NRL Researchers "See" Through Sediment with New CT System

Researchers at the Naval Research Laboratory (NRL) site located at Stennis Space Center, Mississippi, have a new computed tomography system that will enable them to "see" through sediment in ways that have been impossible in the past. The ability to acoustically "see" through sediment can be drastically hindered by gas bubbles and objects as small as the tiniest of sea shells. Additionally, acoustically "seeing" into sand is, at times, influenced by the grain to grain contacts. Since these small scale features are typically unable to be resolved with a medical computed tomography (CT) system, a higher resolution CT system was required.

CT is a specialized x-ray imaging technique that creates images by using an array of individual small x-ray sensors and a computer. Unlike medical CT, where the x-ray source spins around the object collecting data from multiple angles, the industrial CT system projects a fixed x-ray beam onto a rotating object; the x-rays that penetrate the sample are then processed into images. This system has an additional advantage, that is this system produces an image, which appears on a video monitor while the x-rays are being produced.

"To make measurements of density, porosity, and permeability that coincide with the scales of the high frequencies that we currently use, we need to have high-resolution volumetric imaging," said Dr. Allen Reed, the micro-CT operator who works in NRL's Marine Geosciences Division,. Interest is also high in the arrangement, orientation, shape and internal features and constituents (mud, gas, rock, sand) of shells as they are believed to be cause sound to be projected in undetectable ways. The CT has a three-dimensional capability that allows evaluation of the shells and the ability to see through the surrounding materials.

Although the system is capable of a variety of measurements, Dr. Reed has three principal areas of interest for its use: the kinetics of gas bubble formation in mud at varied temperature and pressure; the scalability of constituent processes with gas hydrates; and the mechanics of fluid flow within sand. "Since my work is to apply fundamental principles to actual environments, the theory sets up the basis for the evaluation," said Dr. Reed. "The CT makes it possible to see what is going on in the system and to determine if theory and what we see match up."

The CT should be able to facilitate scientific pursuits in many areas. For instance, specific predictions need to be made about the acoustic processes that are controlled by the gas bubble shape, geometry and interconnectedness in marine mud, the formation of gas hydrates in the presence of varied sediment types, with different gas and fresh water concentrations, and the predictions of fluid flow in pore space. These processes, which are governed by microscopic interactions between components, can be observed, in many cases, with the CT.

Of course there are many other Navy relevant issues that go far beyond marine geosciences, therefore additional materials are currently under evaluation. Evaluations will take place in the deterioration of metal welds - such as those found in the fleet's ships, the correlation of particles and components within polymers and asphalt, and the growth of biofilms on air filters.

Researchers from the Littoral Dynamics Team will be using the CT's capabilities to study sediment transport in the littoral zone. Dr. Tom Drake, Office of Naval Research, and Dr. Joe Calantoni, NRL Marine Geosciences Division, have developed models that track the movement of individual sand grains subject to varied wave and current speeds. These models can directly benefit from having actual grain scale images. "The CT enables the collection of images that can be used in models to make predictions of how morphology can change due to fluid flow over the bed or compression of the bed," remarked Dr. Reed.

A workshop held at Stennis Space Center in late April initiated and promoted collaborative research associated with the CT system and familiarized the potential collaborators with the system's capabilities and limitations. Professor Abraham Grader, Pennsylvania State University, the foremost expert in the scientific applications of micro-CT, led the discussions to explore a variety of scientific problems that can be solved by micro-CT. Dr. Reed led the hands-on experiments with samples provided by the participants.

Twenty-three scientists and engineers representing eleven institutions and companies participated in this workshop with representatives from imaging software companies on hand to help participants explore the various data manipulation options. The researchers were provided the opportunity to evaluate shear banding in sand, multiphase flow or drainage in sand, gas hydrate structure and constituents, volume imhomogeneities, and bubble growth dynamics in a marine mud. Results were discussed during t the course of the workshop, future collaborations were addressed, and participants were furnished with high resolution CT data of their specimens.

"We demonstrated that the CT can work much better than many other CTs currently in operation," said Dr. Reed. "We also collected the highest resolution images of gas hydrates and gas bubbles ever obtained!" While this is exciting in and of itself, these high resolution images also helped aid in the theory of bubble growth in marine mud and the scaling-down of hydrate components.

The workshop also served to develop collaborations and put together planning ideas to submit to research agencies. Collaborations with outside researchers to allow others to benefit from the CT are of prime interest to Dr. Reed. NRL can be contracted through a purchase agreement. For information on forming collaborations, contact Dr. Reed at http://www7430.nrlssc.navy.mil/facilities/CTScanner/index.htm, or by calling 228-688-5473 (office) or 228-688-5433 (CT lab).

Sidebar short story -
Support staff designs exhaust system for CT

When NRL SSC researchers needed a way to maintain a solid state (frozen) for their gas hydrate samples, the NRL support staff came to their rescue. "The trick to scanning the hydrates is to maintain the sample in a solid state long enough for at least an inch of the hydrate to be imaged," said Dr. Allen Reed, micro-CT operator (Code 7431). The support staff of Mr. Grant Bower and Mr. Conrad Kennedy fabricated a environmental chamber from a design made by Reed and Friedrich Abegg of ImF-GEOMAR. This device used an insulated aluminum chamber to exhaust gaseous nitrogen out through a chimney. Samples of gas hydrate placed within the center of the chimney remained frozen due to the cold nitrogen gas flowing around it. This "smoke stack" system allowed gas to be pumped long enough to keep the sample frozen, enabling high-resolution images to be collected.

Gas bubbles (black) within marine mud (light and dark gray) are highly effective acoustic scatterers and as such they inhibit the return of the acoustic signal and the detection of buried objects. An iron rich mineral (white) and the aluminum canister (light gray outer ring) are the other objects displayed.		Gas hydrates (dark gray) are formed in complex marine environments. In this case the hydrates are surrounded by air (black), mud (light gray) and carbonate (white).
The environmental chamber that was used to maintain the gas hydrate samples in a solid state.

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The U.S. Naval Research Laboratory provides the advanced scientific capabilities required to bolster our country's position of global naval leadership. The Laboratory, with a total complement of approximately 2,500 personnel, is located in southwest Washington, D.C., with other major sites at the Stennis Space Center, Miss., and Monterey, Calif. NRL has served the Navy and the nation for over 90 years and continues to advance research further than you can imagine. For more information, visit the NRL website or join the conversation on Twitter, Facebook, and YouTube.

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