Extraction of sedimentological information via computer-based image analyses of gray shales in carboniferous coal-bearing sections of Indiana and Kansas, USA

Fine-scale fabric within shales can be used to interpret environments of deposition. In particular, systematic variations in lamina thicknesses can be used to suggest the presence or absence of specific sedimentological controlling factors, such as tides. Herein, shale fabric is analyzed from Carboniferous coal-bearing and siliciclastic-rich formations in Indiana and Kansas. The shales exhibit well developed, normally graded lamina. Sediment grain sizes range from silt and sand at the base of the lamina up to silt and clay at the top. Computer-based optical scanning of prepared shale samples was undertaken so that the lamina thicknesses could be extracted automatically and then used for thickness-series analysis. The sands and silts tend to be very light in color because they consist of well sorted, quartz-rich sands. Conversely, the silt and clay that cap the laminae tend to be dark because of the small grain size; such fine-grained layers also contain large amounts of macerated and coalified plant material. Grain size and sedimentological differences within the laminae can be detected by optical properties and this facilitates automated image processing. Extraction of laminae thicknesses and harmonic analyses of laminae-thickness series can then be used to test the reasonability of specific environmental processes, such as tidal forcing, that were operative during the formation of these shales.     

The nature and dimensions of regional and local gold-related hydrothermal alteration in tholeiitic metabasalts in the Norseman goldfields: the missing link in a crustal continuum of gold deposits?

The Archaean lode-gold deposits at Norseman, Western Australia, consist of auriferous quartz veins in dextral-reverse ductile-brittle shear zones within tholeiitic metabasalts of upper-greenschist to amphibolite facies metamorphic grade. Three types of deposits (Northern, Central, Southern) are delineated on the basis of their spatial distribution, veining style, alteration mineraloty and metamorphic grade of host rocks. Northern deposits, hosted in upper-greenschist to lower-amphibolite facies rocks, comprise massive to laminated quartz veins with selvedges of quartz-chlorite-calcite-biotite-plagioclase assemblages. Central deposits, hosted in lower-amphibolite facies rocks, consist of laminated to massive quartz veins with selvedges of quartz-actinolite-biotite-plagioclasecalcite assemblages. Southern deposits, hosted in middleamphibolite facies metabasalts, consist of banded quartz-diopside-calcite-microcline-zoisite veins. All deposits exhibit variable ductile deformation of veins and contiguous alteration haloes, consistent with a syn-deformational genesis at high temperatures. From Northern to Southern deposits, the alteration assemblages are indicative of higher temperatures of formation, and there are progressively greater degrees of dynamically recovered textures in alteration and gangue minerals. These observations imply that a thermal variation of gold-related hydrothermal alteration exists within the Norseman Terrane over a distance of 40 km, with T_Northern<T_Central<T_Southern This thermal zonation is corroborated by T−X_{CO 2} phase relations between vein selvedge assemblages, which signify formation temperatures of approximately 420°–475°C, 470°–495°C and >500°C for Northern, Central and Southern deposits, respectively. The sum of structural, petrographic and mineral chemistry data indicates that the alteration assemblages formed in high-temperature, open hydrothermal systems and have not been subsequently metamorphosed. The thermal differences between the deposit groups may reflect (1) a temperature gradient, at relatively constant P, corresponding to the proximity of the deposits to regional granitoid complexes, or (2) formation of the deposits at progressively deeper crustal levels from north to south. In either case the deposits represent a continuum of gold deposition from upper-greenschist to amphibolite facies, now exposed in an oblique section through the Archaean crust at Norseman.     

Plate tectonics and the fusion of Asia

The coincidence of orogenic belts containing zones of high pressure metamorphism, ophiolites and deepwater sediments with faunal province boundaries leads to the postulation of several sutural zones within Asia. About nine blocks are defined and it is suggested on the basis of palaeogeographical, palaeontological and tectonic evidence that Asia did not fuse completely until well into the Mesozoic.     

