Heinrich Horstmeyer

Full-resolution 3D GPR imaging

Noninvasive 3D ground-penetrating radar (GPR) imaging with submeter resolution in all directions delineates the internal architecture and processes of the shallow subsurface. Full-resolution imaging requires unaliased recording of reflections and diffractions coupled with 3D migration processing. The GPR practitioner can easily determine necessary acquisition trace spacing on a frequency-wavenumber (f-k) plot of a representative 2D GPR test profile. Quarter-wavelength spatial sampling is a minimum requirement for full-resolution GPR recording. An intensely fractured limestone quarry serves as a test site for a 100-MHz 3D GPR survey with 0.1 m × 0.2 m trace spacing. This example clearly defines the geometry of fractures in four different orientations, including vertical dips to a depth of 20 m. Decimation to commonly used half-wavelength spatial sampling or only 2D migration processing makes most fractures invisible. The extra data-acquisition effort results in image volumes with submeter resolution, both ...

Using two- and three-dimensional georadar methods to characterize glaciofluvial architecture

Abstract The threat of pollution in the shallow subsurface has led to an increasing need to understand how complex heterogeneities in gravel aquifers influence groundwater transport. To characterize these heterogeneities, we have conducted extensive two-dimensional (2-D) and three-dimensional (3-D) ground-penetrating radar (GPR or georadar) surveys in two quarries within the Rhine valley of northeastern Switzerland. The quarries comprised gravel and sand deposited in a proglacial braided river system that followed the maximum Wurm-stage glaciation. After the surveys, the terrace walls beneath the two study sites were photographed as they were being excavated. By combining information extracted from the 2- and 3-D georadar images with the outcrop photographs, it was possible to correlate georadar facies with the various glaciofluvial architectural elements. The dominant elements were scour pools formed at or near the confluences of two stream channels and horizontally bedded and massive gravel sheets deposited during moderate to high water flow conditions and exposed during low flow conditions. Architectural elements were generally elongated parallel to the mean flow directions of the ancient river system. Variations in the strike of their long axes reflected lateral and vertical changes in the local flow direction. Time slices showed structural trends not evident on 2-D georadar images and photographs. Data interpretation was quicker, more complete and less ambiguous when georadar facies analyses were based on a combination of georadar vertical profiles and time slices than when based on georadar vertical profiles alone.

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.

Mapping the architecture of glaciofluvial sediments with three-dimensional georadar

Three-dimensional (3-D) ground-penetrating radar (georadar) mapping offers new opportunities for determining the geometries and facies of surficial sedimentary units. To investigate the potential of this high-resolution technique and at the same time study the architecture of Quaternary glaciofluvial deposits, georadar data have been collected on a dense grid established across a sequence of braided-river gravels and sands in northeastern Switzerland. Results of this survey are striking 3-D images that provide many more details and much more reliable information on the heterogeneities of the shallow underground than are afforded by conventional georadar profile data. Continuous subhorizontal and oblique reflections can be traced throughout vertical sections and horizontal slices of the georadar data block to a depth of (Approx.)15 m. Clearly defined are the dominant flow direction of the ancient braided-river system, the boundaries between different sedimentary facies, and the level of the ground-water table. Trough-fill sediments and subhorizontal channel deposits observed on 7-m-high quarry walls can be followed confidently in the subsurface. The orientation, shape, and size of the troughs and the strike and dip of the cross-bedding are all well resolved.

Shallow seismic reflection study of a glaciated valley

Shallow seismic reflection data were recorded along two long (>1.6 km) intersecting profiles in the glaciated Suhre Valley of northern Switzerland. Appropriate choice of source and receiver parameters resulted in a high‐fold (36–48) data set with common midpoints every 1.25 m. As for many shallow seismic reflection data sets, upper portions of the shot gathers were contaminated with high‐amplitude, source‐generated noise (e.g., direct, refracted, guided, surface, and airwaves). Spectral balancing was effective in significantly increasing the strength of the reflected signals relative to the source‐generated noise, and application of carefully selected top mutes ensured guided phases were not misprocessed and misinterpreted as reflections. Resultant processed sections were characterized by distributions of distinct seismic reflection patterns or facies that were bounded by quasi‐continuous reflection zones. The uppermost reflection zone at 20 to 50 ms (∼15 to ∼40 m depth) originated from a boundary between...

Shallow seismic surveying of an Alpine rock glacier

To map the internal structure and lower boundary of an alpine rock glacier, we recorded three shallow seismic profiles and drilled four ∼70‐m‐deep holes through to the underlying bedrock. Although analysis of the seismic data using standard reflection processing schemes did not yield conclusive results because of the dominantly low‐frequency returned signals and the presence of strong source‐generated noise, tomographic inversions of first‐arrival times were successful in mapping several critical subsurface features. A thin, low‐velocity layer of loose boulders, air voids, and snow was found to extend across the entire surveyed area. Below this layer, two distinct velocity regimes superimposed on a general increase in velocity with depth were identified. A broad regime of high velocities was interpreted to contain boulders with numerous ice‐filled voids, whereas an adjacent regime of relatively low velocities was explained in terms of boulders with air‐ and water‐filled voids. This latter region of degrad...

