Grain-size Distribution Data Research Articles

New constraints on the Middle-Late Pleistocene Campi Flegrei explosive activity and Mediterranean tephrostratigraphy (∼160 ka and 110–90 ka)

The Campi Flegrei (CF) caldera, in southern Italy, is the source of some of the most powerful Late Pleistocene eruptions of the European sub-continent (e.g., Campanian Ignimbrite, Neapolitan Yellow Tuff eruptions). Although the CF caldera has been continuously and intensively investigated for decades, relatively little is known regarding its earliest volcanic activity. In this work, integrating existing and new tephrostratigraphic data, we provide a comprehensive and updated framework for the CF volcanic activity which has occurred at ∼160 ka and between ∼110 ka and ∼90 ka. The new tephrostratigraphic, geochemical (EMPA + LA-ICP-MS), chronological (40Ar/39Ar dating) and grain-size distribution data relate to CF tephra deposits preserved in mid-proximal (Campanian Plain), distal (Tyrrhenian Sea) and ultra-distal (Lower Danube area) sedimentary archives. Our results allowed us to recognize the presence of at least 13 CF eruptions covering the investigated time frame, with 12 eruptions occurring between 110 and 90 ka. Our high-resolution stratigraphic and chronological investigation also allowed us to recognize that the Triflisco/C-22 tephra, previously considered as a single marker layer, can be actually separated into three different events, sourced from within the CF area in the short time interval of ∼93-90 ka, suggesting a more complex and intense volcanic history than previously thought. Moreover, a Bayesian age-depth model, constrained by previous and new high precision 40Ar/39Ar ages, has led to a reliable estimate of the ages of those undated CF eruptions. Overall, the updated framework on the stratigraphy, chronology, dispersion, and geochemistry of the CF tephra of ∼160 ka and between 110 ka and 90 ka consolidates the notion that the Middle-Late Pleistocene activity in theCF area represents a significant stage of its volcanic evolution, characterised by intense and frequent explosive eruptions.

Source apportionment of suspended sediment using grain-size end-member analysis

End-members mixing analysis (EMMA) was performed on in situ suspended sediment grain-size distribution (GSD) data from Haizhou Bay to estimate suspended sediment provenance and the relative contributions of different sediment sources to total seawater suspended sediment. The results show that the suspended sediment in Haizhou Bay can be expressed as a mixture of three EMs. End-member 1 represents fine-grained suspended sediment that was transported over long distances by advection; this shows a genetic connection with the old Huanghe River delta. End-member 2 represents resuspension of local sediment, while EM3 represents riverine suspended sediment. On average, advection, resuspension, and riverine input contributed 41.92%, 37.83%, and 20.25% of the total suspended sediment, respectively. Temporal changes in advection and resuspension caused considerable variability in these relative contributions. The application of EMMA herein demonstrates that it is a feasible approach to explore suspended sediment sources and their mixing processes using suspended sediment GSD data.

On the genetically meaningful decomposition of grain-size distributions: A comparison of different end-member modelling algorithms

End-member modelling algorithms extract valuable information from grain-size distribution datasets, such as provenance, transport and sedimentation processes, which can subsequently be used to reconstruct past climate variability. Twenty years ago, Weltje (1997) published the algorithm EMMA, first applied to unmixing of grain-size distribution datasets in Prins and Weltje (1999a). In recent years, a range of new non-parametric algorithms developed specifically for unmixing of these datasets has become available. We evaluate whether the new algorithms as well as the original EMMA produce genetically meaningful results. We use artificial datasets and a published dataset from a geological case study. Experiments with the artificial datasets demonstrate that some algorithms are more appropriate for the unmixing of grain-size distribution data than others. Furthermore, the “geologically feasible” number of end members determined by the algorithms depends on the criteria for a statistical goodness of fit and does not necessarily correspond to the true number of end members in a dataset. Our study indicates that when an incorrect number of end members are modelled the shape and modal position of the end member grain-size distributions may deviate. Thus, end-member modelling should be used cautiously and in tandem with geological background information.

