Jens Tronicke

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 ...

Visualization of active faults using geometric attributes of 3D GPR data: An example from the Alpine Fault Zone, New Zealand

Three-dimensional ground-penetrating radar GPR data are routinely acquired for diverse geologic, hydrogeologic, archeological, and civil engineering purposes. Interpretations of these dataareinvariablybasedonsubjectiveanalysesofreflectionpatterns. Such analyses are heavily dependent on interpreter expertiseandexperience.UsingdataacquiredacrossgravelunitsoverlyingtheAlpineFaultZoneinNewZealand,wedemonstratethe utilityofvariousgeometricattributesinreducingthesubjectivity of3DGPRdataanalysis.Weuseacoherence-basedtechniqueto compute the coherency, azimuth, and dip attributes and a graylevel co-occurrence matrixGLCMmethod to compute the texture-basedenergy,entropy,homogeneity,andcontrastattributes. A selection of the GPR attribute volumes allows us to highlight key aspects of the fault zone and observe important features not apparent in the standard images. This selection also provides information that improves our understanding of gravel deposition andtectonicstructuresatthestudysite.Anewdepositional/structuralmodellargelybasedontheresultsofouranalysisofGPRattributes includes four distinct gravel units deposited in three phases and a well-defined fault trace. This fault trace coincides with a zone of stratal disruption and shearing bound on one side by upward-tilted to synclinally folded stratified gravels and on the other side by moderately dipping stratified alluvial-fan gravelsthatcouldhavebeenaffectedbylateralfaultdrag.Whenused in tandem, the coherence- and texture-based attribute volumes can significantly improve the efficiency and quality of 3D GPR interpretation, especially for complex data collected across activefaultzones.

Cooperative inversion of 2D geophysical data sets: A zonal approach based on fuzzy c-means cluster analysis

In many near-surface geophysical applications, it is now common practice to use multiple geophysical methods to explore subsurface structures and parameters. Such multimethod-based exploration strategies can significantly reduce uncertainties and ambiguities in geophysical data analysis and interpretation. We propose a novel 2D approach based on fuzzy c -means cluster analysis for the cooperative inversion of disparate data sets. We show that this approach results in a single zonal model of subsurface structures in which each zone is characterized by a set of different parameters. This finding implies that no further structural interpretation of geophysical parameter fields is needed, which is a major advantage compared with conventional inversions that rely on a single input data set and cooperative inversion approaches.

Quantitative integration of hydrogeophysical data: Conditional geostatistical simulation for characterizing heterogeneous alluvial aquifers

High-resolution geophysical parameter information, as it can be provided, for example, by crosshole georadar and seismic tomography, has proven to provide useful spatial information to complement traditional hydrological methods such as core analyses, logging techniques, and tracer or pumping tests. Quantitative integration of these diverse database components is one of the major challenges in the field of high-resolution hydrogeophysics because of their different scales of measurement and the usually weak petrophysical relations among the measurements. In this study, we systematically explore the usefulness of a conditional stochastic simulation approach based on simulated annealing for this purpose. First, we generate a realistic model of an alluvial aquifer consisting of a 2D scale-invariant porosity field. On the basis of this model, we generate synthetic neutron porosity logs and crosshole georadar tomographic surveys. We then use the proposed geostatistical simulation approach to integrate this hydr...

Integrating surface georadar and crosshole radar tomography: A validation experiment in braided stream deposits

We have used a combination of surface ground‐penetrating radar (GPR) profiling, crosshole radar tomography, and natural gamma‐ray logging to characterize a gravelly braided stream deposit. In a gravel pit, we conducted a survey using 300‐MHz surface GPR, 250‐MHz crosshole radar, and densely sampled gamma‐ray logging at single‐borehole locations. After excavation, we validated the geophysical results by comparison with the sedimentological and hydrogeological information obtained from the corresponding outcrop wall. We found the visual lithofacies boundaries agreed very well with the images provided by applied geophysical techniques. Our results illustrate how GPR reflector images are improved using tomographic velocity information. In addition, the structural interpretation of tomographic velocity fields is guided by the GPR reflector images in combination with natural gamma‐ray logging results.Groundwater flow and transport modeling was also performed on different subsurface models. The hydrogeological r...

Morphology and sedimentology of a giant supraglacial, ice-walled, jökulhlaup channel, Skeiðarárjökull, Iceland: implications for esker genesis

Abstract This paper examines the sedimentary infill of a spectacular, 500-m-long, 100-m-wide ice-walled supraglacial channel, excavated into the snout of Skeiðararjokull, Iceland during the November 1996 jokulhlaup. The ice-walled channel developed in an area of the glacier, which was extensively fractured during the jokulhlaup. Sculpting of the ice-walled channel into the active snout of Skeiðararjokull suggests that the presence of stagnating glacier ice is not a prerequisite for the development of ice-walled channels. The ice-walled channel occupied an inter-lobate location, which acted as a focus for meltwater during the November 1996 jokulhlaup. The geometry of the supraglacial ice-walled channel system acted as a major control on the morphology and sedimentology of jokulhlaup deposits, through the tremendous spatial variability of resultant flow conditions. Maximum calculated jokulhlaup powers and shear stresses for the supraglacial ice-walled channel reached 40,000 W m −2 and 5000 N m −2 , respectively, with associated mean flow velocities between 7 and 11 m s −1 . Within the main ice-walled channel, Ground Penetrating Radar and outcrop exposure provide evidence of an ∼8-m-thick progradational and aggradational gravel macroform succession. The supraglacial ice-walled channel system is therefore analogous to a bedrock-confined fluvial system. This study provides a new analogue for the interpretation of ice-contact glaciofluvial deposits associated with former ice margins in Iceland and other areas subject to high magnitude discharges. Former supraglacial ice-walled channels resulting from tunnel collapse and ice margin break-up during high magnitude jokulhlaups will be associated with extensive coarse-grained, heavily kettled proglacial outwash surfaces. It is clear that the relationship between the characteristics of former ice-walled channels labeled as eskers and the prevailing glaciological and hydrological conditions needs to be modified in light of our knowledge of a modern flood-related large-scale supraglacial channel and its sedimentary infill. Such re-evaluation may provide a valuable new insight into former ice margin positions, modes of glacier retreat, and the role of high magnitude floods within the sedimentary record of former proglacial areas. This study therefore improves our understanding of the meltwater magnitude and frequency regime of former glaciers.