Acquisition and processing strategies for 3D georadar surveying a region characterized by rugged topography

Efficiently performing 3D ground-penetrating radar (GPR or georadar) surveys across rugged terrain and then processing the resultant data are challenging tasks. Conventional approaches using unconnected GPR and topographic surveying equipment are excessively time consuming for such environments, and special migration schemes may be required to produce meaningful images. We have collected GPR data across an unstable craggy mountain slope in the Swiss Alps using a novel acquisition system that records GPR and coincident coordinate information simultaneously. Undulating topography (dips of 8 ◦ to 16 ◦ ) and boulders with diameters up to about 2 m complicated the field campaign. After standard processing, the data were found to be plagued by time shifts associated with minor coordinate inaccuracies, uneven antenna-ground coupling, and numerous small gaps in data coverage. These problems were resolved by passing the data sequentially through an adaptive f -xy deconvolution routine and f -kx and f -ky filters. This filtering also reduced incoherent noise. Finally, the data were migrated using a 3D algorithm that accounted for the undulating topography. The nonmigrated and migrated images contained gently and moderately dipping reflections from lithological boundaries and actively opening fracture zones. A suite of prominent diffraction patterns was generated at a steeply dipping fracture zone that projected to the surface. Through this case history we introduce a general strategy for 3D GPR studies of topographically rugged land.

Three-dimensional ground-penetrating radar imaging of sedimentary structures, fractures, and archaeological features at submeter resolution

Contemporary geoscientific shallow-subsurface assessment chiefly relies on outcrops, drilling, excavations, and sometimes geophysics. Often the information gathered is insufficient to accurately characterize the archaeological and/or geologic record and ongoing shallow-subsurface processes that affect a variety of economic and environmental aspects of our society. The extra effort of acquiring very dense ground-penetrating radar (GPR) survey grids and three-dimensional (3D) data processing transforms uncorrelatable and uninterpretable GPR signals into clear images of complex shallow-subsurface anatomy with an unprecedented resolution. Here we present two examples of noninvasive 3D shallow-subsurface imaging. Example 1 images decimeter- to meter-scale sedimentary structures in a Pleistocene oolite shoal-barrier bar setting. Example 2 images the fracture network in a Triassic limestone quarry. Denser-than-quarter-wavelength grid acquisition in combination with 3D migration processing focuses scattered energy and removes out-of-plane reflections. In addition to conventional vertical cross sections, horizontal depth slices and data volume animations reveal previously unseen diagnostic patterns of past human activities, laterally changing depositional processes, and fracture networks including near-vertical joints with millimeter apertures.

Results of 3-D georadar surveying and trenching the San Andreas fault near its northern landward limit

Abstract As part of a program to determine the location and geometry of the San Andreas Fault (SAF) buried beneath shallow sediments near its northern landward limit, three >20-m-long parallel trenches were constructed at positions distributed over a distance of ∼55 m. The majority of excavated material comprised unconsolidated fluvial sediments deposited in a number of paleochannels. Single zones of active faulting identified in each of the trenches were initially interpreted in terms of a solitary strand of the SAF. To map the SAF between and beyond the trenches and to detect other active fault zones hidden by the young sedimentary cover, we collected a dense ground-penetrating radar (georadar) data set across a 23.2×72 m area. The data were recorded using a semi-automated acquisition system that included a conventional georadar unit coupled to a self-tracking laser theodolite with automatic target recognition capabilities. Since these data were plagued by system ringing as a result of the moderate-to-high electrical conductivities of the surficial sediments, an extensive data processing scheme was required to extract meaningful subsurface information. The final processed georadar volume (cuboid) contained numerous subhorizontal and trough-shaped reflections that originated from the fluvial paleochannels. Using the geological interpretation of the trench walls as a guide to pick semi-automatically the times of the most important reflecting horizons, we discovered that alignments of the nearly linear boundaries of these horizons defined two NW–SE trending strands of the SAF within the survey area. The georadar expression of the eastern SAF strand could only be traced over a distance of ∼38 m. It had been intersected in the northern trench. In contrast, the western SAF strand extended over the entire length of the georadar volume and had been intersected in the central and southern trenches. Prominent reflections on georadar cross sections were found to be vertically displaced by 0.2–0.3 m across both SAF strands. A conspicuous linear-trending feature observed on horizontal sections at 3.3–3.6 m depth was laterally offset by 4.5–5.5 m along the eastern SAF strand. The interpreted vertical and horizontal offsets could have been generated by the 1906 San Francisco earthquake and/or earlier events. Undetermined amounts of aseismic slip may also have occurred along the newly defined SAF strands.

Results of 2- and 3-D High-Resolution seismic reflection surveying of surficial sediments

To investigate the potential of seismic reflection methods for studying surficial sediments, 2- and 3-D high-resolution surveys have been conducted across a plot of land adjacent to a landfill in northern Switzerland. Results of the reflection profiling demonstrated that closely spaced common-reflection points (≤ 2.5 m) and high-fold (≥ 12) data were prerequisites for producing clear images at this site. For the 3-D survey, an area of 115×160 m² was covered with common-reflection points every 2.5×2.5 m². Three-dimensional migration of the resultant 18- to 70-fold data proved to be a critical step in correctly locating out-of-plane reflections and collapsing diffractions that were prominent features in the 2-D profile data. Within the 3-D data volume a conspicuous reflective zone and a marked decrease in reflection amplitudes at ∼200 m depth delineates the bedrock surface. Immediately above this level are continuous reflections from flat-lying lacustrine sediments. These are in turn overlain by regions of strong reflected energy interpreted as either large erratic blocks, anomalous compaction zones, or gravel channels/lenses.