Mid-late Holocene climate variability in the Indian monsoon: Evidence from continental shelf sediments adjacent to Rushikulya river, eastern India

We present elemental and grain-size distributions obtained from the sediment core of the continental shelf adjacent to the Rushikulya river mouth, eastern India to quantify the paleoclimatic changes. The retrieved 1.60 m long well dated core spans the past ca. 6800 cal BP. The modern spatial distribution of grain size and geochemistry of the inner-mid shelf sediments has been carried out to understand the seafloor morphology and sedimentary processes. Based on the modern investigations, the proportion of particle size (clay vs sand) and variation in elemental values (TiO2 vs Al2O3) has been used to interpret the changes in terrigenous supply. The grain-size and elemental distribution data from the core sediments indicates a period of enhanced surface water runoff from 6800 to 3100 cal BP followed by a drier condition (3100 cal BP to present) suggesting weakening of monsoon. The weakening of the monsoonal strength is coeval with other records from the Indian sub-continent and suggests response of Indian monsoon to changing solar insolation during late Holocene.

Long-term coastal openness variation and its impact on sediment grain-size distribution: a case study from the Baltic Sea

Abstract. We analysed the long-term variations in grain-size distribution in sediments from Gåsfjärden, a fjord-like inlet in the southwestern Baltic Sea, and explored potential drivers of the recorded changes in the sediment grain-size data. Over the last 5.4 thousand years (ky) in the study region, the relative sea level decreased 17 m, which was caused by isostatic land uplift. As a consequence, Gåsfjärden was transformed from an open coastal setting to a semi-closed inlet surrounded by numerous small islands on the seaward side. To quantitatively estimate the morphological changes in Gåsfjärden over the investigated time period and to further link the changes to the grain-size distribution data, a digital elevation model (DEM)-based openness index was calculated. The largest values of the openness indices were found between 5.4 and 4.4 cal ka BP, which indicates relatively high bottom water energy. During the same period, the highest sand content (∼ 0.4 %) and silt / clay ratio ( ∼ 0.3) in the sediment sequence were also recorded. After 4.4 cal ka BP, the average sand content was halved to ∼ 0.2 % and the silt / clay ratio showed a significant decreasing trend over the last 4 ky. These changes were found to be associated with the gradual embayment of Gåsfjärden, as represented by the openness indices. The silt / clay ratios exhibited a delayed and relatively slower change compared with the sand content, which indicates different grain-size sediment responses to the changes in hydrodynamic energy. Our DEM-based coastal openness indices have proved to be a useful tool for interpreting the temporal dynamics of sedimentary grain size.

Holocene climatic events recorded in palaeoflood slackwater deposits along the middle Yiluohe River valley, middle Yellow River basin, China

Palaeohydrological investigations were carried out in the middle reaches of the Yiluohe River, a major tributary in the lower–middle Yellow River basin. Typical palaeoflood slackwater deposits (SWDs) were identified in the Holocene pedostratigraphy on the cliffy river banks. Analytical results, including magnetic susceptibility and grain-size distribution data, indicated that these SWDs were deposited from the suspended sediment load in flood water. These SWDs are different from eolian loess, soils and aeolian sands in the riverbank profile. They recorded several episodes of extraordinary palaeoflood events. In the Longmenxia reaches of the Yihe River valley, these flood events were dated at 3100–3000a, 1800–1700a, 770–610a, and 420–340a using the optically stimulated luminescence method in combination with the pedostratigraphic correlations. In the Longhutan reaches of the Luohe River valley, the palaeoflood events were dated at 1975–1466a, i.e., from the Han to Wei dynasties (AD 25–534), during which the capital city on the river banks was flooded many times, as recorded in the literature. These extraordinary flood events are well correlated chronologically with the known Holocene climatic events that occurred in the Northern Hemisphere. Thus, the monsoonal climate was highly variable with both floods and droughts occurring frequently during these episodes. These results are important for understanding the response of river systems in eastern Asia to global changes.

