Pacific Coastal and Marine Science Center

Core X-Ray: 3-D CT Core Imaging Laboratory

Learn more about the Rotating X-ray Computed Tomography (RXCT) system at the USGS Pacific Coastal and Marine Science Center in Santa Cruz, California.

Photograph of a laboratory with a big x-ray machine and two computers for calibration and visualization.

The Geotek RXCT

The Geotek RXCT, a "rotating x-ray computed tomography" system, creates ultra high-resolution imagery of sediment cores. The system resides at the USGS Pacific Coastal and Marine Science Center in Santa Cruz, California. It requires the operator to take specialized training and hold X-ray radiation and safety certifications.

In medicine, radiologists use computed tomography (CT) scans to collect highly detailed images of your body. Similarly, the RXCT creates a complete 3-D image rendering of a sediment core by combining a series of X-ray images taken from different angles around it (thus, the "rotating" part of the name). The system then uses computer processing to create cross-sectional images (slices) of the core. Thus, a CT scan provides more detailed information than a simple 2-D X-ray image.

The images created by the RXCT allow scientists to look into the core at any area and at any angle, without having to carve into it. Looking at the structure and composition of cores in this fashion helps scientists determine the history of the location where the core was collected, such as the seafloor, a lake bed, or a marshy area. For example, if they find a sandy layer in an otherwise calm environment, like a coastal marsh which is normally just peat and mud, this may be evidence of a big wave event that carried sand from the beach and nearshore back into the marsh area. Further inspection of the CT image may reveal subtle sedimentary characteristics of the sandy deposit such as changes in grainsize, heavy mineral layers, and rip-up clasts that may help researchers determine whether deposition occurred during a tsunami or a storm.

An offshore map shows locations of ROV dives and a star where a sediment core was collected, with photos of the core.

The figure shows the location of remotely operated vehicle (ROV) Doc Ricketts dives conducted in September 2020, plus three CT scan images from one of the collected cores. The photograph of a vertical slice through the upper 80 cm of the core "DR1281 VC-870," with a zoomed-in section to the right, outlined in red, are examples of image output from the RXCT. Lighter colors in the CT scan indicate sandy sediment where darker colors indicate fine sediment or mud. The sandy layers suggest horizons associated with 4 turbidites in the Cascadia Margin.

The CT imagery can be manipulated to look at cross-sections of the core scan. In the lower right is a perpendicular slice through a horizon (marked in orange) showing coarse grains that are characteristic of turbidites. Previously published work identified a sand layer in a core at this same site and subbottom depth, which was determined to be associated with the January 1700 Cascadia mega-earthquake and subsequent tsunami.

Credit: Jenna Hill, USGS Pacific Coastal and Marine Science Center

Various images of a long and narrow core of sediment that show various features within the sediment.

Images from a sediment core taken from Floras Lake, a coastal lake in southern Oregon. At the far left is a false color CT scan of a sediment core, and next to it is an actual photograph of the core. The box shows the area of an expanded, zoomed-in section of these images, which is displayed in the center. In both the CT scan and the photograph, the lighter, brighter colors are dense sand and the darker colors are mud layers. At far right is another false color image of the CT scan. Scientists use the CT scan in order to see unique aspects of the layers of sediment that may otherwise not be visible.

Credit: SeanPaul La Selle, USGS Pacific Coastal and Marine Science Center

 

A view of a thin slab of sediment as if cut from a core, the image rotates around to show front and back and internal structure.

Once a CT scan is obtained, scientists can use visualization software to manipulate a 3-D section of the scan to highlight certain features. Here, the same CT scan from the Floras Lake core rotates so that features can be seen from all angles. The beige (light) color is sand, which is more dense than the mud (brown, dark) surrounding it.

As the animation continues, the darker colors are hidden in order to highlight the lighter colors of the sand deposits. This helps scientists see amazing detail, which (along with other analyses of the sediment) helps them develop a sedimentologic "story" about how the sand was deposited.

The oval fragments of mud in the upper part of the sandy deposit appear to be rip-up clasts, indicative of an erosive, high flow into the lake that deposited beach and dune sand in the basin. This particular deposit is about 8 cm thick, and radiocarbon dating of sediments above and below the sand suggest that the tsunami was generated by a Cascadia megathrust earthquake roughly 2000 years ago.

Credit: SeanPaul La Selle, USGS Pacific Coastal and Marine Science Center

 

Contacts

SeanPaul La Selle

Geologist
Pacific Coastal and Marine Science Center
Phone: 831-460-7574

Jenna C Hill

Research Geologist
Pacific Coastal and Marine Science Center
Phone: 831-460-7463