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The Federal Office for the Safety of Nuclear Waste Management (BASE – Bundesamt für die Sicherheit der nuklearen Entsorgung) is the German federal regulatory authority for radioactive waste disposal. It supervises the German site selection process and is responsible for the accompanying public participation. Task related research is an integral part of BASE's activities. The projects MessEr and übErStand compiled the state-of-the-art science and technology regarding surface based exploration methods suitable for addressing the criteria and requirements specified in the German Site Selection Act. The results support BASE to review and define the surface-based exploration programs to be executed by the national implementer BGE (Bundesgesellschaft für Endlagerung mbH). To support BASE in reviewing the application of the exclusion criteria “active fault zones” according to the Site Selection Act, the project KaStör reviewed the current knowledge on active faults and fault zones in Germany and recommends methodological approaches to date and identify the activity of faulting. For the time being, the Site Selection Act defines 100 ∘C as a draft limit on the temperature at the outer surface of a repository container for all host rocks. The project Grenztemperatur studied the temperature dependency of the different thermal-hydraulic-mechanical-chemical/biological (THMC/B) processes according to available features-events-processes (FEP) catalogues for rock salt, clay stone, and crystalline rock and describes ways to defining host rock specific maximum temperatures based on specific disposal and safety concepts. Safety oriented weighting of different criteria and comparison of different potential regions and sites are key challenges during the siting process. The project MABeSt studied and reviewed methodological approaches to this weighting and comparison problem with special emphasis on multi criteria analysis (MCA) and multi criteria decision analysis (MCDA). A key requirement for safe geological disposal of nuclear waste is barrier integrity. The project PeTroS performed the first triaxial flow-through experiments on natural rock salt samples at disposal relevant p−T conditions and studied potential percolation mechanisms of fluids within rock salt. The data substantiate that the minimum stress criterion and/or the dilatancy criterion are the prime “percolation thresholds” in rock salt. The research results support BASE in fulfilling its tasks as national regulator according to state-of-the-art science and technology and are also relevant to other stakeholders of the siting process.

This contributed poster shows the current state of development of a finite element implementation as part of an open source software library (OSSL) for the simulation of thermo-hydro-mechanical (THM) coupled processes. The reliable handling of numerical methods is fundamental for the understanding of scientific interrelationships and thus, a crucial prerequisite for modeling THM scenarios, as well as for the understanding and evaluation of preliminary safety investigations during the site selection process for the storage of high-level radioactive waste. There are several motivations for developing an in-house OSSL, which will allow us to: Build capacity and maintenance within BASE (Federal Office for the Safety of Nuclear Waste Management) regarding issues of the numerical modeling of safety-relevant aspects on the long-term safety analyses specified by the German legislator in the site selection process. Develop a collection of known benchmarks and evaluation examples for the comparison of different software tools, applying a uniform interface to simplify the use of the available highly specialized open source codes. Diversify the testing possibilities regarding the preliminary safety investigations by means of our own, independent modeling software. Document basic THM scenarios for internal or, if necessary, public technical training, e.g., density-driven fluid flow (Fig. 1), convergence in salt, temperature propagation in the repository area, crack development, diffusive or advective mass transport. Ensure transparency and, in principle, might allow for appropriately proven-quality (validated) and documented simulation tools for the public regarding questions about the preliminary safety investigations during the site selection process. The development of the OSSL is mainly based on the scripting language Python, which allows the necessary flexibility for the diverse fields of application and at the same time enables maximum transparency for all aspects of the source code. To ensure the high quality of the software, state of the art development tools are used (e.g., version control, automated tests, and documentation generation). Figure 1 shows our preliminary simulation results of the so-called Elder problem (Elder, 1967), a popular standard benchmark for thermo-hydrogeological coupling in which fluid motion in a porous medium is driven by buoyancy forces.