The dynamics of the Late Pleistocene loess formation, Lozhok section, Ob loess Plateau, SW Siberia

There are unique loess-paleosol sections that fully reflect paleoclimate changes during the Late Pleistocene in southern West Siberia. This record of climate shows a good correlation with the marine oxygen isotope curve and other global climate records, and consequently the loess-paleosol sequence of West Siberia is a detailed reference scale. Lozhok is a key section for the Late Pleistocene loess-paleosol sequence of the Novosibirsk Ob' Region. Average loess accumulation rates of each studied loess layer were calculated on the basis of previous 14C and TL dating results and on correlation of the loess-paleosol sequence of southern West Siberia with MIS and the Middle Siberia loess-paleosol sequence. The grain-size distribution data indicates intensification of environmental dynamics until the Last Glacial. Bulk chemical composition reveals the intensification of aridization from the Tulino to the Bagan loess, and that this trend was not constant. Greater accumulation loess activity during Bagan time (MIS-2) is also confirmed by loess accumulation rate calculations. Results of sand quartz grain morphoscopy show that loess layers were formed by aeolian processes in periods of deep cooling and aridization. The Late Pleistocene loess layers were formed in periglacial environments, in arid cold deserts.

Controls on the basin-scale distribution of hydraulic conductivity of superficial deposits: a case study from the Thames Basin, UK

Controls on the basin-scale distribution of hydraulic conductivity of superficial deposits are assessed in the context of hydrological setting and basin evolution and are investigated using a case study from the Thames Basin, UK. A conceptual model of superficial deposits across the Thames Basin is used to define six lithostratigraphic classes of superficial deposits: pre-Anglian Clay-with-Flint deposits; pre-Anglian River Terrace Deposits associated with the ancestral River Thames and its tributaries; Tills formed during the Anglian glaciations; glacio-fluvial sand and gravel deposits formed during the Anglian; post-Anglian River Terrace Deposits associated with the modern-day River Thames and tributaries; post-Anglian alluvium associated with the modern-day River Thames and tributaries. Hydraulic conductivity of the superficial deposits has been estimated from grain-size distribution data, originally collected for mineral resource assessments, using the Kozeny–Carman method. Based on 6411 samples from 1416 boreholes, estimated hydraulic conductivity ranges from 0.2 to 5942 m day –1 , median and mean hydraulic conductivities are 1.67 and 26.72 m day –1 respectively, and the overall distribution of hydraulic conductivity values has a strong positive skew An apparent reduction in mean hydraulic conductivity with increasing age of the deposit is observed, particularly for the River Terrace Deposits. A reduction in maximum hydraulic conductivity at depths >10 m is also observed and the relationship between hydraulic conductivity and depth is controlled by the type of superficial deposit. At the catchment to basin scale, variation in hydraulic conductivity with depth may be explained with reference to both the deposit types and the age of the deposits. Where hydraulic conductivity is found to be intimately linked to the Quaternary evolution of the basin, through contrasts in age and deposit type, permeability variations at the basin scale may be constrained by applying a suitably refined conceptual model of the superficial deposits.

Estimation of Hydraulic Conductivity and Its Uncertainty from Grain-Size Data Using GLUE and Artificial Neural Networks

