Propagation of a 18F-labelled salt solution in the fracture system of a mechanical stressed salt core (injection from the left). The fracture system was characterized with µCT. (Staßfurt-Projekt)
In the Reactive Transport Division of the HZDR we apply advanced imaging methods to visualize the spatiotemporal evolution of complex geochemical processes in opaque media.
GeoPET was developed for direct non-destructive quantitative visualization of transport processes in opaque media, in particular in natural geological materials on the scale of drill cores [1-7,11-13].
We apply positron emission tomography (PET), which originally is a nuclear medical imaging method. PET is characterized by its unrivalled sensitivity and selectivity for positron emmitting radiotracers. In this respect it is superior to all other imaging methods that are applied in geosciences, like microfokus X-ray CT (µCT), electrical resistivity tomography (ERT) and magnetic resonance imaging (MRI). This high sensitivity for tracer concentrations provides an observation method for (reactive) transport processes without retroactions on the process. Utilization of a high-resolution PET-camera, like our ClearPET scanner (Elysia-Raytest) is a prerequisite for obtaining an image resolution around 1 mm, at the fundamental physical limit of the method.
In a typical GeoPET experiment the PET tracer (a substance labelled with a positron-emitting radionuclide) is injected into the sample, similar to conventional column experiments. The photon radiation, which is emitted from the tracer, is detected spatially resolved. The momentary spatial distribution of the tracer concentration is then computed from these data of detection positions.
Compared to conventional column experiments, GeoPET delivers much more detailled insights into the transport processes and delivers comprehensive parameter sets, like velocity distribution, distribution of the effective pore volume, an estimate of the effective internal surface area, and frequently reveals preferential transport pathways. GeoPET images the concentration of the tracer, which is a key parameter of geochemical transport, with molecular sensitivity and with mesoscale resolution on the millimetre scale. In contrast, microscale images of the internal structure with micrometer resolution are provided by µCT.
Propagation of a 18F-labelled salt solution in a dense sandstone. (Staßfurt-Projekt)
What is GeoPET?
Insertion of a typical drill core (diameter and length ca. 10 cm) into the ClearPET-Scanner. The 20 concentric cassettes contain each four 8x8 detectors, consisting of LYSO/LuYAP szintillator crystals and photomultipliers.
Positron emitting radiotraceres with decay times between minutes and years are produced with a cyclotron (e.g. our IBA Cyclone 18/9). These nuclides are applied for labelling or directly as tracers and injected into a sample. The emitted positrons travel a slowing-down length of about 1 mm - which limits the physical resolution - until they annihilate in contact with an electron, transmitting two antiparallel photons with 511 keV. These photons are detected as coincidences in the scintillation crystals of detector ring of the PET-scanner, each pair of events defining a line-of-response (LOR). Depending on the activity in the field-of-view (FOV), after a period of 1 to 60 min sufficient coincidences (about 107) are registered for tomographic reconstruction, which after extensive computations yields the spatial distribution of the tracer concentration C(x,y,z,t).
Our in-house ClearPET scanner was originally dedicated for medical research on small animals. We apply this scanner exclusively to geomaterial samples and benefit from its miniaturisation in comparison to clinical PET scanners, Its small gantry diameter (adjustable to 13 or 21 cm) and the small-sized detector crystals enable to reach the physical spatial resolution limit of about 1 mm on standard drill cores (in contrast to the physical resolution limit of medical PET-scanners of 3-5 mm). PET has a sensitivity for tracer concentrations to some 107 tracer atoms/µl, assuming a detection limit of activity of 1 kBq/mm3, or even better, which is several orders higher than CT or MRI. PET is ideally suited for direct flow and transport process observations in geomaterials. However, we had to optimize the reconstruction process for measurements on geological materials, taking into account the high density and thus strong attenuation and scattering effects.
Matching observations with numerical model simulation
Diffusional spreading of 22Na+ in a Opalinus clay drill core (diameter and length ca. 10 cm). The tracer was applied into an axial blind hole. Compare [12,13].
