12 research outputs found
Deposits of the sandy braided South Saskatchewan River: Implications for the use of modern analogs in reconstructing channel dimensions in reservoir characterization
Estimation of the dimensions of fluvial geobodies from core data is a
notoriously difficult problem in reservoir modeling. To try and improve
such estimates and, hence, reduce uncertainty in geomodels, data on
dunes, unit bars, cross-bar channels, and compound bars and their
associated deposits are presented herein from the sand-bed braided South
Saskatchewan River, Canada. These data are used to test models that
relate the scale of the formative bed forms to the dimensions of the
preserved deposits and, therefore, provide an insight as to how such
deposits may be preserved over geologic time. The preservation of
bed-form geometry is quantified by comparing the Alluvial architecture
above and below the maximum erosion depth of the modem channel deposits.
This comparison shows that there is no significant difference in the
mean set thickness of dune cross-strata above and below the basal
erosion surface of the contemporary channel, thus suggesting that
dimensional relationships between dune deposits and the formative
bed-form dimensions are likely to be valid from both recent and older
deposits.
The data show that estimates of mean bankfull flow depth derived from
dune, unit bar, and cross-bar channel deposits are all very similar.
Thus, the use of all these metrics together can provide a useful check
that all components and scales of the alluvial architecture have been
identified correctly when building reservoir models. The data also
highlight several practical issues with identifying and applying data
relating to cross-strata. For example, the deposits of unit bars were
found to be severely truncated in length and width, with only
approximately 10% of the mean bar-form length remaining, and thus
making identification in section difficult. For similar reasons, the
deposits of compound bars were found to be especially difficult to
recognize, and hence, estimates of channel depth based on this method
may be problematic. Where only core data are available (i.e., no outcrop
data exist), formative flow depths are suggested to be best
reconstructed using cross-strata formed by dunes. However, theoretical
relationships between the distribution of set thicknesses and formative
dune height are found to result in slight overestimates of the latter
and, hence, mean bankfull flow depths derived from these measurements.
This article illustrates that the preservation of fluvial cross-strata
and, thus, the paleohydraulic inferences that can be drawn from them,
are a function of the ratio of the size and migration rate of bed forms
and the time scale of aggradation and channel migration. These factors
must thus be considered when deciding on appropriate length:thickness
ratios for the purposes of object-based modeling in reservoir
characterization
Quantification of the relation between surface morphodynamics and subsurface sedimentological product in sandy braided rivers
This study uses digital elevation models and ground-penetrating radar to
quantify the relation between the surface morphodynamics and subsurface
sedimentology in the sandy braided South Saskatchewan River, Canada. A
unique aspect of the methodology is that both digital elevation model
and ground-penetrating radar data were collected from the same locations
in 2004, 2005, 2006 and 2007, thus enabling the surface morphodynamics
to be tied explicitly to the associated evolving depositional product.
The occurrence of a large flood in 2005 also allowed the influence of
discharge to be assessed with respect to the processproduct
relationship. The data demonstrate that the morphology of the study
reach evolved even during modest discharges, but more extensive erosion
was caused by the large flood. In addition, the study reach was
dominated by compound bars before the flood, but switched to being
dominated by unit bars during and after the flood. The extent to which
the subsurface deposits (the product') were modified by the surface
morphodynamics (the process') was quantified using the changes in
radar-facies recorded in sequential ground-penetrating radar surveys.
These surveys reveal that during the large flood there was an increase
in the proportion of facies associated with bar margin accretion and
larger dunes. In subsequent years, these facies became truncated and
replaced with facies associated with smaller dune sets. This analysis
shows that unit bars generally become truncated more laterally than
vertically and, thus, they lose the high-angle bar margin deposits and
smaller scale bar-top deposits. In general, the only fragments that
remain of the unit bars are dune sets, thus making identification of the
original unit barform problematic. This novel data set has implications
for what may ultimately become preserved in the rock record
A note on political science and the metaphorical imagination
This article discusses the role of metaphor in political science, highlighting the centrality of the metaphor historically for understanding human societies. It notes how the contemporary emphasis on global interconnectedness has generated an influx of new metaphors influenced by technological developments, such as networks and the Internet. The article highlights questions that are of enduring interest for political scientists: how to make sense of metaphor itself, and the place of metaphor in political analysis