Ralf J. Weger

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 ...

Quantification of pore structure and its effect on sonic velocity and permeability in carbonates

Carbonate rocks commonly contain a variety of pore types that can vary in size over several orders of magnitude. Traditional pore-type classifications describe these pore structures but are inadequate for correlations to the rocks physical properties. We introduce a digital image analysis (DIA) method that produces quantitative pore-space parameters, which can be linked to physical properties in carbonates, in particular sonic velocity and permeability. The DIA parameters, derived from thin sections, capture two-dimensional pore size (DomSize), roundness (), aspect ratio (AR), and pore network complexity (PoA). Comparing these DIA parameters to porosity, permeability, and P-wave velocity shows that, in addition to porosity, the combined effect of microporosity, the pore network complexity, and pore size of the macropores is most influential for the acoustic behavior. Combining these parameters with porosity improves the coefficient of determination (R2) velocity estimates from 0.542 to 0.840. The analysis shows that samples with large simple pores and a small amount of microporosity display higher acoustic velocity at a given porosity than samples with small, complicated pores. Estimates of permeability from porosity alone are very ineffective (R2 = 0.143) but can be improved when pore geometry information PoA (R2 = 0.415) and DomSize (R2 = 0.383) are incorporated. Furthermore, results from the correlation of DIA parameters to acoustic data reveal that (1) intergrain and/or intercrystalline and separate-vug porosity cannot always be separated using sonic logs, (2) P-wave velocity is not solely controlled by the percentage of spherical porosity, and (3) quantitative pore geometry characteristics can be estimated from acoustic data and used to improve permeability estimates.

Changes of shear moduli in carbonate rocks: Implications for Gassmann applicability

In laboratory experiments we measured the saturation effects on the acoustic properties in carbonates and the results question some theoretical assumptions. In particular, these laboratory experiments under dry and wet conditions show that shear moduli do not remain constant during saturation. This change in shear modulus puts Gassmanns assumption of a constant shear modulus into question and also explains why velocities predicted with the Gassmann equation can be lower or higher than measured velocities.

Effects of microporosity on sonic velocity in carbonate rocks

The elastic moduli of a rock are affected by three main factors: pore fluid, rock framework, and pore space. In carbonate rocks, the latter two factors are a function of the depositional environment and the diagenetic history. Cementation, recrystallization, and dissolution processes can change the mineralogy and texture of the original framework and thereby alter the original grain-to-grain contacts and/or occlude pore space. Dissolution processes can enlarge interparticle pore space or dissolve grains entirely, thereby increasing porosity. These diagenetic alterations and associated changes in the rock frame and pore structure result in a wide velocity range at a given porosity.

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.

Effect of pore structure on electrical resistivity in carbonates

The electrical resistivity log has proven to be a powerful tool for lithology discrimination, correlation, porosity evaluation, hydrocarbon indication, and calculation of water saturation. Carbonate rocks develop a variety of pore types that can span several orders of magnitude in size and complexity. A link between the electrical resistivity and the carbonate pore structure has been inferred, although no detailed understanding of this relationship exists. Seventy-one plugs from outcrops and boreholes of carbonates from five different areas and ages were measured for electrical resistivity properties and quantitatively analyzed for pore structure using digital image analysis from thin sections. The analysis shows that in addition to porosity, the combined effect of microporosity, pore network complexity, pore size of the macropores, and absolute number of pores are all influential for the flow of electric charge. Samples with small pores and an intricate pore network have a low cementation factor, whereas samples with large pores and a simple pore network have high values for cementation factor. Samples with separate-vug porosity have the highest cementation factor. The results reveal that (1) in carbonate rocks, both pore structure and the absolute number of pores (and pore connections) seem more important in controlling the electrical resistivity, instead of the size of the pore throats, as suggested by previous modeling studies; (2) samples with high resistivity can have high permeability; large simple pores facilitate flow of fluid, but fewer numbers of pores limit the flow of electric charge; and (3) pore-structure characteristics can be estimated from electrical resistivity data and used to improve permeability estimates and refine calculations of water saturation.