We apply COMSOL Multiphysics and interface Comsol Phreeqc (iCP: image-modelling.net/icp) for aligning experimental GeoPET results with computer simulations. In the example, we recorded the diffusional spreading of 22Na+ in an Opalinus drill core over a period of 6 months. The same scenario was simulated by implementing the differential equation for anisotropic diffusion and adsorption as finite element model with Comsol. These simulation data were fitted to the measured spatiotemporal concentration distributions with the help of the Comsol Optimization Module. Thus we could determine the optimum value for the diffusion tensor [8-10,14].
In the same manner, it is possible to derive comprehensive parametrizations of complex scenarios, like reactive-advective-diffusive transport in porous or fractured geomaterials.
We are focusing on the investigation of (reactive) transport processes in heterogeneous geological material by means of radiotracer applications. Our activities address two themes in the research field Energy:
- Long-lived radionuclides at permanent disposal sites (Nuclear Safety Research) and
- Particle-mediated transport in geosystems (Energy Efficiency, Materials and Resources).
-  Richter, M., et al., Positron Emission Tomography for modelling of geochmical transport processes in clay. Radiochim. Acta, 2005. 93: p. 643-651.
-  Gründig, M., et al., Tomographic radiotracer studies of the spatial distribution of heterogeneous geochemical transport processes. Appl.Geochem., 2007. 22: p. 2334-2343.
-  Kulenkampff, J., et al., Evaluation of positron emission tomography for visualisation of migration processes in geomaterials. Phys. Chem. Earth, 2008. 33: p. 937-942.
-  Wolf, M.: Visualisation and quantification of fluid dynamics in drilling cores from the salinar and overburden of the area of Staßfurt by means of positron emission tomography PhD thesis, Leipzig University, Germany, 2011
-  Zakhnini, A., et al.: Monte Carlo simulations of GeoPET experiments: 3D images of tracer distributions 18F, 124I and 58Co in Opalinus Clay, anhydrite and quartz. doi:10.1016/j.cageo.2013.03.023 Comp. Geosci., 57, 183-196
-  Barth, T., et al.: Positron emission tomography in pebble beds. Part 1: Liquid particle deposition. Nucl. Engin. Design, 267 (2014) 218-226
-  Barth, T., et al.: Positron emission tomography in bebble beds. Part 2: Graphite particle desposition and resuspension. Nucl. Engin. Desing, 267 (2014) 227-237
-  Schikora, J., Simulation of diffusion-adsorption processes in natural geological media by means of COMSOL Multiphysics, in Faculty of mechanical Science and Engineering. 2012, Dresden Technical University: Dresden, Germany. p. 95.
-  Schikora, J., Kulenkampff, J., Gründig, M., Lippmann-Pipke, J. Modelling and simulation of GeoPET experiments with COMSOL Multiphysics. Geophysical Reasearch Abstracts. EGU2012-10965
-  R. Gerasch, J. Kulenkampff, J. Lippmann-Pipke: Parameter Estimation of anisotropic diffusion in Clay with COMSOL Multiphysics. COMSOL Conference 2014, 17.-19.09.2014, Cambridge UK
-  Kulenkampff, J., Gründig, M., Zakhnini, A., and Lippmann-Pipke, J.: Geoscientific process monitoring with positron emission tomography (GeoPET), Solid Earth, 7, 1217-1231, 2016
-  Kulenkampff, J., Zakhnini, A., Gründig, M., and Lippmann-Pipke, J.: Quantitative experimental monitoring of molecular diffusion in clay with positron emission tomography, Solid Earth, 7, 1207-1215, 2016
-  Kulenkampff, J., Gründig, M., Zakhnini, A., Lippmann-Pipke, J.: Observation of 22Na+ - Diffusion in Opalinus Clay using Positron Emission Tomography (GeoPET) (mpeg-movie), https://doi.org/10.5281/zenodo.166509, 2016.
-  Lippmann-Pipke, J., Gerasch, R., Schikora, J., and Kulenkampff, J.: Benchmarking PET for geoscientiﬁc applications: 3D quantitative diffusion coefﬁcient estimation in clay rock, Comput. Geosci.,101, 21-27, 2017