Hansruedi Maurer

Application of a new 2D time-domain full-waveform inversion scheme to crosshole radar data

Crosshole radar tomography is a useful tool in diverse investigations in geology, hydrogeology, and engineering. Conventional tomograms provided by standard ray-based techniques have limited resolution, primarily because only a fraction of the information contained in the radar data i.e., thefirst-arrivaltimesandmaximumfirst-cycleamplitudesis included in the inversion. To increase the resolution of radar tomograms,wehavedevelopedaversatilefull-waveforminversion scheme that is based on a finite-difference time-domain solution of Maxwell’s equations. This scheme largely accountsforthe3Dnatureofradar-wavepropagationandincludes an efficient method for extracting the source wavelet from the radar data.After demonstrating the potential of the newschemeontworealisticsyntheticdatasets,weapplyitto two crosshole field data sets acquired in very different geologic/hydrogeologic environments. These are the first applications of full-waveform tomography to observed crosshole radar data.The resolution of all full-waveform tomograms is showntobemarkedlysuperiortothatoftheassociatedraytomograms. Small subsurface features a fraction of the dominant radar wavelength and boundaries between distinct geological/hydrological units are sharply imaged in the fullwaveformtomograms.

Integration of diverse physical-property models: Subsurface zonation and petrophysical parameter estimation based on fuzzy c-means cluster analyses

Inversions of an individual geophysical data set can be highly nonunique, and it is generally difficult to determine petrophysical parameters from geophysical data. We show that both issues can be addressed by adopting a statistical multiparameter approach that requires the acquisition, processing, and separate inversion of two or more types of geophysical data. To combine information contained in the physical-property models that result from inverting the individual data sets and to estimate the spatial distribution of petrophysical parameters in regions where they are known at only a few locations, we demonstrate the potential of the fuzzy c -means (FCM) clustering technique. After testing this new approach on synthetic data, we apply it to limited crosshole georadar, crosshole seismic, gamma-log, and slug-test data acquired within a shallow alluvial aquifer. The derived multiparameter model effectively outlines the major sedimentary units observed in numerous boreholes and provides plausible estimates ...

Ray-based amplitude tomography for crosshole georadar data: a numerical assessment

Analyses of travel times and amplitudes of crosshole georadar data provide estimates of the electromagnetic velocity and attenuation of the probed media. Whereas inversions of travel times are well established and robust, ray-based inversions of amplitudes depend critically on the complex directive properties of the georadar antennae. We investigate the variations of radiation patterns in the presence of water-filled boreholes and/or changes of electrical material properties in the vicinity of the transmitters or receivers. To assess the implications of such complicating factors for ray-based georadar amplitude tomography, we generate crosshole georadar data for a suite of canonical models using a finite difference time domain (FDTD) solution of Maxwells equations in cylindrical coordinates. The emitting dipole-type antenna is approximated by an infinitesimal vertical electric dipole, whereas a corresponding receiving antenna is emulated by recording the vertical component of the transmitted electric field. Inversions of the amplitudes of these synthetic data demonstrate that the presence of water-filled boreholes as well as changes in the material properties along the boreholes may cause substantial artifacts in the estimated attenuation structure. Furthermore, our results indicate that ray-based amplitude tomography of crosshole georadar data is unable to constrain absolute values of attenuation. Despite these inherent limitations, the method is surprisingly robust at detecting and constraining relative changes in attenuation. In particular, we find the method to be highly effective for locating conductivity contrasts that are not associated with corresponding changes in dielectric permittivity, and hence, cannot be located by travel time tomography alone.

Characterizing an unstable mountain slope using shallow 2D and 3D seismic tomography

As transport routes and population centers in mountainous areas expand, risks associated with rockfalls and rockslides grow at an alarming rate. As a consequence, there is an urgent need to delineate mountain slopes susceptible to catastrophic collapse in a safe and noninvasive manner. For this purpose, we have developed a 3D tomographic seismic refraction technique and applied it to an unstable alpine mountain slope, a significant segment of which is moving at 0.01–0.02 m∕year toward the adjacent valley floor. First arrivals recorded across an extensive region of the exposed gneissic rock mass have extraordinarily low apparent velocities at short (0.2 m) to long (>100 m) shot-receiver offsets. Inversion of the first-arrival traveltimes produces a 3D tomogram that reveals the presence of a huge volume of very-low-quality rock with ultralow to very low P-wave velocities of 500–2700 m∕s . These values are astonishingly low compared to the average horizontal P-wave velocity of 5400 m∕s determined from labora...

Realistic FDTD modelling of borehole georadar antenna radiation: methodolgy and application

High-frequency electromagnetic-wave propagation phenomena associated with borehole georadar experiments are complex. To improve our understanding of the governing physical processes and radiative properties of borehole georadar antenna systems, we have developed a modelling toolbased on a finite-difference time-domain (FDTD) solution of Maxwell’s equations in cylindrical coordinates. The computational domain is bounded by cylindrical symmetry conditions along the left edge of the model and uniaxial perfectly matched layer (UPML) absorbing boundary conditions along the top, bottom and right model edges. An accurate and efficient grid-refinement technique allows us to account for detailed aspects of borehole georadar antenna systems, slim boreholes and materials with very high dielectric permittivities, such as water. Numerical experiments reveal that the radiation patterns of finite-size Wu–King-type antennae and infinitesimal electric dipoles in dry boreholes differ only slightly from the analytic solution of an infinitesimal electric dipole in a homogeneous full-space. In contrast, there are substantial differences between the radiation patterns of antennae placed in water-filled boreholes and their analytic full-space equivalents without bore-holes. The effects of placing the antennae in air- and water-filled boreholes are explored using data acquired in crystalline rock and alluvial sediments, respectively. In both cases, simulations based onrealistic transmitter antennae located in boreholes and spatially corrected receiver radiation patterns provide better agreement between the observed and modelled data than simulations based on infinitesimal transmitter and receiver dipoles.

