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 fullresolution 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 in the vertical and horizontal directions. Such 3D data sets accurately image fractured rock, sedimentary structures, and archeological remains in previously unseen detail. This makes full-resolution 3D GPR imaging a valuable tool for integrated studies of the shallow subsurface.
Three‐dimensional, ground‐penetrating radar (georadar) techniques suitable for geological engineering applications have been developed and tested. Initial experiments were conducted on the floor of a quarry in southern Switzerland from which ornamental gneissic rock is extracted. During a brief two‐day period, constant‐offset georadar data were recorded over a [Formula: see text] area with a grid cell size of 0.1 m × 0.2 m. Georadar velocities were estimated from the results of expanding spread surveys. All georadar data and associated geometry files were recorded automatically in seismic industry formats. The experimental georadar data set was processed, image‐enhanced, and interpreted using [Formula: see text] seismic reflection software operating on a workstation. Arbitrary vertical sections, time slices, [Formula: see text] images, and animated movies in which the observer “travels” through the entire data volume were constructed from the resultant migrated georadar data. Semi‐automatic tracking routines allowed continuous subhorizontal reflections to a maximum depth of 30 m to be mapped through the rock mass. These reflections, which are characterized by negative polarity onsets, are probably caused by a system of ubiquitous water‐filled fractures, 2–4 cm thick. Volumes of rock bounded by the subhorizontal fractures were estimated from isopach maps and rock quality was assessed on the basis of root‐mean‐square (rms) amplitudes of reflections. An extension of a steep‐dipping fault exposed on a nearby quarry wall was best delineated on maps representing the horizontal gradients of reflection times. To synthesize in a single figure the principal geological results of the study, picked reflection times were presented in the form of shaded relief surfaces, in which remarkably vivid structural details of the subhorizontal fractures and intersecting near‐vertical fault could be discerned. It is concluded that 3-D georadar methods have the potential to resolve a wide range of engineering and environmental problems.
[1] To make progress in understanding the distribution and genesis of coral mounds in cold and dark water, maps of morphology and oceanographic conditions resolving features at the 1 -10 m scale are needed. An autonomous underwater vehicle (AUV) cruising 40 m above the seafloor surveyed a 48 km 2 coral mound field in 600-800 m water depth at the base of slope of Great Bahama Bank. The AUV acquired 1 -3 meter resolution acoustic backscatter and bathymetry together with current vectors, salinity, and temperature. The multibeam bathymetry resolved more than 200 coral mounds reaching up to 90 m height. Mound morphology is surprisingly diverse and mound distribution follows E-W oriented off-bank ridges. Bottom currents reverse every 6 hours indicating tidal flow decoupled from the north flowing surface current. The AUV data fill the gap between low-resolution surface-based mapping and visual observations on the seafloor, revealing the dynamic environment and spatial relationships of an entire coral mound field.
[1] The vadose zone of the Miami limestone is capable of draining several centimeters of rainfall within a fraction of an hour. Once the water enters the ground, little is known about the flow paths in the oolitic rock. A new rotary laser-positioned ground-penetrating radar (GPR) system enables centimeter-precise and rapid acquisition of time-lapse surveys in the field. Two-dimensional (2-D) GPR time-lapse surveying at a 3-min interval before, during, and after rainfall shows how buried sand-filled dissolution sinks efficiently drain the bulk of the rainwater. Hourly repeated 3-D imaging of a dissolution sink in response to surface infiltration shows how the wetting front propagates at a rate of 0.6-1.2 m/h traversing the 5-m-thick vadose zone within hours. At the same time, some of the water migrates laterally into the host rock guided by stratigraphic unit boundaries. Average lateral propagation measured over a 28-hour period was of the order of 0.1 m/h. On a seasonal time frame, redistribution involves the entire rock volume. Comparing 3-D surveys acquired after wet summer and dry winter conditions shows good GPR event correspondence, but also time shifts up to 20 ns caused by the change of overall water content within the vadose zone. High-precision time-lapse GPR imaging can therefore be used to noninvasively characterize natural drainage inside the vadose zone ranging from transient loading to seasonal variation.
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