Hydrogeophysics is interdisciplinary by definition. As researchers strive to gain quantitative information on process-relevant subsurface parameters while integrating non-geophysical measurements, multi-physical geoscientific models are often developed that simulate the dynamic process and its geophysical response. Such endeavors are associated with considerable technical challenges due to coupling of different numerical models, which represents an initial hurdle for students and many practitioners. Even technically versatile users often end up with individually tailored solutions at the cost of scientific reproducibility.We argue that the reproducibility of studies in computational hydrogeophysics, and therefore the advancement of the field itself, needs versatile open-source software. One example is pyGIMLi - a flexible and computationally efficient framework for modeling and inversion in geophysics. The library provides management for structured and unstructured 2D and 3D meshes, finite-element and finite-volume solvers, various geophysical forward operators, as well as a generalized Gauss-Newton based inversion framework.In this contribution, we highlight some of the recent advances and use cases in research and education since its 1.0 release in 2017 (Rücker et al., 2017) including:generalized modeling and inversion frameworks for conventional, joint, time-lapse and process-based inversion geostatistical regularization operators for unstructured meshes (Jordi et al., 2018) improved constraints in the presence of petrophysical parameter transformations demonstrated by an estimation of water, ice, and air in partially frozen systems (Wagner et al., 2019) 3D visualization leveraging upon PyVista (Sullivan and Kaszynski, 2019) simulation of electrical streaming potentials complex-valued forward modeling and inversion of induced polarization forward modeling with anisotropic parameters availability for Mac OS improved API and documentation Since the library is freely available and platform-compatible, it is also well suited for teaching. We demonstrate examples from Master level university courses and public outreach, where learners can interactively change model and acquisition parameters to study their influence on a hydrogeophysical process simulation. Finally, we would like to use this opportunity to discuss future developments with the community.ReferencesJordi, C., Doetsch, J., Günther, T., Schmelzbach, C., & Robertsson, J. O. (2018). Geostatistical regularization operators for geophysical inverse problems on irregular meshes. Geophysical Journal International, 213(2), 1374–1386. Rücker, C., Günther, T., Wagner, F.M., 2017. pyGIMLi: An open-source library for modelling and inversion in geophysics, Computers and Geosciences, 109, 106-123. Sullivan, C., & Kaszynski, A. (2019). PyVista: 3D plotting and mesh analysis through a streamlined interface for the Visualization Toolkit (VTK). Journal of Open Source Software, 4(37), 1450. Wagner, F. M., Mollaret, C., Günther, T., Kemna, A., & Hauck, C. (2019). Quantitative imaging of water, ice and air in permafrost systems through petrophysical joint inversion of seismic refraction and electrical resistivity data. Geophysical Journal International, 219(3), 1866–1875.

Scientists working with numerical models may notice that their presentations of numerical results to non-specialists sometimes unfold substantial persuasive power. It seems obvious that someone has worked intensively on a topic, bundled information and solved complicated equations on a high-performance computer. The final result is a number, a curve or a three-dimensional representation. The computer has made no mistake, so the result can certainly be trusted. But can it? Those who do the modelling often know the weak points of their models and invest time in increasing the reliability of the model calculation. Trust in model calculations is usually based on rigorous quality assurance of data, programs, simulation calculations and result analyses. It requires appropriate handling of uncertainties. In view of the simplifications and idealizations of models it is also necessary to assess which model results are actually meaningful. Additionally, in most cases simplified or idealised models have been used and it is necessary to assess which model results are actually meaningful. We want to discuss what it takes to generate simulation results that can be considered reliable and how scientists can appropriately convey their confidence in their own models in discussions with the public. The framework of the discussion is provided by an introduction from Martin Navarro und Ingo Kock (BASE) and we are happy to have brief input from Thomas Nagel (TUBAF), Klaus-Jürgen Röhlig (TUC) and Wolfram Rühaak (BGE).