Various approaches exist to relate saturated hydraulic conductivity (K s) to grain-size data. Most methods use a single grain-size parameter and hence omit the information encompassed by the entire grain-size distribution. This study compares two data-driven modelling methods—multiple linear regression and artificial neural networks—that use the entire grain-size distribution data as input for K s prediction. Besides the predictive capacity of the methods, the uncertainty associated with the model predictions is also evaluated, since such information is important for stochastic groundwater flow and contaminant transport modelling. Artificial neural networks (ANNs) are combined with a generalised likelihood uncertainty estimation (GLUE) approach to predict K s from grain-size data. The resulting GLUE-ANN hydraulic conductivity predictions and associated uncertainty estimates are compared with those obtained from the multiple linear regression models by a leave-one-out cross-validation. The GLUE-ANN ensemble prediction proved to be slightly better than multiple linear regression. The prediction uncertainty, however, was reduced by half an order of magnitude on average, and decreased at most by an order of magnitude. This demonstrates that the proposed method outperforms classical data-driven modelling techniques. Moreover, a comparison with methods from the literature demonstrates the importance of site-specific calibration. The data set used for this purpose originates mainly from unconsolidated sandy sediments of the Neogene aquifer, northern Belgium. The proposed predictive models are developed for 173 grain-size K s-pairs. Finally, an application with the optimised models is presented for a borehole lacking K s data.

Feasibility of grain-size analysis methods for determination of vertical hydraulic conductivity of streambeds

Summary Accurate estimation of streambed vertical hydraulic conductivity ( K v ) is of great importance in the analysis of water quantity exchange and solute transfer between a stream and its sediments. The paper analyzed the inaccuracy of hydraulic conductivity values of sediments derived from grain-size distribution ( K g ), which were determined from eight empirical grain-size methods to represent streambed K v . In this study, the values of K v for a streambed were derived using falling-head standpipe permeameter tests conducted at eight study sites in the Elkhorn River, Nebraska, and the tested streambed columns were then collected for grain-size analysis by sieving. These empirical methods were used to calculate the K g values of the streambed from grain-size distribution data of sediments. Unlike many other studies, this study verifies K g from grain-size distribution with K v from permeameter tests on the basis of the same samples of streambed sediments. The K g values derived from the eight empirical methods were larger than the K v from permeameter tests; there are five methods that give K g values of about 3–6 times larger than these K v . The K g values from the Kozeny formula followed by the Hazen formula give the largest overestimation error if they are used to represent the K v of the streambed. The USBR and Shepherd formulas generated K g values close to K v , but these K g values are still larger in general than the K v values. Moreover, the new values of coefficient C for the empirical formulas were revised so that they can be used to calculate the approximate K v of a streambed. Among the eight methods, the ratios of the original C values to the average new C range from 1.3 to 5.9. It can be hypothesized that smaller C values must be used in the estimation of K v for general soil samples if these empirical formulas are used to calculate K v .

Windows-based software for optimising entropy-based groupings of textural data

EntropyMax is a new 32-bit Windows-based software that groups large matrices of grain-size distribution data into a finite number of groups. The software utilises statistical algorithms that minimise the entropy within a group while maximising the entropy between groups. EntropyMax builds upon an existing DOS–BASIC program through the addition of a more robust computational routine, a graphical user interface, incorporation of an improved and more statistically meaningful method for identifying the optimal number of groupings, and better graphical presentation of results. These refinements add significantly to the functionality and statistical rigour of the program on modern computer platforms and large data sets.

Grain-size analysis of volcanic ash for the rapid assessment of respiratory health hazard

Volcanic ash has the potential to cause acute and chronic respiratory diseases if the particles are sufficiently fine to enter the respiratory system. Characterization of the grain-size distribution (GSD) of volcanic ash is, therefore, a critical first step in assessing its health hazard. Quantification of health-relevant size fractions is challenging without state-of-the-art technology, such as the laser diffractometer. Here, several methods for GSD characterization for health assessment are considered, the potential for low-cost measurements is investigated and the first database of health-pertinent GSD data is presented for a suite of ash samples from around the world. Methodologies for accurate measurement of the GSD of volcanic ash by laser diffraction are presented by experimental analysis of optimal refractive indices for different magmatic compositions. Techniques for representative sampling of small quantities of ash are also experimentally investigated. GSD results for health-pertinent fractions for a suite of 63 ash samples show that the fraction of respirable (<4 microm) material ranges from 0-17 vol%, with the variation reflecting factors such as the style of the eruption and the distance from the source. A strong correlation between the amount of <4 and <10 microm material is observed for all ash types. This relationship is stable at all distances from the volcano and with all eruption styles and can be applied to volcanic plume and ash fallout models. A weaker relationship between the <4 and <63 microm fractions provides a novel means of estimating the quantity of respirable material from data obtained by sieving.

