Recently, Nikon Metrology installed an XT H system at NGI – Norwegian Geotechnical Institute – an internationally leading center for geosciences research and consulting. NGI researchers use this high-performance industrial computed tomography (CT) scanner to run in-depth non-destructive investigation of large rock and soil samples. Voids, inclusions and disturbances in soil samples are key characteristics when investigating stability issues in the design and construction phases of large infrastructure, such as buildings, pipelines and offshore platforms.
CT scanning serves a wide application spectrum
Companies and public authorities seek NGI expertise on the design and construction of infrastructure to obtain maximum stability on local soil structure and rock formations. NGI assists the oil, gas and energy industries with expertise regarding exploration, development and operation of offshore fields. Soil geotechnical expertise also supports international building and construction markets.
CT scanning results will complement the characterization of geological heterogeneities and fractures in rocks. By monitoring aspects like evolution of fractures and fluid flow inside rock samples, the stability and operation of reservoirs and wells can be addressed.
“To closely examine samples of soil and rock in the laboratory, we chose the industrial Nikon Metrology XT H225 CT system,” explains Magnus Soldal, laboratory technician at NGI headquarters in Oslo, Norway. “The purchase of this premium equipment fits with our strategy to acquire as much information as possible on expensive onshore and offshore rock and soil samples using non-destructive inspection technologies. Our choice for Nikon Metrology was based on the system’s fine image resolution along with the optional convertible roof and extra-large cabinet that allow additional instrumentation to be placed inside the machine.”
Capturing dynamic tests at high image resolution
Cylindrical rock and soil samples in plastic or metal tubes come in different sizes. They typically have diameters between 50 – 150 mm and lengths up to 1 meter. Magnus Soldal says that for the longer samples, the system is equipped to move the rotating sample up or down to subject different parts of the sample to three-dimensional CT scanning. “For the smaller samples, the plan is to be able to use the CT scanner in combination with a triaxial loading cell to monitor the evolution of fractures and fluid flow”.
“For large soil samples as well as detailed rock investigations, the intrinsic image quality of the Nikon Metrology XT H system is impressive,” states Magnus Soldal. “We opted for a panel detector that features a larger size and higher image resolution. This is particularly important when performing detailed investigations of inhomogeneous sample sections and areas that can only be detected through CT scanning. Voids, inclusions and fracture planes are key features when characterizing the sample. On the basis of the inspection results, we pick the sample sections that will be subjected to geotechnical testing for stability evaluation.”
A truly safe system requiring limited training
Magnus Soldal testifies that inspection times are relatively short. “On average, detailed rock samples take approximately 30 minutes for X-ray and 15 minutes to generate the three-dimensional CT reconstruction of the sample. The inspection of offshore clay or other larger samples go faster, as less attention is given to the reconstruction of 3D volumes.” After calibrating the relation between material density and image grey scales, NGI researchers will be able to map various types of features and materials in the rock and assess their importance.
Overall, NGI operators perceive the use of the XT H system as fairly straightforward. “To be able to activate the optimum settings for a particular inspection task, we received three days of training, with two more to follow shortly. In addition to intuitive system operation, the Nikon Metrology CT scanner is an instrument that can be installed without requiring special floor conditions. Even more important is operator safety with regard to radiation. Although the system already met strict safety standards, we had its radiation measured here in Norway, confirming the excellent safety status of the equipment.”