3-D GPR Providing A New Field Tool

Seeing Things Differently

Legions of geologists have ventured forth to study the outcrop armed only with a trusty hammer and a pack full of sample bags.

But technology encroaches on everything, including that outcrop of soil and rocks in the far reaches of the countryside.

In fact, the emergence of 3-D ground penetrating radar (GPR) technology has the potential to drastically alter the traditional approach to field work.

Where conventional outcrop observations, digital photography, laser scanning and remote sensing capture only the characteristics of the exposed geology, 3-D GPR imaging can quickly yet non-destructively extract information at sub-meter resolution from inside the rock volume.

Instead of relying on the best guesstimate of where to drill holes and take samples of the outcrop, 3-D radar data helps to accurately determine the best spot, some believe.

Image Caption

Figure 1

Palentine Bridge limestone quarry, Mohawk Valley, N.Y. In the foreground the excavated dolomitized en-echelon fault zone is visible; limestones are grey and the dolomites have a brown tan. The 3-D GPR survey (figure 2) is located in the background of this view, where the fault was still buried under rubble.

Photo courtesy of Mark Grasmueck

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Legions of geologists have ventured forth to study the outcrop armed only with a trusty hammer and a pack full of sample bags.

But technology encroaches on everything, including that outcrop of soil and rocks in the far reaches of the countryside.

In fact, the emergence of 3-D ground penetrating radar (GPR) technology has the potential to drastically alter the traditional approach to field work.

Where conventional outcrop observations, digital photography, laser scanning and remote sensing capture only the characteristics of the exposed geology, 3-D GPR imaging can quickly yet non-destructively extract information at sub-meter resolution from inside the rock volume.

Instead of relying on the best guesstimate of where to drill holes and take samples of the outcrop, 3-D radar data helps to accurately determine the best spot, some believe.

Today, 3-D imaging is ubiquitous within the oil industry. On an entirely different level, it's also widely used in such diverse areas as medical imaging and airport luggage screening.

"For the scale in between these — the sub-meter-to-meter shallow subsurface — there's been nothing available which produces images of equal quality and resolution like where you can slice a volume in any direction and see features in high resolution resolved," said Mark Grasmueck, assistant professor marine geology and geophysics University of Miami, RSMAS.

"If it is available, it exists as academic prototypes, and then it takes weeks to months to process the data," Grasmueck said. "This is what's driving the effort to make this particular 3-D imaging technology simple and efficient in acquiring and processing information."

New York Case Study

Grasmueck began working on 3-D GPR in the early 1990s. For the last year or so he's been busy developing the new, next generation 3-D GPR imaging tool.

The tool was used earlier this year in a field test at the Palentine Bridge limestone quarry in New York. The effort was a collaboration between the Comparative Sedimentology Laboratory at University of Miami and the Reservoir Characterization Group at New York State Museum.

The 3-D GPR survey's objective was to delineate the rubble-covered part of the fractured and hydrothermally-altered reservoir analog. Grasmueck noted the quarry could serve as a mini-analog to develop improved understanding of structural, hydrothermal and stratigraphic relationships in difficult to find and produce hydrocarbon reservoirs, e.g., the Texas Ellenburger and Michigan Albion Scipio.

Grasmueck presented a poster on the project titled "3-D Vision GPR: Reservoir Anatomy Beyond the Outcrop Surface," during the AAPG Annual Convention in Calgary.

The poster was co-authored by Mark Grasmueck and David Viggiano of the University of Miami (Fla.); and Langhorne Smith Jr., and Richard Nyahay, both of the New York State Museum, Albany, N.Y.

A key finding of the Palentine Bridge project: Even with low penetration depth, full-resolution 3-D GPR can still provide a clear structural "floor plan" of an outcrop covered with rubble. In this particular project, the 3-D cubes precisely defined the "floor plan" of a buried en-echelon fault geometry in map view.

During the project, the group developed a suite of software tools to fuse and process 3-D GPR and position data into interpretable cubes and animations within one-two hours after the last trace is acquired — an effort that previously required several weeks.

"This opens the opportunity to start using 3-D GPR as an onsite imaging tool for outcropping reservoir analog field studies supporting ongoing structural and stratigraphic field work and sample collection," Grasmueck said. "The new 3-D GPR offers near-instant views inside the rock volume when outcrop exposure is limited and can be used to test geological hypotheses while still in the field.

"It used to be you went to the field and acquired a 3-D data set and months later you would get the final 3-D volume and then go back to the field," Grasmueck noted. "There was a disconnect between field work and having the 3-D result."

The Next Dimension

Because the technology can acquire 3-D volumes so quickly, the team is starting to do repeat volumes to perform 4-D imaging of fluid flow to determine how the fluid moves in the near-surface. This has not been possible before because fluid flow is not observable with boreholes.

"We can see what's happening with fluids around the borehole as they migrate toward the hole," Grasmueck said. "We're bridging the scale between borehole observation and conventional 3-D seismic information."

The impact of 3-D GPR imaging in the oil industry could be substantial.

"The use of 3-D GPR will help develop better reservoir models in less time with fewer but more critical samples," Grasmueck said, "integrating sedimentology, stratigraphy, structure and geochemistry in an accurate 3-D spatial framework."

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