Pore space properties of sedimentary rocks are of fundamental importance with regard to reservoir evaluation and fluid flow modelling or special geotechnical applications such as stability considerations for reservoirs, dams, or embankments. Processes such as compaction due to pressure drawdown in weakly consolidated reservoirs, effects caused by stimulation of reservoirs with decreasing productivity, or the compaction of building ground can be effectively studied by verifying and monitoring changes in porosity. The close connections between electrical resistivity or conductivity and pore space geometry as well as a high sensitivity to even slight changes in pore space characterising parameters are well-established. Therefore, the results of complex electrical conductivity measurements in the frequency range from 0.05 Hz to 1 kHz on a set of heterogeneous shaly sand samples during increasing compaction are presented in this study. The major objective of these investigations was to quantify the porosity reduction in shaly sands during the step-wise compaction of the samples in a specially designed measurement cell. Overall, ten unconsolidated shaly sand samples with varying grain size distribution were analysed. In addition, selected shaly sandstone data are presented to enhance the observations made for the sand samples with regard to the potential influence of cementation as expressed by the cementation exponent. With increasing compaction, the measured complex conductivity data of the fully water-saturated samples show two co-occurring effects. On the one hand, the real part decreases due to the dominating effect of Archie’s law. On the other hand, the imaginary part increases due to the increasing contribution of interface conductivity. This effect is due to an increase in the internal surface-area-to-porosity ratio. Cementation exponent and the considered porosity range seem to be controlling the magnitude of this effect. These observations may be explained by using a simple complex conductivity model that relates conductivity components to porosity, specific surface area, and cementation exponent. An interpretation algorithm is proposed that allows determining relative porosity variations based on a baseline and a single repeat measurement without prior knowledge of further rock characteristics. To demonstrate the applicability of the algorithm on field measurements, data from spectral induced polarization soundings obtained at a test site for compaction techniques were interpreted. It could be shown that these observations bear further potential for enhancing the prediction of porosity, changes of compaction, and, hence, changes in hydraulic permeability.

Electric resistivity tomography (ERT) is a widespread technique used for geologic and hydrological subsurface characterization. The 3D application is limited to small scales due to the enormous effort required for field surveys. Long-electrode (LE)-ERT using steel-cased boreholes can overcome small-scale limitations and has recently gained significant attention. However, no systematic investigation concerning the performance of long electrodes has been performed. We have conducted synthetic studies of LE-ERT to understand its advantages and limitations. We have compared three different approaches of modeling long electrodes, placing particular emphasis on the complete electrode model, to determine which reflects the physical reality best and thus can be used as a benchmark. The conductive cell model led to comparable results but was numerically more expensive. An interesting alternative approach was the shunt electrode model, which does not require discretizing the lateral extension of the casing. Systematic sensitivity studies revealed that model resolution could be enhanced in the borehole depth range through the use of electrodes of different lengths or surface electrodes. Varying contact impedances along different sections of the borehole can change the electric field visibly and influence four-point measurements. Even if large electrode segments show reduced coupling, the influence on voltage measurements was below 4% for realistic contact impedances. We have proven the applicability of LE-ERT for imaging lateral saltwater movement through the simulation of simple scenarios in the context of saltwater intrusion. Our synthetic examples determined the advantage of long electrodes compared with simulations using surface electrodes. The 3D inversion of synthetic data sets revealed that the modeled anomalies could be imaged in most cases. Different models revealed limitations due to poor vertical resolution and supported the usage of casings with different lengths or in combination with surface electrodes to improve resolution.

Electrical resistivity tomography (ERT) and refraction seismics are among the most frequently used geophysical methods for site-investigations and the combined results can be very helpful to fill in the gaps between the point measurements made by traditional geotechnical methods such as Cone Penetration Test (CPT), core-drilling and geophysical borehole logging. The interpretation of the results from a geophysical investigation constituting a single method often yields ambiguous results. Hence, an approach utilizing multiple techniques is often necessary. To facilitate interpretation of such a combined dataset, we propose a more controlled and objective approach and present a method for a structurally coupled inversion of 2D electrical resistivity and refraction seismic data using unstructured meshes. Mean shift clustering is used to combine the two images and to compare the separate and coupled inversion methodologies. Two synthetic examples are used to demonstrate the method, and a field-data example is included as a proof of concept. In all cases a significant improvement by the coupling is visible. The methodology can be used as a tool for improved data interpretation and for obtaining a more comprehensive and complete picture of the subsurface by combining geophysical methods.

