Intermediate-scale Heterogeneity Research Articles

Local PCA Shows How the Effect of Population Structure Differs Along the Genome.

Population structure leads to systematic patterns in measures of mean relatedness between individuals in large genomic data sets, which are often discovered and visualized using dimension reduction techniques such as principal component analysis (PCA). Mean relatedness is an average of the relationships across locus-specific genealogical trees, which can be strongly affected on intermediate genomic scales by linked selection and other factors. We show how to use local PCA to describe this intermediate-scale heterogeneity in patterns of relatedness, and apply the method to genomic data from three species, finding in each that the effect of population structure can vary substantially across only a few megabases. In a global human data set, localized heterogeneity is likely explained by polymorphic chromosomal inversions. In a range-wide data set of Medicago truncatula, factors that produce heterogeneity are shared between chromosomes, correlate with local gene density, and may be caused by linked selection, such as background selection or local adaptation. In a data set of primarily African Drosophila melanogaster, large-scale heterogeneity across each chromosome arm is explained by known chromosomal inversions thought to be under recent selection and, after removing samples carrying inversions, remaining heterogeneity is correlated with recombination rate and gene density, again suggesting a role for linked selection. The visualization method provides a flexible new way to discover biological drivers of genetic variation, and its application to data highlights the strong effects that linked selection and chromosomal inversions can have on observed patterns of genetic variation.

Facies- to sandbody-scale heterogeneity in a tight-gas fluvial reservoir analog: Blackhawk Formation, Wasatch Plateau, Utah, USA

Using photomosaics and measured sections, this outcrop study characterizes facies- to sandbody-scale heterogeneity in the fluvial and coastal-plain deposits of the Blackhawk Formation of the Wasatch Plateau, Utah, USA, as an outcrop analog for the fluvial tight-gas reservoirs of the adjacent greater western Rocky Mountain basins as well as for conventional fluvial reservoirs elsewhere. Analysis on eight contiguous, vertical cliff-faces comprising both depositional-dip- and -strike-oriented segments provides field-validation and calibration of the entire range of fluvial heterogeneity, where: 1) large-scale heterogeneity (10's of m vertically and 100's of m laterally) is associated with stacking of channelized fluvial sandbodies encased within coastal-plain fines, 2) intermediate-scale heterogeneity (1's of m vertically and 10's of m laterally) is related to type and distribution of architectural elements like bar-accretion and crevasse-splay units within individual sandbodies, and 3) small-scale heterogeneity (10's of cm vertically and 1's of m laterally) is attributed to facies spatial variability within individual architectural elements.At a reservoir-scale (∼6 km strike-transect), impact of these heterogeneities has resulted in potential stratigraphic compartmentalization in varied patterns and scales within and among three zones, which have similar lateral extents. Distinct vertical or lateral compartmentalization, contrasting net-to-gross pattern, width-constraint by either large- or intermediate-scale heterogeneity, disparity in communication between principal reservoir compartments by intermediate-scale heterogeneity, and reservoir-quality segregation to barrier styles rendered by small-scale heterogeneity are documented in an array of trends. These intriguing trends are challenging to correlate across the reservoir-scale dataset, contributing to multiple, analogous exploration and production uncertainties. For improved tight-gas exploration and production strategy of the western Rocky Mountain basins, study results were also used in developing potential predictive tools: 1) thickness threshold of individual channelized sandbody favoring multiple well intersection, 2) aspect ratio in performing probabilistic sandbody-width estimation, and 3) prediction of sandbody amalgamation using underlying coal thickness.

Analysis and modeling of intermediate-scale reservoir heterogeneity based on a fluvial point-bar outcrop analog, Williams Fork Formation, Piceance Basin, Colorado

This study presents results of outcrop characterization and modeling of lithologic heterogeneity within a well-exposed point bar of the Williams Fork Formation in Coal Canyon, Piceance Basin, Colorado. This deposit represents an intermediate-scale depositional element that developed from a single meandering channel within a low net-to-gross ratio fluvial system. Williams Fork outcrops are analogs to petroleum reservoirs in the Piceance Basin and elsewhere. Analysis and modeling of the point bar involved outcrop measurements and ground-based high-resolution light detection and ranging data; thus, the stratigraphic frameworks accurately represent the channel-fill architecture. Two- and three-dimensional (2-D and 3-D) outcrop models and streamline simulations compare scenarios based on different lithologies, shale drapes, observed grain-size trends, petrophysical properties, and modeling methods. For 2-D models, continuous and discontinuous shale drapes on lateral-accretion surfaces result in a 79% increase and 24% decrease in breakthrough time (BTT), respectively, compared to models without shale drapes. The discontinuous shale drapes in the 2-D and 3-D models cause a 30% and 107% decrease, respectively, in sweep efficiency because they focus fluid flow downward to the base of the point bar. For similar reasons, 2-D models based on grain size exhibit 67–267% shorter BTT and 44–57% lower sweep efficiency compared to other model scenarios. Unlike the 2-D models, the continuous shale drapes in the 3-D models cause the fluid front to spread out and contact more of the reservoir, resulting in 42–53% longer BTT and 41–52% higher sweep efficiency compared to the other models. These results provide additional insight into the significance of intermediate-scale heterogeneity of fluvial reservoirs.

