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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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Understanding the effects of saturation on the acoustic properties of porous media is paramount for using amplitude versus offset (AVO) technique and 4-D seismic. Most laboratory research on saturation effects has been carried out in sandstones, despite the fact that about half of the world's oil and gas reserves are in carbonates. We conducted saturation experiments in carbonates with the intention to fill this gap. These experimental data are used to test theoretical assumptions in AVO and seismic analysis in general. Earlier studies have shown that the complex pore structures of carbonates produce poorly defined porosity-velocity trends. Although porosity is the most important factor to control sonic velocity, our data document that pore type, pore fluid compressibility and variations in shear modulus due to saturation are also important factors for velocities in carbonate rocks. Complete saturation of the pore space separated our samples into two groups: one group showed decreases in shear bulk modulus of the rock by up to 2 GPa, the other group showed increase by up to 3 GPa. This change in shear modulus questions Gassmann's assumption of constant shear modulus in dry and saturated rocks. It also explains our observation that velocities predicted with by the Gassmann equation under-and overestimates the measured velocities of saturated carbonate samples. In addition, the Vp/Vs ratio shows an overall increase with saturation. In particular, rocks displaying shear weakening have distinct higher Vp/Vs ratios.

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