Isotopic insights into smoothening of abandoned fan surfaces, Southern California

Cosmogenic nuclide concentrations measured on abandoned fan surfaces along the Mojave section of the San Andreas Fault suggest that sediment is generated, transported, and removed from the fans on the order of 30–40 kyr. We measured in situ produced cosmogenic 10Be, and in some cases 26Al, in boulders ( n = 15), surface sediment ( n = 15), and one depth profile ( n = 9). Nuclide concentrations in surface sediments and boulders underestimate fan ages, suggesting that 10Be accumulation is largely controlled by the geomorphic processes that operate on the surfaces of the fans and not by their ages. Field observations, grain-size distribution, and cosmogenic nuclide data suggest that over time, boulders weather into grus and the bar sediments diffuse into the adjacent swales. As fans grow older the relief between bars and swales decreases, the sediment transport rate from bars to swales decreases, and the surface processes that erode the fan become uniform over the entire fan surface. The nuclide data therefore suggest that, over time, the difference in 10Be concentration between bars and swales increases to a maximum until the topographic relief between bars and swales is minimized, resulting in a common surface lowering rate and common 10Be concentrations across the fan. During this phase, the entire fan is lowered homogeneously at a rate of 10–15 mm kyr −1.

Aeolian process and pedogenesis under the influence of the East Asian monsoon: A statistical approach to particle-size distribution variability

The grain size of Chinese loess has been widely regarded as a sensitive proxy for the intensity of the wind strength associated with the winter component of the East Asian monsoon. The majority of papers concerned with loess particle size examine variations in moments of the particle size distribution. However, in coastal geomorphological research parametric models have been used and the samples represented by resulting parameter estimates. Here we investigate the use of log-hyperbolic and log-skew-laplace models to distinguish between loess, loess subgroups and palaeosols in a particularly high accumulation rate profile from the NW Chinese loess plateau area. The aim is to define narrow units of ‘pristine’ loess that may indicate brief returns to dust storm conditions. With this approach, units within the loess, previously ascribed to Heinrich events, can be seen to have a particular particle size distribution signature, thus allowing them to be stratigraphically and environmentally constrained. Parametric models applied to grain-size distribution data in Chinese loess make a useful addition to previously used particle size techniques that have demonstrated cyclicity on a scale similar to that of Dansgaard–Oeschger events in ice cores and those designed to determine the degree of loess pedogenesis such as micromorphology and magnetic enhancement. The aim of our work is to make full use of particle size distribution data in the wider context of palaeoenvironmental reconstruction.

Significance of porosity for stratigraphy and textural composition in subsurface, coarse fluvial deposits: Boise Hydrogeophysical Research Site

