edge.

And there’s a reason why USArray started west before moving east: “All the action is in California,” Woodward said, laughing.

And because of the data that’s been collected scientists now can link geologic structures detected from earlier (and earliest) stages of continental formation to current known and potential geologic hazards (e.g., earthquakes, volcanoes, landslides).

When completed, nearly 2,000 locations will have been occupied during this program.

Where are these locations?

Some are on government facilities, but mostly the stations are located on private property. They’re unobtrusive – and like those who place Neilson boxes on home televisions to monitor TV programming, USArray officials want people to forget they’re even there.

And even though there’s not a lot to see, Woodward says, “People take care of them.”

The approach must be working.

“We have had essentially no vandalism,” he says. “In fact, people have a certain pride of ownership involving the stations.”

And, incidentally, nobody receives a fee for hosting the equipment.

On the Move 

The entire USArray observatory was acquired with funds from the National Science Foundation (the Transportable Array part of the budget alone was $30 million).

The program had three phases:

  • The construction, which included buying station components, equipment and moving them to their initial locations.
  • Maintenance.
  • Transportation – as in, moving them eastward.

And while the process began in the early part of the last decade, it wasn’t until August 2007 that the first footprint of 400 stations was completed.

Of his total budget – of that $30 million from NSF – Woodward says he’s satisfied: “It was enough.”

He then immediately laughs when reminded he may be the only scientist to ever say that about funding on a project from the federal government.

The Transportable Array stations consist of a three-component broadband seismometer with associated signal processing, power and communications equipment. In the early phase of the experiment, significant effort was devoted to the design of the temporary vaults to house the instruments, which resulted in a configuration that provides both high-quality data and a data return of greater than 98 percent.

Data from each station are continuously transmitted in near real-time to the Array Network Facility at the University of California, San Diego, where initial operational and quality checks are performed, and then sent to the IRIS Data Management Center, where all the data and associated metadata are archived and then distributed completely openly and without restriction.

The costs, Woodward said, could have been prohibitive if not for something as simple as the market place.

“When designed, the cellphone and data plans were not available.”

But now they are, and USArray uses the same technology to move its information as you do when using your phone or tablet to check movie times.

Data in 3-D 

So what’s being uncovered?

Since the Transportable Array is providing a consistent, synoptic image of the crust and upper mantle, Woodward says of particular interest are the three-dimensional structures within geologic terrains and nature of the boundaries and interactions between them.

That would include:

  • The San Andreas system.
  • The West’s Basin and Range region.
  • High Lava Plain.
  • Colorado Plateau.
  • Snake River Plain.
  • Yellowstone National Park region.
  • The Rocky Mountain front range.
  • The Colorado Plateau.

“Seismic combined with magnetotelluric and geodetic data from EarthScope’s Plate Boundary Observatory are providing new insights into deformation and fluid content in Pacific Northwest, as well as higher resolution Flexible Array studies of targets of special geological interest that can be carried out with more dense arrays of instrument deployments,” he added.

“In contrast to the Transportable Array’s fixed geometry of 70-kilometer spacing and two-year recording with broadband instruments, the geometry, duration and instrument type used in these Flexible Array experiments can be tuned to fit the specific geological targets under study.

“It is giving us,” he concluded, “a much more complete picture of the continent.” 

Surprises Yet to Come 

The project is unique in many respects – not the least of which is how it began.

Usually when a funding proposal is presented for a project, Woodward said, the proposal includes a hypothesis.

“What’s different about EarthScope – and the Transportable Array, in particular – is that the project laid out a systematic survey,” he said. “People didn’t know what they would find.”

In the future, Woodward said he’s looking forward to the Transportable Array’s move from along the eastern side of the Mississippi River to the mid-Atlantic and northeastern regions, as well as the work planned for Alaska and Canada, which should start in 2014.

“On second thought,” he says, “if we had more money, we’d be buying more instruments and leaving them in place.”

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EarthScope facilities are funded by the National Science Foundation and are being operated and maintained as a collaborative effort by UNAVCO Inc. and the Incorporated Research Institutions for Seismology, with contributions from the U.S. Geological Survey and several national and international organizations.

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