Rock Glacier Degradation and Instabilities in the European Alps: A Characterisation and Monitoring Experiment in the Turtmanntal, CH

Global climate change is impacting sensitive alpine cryogenic regions, through slope instabilities in rocks and soils. Significant temperature increase at the air-ground surface interface may be accompanied by increased rainfall, more extreme storms and additional severe rise in mean global temperatures in the coming decades, enhancing risk of mass movement hazards to human life and infrastructure. Rock glaciers and degrading permafrost on steep Alpine slopes are particularly susceptible to warming and phase change in either massive or interstitial ground ice, which may lead to release of water, accelerated motions, initiation of landslides and instabilities. Accumulated failure in soil elements, determined on artificial frozen specimens of rock glacier materials at temperatures below 0 °C, is linked to these processes at field scale. A geophysical and geotechnical field characterisation and monitoring experiment is being conducted on a rock glacier that is undergoing thermally induced creep and growth of thermokarst. Preliminary investigations are described in this contribution.

Real-time experimental design applied to high-resolution direct-current resistivity surveys

Designing optimized survey layouts for DC resistivity data is a difficult task. This is primarily due to the non-linear relationship between observed data and the subsurface resistivity structure. A possible solution is provided by real-time experimental design, a novel approach for acquiring geophysical data. Technical realization of real-time experimental design requires versatile recording facilities. We have developed a new multi-electrode system, that offers the necessary flexibility. Furthermore, our concept of distributed acquisition units with a digital layout enables the recording of high quality data.

Integrating Multi-scale Geophysical Data for the 3D Characterization of an Alluvial Aquifer

Understanding groundwater flow and contaminant transport within alluvial aquifers requires the detection and detailed characterization of preferential flow paths. We present an integrated interpretation of a 3D geoelectric data set, 2D crosshole radar and seismic tomograms and the results of sparse direct-push slug tests that allows us to identify potential preferential flow paths within an alluvial aquifer in northwest Switzerland. The 3D electrical resistivity model is divided into clusters of high and low electrical resistivity using a fuzzy c-means cluster analysis technique. Despite significant differences in resolution, dominant structures identified in a 2D cross-section extracted from the clustered 3D resistivity model largely coincide with those obtained from an independent fuzzy c-means cluster analysis of crosshole radar and seismic tomograms. By linking the 3D electrical resistivity clusters to hydraulic conductivity measurements obtained from a limited number of slug tests, we derive a 3D aquifer model distinguished by zones of increased hydraulic conductivity that may act as preferential flow paths.

Effects of small-scale stochastic heterogeneity on the tomographic inversion of cross-hole georadar data

Analyses of traveltimes and amplitudes of crosshole georadar data provide estimates of the electromagnetic velocity and attenuation of the probed media. In contrast to inversions of traveltimes, which are well established and robust, ray-based inversions of amplitudes depend critically on a priori assumptions about the directive properties of the antennas. In particular, the influence of electric material property fluctuations on antenna performance may lead to serious distortions of the radiation pattern. Such distortions cannot be accounted for by currently employed ray-based amplitude inversion algorithms. To explore the problem of antenna coupling to local heterogeneities, we generate synthetic crosshole georadar data for a suite of stochastic models using a finite-difference time-domain (FDTD) solution of Maxwells equations in cylindrical coordinates. Analyses of the radiation patterns extracted from the synthetic data indicate that distortions of the radiation patterns are primarily due to propagation effects and not to dipolecoupling effects. We do, however, find that the quality of the amplitude tomograms diminishes rapidly with increasing heterogeneity, probably because of inherent inadequacies in ray-based inversion methods. In contrast, the quality of ray-based traveltime tomograms is surprisingly high, even in the presence of strong heterogeneity.

Testing vertical seismic profiling (VSP) as a subsurface mapping method at the Krafla volcanic geothermal field in Iceland

Summary Vertical seismic profiling (VSP) was tested for mapping volcanic stratigraphy, fractures, dykes, fluid and steam in the geothermal area of Krafla in Iceland. Seismic imaging in magmatic environments is very challenging, largely due to the intense scattering of seismic waves traveling through the highly heterogeneous volcanic rocks. VSP offers means to image structures beneath and away from the well in complex volcanic environments. The VSP survey at Krafla was carried out in two wells, for each of which a zero offset, a far offset and a walk-away experiment were recorded. The zero offset data is of good quality, with the observed reflections corresponding to stratigraphic boundaries that can be explained by a simple 1D velocity model. The corridor stacks of the synthetic and field data look similar to each other, apart from a constant time shift and amplitude differences. High scattering in the subsurface leads to low amplitude reflections from deeper horizons. The walk-away data shows little coherent reflectivity. Furthermore, a complex 2D velocity model involving heterogeneities in the horizontal as well as vertical directions will be required to explain the observed seismograms.