Recognition and quantitative characterization of subsurface stratigraphic units in coarse unconsolidated fluvial deposits are difficult because large grain size and the large scale of sedimentary structures make direct interpretation from core difficult or impossible. In this paper, we use porosity data from well logs and grain-size distribution (GSD) data from core to investigate four pebble- and cobble-dominated units that have been identified in porosity logs in deposits at a research well field (Boise Hydrogeophysical Research Site). Lacking direct observation at an appropriate scale, questions about distribution of parameters and textural composition in these units are analyzed with statistical tests. The four pebble- and cobble-dominated “porosity stratigraphic” units may be grouped into two types: (1) Units 1 and 3 have low porosity (mean ∼0.17–0.18) and low porosity variance; and (2) Units 2 and 4 have higher porosity (mean ∼ 0.23–0.24) and higher porosity variance. Based on GSD data, core samples are subdivided into five lithotypes. The five lithotypes occur in different proportions and have different vertical transition probability characteristics in the two types of units: (1) Units 1 and 3 have only framework-gravel–dominated lithotypes and have random vertical transition probability between these two lithotypes; and (2) Units 2 and 4 consist of both framework-gravel–dominated and sand- or matrix-dominated lithotypes and have structured vertical transition probability. The two framework-gravel–dominated lithotypes occur in all four stratigraphic units but have distinctly lower porosity in Units 1 and 3 (i.e., tighter packing) than in Units 2 and 4 (looser packing). Considering the repeated stratigraphic occurrence of (and the statistical significance of differences between) the two types of units, both the individual unit distinctions and the two unit groupings appear to be valid. It is reasonable to interpret that the observed packing differences associated with Units 1 and 3 compared with Units 2 and 4 are related to different sedimentary processes that produce different bedforms or grain fabrics, perhaps under different bedload transport rates.

An equation to represent grain-size distribution: Discussion

Hansen 1436 Statistically summarizing and prese ting grain-size distribution (GSD) data would appear to be a simple, even mundane task, but actually presents subtle nontrivial challenges. The authors are to be commended for bringing this to the attention of the geotechnical community. The goal of summarizing GSD information in a soil characteristics database will also be a useful outcome. Perhaps because of the hidden difficulties in concisely characterizing such data, the geotechnical community invented their own way of presenting GSDs long ago. To some extent I feel that this paper represents a continued divergence from available approaches. When deciding how to attach a given statistical model (distribution) to a given set of data, statisticians appear to use one of the following two approaches: (1) They compare theoretical probability density functions (PDFs) to histograms describing the observed data (this is the basis for the well-known χ2 test), or (2) They compare cumulative distribution functions (CDFs) to observed percentiles, or to points found using so-called plotting position formulae. In either case, paper with a probability scale is usually used. This scale is most often the probability of nonexceedance of the standard normal deviate. This approach is common in hydrology. However, practicing geotechnical engineers have historically taken neither approach when attempting to present GSDs, and few practitioners would take the time to obtain a statistical model of a given dataset. In addition, rather than using statistical measures that are almost universally understood, such as the standard deviation, the coefficient of variation, and the coefficient of skewness, various uniformity coefficients are used in geotechnical engineering. In my opinion this has tended to make the entire “GSD process” more arcane. Both geotechnical engineers and statisticians do, however, frequently use percentiles, especially the median (“D50” to geotechnical engineers). The median particle diameter almost never has the same value as the mean particle diameter for soils because soil PDFs tend to be skewed. An estimate of the mean particle size from a sieve analysis is (compare eq. [3] in Wagner and Ding 1994)