Globally increased pelagic carbonate production during the Mid-Brunhes dissolution interval and the CO2 paradox of MIS 11

The Mid-Brunhes dissolution interval (MBDI) represents a period of global carbonate dissolution, lasting several hundred thousand years, centred around Marine Isotope Stage (MIS) 11. Here we report the effects of dissolution in ODP core 982, taken from 1134 m in the North Atlantic. Paradoxically, records of atmospheric CO₂ from Antarctic ice-cores reveal no long term trend over the last 400 kyr and suggest that CO₂ during MIS 11 was no higher than during the present interglacial. We suggest that a global increase in pelagic carbonate production during this period, possibly related to the proliferation of the Gephyrocapsa coccolithophore, could have altered marine carbonate chemistry in such a way as to drive increased dissolution under the constraints of steady state. An increase in the production of carbonate in surface waters would cause a drawdown of global carbonate saturation and increase dissolution at the seafloor. In order to reconcile the record of atmospheric CO₂ variability we suggest that an increase in the flux of organic matter from the surface to deep ocean, associated with either a net increase in primary production or the enhanced ballasting effect provided by an increased flux of CaCO₃, could have countered the effect of increased calcification on CO₂.     

Velocities of compressional and shear waves in limestones

Carbonate rocks are important hydrocarbon reservoir rocks with complex textures and petrophysical properties (porosity and permeability) mainly resulting from various diagenetic processes (compaction, dissolution, precipitation, cementation, etc.). These complexities make prediction of reservoir characteristics (e.g. porosity and permeability) from their seismic properties very difficult. To explore the relationship between the seismic, petrophysical and geological properties, ultrasonic compressional‐ and shear‐wave velocity measurements were made under a simulated in situ condition of pressure (50 MPa hydrostatic effective pressure) at frequencies of approximately 0.85 MHz and 0.7 MHz, respectively, using a pulse‐echo method. The measurements were made both in vacuum‐dry and fully saturated conditions in oolitic limestones of the Great Oolite Formation of southern England. Some of the rocks were fully saturated with oil. The acoustic measurements were supplemented by porosity and permeability measurements, petrological and pore geometry studies of resin‐impregnated polished thin sections, X‐ray diffraction analyses and scanning electron microscope studies to investigate submicroscopic textures and micropores. It is shown that the compressional‐ and shear‐wave velocities (V_p and V_s, respectively) decrease with increasing porosity and that V_p decreases approximately twice as fast as V_s. The systematic differences in pore structures (e.g. the aspect ratio) of the limestones produce large residuals in the velocity versus porosity relationship. It is demonstrated that the velocity versus porosity relationship can be improved by removing the pore‐structure‐dependent variations from the residuals. The introduction of water into the pore space decreases the shear moduli of the rocks by about 2 GPa, suggesting that there exists a fluid/matrix interaction at grain contacts, which reduces the rigidity. The predicted Biot–Gassmann velocity values are greater than the measured velocity values due to the rock–fluid interaction. This is not accounted for in the Biot–Gassmann velocity models and velocity dispersion due to a local flow mechanism. The velocities predicted by the Raymer and time‐average relationships overestimated the measured velocities even more than the Biot model.     