Surface-downhole electrical resistivity tomography (SD-ERT) surveys were repeatedly carried out to image [Formula: see text] injected at the pilot storage Ketzin, Germany. The experimental setup combines surface with downhole measurements by using a permanent electrode array that has been deployed in three wells. Two baseline experiments were performed during the site startup and three repeat experiments were performed during the first year of CO2 injection. By the time of the third repeat, approximately 13,500 tons of [Formula: see text] had been injected into the reservoir sandstones at about 650 m depth. Field data and inverted resistivity models showed a resistivity increase over time at the [Formula: see text] injector. The lateral extent of the related resistivity signature indicated a preferential [Formula: see text] migration toward the northwest. Using an experimental resistivity-saturation relationship, we mapped [Formula: see text] saturations by means of the resistivity index method. For the latest repeat, [Formula: see text] saturations show values of up to 70% near the injection well, which matches well with [Formula: see text] saturations determined from pulsed neutron-gamma logging. The presence of environmental noise, reservoir heterogeneities, and irregularities in the well completions are the main sources of uncertainty for the interpretations. The degradation of the permanently installed downhole components is monitored by means of frequently performed resistance checks. In consistency with the SD-ERT data, these resistance checks indicate a long-term resistivity increase near the [Formula: see text] injector. In conclusion, the investigations demonstrate the capability of surface-downhole electrical resistivity tomography to image geologically stored [Formula: see text] at the Ketzin site.

The major task of a safety surveillance concept is to support the closed loop of reservoir management, i.e. to guarantee an iterative interaction between reservoir monitoring and numerical simulation. The presented research is based on the experience gained on the permanent geoelectrical monitoring at the Ketzin CO2 pilot storage site continuously operating since 2007. We have structured this experience in a modular monitoring workflow which offers well prepared links to actual process data, petrophysical data, and reservoir simulation. First results have been reached in major topics such as site-specific customization of technical tools, data acquisition and processing, and data evaluation. Structuring this approach in a modular manner allows the geoelectric monitoring at the Ketzin site to be efficiently scaled up and adapted to the fit-for-purpose requirements of future CCS demonstrations.

Direct-current resistivity surveys usually are performed using steel rods of finite extent and grounding resistance. However, in modeling, electrodes are commonly treated as ideal point sources. We present an approach for numerical computation applying the complete electrode model (CEM), which is known from medical imaging. The electrode surface was discretized, and the partial-differential equations were extended by additional relations incorporating a contact impedance and a condition for the current flow through the electrode surface. We verified the modeling of the electrical potential using an analytical solution for a perfectly coupled half-ellipsoid current source. To quantify the influence of a finite electrode, we computed the electrode effect as the ratio between CEM and point-source solution and investigated its dependence on geometry and contact impedance. Surface measurements using rods of typical spatial extent showed electrode effects on the order of the measuring accuracy for an electrode length/spacing ratio lower than 0.2. However, the effects are more significant for closed geometries such as experimental tanks. A comparison with a point approximation for finite electrodes using point-source locations along the electrode axis showed the best agreement, with points at about 60% of the electrode extension. The contact impedance played a minor role for four-point measurements, contributing only a few percent to the electrode effect. In addition to penetrating electrodes, we investigated surface electrodes with galvanic or capacitive coupling, showing electrode effects on the same order as for penetrating electrodes. An inhomogeneous resistivity distribution clearly increased the size of the effects. We also investigate the use of CEM to simulate current injected through steel-cased boreholes. Finally, we applied the approach with buried ring electrodes to calculate effects caused mainly by geometric disturbances from the borehole.