Changes in dynamic shear moduli of carbonate rocks with fluid substitution

To assess saturation effects on acoustic properties in carbonates, we measure ultrasonic velocity on 38 limestone samples whose porosity ranges from 5% to 30% under dry and water-saturated conditions. Complete saturation of the pore space with water causes an increase and decrease in compressional- and shear-wave velocity as well as significant changes in the shear moduli. Compressional velocities of most water-saturated samples are up to 500 m∕s higher than the velocities of the dry samples. Some show no change, and a few even show a decrease in velocity. Shear-wave velocity ( VS ) generally decreases, but nine samples show an increase of up to 230 m∕s . Water saturation decreases the shear modulus by up to 2 GPa in some samples and increases it by up to 3 GPa in others. The average increase in the shear modulus with water saturation is 1.23 GPa ; the average decrease is 0.75GPa . The VP ∕ VS ratio shows an overall increase with water saturation. In particular, rocks displaying shear weakening have disti...

Estimating permeability of carbonate rocks from porosity and vp ∕ vs

We present a method for predicting permeability from sonic and density data. The method removes the porosity effect on the ratio vp ∕ vs of dry rock, and it addresses the specific surface as an indirect measure of permeability. We look at ultrasonic data, porosity, and the permeability of 114 carbonate core plugs. In doing so, we establish an empirical relationship between the specific surface of the solid phase (as calculated by Kozeny’s equation) and vp ∕ vs (linearly transformed to remove the porosity effect). One must view the specific surface derived by using Kozeny’s equation as an effective specific surface because Kozeny’s equation only holds for homogeneous rock with interconnected pores. The ratio vp ∕ vs of dry rocks, on the other hand, seems to be controlled by the true specific surface, pointing to an inherent limitation in the method. The 114 carbonate plugs originate in three geological settings and comprise 83 calcitic and 31 dolomitic samples. Their depositional texture varies from mud-do...

Effect of carbonate pore structure on dynamic shear moduli

Elastic shear moduli from ultrasonic measurements of carbonate rocks show an increase or decrease when saturated from the dry state with brine. The induced changes on the moduli have been attributed to several rock-fluid interaction effects, including viscous coupling, reduction in free surface energy, and dispersion caused by local flow with a subsequent effect on acoustic velocities. As many studies have recognized that acoustic velocity in carbonate rocks is dependent on pore structure, a relationship between the pore structure and observed changes must, to a certain extent, exist. We make use of quantitative digital image analysis parameters, such as dominant pore size and perimeter over area, to demonstrate how changes in dynamic shear moduli upon saturation relate to the geologic make up of the rock. Samples with a small dominant pore size and large perimeter over area show abnormal decrease in shear moduli with saturation from the dry state. This is attributed to the large surface area available for matrix-fluid interaction. Samples with large dominant pore size and small perimeter over area show an increase in shear moduli with saturation. This may be a dispersion effect, as high frequency and high permeability may cause the fluid in the saturated samples to move out-of-phase with the solid during propagation of the acoustic wave and thus cause a stiffening effect.

Modeling Velocity In Carbonates Using a Dual Porosity DEM Model

The differential effective medium theory is used to model the velocity of carbonates with two predefined end-member pore types and under dry and water saturated conditions. The dual porosity DEM takes into account input parameters derived from digital image analysis of thin sections. In particular the respective amount of microporosity and macroporosity and the aspect ratio of the macropores are incorporated. A conceptual aspect ratio of 0.1 for micropores and a measured aspect ratio of 0.5 for macropores is used as input parameters for the differential effective medium (DEM) model. The model predicts that the compliant micropores have a strong influence on the sonic velocity of porous carbonates because increasing concentrations of micropores reduce the rock stiffness. The model values are compared to high frequency (1MHz) laboratory velocity measurements. These velocity predictions with the dual porosity DEM model show significant better velocity prediction than empirical models, e.g. the Wyllie times average equation. We obtain a rootmean-square-error of 392 m/s when comparing predicted with measured velocity values. Our results also show that a differential effective medium model that uses measured input parameters from quantitative digital image analysis improves estimates of acoustic properties of carbonates.