Mark Grasmueck

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 ...

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.

3D GPR reveals complex internal structure of Pleistocene oolitic sandbar

Active oolitic sandbars like those in the Bahamas (Figure 1) exhibit complex internal architecture with a multitude of stacked sedimentary structures. Ooids are round, carbonate-coated grains that form in tropical climates. The internal anatomy of carbonate sandbars is often too complex to be captured in one- and two-dimensional data. Outcrops, cores, and 2D geophysical profiles provide a limited vertical view of the geologic record. Depositional processes are confined to the momentary subhorizontal boundary surface between sediment and water or air. Vertical 2D views limit the visibility of features developed on subhorizontal surfaces, making interpretation of 3D internal anatomy and reconstruction of related depositional parameters difficult. Closely spaced 3D data are needed to accurately map sedimentary structures and improve fluid flow modeling used in water and hydrocarbon resource management. 3D reflection seismic imaging has successfully been used to delineate oolitic bars in sub-surface oil field...

Diffraction imaging of sub-vertical fractures and karst with full-resolution 3D Ground-Penetrating Radar

ABSTRACT Vertical fractures with openings of less than one centimetre and irregular karst cause abundant diffractions in Ground‐Penetrating Radar (GPR) records. GPR data acquired with half‐wavelength trace spacing are uninterpretable as they are dominated by spatially undersampled scattered energy. To evaluate the potential of high‐density 3D GPR diffraction imaging a 200 MHz survey with less than a quarter wavelength grid spacing (0.05 m × 0.1 m) was acquired at a fractured and karstified limestone quarry near the village of Cassis in Southern France. After 3D migration processing, diffraction apices line up in sub‐vertical fracture planes and cluster in locations of karstic dissolution features. The majority of karst is developed at intersections of two or more fractures and is limited in depth by a stratigraphic boundary. Such high‐resolution 3D GPR imaging offers an unprecedented internal view of a complex fractured carbonate reservoir model analogue. As seismic and GPR wave kinematics are similar, improvements in the imaging of steep fractures and irregular voids at the resolution limit can also be expected from high‐density seismic diffraction imaging.

Diffraction signatures of fracture intersections

Fractured rock causes diffractions, which are often discarded as noise in ground-penetrating radar (GPR) and seismic data. Most fractures are too thin, too steep, and their displacement is too small to be imaged by reflections, and diffractions are the only detectable signal. To decipher the information about fracture geometry and distribution contained in diffractions, we compare 3D synthetic ray-Born modeling with high-density 3D GPR data and outcrop observations from the Cassis Quarry in Southern France. Our results reveal how the intersection between two fractures is the basic geologic element producing a recordable diffraction. In this new model, two intersecting fractures are represented by one finite-length line diffractor. The intersection of three fractures is a 3D cross composed of three line diffractors. Fractures extending over several meters in the outcrop display linear clusters of diffraction circles in unmigrated GPR time slices. Such large-scale fracture intersections are composed of many aligned short subwavelength line diffractors due to fracture roughness and variations of fracture opening. The shape irregularities and amplitude variations of composite diffraction signatures are a consequence of the geometry and spacing of the intersecting fractures generating them. With three simple base-type intersecting fracture models (horizontal dip, gentle dip, and steep dip), the fracture network geometry can be directly deciphered from the composite diffraction signatures visible on unmigrated time slices. The nonrandom distribution of diffractions is caused by fracture trends and patterns providing information about fracture dip, spacing, and continuity of fractured domains. With the similarity law, the diffraction phenomena observed in GPR data are very similar in character to those seen on the seismic scale with the wavelength as the scaling link. GPR data serve as a proxy to decipher seismic diffractions.

How dense is dense enough for a ‘real’ 3D GPR survey?

Unaliased 3D Ground Penetrating Radar (GPR) images of the shallow subsurface require dense data sampling. Trace spacing has to be close enough to sample entire diffractions. The steep tails of diffraction hyperbolae require trace spacing of a quarter wavelength or less. Using frequency-wavenumber analysis, the necessary spatial sampling intervals can be easily determined. With proper sampling and 3D migration, shallow subsurface features such as steep fractures can be imaged in great detail.