The Cheb Basin, located in the western Eger (Ohře) Rift, is part of the European Cenozoic Rift system. Although presently non-volcanic, it is the most active area within the European Rift with signs of recent geodynamic activity like emanations of mantle derived CO2, and the repeated occurrence of swarm earthquakes, which are common features in active volcanic regions. It is assumed that the fluids, uprising in permeable channels, play a key role for the genesis of these earthquake swarms.

PyGIMLi - A Python Library for Inversion and Modelling in Geophysics is a versatile open-source tool for modelling and inversion of various geophysical methods, based on finite element modelling and inversion frameworks that allow flexible regularization strategies, time-lapse processing and petrophysical or structural coupling of multi-physical data sets.

SUMMARY We present a novel technique for the determination of resistivity structures associated with arbitrary surface topography. The approach represents a triple-grid inversion technique that is based on unstructured tetrahedral meshes and finite-element forward calculation. The three grids are characterized as follows: A relatively coarse parameter grid defines the elements whose resistivities are to be determined. On the secondary field grid the forward calculations in each inversion step are carried out using a secondary potential (SP) approach. The primary fields are provided by a one-time simulation on the highly refined primary field grid at the beginning of the inversion process. We use a Gauss‐Newton method with inexact line search to fit the data within error bounds. A global regularization scheme using special smoothness constraints is applied. The regularization parameter compromising data misfit and model roughness is determined by an L-curve method and finally evaluated by the discrepancy principle. To solve the inverse subproblem efficiently, a least-squares solver is presented. We apply our technique to synthetic data from a burial mound to demonstrate its effectiveness. A resolution-dependent parametrization helps to keep the inverse problem small to cope with memory limitations of today’s standard PCs. Furthermore, the SP calculation reduces the computation time significantly. This is a crucial issue since the forward calculation is generally very time consuming. Thus, the approach can be applied to large-scale 3-D problems as encountered in practice, which is finally proved on field data. As a by-product of the primary potential calculation we obtain a quantification of the topography effect and the corresponding geometric factors. The latter are used for calculation of apparent resistivities to prevent the reconstruction process from topography induced artefacts.

The Mýtina maar is the first known Quaternary maar in the Bohemian Massif. Based on the results of Mrlina et al. (J Volcanol Geother Res 182:97–112, 2009), a multiparametric geophysical (electrical resistivity tomography, gravimetry, magnetometry, seismics) and geological/petrochemical research study had been carried out. The interpretation of the data has provided new information about the inner structure of the volcanic complex: (1) specification of the depth of post-volcanic sedimentary fill (up to ~100 m) and (2) magnetic and resistivity signs of one (or two) hidden volcanic structures interpreted as intrusions or remains of a scoria cone. The findings at the outer structure of the maar incorporate the (1) evidence of circular fracture zones outside the maar, (2) detection and distribution of volcanic ejecta and tephra-fall deposits at the surface, and (3) indications from electrical resistivity tomography and gravity data in the area between the Mýtina maar and Železna hůrka scoria cone, interpreted as a palaeovalley, filled by volcaniclastic rocks, and aligned along the strike line (NW–SE) of the Tachov fault zone. These findings are valuable contributions to extend the knowledge about structure of maar volcanoes in general. Because of ongoing active magmatic processes in the north-east part of the Cheb Basin (ca. 15–30 km north of the investigation area), the Mýtina maar-diatreme volcano and surroundings is a suitable key area for research directed to reconstruction of the palaeovolcanic evolution and assessment of possible future hazard potential in the Bohemian Massif.