The acoustic signature of fluid flow in complex porous media

Effective medium approximations for the frequency-dependent and complex-valued effective stiffness tensors of cracked/porous rocks with multiple solid constituents are developed on the basis of the T-matrix approach (based on integral equation methods for quasi-static composites), the elastic–viscoelastic correspondence principle, and a unified treatment of the local and global flow mechanisms, which is consistent with the principle of fluid mass conservation. The main advantage of using the T-matrix approach, rather than the first-order approach of Eshelby or the second-order approach of Hudson, is that it produces physically plausible results even when the volume concentrations of inclusions or cavities are no longer small. The new formulae, which operates with an arbitrary homogeneous (anisotropic) reference medium and contains terms of all order in the volume concentrations of solid particles and communicating cavities, take explicitly account of inclusion shape and spatial distribution independently. We show analytically that an expansion of the T-matrix formulae to first order in the volume concentration of cavities (in agreement with the dilute estimate of Eshelby) has the correct dependence on the properties of the saturating fluid, in the sense that it is consistent with the Brown–Korringa relation, when the frequency is sufficiently low. We present numerical results for the (anisotropic) effective viscoelastic properties of a cracked permeable medium with finite storage porosity, indicating that the complete T-matrix formulae (including the higher-order terms) are generally consistent with the Brown–Korringa relation, at least if we assume the spatial distribution of cavities to be the same for all cavity pairs. We have found an efficient way to treat statistical correlations in the shapes and orientations of the communicating cavities, and also obtained a reasonable match between theoretical predictions (based on a dual porosity model for quartz–clay mixtures, involving relatively flat clay-related pores and more rounded quartz-related pores) and laboratory results for the ultrasonic velocity and attenuation spectra of a suite of typical reservoir rocks.     

Three-Dimensional Geopositioning Accuracy of Ikonos Imagery

An investigation of the accuracy potential of Ikonos 1m satellite imagery is reported. Three sensor orientation/triangulation models are applied to stereo- and three-image configurations of "Geo" imagery with the aim of achieving 3D geopositioning to sub-metre accuracy. The models considered comprise rational functions with bias compensation, affine projection and the direct linear transformation. Test results from the Melbourne Ikonos Testfield are reported and these show that with modest provision of good quality ground control, Ikonos "Geo" imagery can yield 3D object-point determination to an accuracy of 0.5m in planimetry and 0.7m in height. The accuracy achieved is not only consistent with expectations for rigorous sensor orientation models, but is also readily attainable in practice with only a small number of ground control points being required     

Modelling the seismic attributes of overpressured siliciclastic rocks, with a genetic annealing (GAN) algorithm

ABSTRACT A genetic annealing (GAN) algorithm is used to derive an empirical model which predicts compressional-wave velocity values for overpressured siliciclastic rocks. The algorithm involves non-linear random searching and mutation techniques and its annealing component imposes a very strict control over the rate of convergence of the search. This technique provides an alternative to the standard calculations involving the effective stress coefficient ( n ). The pore pressure is introduced into the model as an explicit variable and as part of an overpressure coefficient, ( P _p/ P _c) − the ratio of pore to confining pressure. Empirical model-derived data and known laboratory data are compared and their differences are shown to be within statistically acceptable error limits. The empirical equation fits all under- and overpressured data simultaneously, irrespective of pore fluid pressure level, with the same parameters. It is used to predict seismic velocities very accurately for extreme levels of overpressure, starting from normally pressured experimental data. The model highlights the effect of pore pressure on the compressional-wave velocity of fully saturated samples with different clay contents. It can be used when the experimental data available are sparse and particularly when a prediction of material behaviour is necessary at specific pore fluid pressure and depth conditions.     

The influence of sampling design on spatial data quality in a geographic citizen science project

Geographic citizen science has much potential to assist in wildlife research and conservation, but the quality of observation data is a key concern. We examined the effects of sampling design on the quality of spatial data collected for a koala citizen science project in Australia. Data were collected from three samples—volunteers (n = 454), an Internet panel (n = 103), and landowners (n = 35)—to assess spatial data quality, a dimension of citizen science projects rarely considered. The locational accuracy of koala observations among the samples was similar when benchmarked against authoritative data (i.e., an expert‐derived koala distribution model), but there were differences in the quantity of data generated. Fewer koala location data were generated per participant by the Internet panel sample than the volunteer or landowner samples. Spatial preferences for land uses affecting koala conservation were also mapped, with landowners more likely to map locations for residential and tourism development and volunteers less likely. These spatial preferences have the potential to influence the social acceptability of future koala conservation proposals. With careful sampling design, both citizen observations and land use preferences can be included within the same project to augment scientific assessments and identify conservation opportunities and constraints.