Generation of Aquifer Heterogeneity Maps Using Two-Dimensional Spectral Texture Segmentation Techniques

Numerical models that solve the governing equations for subsurface fluid flow and transport require detailed quantitative maps of spatially variable hydraulic properties. Recently, there has been great interest in methods that can map the spatial variability of hydraulic properties such as porosity and hydraulic conductivity (permeability). Presently, only limited data on natural permeability spatial structure are available. These data are often based on extensive discrete sampling in outcrops or boreholes. Then methods are used to interpolate between data values to map aquifer heterogeneity. Interpolation methods often mask critical local or intermediate scale heterogeneities. As sediment texture is directly correlated with many hydraulic properties we developed two new texture segmentation algorithms based on a space-local two-dimensional wavenumber spectral method known as the S-Transform. Existing texture segmentation algorithms could not delineate the subtle and continuous texture variations that exist in natural sediments. The S-Transform algorithms successfully delineated geologic structures and grain size patterns in photographs of outcrops in a glacial fluvial deposit; thus, no interpolation methods were required to produce continuous two-dimensional maps of texture facies. The S-Transform method is robust and is insensitive to changes in light intensity, and moisture variations. This makes the algorithm particularly applicable to natural sedimentary outcrops. The effectiveness of our methods are tested by correlating measured relative grain sizes in the images with actual grain size measurements taken from the sedimentary outcrops.

Activity and mobility of Corophium volutator: A field study

The distribution of the intertidal amphipod Corophium volutator is heterogeneous at scales of cm to km. Whilst habitat preferences, interspecific and intraspecific interactions are factors affecting patterns of heterogeneity, these are moderated by mobility. A field investigation of small‐scale (cm to m) mobility was carried out on the Ythan estuary, Aberdeenshire, during immersion and after the ebb of the tide. Observations on immigration to, emigration from and activity within small areas of sediment indicated that mobility is low, but animals that do move are adult males which crawl over the sediment rather than swim. These individuals moved only short distances and they frequently re‐used existing burrows. This movement is probably related to reproductive behaviour. The low mobility and the short distances moved indicated that mobility has little impact on at least large and intermediate scale heterogeneity, but shows that small‐scale patterns are constantly dynamic in time and space.

On the presence of intermediate-scale heterogeneity in the upper mantle

SUMMARY Power spectra of tomographic models of the entire earth and of models of Europe and the Mediterranean area are compared. This comparison reveals a discrepancy (a factor 15–30) between the power of both kinds of models at intermediate spherical-harmonic degrees (around l = 30). Two possible explanations for this discrepancy are given: first, the fact that Europe is more complex than the global average; and second, the lack of resolution in the inversions. Another approach to the problem of the power spectrum of heterogeneities of intermediate length scale is the study of multipathing of long-period surface waves. The multipathing observed in the recordings of these waves, which manifests itself as an increasing complexity of their amplitude spectra due to increasing interference of spectral components with increasing epicentral distance, cannot be explained by present-day global earth models. The gradients in the models are simply not strong enough. However, random models with more power at intermediate scales, and consequently stronger gradients, are able to explain the observed multipathing. It is concluded that the global models underestimate the power in the heterogeneities of intermediate-scale structure in the earth. Finally, an explanation is given why current global models, although they do not contain sufficient power at shorter scales, do explain the traveltime and dispersion data used for the construction of the models.

Finite difference modeling of geoacoustic interaction at anelastic seafloors

A major problem in understanding sound propagation in the seafloor is to distinguish between the loss of energy due to intrinsic attenuation (anelasticity) and the loss of energy due to scattering from intermediate scale heterogeneities and bottom roughness. Energy lost to intrinsic attenuation (heat) disappears entirely from the system. Energy lost to scattering is conserved in the system and can effect observations as incoherent noise (time spread, angle spread) and/or mode converted waves. Dougherty and Stephen (1988, 1991) showed that the finite difference synthetic seismogram method can be applied to the seafloor scattering problem. Recently, this two-dimensional finite difference code has been extended to include intrinsic attenuation using the formulation of Day and Minster (1984). The formulation assumes a stress–strain relation for which Q is independent of frequency over a specified bandwidth. For each node on a two-dimensional grid arbitrary values of compressional and shear Q can be specified in addition to the usual values of compressional and shear velocity and density. Fluids are represented by setting the shear modulus to zero. Q’s computed from the output time series are in good agreement with the input values for homogeneous media. Now by forward modeling the trade-off between scattering and intrinsic attenuation in seafloor models with both surface roughness and volume heterogeneities can be studied. Results can be compared with field observations from vertical seismic profiles (VSP’s) in the seafloor and marine refractions experiments.

The Dispersive Effect of Variable Layer Lengths on Miscible Displacements in Layered Heterogeneous Porous Media

Summary The effect of small- and intermediate-scale heterogeneities on miscible displacements is investigated by Monte Carlo simulations in a two-dimensional (2D) flow field containing random permeabilities and random layering. The effect of the variable layer lengths on the level of dispersion is calculated for various well-to-well distances. It was determined that the variable layer lengths add a significant amount of dispersion compared with constant layer lengths for well-to-well distances on the order of 1,000 ft [305 m] or less.