A combination of seismic and geoelectric processing was studied by means of a structurally constrained inversion approach. Structural constraints were interpreted from the seismic data and integrated into the geoelectric inversion through a local regularization, which allowed inverted resistivities to behave discontinuously across defined boundaries. This arranged seismic processing and constrained resistivity inversion in a sequential workflow, making the generic assumption that the petrophysical parameters of both methods change across common lithostructural boundaries. We evaluated the approach using a numerical example and a real data example from the Ketzin [Formula: see text] pilot storage site, Germany. The latter demonstrated the efficiency of this approach for combining 4D seismic and surface-downhole geoelectric data. In consistence with the synthetic example, the constrained resistivity inversions produced clearer delineated images along the boundary between caprock and reservoir formation. Near the [Formula: see text]-flooded reservoir, the seismic and geoelectric time-lapse anomalies correlated well. At some distance to the downhole electrodes, however, the geoelectric images conveyed a notably lower resolution in comparison to the corresponding seismic images. Both methods confirm a northwesterly trend for the [Formula: see text] migration at the Ketzin site, although a rather northerly direction was initially expected. The results demonstrate the relevance of the presented approach for the combination of both methods for integrated geophysical [Formula: see text] storage monitoring.

A combination of seismic and geoelectric processing is presented using a structurally constrained inversion approach. Structural constraints are interpreted from seismic data and inserted into the geoelectric inversion through a local regularisation, which allows inverted resistivities to behave also discontinuously across these a priori constraints. This sequential arrangement of seismic processing and constrained resistivity inversion makes the generic assumption that the petrophysical parameters of both methods change across common lithostructural boundaries. We evaluate the approach using a numerical example and a real 4D data example from the CO2 pilot storage site, Ketzin, Germany. The real data case shows that the time-lapse anomalies, which have independently been imaged by surface seismic and surface-downhole geoelectric methods, correlate well at the CO2-flooded reservoir. However, at some distance to the downhole electrodes the geoelectric images provide a notably lower resolution in comparison to the corresponding seismic images. The results demonstrate the relevance of the presented approach for the combination of both methods in geophysical CO2 storage monitoring operations.

The LArge Reservoir Simulator (LARS) was developed to investigate various processes during gas hydrate formation and dissociation under simulated in situ conditions of relatively high pressure and low temperature (close to natural conditions). To monitor the spatial hydrate distribution during hydrate formation and the mobility of the free gas phase generated during hydrate dissociation, a cylindrical Electrical Resistivity Tomography (ERT) array was implemented into LARS. The ERT contains 375 electrodes, arranged in 25 circular rings featuring 15 electrodes each. The electrodes were attached to a neoprene jacket surrounding the sediment sample. Circular (2D) dipole-dipole measurements are performed which can be extended with additional 3D cross measurements to provide supplemental data. The data quality is satisfactory, with the mean standard deviation due to permanent background noise and data scattering found to be in the order of 2.12%. The measured data are processed using the inversion software tool Boundless Electrical Resistivity Tomography to solve the inverse problem. Here, we use data recorded in LARS to demonstrate the data quality, sensitivity, and spatial resolution that can be obtained with this ERT array.

Geophysical monitoring at the CO2 pilot storage Ketzin comprises time-lapse seismic and geoelectric (ERT) measurements. Both types of measurements are known to image geological structures differently, in particular for a variable saturation of CO2. We present a combination of both methods by means of a structural constrained inversion approach. Structural constraints are implemented in the geoelectric inversion by a local regularization. This allows model parameters to behave discontinuously across prominent geological boundaries. A tetrahedral finite-element parametrization is used to closely follow geometric a priori structures interpreted from seismic reflection data. Thus, seismics and geoelectrics are arranged in a sequential workflow which is based on a structural similarity. Practical application is performed on the Ketzin datasets which comprise repeated 3D seismic surveys, as well as repeated surface-downhole geoelectric surveys, providing different illumination/coverage and time-lapse signals. However, application of a local regularization for the caprock-reservoir boundary yields a more consistent image and reduces some of the ill-posedness in the ERT inversion. We compare the result with that of an unconstrained inversion and discuss its potential for CO2 migration imaging.

The permanently installed vertical electrical resistivity array (VERA) as part of the multi-disciplinary surveillance concept of the Ketzin pilot site represents an interesting object for long-term monitoring and verification studies. A practical workflow has been established to qualify the acquired data considering site specific, operational and technical circumstances. An independently performed electrode test was carried out in order to investigate the behaviour of the individual borehole electrodes. In the present stage, consolidated data sets are available from which preliminary CO2 saturation maps have been derived.