On the Potential of Kinematic GPR Surveying Using a Self-Tracking Total Station: Evaluating System Crosstalk and Latency

In this paper, we present an efficient kinematic ground-penetrating radar (GPR) surveying setup using a self-tracking total station (TTS). This setup combines the ability of modern GPR systems to interface with Global Positioning System (GPS) and the capability of the employed TTS system to immediately make the positioning information available in a standardized GPS data format. Wireless communication between the GPR and the TTS system is established by using gain variable radio modems. Such a kinematic surveying setup faces two major potential limitations. First, possible crosstalk effects between the GPR and the positioning system have to be evaluated. Based on multiple walkaway experiments, we show that, for reasonable field setups, instrumental crosstalk has no significant impact on GPR data quality. Second, we investigate systematic latency (i.e., the time delay between the actual position measurement by TTS and its fusion with the GPR data) and its impact on the positional precision of kinematically acquired 2-D and 3-D GPR data. To quantify latency for our kinematic survey setup, we acquired forward-reverse profile pairs across a well-known subsurface target. Comparing the forward and reverse GPR images using three fidelity measures allows determining the optimum latency value and correcting for it. Accounting for both of these potential limitations allows us to kinematically acquire high-quality and high-precision GPR data using off-the-shelf instrumentation without further hardware modifications. Until now, these issues have not been investigated in detail, and thus, we believe that our findings have significant implications also for other geophysical surveying approaches.

Joint application of surface electrical resistivity- and GPR-measurements for groundwater exploration on the island of Spiekeroog-northern Germany

Geophysical surveys have been carried out on the East Frisian island of Spiekeroog at the German North Sea coast in 1996. So-called “freshwater lenses”, lying on top of saltwater-saturated layers because of their lower density, provide most of the freshwater supply on the Frisian Islands. The major goal of the present study is to show the possibilities of a joint application of surface electrical resistivity soundings and ground-penetrating radar (GPR) for the hydrogeological investigation of the island. It is shown that a comprehensive interpretation of large-scale GPR-profiling data in a sandy aquifer provides valuable hydrogeological information, especially if there is not any other information available, such as from boreholes or groundwater measuring points. Applying the techniques of surface electrical resistivity and ground penetrating radar jointly, we were able to construct a groundwater contour map for the entire island of Spiekeroog. In addition, we were able to delineate hydrogeologically important changes in grain size and stratigraphical features and to indicate temporal changes of the spatial extent of the freshwater reservoir in comparison to earlier (less complete) investigations. Finally, we located an area, previously unknown, in the eastern part of the island where precipitation appears to be developing a substantial new freshwater lens.

Improved crosshole radar tomography by using direct and reflected arrival times

Abstract Traveltime tomography by inverting direct arrival times is the most common technique in crosshole radar surveying. In a standard crosshole experiment, the inversion is a mathematically nonunique process due to the limited range of viewing angles producing ambiguous and smeared results. If possible, additional information should be extracted from the data or added from the outside to improve and constrain the inversion and the interpretation. In low attenuation media, reflected wave energy can also be present in the crosshole data. Reflections are often caused by well-known interfaces, such as the free earth surface or the groundwater table. Because of their differing travel paths, these reflected events contain additional information about subsurface structure and parameter distribution. We present a technique to include reflection traveltimes associated with known interfaces into tomographic traveltime inversion of crosshole radar data. If in a crosshole radar survey such reflections can be clearly identified, the total number of rays and the range of raypath angles are increased by the presented inversion strategy. Applications to a synthetic and a real field data set show the practicability and the improvements offered by this method. The results show that horizontal smearing is reduced and local anomalies are better contoured compared to standard crosshole tomography using only direct arrivals.

Subsurface Utility Extraction and Characterization: Combining GPR Symmetry and Polarization Attributes

Polarization of the electromagnetic wavefield has significant implications for the acquisition and interpretation of ground-penetrating radar (GPR) data. Based on the geometrical and physical properties of the subsurface scatterer and the physical properties of its surrounding material, strong polarization phenomena might occur. Here, we develop an attribute-based analysis approach to extract and characterize buried utility pipes using two broadside antenna configurations. First, we enhance and extract the utilities by making use of their distinct symmetric nature through the application of a symmetry-enhancing image-processing algorithm known as phase symmetry. Second, we assess the polarization characteristics by calculating two attributes (polarization angle and linearity) using principal component analysis. Combination of attributes derived from these steps into a novel depolarization attribute allows one to efficiently detect and distinguish different utilities present within 3-D GPR data. The performance of our analysis approach is illustrated using synthetic examples and evaluated using field examples (including a dual-configuration 3-D data set) collected across a field site, where detailed ground-truth information is available. Our results demonstrate that the proposed approach allows for a more detailed extraction and combination of utility relevant information compared to approaches relying on single-component data and, thus, eases the interpretation of multicomponent GPR data sets.