Estimation of biomass of tree roots by GPR with high accuracy positioning system

A new approach for estimation of the tree root biomass is proposed by using GPR system with high accuracy positioning system. The 3D images of subsurface can be obtained clearly with this system, which we refer as “3DGPR”. We try to estimate the biomass of tree roots quantitatively by measuring the volume of the tree roots. We tested this system for tree roots measurement, and the broadening of the tree root with horizontal direction could be detected with 500 MHz and 800 MHz antenna. We excavated this tree every 10 cm from 0 cm to 50 cm to validate the accuracy of the result. Compared with the measurement result and the excavated one, the tree root whose diameter is more than 5 cm could be detected correctly. Some tree roots whose diameter is 3 cm also could be detected. The diameter in most of the tree roots is more than 2cm. In this result, we expect that the volume is supposed to be estimated within 30 percent error by using the excavated result for calibration data.

CCD camera and IGPS tracking of geophysical sensors for visualization of buried explosive devices

High-resolution Ground Penetrating Radar (GPR) images of the ground surface and shallow subsurface are needed in order to detect and identify small buried explosive materials such as Anti-Personnel (AP) landmines. A key requirement to produce sharp visualizations is centimetre-precise sensor positioning with real-time imaging results. We are pursuing two complementary approaches to accomplish this task: 1) Sensor tracking with a CCD camera, 2) and large work volume Indoor GPS. In outdoor field tests both methods have successfully imaged small landmine targets, which has a plastic body of less than 10cm diameter.

The impact of high-density spatial sampling versus antenna orientation on 3D GPR fracture imaging

Three-dimensional Ground Penetrating Radar (3D GPR) surveys are necessary to reconstruct complex fracture geometries in the subsurface. Two of the most important factors controlling image quality are antenna orientation relative to fractures and density of acquisition grids. This study, conducted in the Madonna della Mazza quarry (Italy), compares two acquisition methods with the goal of optimizing the imaging of fractures and related 3D fracture networks. We acquired two very dense, orthogonal 3D GPR surveys with a 250 MHz antenna and 5 cm trace spacing on the same day, covering the same area of 20 x 20 m. By decimation of the original raw datasets, reduced survey densities of 10 cm and 20 cm spacing are simulated. The results show differences in the imaging quality of the two methods to depths of 75 cm, while for a depth of 130 cm and deeper, image quality is similar. At the same trace density/m², a single, unidirectional survey with a densely sampled grid is the preferred method rather than two surveys with orthogonal antenna orientations but larger profile spacing. The extra effort of conducting surveys with an acquisition grid of an eighth of wavelength of the antenna centre frequency guarantees that we can properly sample the high-frequency content of the GPR signal spectrum and results in optimum image quality regardless of fracture orientation. The simple survey design principle found in this study is universally applicable to any field condition, target geometry, and antenna frequency. Such high-density 3D GPR survey design enables the high-resolution characterization of 3D fracture networks on subsurface timeslices in near photographic quality.

Impact of spatial sampling and antenna polarization on 3D GPR fracture detection

Three-dimensional Ground Penetrating Radar (3D GPR) surveys are needed to reconstruct subsurface fracture networks in terms of strike, dip and interconnectivity. This particular GPR study conducted in the Madonna della Mazza quarry (Italy) compares the impact of a dense acquisition grid versus antenna polarization on the characterization of subvertical fractures. 3D GPR data were acquired using a 250 MHz antenna over a 20 × 20 m area with a grid bin size of 5 × 5 cm. According to the quarter-wavelength criterion, profile spacing of 5 cm allows to properly sample also the high-frequency content of the signal spectrum. The results demonstrate that, rather than repeated surveys with different antenna orientation but larger profile spacing, a single survey with a regular and highly sampled grid is a preferred approach for high-resolution characterization of three-dimensional fracture networks.