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Abstract. In this study, we apply the double-difference tomography to investigate the detailed 3-D structure within and around the Nový Kostel Seismic Zone, an area in the Czech Republic known for frequent occurrences of earthquake swarms. We use data from the 2008 swarm since it has already been analysed in terms of earthquake focal mechanisms, principal faults, tectonic stress and foci migration. We selected about 500 microearthquakes recorded at 22 local seismic stations of the West Bohemia seismic monitoring network (WEBNET). Applying double-difference tomography, combined with weighted average model (WAM) post-processing to correct for parameter dependence effects, we produce and interpret 3-D models of the Vp-to-Vs ratio (Vp/Vs) in and around the focal zone. The modelled Vp/Vs ratio shows several distinct structures, namely an area of high Vp/Vs ratio correlating with the foci of the microearthquakes, and a layer of low values directly above it. These structures may reflect changes in lithology and/or fluid concentration. The overlaying low Vp/Vs ratio layer coincides with the base of the Fichtelgebirge (Smrčiny) granitic intrusion. It is possible that the base of the layer acts as a fluid trap and an upper limit to the seismicity, resulting in observed periodic swarms.

Characterizing heterogeneity in reservoir rocks at the pore scale is crucial to understand the flow patterns better, and estimate reservoir petrophysical properties such as porosity and permeability. This study introduces multifractals as descriptors for rock samples' heterogeneity at the pore scale. We analyzed twenty rock samples from sandstone and carbonate reservoirs using their 3D X-ray micro-computed tomography images. In addition, we simulated porosity and permeability properties and examined their correlation with multifractal parameters. The results show that the capacity dimension D 0 and the information dimension D 1 correlate with porosity and permeability simulated from images, respectively. Finally, we calculated several multifractal parameters such as the width of the spectrum, the asymmetry degree of the spectrum in the horizontal direction and the value of the vertical difference between the two branches of the spectrum. Results illustrate the ability of multifractal parameters to classify groups of rock samples according to their degree of heterogeneity.INDEX TERMS Heterogeneity, micro computed tomography, multifractals.