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.

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Three-dimensional ground-penetrating radar ͑GPR͒ data are routinely acquired for diverse geologic, hydrogeologic, archeological, and civil engineering purposes. Interpretations of these data are invariably based on subjective analyses of reflection patterns. Such analyses are heavily dependent on interpreter expertise and experience. Using data acquired across gravel units overlying the Alpine Fault Zone in New Zealand, we demonstrate the utility of various geometric attributes in reducing the subjectivity of 3D GPR data analysis. We use a coherence-based technique to compute the coherency, azimuth, and dip attributes and a graylevel co-occurrence matrix ͑GLCM͒ method to compute the texture-based energy, entropy, homogeneity, and contrast attributes. 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 and tectonic structures at the study site.Anew depositional/structural model largely based on the results of our analysis of GPR attributes 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 gravels that could have been affected by lateral fault drag. When used 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 active fault zones.