International audience; This paper analyzes simultaneous self-potential and gas temperature variations recorded at Merapi volcano in spring 2001, the dry season shortly after the volcanic crisis 2001. Temporal variations of fumarole gas temperature show characteristic quasi periodic signals at scales 1-8 hours and amplitudes up to ten degrees. We propose a simple graphical technique combining a wavelet scalogram and a cross-correlation analysis to demonstrate that the variations of gas temperature are systematically preceded by self-potential variations at the same scales. The influence of meteorological variations on these correlated signals can be ruled out. Rather, we suggest them to be related to the magma degassing in the upper conduits of the volcano. We discuss a semi-qualitative model to explain this correlation and the observed phase shift of about two hours.

[1] Many natural bodies as well as materials inside industrial installations, such as the Earth's mantle and the glass inside melting furnaces, exchange matter through convection. These processes result from differences in temperature, density, and chemical concentration. In this analysis, we focus on the visualization of thermally driven flows in the laboratory. In nature and in industrial installations, it is difficult to measure the temperature inside the object of interest directly. We benchmark a new DC-geoelectrical 3-D tomography method for temperature measurements that allows obtaining temperature values without influencing the flow pattern. For verification of the method, we use dissolved polymer polyethylene glycol heated in a tank as a laboratory analogue. The temperature inside the viscous material is measured indirectly by DC electrodes placed on the sides of the tank. The principle for conversion of the 3-D inverted geoelectrical data into temperatures is based on the adjusted Arrhenius temperature dependence of the electrical conductivity. In this way, the 3-D temperature distribution is calculated from the distribution of electrical conductivity in the tank. To verify the validity of the method, the calculated temperatures are compared with the temperatures recorded by the 29 thermometers on the sidewalls of the tank. The minor deviations obtained demonstrate the suitability of this method.

We give an overview on possible transform functions (TF) used in geophysical inversion. For three different geophysical methods - dc resistivity sounding, surface nmr sounding and travel time tomography - we investigate how TF's affect the results of synthetic data, the singular value spectrum and resolution properties.

For image reconstruction in absolute EIT (a-EIT) on the human thorax, often a simple body geometry as a circular or elliptical cylinder is assumed. However, deviations from this assumption lead to distortions or artefacts in the resulting tomograms. Furthermore, inexact electrode positions affect the results. In order to investigate the influence of these factors on image reconstruction, synthetic data sets were calculated using 3D-FE modelling (COMSOL Multiphysics, COMSOL AB) with varying body shape and electrode positions. Additionally, mea-surements on a healthy volunteer were performed accompanied by exact determination of individual body shape and slight variation of electrode positions. Two different reconstruction algorithms were used: one based on an unstructured triangular parametrisation with FE forward modelling in combination with a Gauss-Newton inversion and another one based on a modified SIRT-algorithm with FD forward modelling using cylindric segments. The results show, that the ignorance of individual body shape and electrode positioning errors lead to distortions or artefacts with both algorithms. If the geometrical peculiarities are known, they easily can be included in the inversion process by the use of the unstructured parametrisation and so their influences are mainly diminished.

We present a more general inversion and regularization method based on constrained minimization on arbitrary grids. Weighting matrices are used to control both model cells and their boundaries. This allows for involving additional information as bore hole data, known boundaries or other geophysical methods, e.g., in form of a joint inversion. We show different examples, each with additional information that is incorporated into the inversion and, thus, enhances the interpretation significantly.

We present a novel technique for the determination of resistivity structures associated with arbitrary surface topography. The approach represents a triple-grid dc resistivty inversion technique which is based on unstructured tetrahedral meshes and a finite element forward operator applied to the secondary potential. We use a Gauss-Newton method with inexact line search to fit the data within error bounds. A global regularization scheme using special smoothness constraints is applied. The regularization parameter compromising data misfit and model roughness is determined by an L-curve method and finally evaluated by the discrepancy principle.

