Extremely deep freshwater filled cave systems are common in karst systems globally. The origin and evolution of such caves are usually attributed to hypogenic (bottom-up) processes, in which acidic groundwater dissolves limestone from below. However, these deep cave systems can form by epigenic (top-down) processes, with meteoric waters descending from the surface underground. The Hranice Abyss (Czech Republic), with a reached depth of 473.5 m, is the deepest mapped extent of such a system globally, although its maximum depth is unknown. Multiple geophysical data sets (gravity, electrical resistivity tomography, audiomagnetotellurics, and seismic refraction and reflection) are used to investigate the extent and formation of the Hranice Abyss. The geophysical results suggest the Hranice Abyss extends to depths of~1 km. Further, we identify structures within the karst, including buried cockpit karst towers with several NW-SE-oriented valleys. The new geophysical results from the Hranice Abyss, considered in combination with geological constraints of the region (tectonic evolution and morphology of karst structure), suggest an epigenic formation process, rather than the traditionally invoked hypogenic origin. Formation by epigenic rather than hypogenic processes has implications for local and regional karst history associated with areas hosting deep karst systems.

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In the 1-D magnetotelluric theory, a Riccati equation governs the depth variation of the impedance, or admittance, for a given distribution of the electrical conductivity. This equation can be used to compute the surface magnetotelluric functions for generally piecewise continuous conductivity profiles. In case of a simple layered medium, it provides the classical formulae for recalculating recursively the impedances between the individual layer boundaries. We present an extended version of the Riccati differential equations for generally anisotropic 1-D structures for the case of a plane wave incident field. Relation between the standard matrix propagation procedure for a layered medium and the Riccati equation approach, as a limiting case of the former, is demonstrated. In the anisotropic case, all elements of the 2 × 2 impedance tensor are present and, consequently, a system of four coupled Riccati equations has to be considered. Standard methods for the numerical solution of systems of ordinary differential equations are applied to the Riccati system, which gives an efficient alternative to the current matrix propagation procedures for the numerical evaluation of 1-D magnetotelluric impedances in anisotropic media. As an application, a synthetic study on the influence of a depth-variable regional strike on magnetotelluric decomposition results is presented, with the variable strike simulated by a variable anisotropy within the 1-D section.

Magnetic variation data recorded at 143 sites in the period range of 1000−6000 seconds were used to model the conductance distribution in the eastern part of the Bohemian Massif (BM) and the West Carpathians (WCP). The region represents a contact zone of the Palaeozoic Hercynian and Tertiary Carpathian orogenic systems. Two anomalies in the distribution of the electrical conductivity were found. One with a simple two-dimensional feature is located approximately near the boundary of the Inner and Outer West Carpathians, but the other of a complicated three-dimensional character is at the eastern margin of the BM. We inverted the observed geomagnetic data for the conductance in a unimodal thin sheet embedded at a specific depth. The inversion algorithm minimises the parametric functional that sums the squared norm of the misfit and the stabilising functional, and employs conjugate gradient optimisation. To express the sharp tectonic boundaries, we employed a minimum gradient support functional, which is applied in areas with strong variations of the model parameters. The inversion results confirmed a quasi-linear character of the regional anomaly distinguishing the Carpathian plate and led to new insight into the anomaly over the eastern margin of the BM. According to the results of the modelling the anomaly is formed by several conductive belts intersecting the anomalous zone. These belts follow important faults, dividing the transition zone between the BM and the WCP into individual blocks.

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We present results of a classical global induction analysis of the geomagnetic variation data in the range of daily Sq variations, as well as for long period variations within the period range of about 8 to 400 days. The Sq data from 88 to 94 world observatories are processed in two ways, first by constructing and analyzing average monthly daily variations for the whole months of the International Quiet Sun Year (IQSY) 1995, and second by analyzing the individual, especially quiet Q* daily records from the same year. The electrical images of the Sq response functions obtained via the Schmucker's ρ* − z* procedure show a good fit with results of other induction studies, though especially our global impedance phases show a larger scatter than two other published data sets used for comparison.The long period variations from three 3-years' intervals with different solar and geomagnetic activities and for 44 to 57 world observatories have been processed by power spectral and Fourier analyses, as well as by a simplified GDS approach. The induction response functions show a good correspondence with other deep induction studies, the seasonal processing did not, however, allow us to detect any significant effects of the solar/geomagnetic activity on the transfer functions.The obtained global geomagnetic induction functions along with other two published data sets are analyzed by a bayesian Monte Carlo analysis for the mantle conductivity distribution. We use a modified version of the Monte Carlo method with Markov chains based on an effective, data adaptive Metropolis sampling approach, and simulate samples from the posterior probability distribution of the resistivities in the mantle. Stochastic sampling provides comprehensive maps of the parameter space based on fairly ranking the models according to their ability to explain the experimental data, as well as on respecting the prior information on the model parameters. From four generally formulated and tested priors for the mantle resistivities, the non-informative distribution on strictly increasing conductance is the most non-restricting prior that, at the same, avoids the non-likely high-resistivity tails in the marginal resistivity distributions. O. Praus et al. 242 Stud. Geophys. Geod., 55 (2011) A prediction power of the Monte Carlo sampling approach is demonstrated by a comparison of published maximum likelihood bounds on average conductivities in specific mantle zones with those produced simply by computing the average conductivities from the Markov chain of models.

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