An equation to represent grain-size distribution: Reply

The authors wish to thank the discusser for putting together his case for the possibility of using a log-normal type representation to characterize particle size distributions. Log-normal type plots have found a degree of acceptability in the area of hydrology; however, similar attempts to use log-normal distributions have not been met with the same enthusiasm in the soil mechanics discipline. Geotechnical engineers do not view particle size distribution as statistical or probability-based information related to the number of particles associated with each particle size range. In the original paper, the authors point out that “Gardner (1956) proposed a two-parameter, log-normal distribution to provide representation of grain-size distribution data.” The idea of using log-normal distributions to characterize grainsize distribution curves has been around for a long time. It does not appear to have gained significant acceptability in geotechnical engineering nor engendered further research interest. It was against this background that the present research study was formulated. It was decided to further investigate the use of an empirical, mathematically based equation where the fitting parameters had some physical meaning. The equations were focused on the generally accepted “percent passing” versus the logarithm of the particle size plots. There has been a recent proliferation of studies related to the estimation of the soil-water characteristic curve from a grain-size distribution curve in the soil science and related disciplines. This also encouraged further attempts to mathematically characterize the grain-size distribution curve. The use of a continuous mathematical equation that can be differentiated and integrated has appeal because it can be used for further analysis in soil mechanics. There appears to be a fundamental difficulty in viewing a grain-size distribution curve as a purely statistical phenomenon since the grain-size distribution is commonly plotted as a “percent passing” by mass (as opposed to a cumulative distribution based on the number of particles) versus the logarithm of the particle size. The “percent passing” by mass does not lend itself to a statistical evaluation in the same way as if the number of particles was being assessed (Ang and Tang 1975). The largest particles have the dominance with respect to mass but in general, have the least influence on soil behavior. For example, only 30% clay size particles render the behavior of the soil like that of a clay soil. Hazen (1930) suggested that the D10 size of particles best represented the particle size for the assessment of the coefficient of permeability of a soil. In other words, conventional statistical measures based strictly on frequency do not provide the geotechnical engineer with the most meaningful parameters required when attempting to foresee the mechanical and hydraulic behavior of a soil. As a result, D50 does not have special meaning over other “percent passing” values in soil mechanics. Part of the natural bias towards the coarse end of the particle size spectrum (i.e., because of the mass basis for presentation of the data) is overcome in soil mechanics by plotting particle size on a logarithm scale. The differentiation of the percent passing grain size curve gives a log-normal type distribution, but this is not often used by geotechnical engineers. The particle size scale is equally spaced on a logarithm scale, and while this presents some difficulties in interpretation, it still appears to be a meaningful way to visualize the particle sizes in the distribution. The discusser states that he feels “that this paper represents a continued divergence from available approaches.” The authors maintain that it might represent a divergence from approaches quite common to hydrology; however, the approach proposed is consistent with the historical and present interpretive procedures used in soil mechanics. The authors used a four-parameter equation in the case of a unimodal grain-size distribution curve and a sevenparameter equation in the case of a bimodal grain-size distribution curve. This may appear to be a large number of parameters, but the parameters come out of a regression analysis and allow a consistently accurate representation of the entire grain-size curve. When used in conjunction with a

Mode of Emplacement of the Ikeda Pyroclastic-flow Deposit Inferred from Grain-size Distribution and Component Data

Mode of Emplacement of the Ikeda Pyroclastic-flow Deposit Inferred from Grain-size Distribution and Component Data

Effects of Laboratory Consolidation on Petrophysical Properties of Fine-Grained Marine Sediments: Electron Microscopic Observations

Pliocene-Recent marine sediments, recovered at site 1125 by ODP Leg 181 on the eastern New Zealand margin, were subjected to laboratory consolidation tests and analyzed by scanning electron microscopy (SEM) before and after consolidation test. The primary objective of this study is to document the effects of consolidation on microfabric and petrophysical properties. X-ray diffraction and grain-size distribution data indicate that the samples are mineralogically and texturally similar and thus are ideal for the present study on the role of consolidation solely on petrophysical properties. Porosity was measured before and after each consolidation test, and permeability was estimated indirectly based on the theoretical method. SEM photomicrographs show details of changes of pore geometry and distribution after the consolidation test that account for the porosity loss.

Grain-Size Distribution of Granular Soils by Computer Vision

Abstract A semi-automated technique for obtaining the grain-size distribution (GSD) of granular soils using computer vision is presented. Backlighted digital images of a soil specimen dispersed over a glass specimen plate are acquired at three different magnifications. Images of the specimen are acquired by placing the specimen plate randomly beneath the field of view of a charged-coupled device (CCD) video camera. The size of particles with projected areas from 50 to 2000 pix2 is measured in each image. Multiple images are acquired at each magnification until the measured size distribution of particles counted at that magnification stabilizes. Probabilistic corrections are then used to obtain a statistically unbiased GSD from the image data obtained at all three magnifications. A comparison of GSD data for two uniform and two nonuniform soils using both computer vision and sieving is presented.