In the inversion of ERT measurements we usually treat the electrodes as points. However, for small-scale arrays, e.g. lysimeter studies, this assumption may lead to severe misinterpretations. We present a method to incorporate both the electrode shape and its contact impedance in the finite element simulation using the complete electrode model. We show that for small-scale measurements the electrode shape can affect the measurements significantly.

The upward surge of saltwater due to a high extraction rate of water plants, tectonic weakness zones or eroded clay layers threatens the fresh water supply in some regions of northern Europe. For monitoring the movement of salt water, the usage of borehole measurements is insufficient as only point information can be retrieved. Our aim is to develop a monitoring system that uses boreholes as electrodes (Long Electrode Electrical Resistivity Tomography - LEERT) in conjunction with surface-electrodes. As a first step we conducted synthetic modelling using the Finite Element Method and the Complete Electrode Model in order to obtain a better insight on the influence of long electrodes ERT measurements. Sensitivity and resolution measures show a reasonable capability of imaging saltwater intrusions if surface electrodes are used in combination with boreholes.

Geophysical inverse problems often suffer ambiguity and yield fuzzy subsurface images. Often satisfactory results can only be obtained if additional information is incorporated in the inversion. The latter can be structural information about known boundaries or information about the parameters or their limits. However, this is rarely done by the available inversion software packages. We present an extremely versatile inversion and modelling framework for solving inverse problems on arbitrary geometries. Irregular meshes are used to incorporate known discontinuities. The generalized minimisation scheme allows for controlling every model cell and every cell boundary individually. Moreover the subsurface can be subdivided into regions that represent different geological units or different physical properties. For each region the model transformation function can be set, e.g. for incorporating petrophysical relations, logarithmic barriers or a combination of it. The constraint type, e.g. smoothness, and the strength and characteristics can easily by varied in the course of iterations. Different joint inversion schemes are easily derived from that. By hand of a synthetic ERT study we show how the approach can be used to imagine small contrasts within a contrasted environment and to monitor small changes with time-lapse inversion.

We present a new joint inversion approach that allows for a combined inversion of independent physical parameters by exchanging structural information. The technique is based on the ideas of robust modeling. Occurring gradients of one parameter facilitate the development of gradients in the other but does not enforce this. In the presence of boundaries that can be seen by both methods it leads to sharp contrasted models. Finally, a combined image of the subsurface is obtained by cluster analysis. The technique is applied to the inversion of dc resistivity and seismic refraction data. Two synthetic data sets show how different boundary types are resolved with and without structural coupling. It is demonstrated how the quality of the inversion results is improved by the new approach. Introduction The joint interpretation of different measurement types is a basic principle to confine the ambiguity of the inverse problems in geophysics. In the existing approaches one inverts for one parameter using an augmented data vector (Vozoff and Jupp, 1975). This is only possible if all measurements depend on the same parameter or if the parameters are interconnected by some petrophysical relationship. However, often such a relationship does not exist, as for electrical conductivity and seismic velocity. Nevertheless, we expect at least similar structures in the resulting models. Therefore we like to combine the otherwise independent inversions to allow for structural similarities. The challenge is to facilitate similar structures without enforcing it. Gallardo and Meju (2004) presented an algorithm where a combined data functional based on the cross-gradients of both models is minimized. However, one problem is the weighting of the individual data and model updates for different data numbers and convergence properties. Here we present an approach where two inversion runs are carried out separately. The combination of both models is accomplished by mutually controlled structural weights based on the principles of robust modeling. First we describe the minimization problem and the regularization procedure. Thereafter we introduce robust modeling and depict how structural information may be interchanged. We apply this technique to the dc resistivity and seismic refraction data on unstructured meshes. Two synthetic models using different model boundaries shows how the joint inversion improves the model concept. Inversion Minimization procedure Assume D data points di subsumed in a data vector d = (d1, . . . , dD). The parameter distribution p(~r) is discretized byM basis functions ψj and their coefficientsmj