bluuebunny/crossref_metadata_2025_split huggingface dataset:
Petrophysical estimation of permeability as a function of scale
Authors: Worthington Paul F. 
Published: September 2007

Abstract

The prediction of reservoir intergranular permeability from core and log measurements of other physical properties is demonstrably refined by taking a more scientific account of the scale of the predictors. Further, the expression of an empirical algorithm that relates permeability to a measured predictor can vary significantly from the core through the log to the well-test scales, with a reduction in data scatter as scale increases. In the most common case of porosity-permeability relationships, the coefficients and exponents that define the petrophysical algorithms are themselves functions of scale. The axial scale dependence of these predictive algorithms is quantified through filtering of regularly sampled core data for the specific case of primary flow and for four different distributions of characterized strata. The algorithms initially diverge as the scale of the processed data increases beyond the spatial resolution of the input core measurements, and they reconverge as the running means become more generally representative. The alternative method of predicting intergranular permeability from measurements of apparent formation factor also shows a scale dependence, which here is quantified through porosity-governed averaging of irregularly sampled core data. The algorithm so generated shows encouraging agreement with that established from a correlation of permeability inferred from drawdown tests with field-measured apparent formation factor, normalized to a reference water resistivity. The procedures used to investigate the scale dependence of petrophysical relationships are applicable to all primary clastic reservoirs that require to be characterized at the flow unit scale. They offer a
modus operandi
for establishing a fit-for-purpose, scale-reconciled relationship between permeability and a selected petrophysical predictor for a given reservoir situation. This approach should therefore lead to more reliable estimates of field permeability for input to reservoir simulation.
Prediction of petrophysical parameter logs using a multilayer backpropagation neural network
Authors: Gonçalves C. A. , Harvey P. K. , Lovell M. A. 
Published: September 2007

Abstract
Quantitative petrophysical characterization is one of the principal tasks of a reservoir analyst and is generally affected by the methods used. For example, different theoretical and empirical formulas are in most cases restricted to the specific areas where they were developed. Prediction of continuous petrophysical parameters is often time consuming and complicated because of geological variability such as facies changes due to sedimentary and structural changes.
In this work we propose a neural network approach which is used to predict quantitative petrophysical parameters from wireline logs of cored intervals. We then apply the knowledge learned during training to uncored intervals or other holes. Data from the Ocean Drilling Program and from two South American oilfield holes are used to test this technique.
The results show a good match between the neural network-derived petrophysical parameter logs and the actual core measurements. Problematic petrophysical measurements can be identified by a mismatch between the responses.
Electrical conductivity, spontaneous potential and ionic diffusion in porous media
Authors: Revil A. , Pezard P. A. , Darot M. 
Published: September 2007

Abstract
A consistent set of equations describing electrical conduction, spontaneous potential and ionic diffusion phenomena in porous media, with a particular emphasis given to surface electrical phenomena, is presented. The porous medium is assumed to consist of an insulating matrix and an inter-connected pore volume that is saturated with an electrolyte. When in contact with an electrolyte solution, mineral surfaces have an excess of charge that is balanced by mobile ions in an electrical diffuse layer (EDL). Electrical conduction and ionic diffusion in this diffuse layer can contribute substantially to the effective electrical conductivity of the porous medium, and can play an important role in the strength of several spontaneous electrical potentials (i.e., membrane potential, streaming potential, and thermo-electric potential) and ionic diffusion phenomena. Our surface electrical properties model is based on a thermodynamic description of surface chemical reactions and electrical diffuse layer processes. As an example, we consider an amphoteric mineral surface described by a three-site-type model. We derive the fractional occupancies of positive, negative and neutral sites, and the fractional ionic diffuse layer densities on the surface as a function of the salinity and the pH. The surface charge density at a given pH, is found to be dependent on the electrolyte concentration. Finally, parameters of interest in describing macroscopic effects (effective electrical conductivity, spontaneous potential coupling coefficients and effective diffusion coefficient), are related to the previously mentioned properties, via the mineral surface electrical potential (called the Stern potential).
Fractal Geometry, porosity and complex resistivity: from hand specimen to field data
Authors: Da Rocha Brígida Ramati P. , Habashy Tarek M. 
Published: September 2007

Abstract
In the induced polarization (IP) method of geophysical prospecting, the frequency dependence of the constitutive parameters (conductivity and permittivity) of rocks is exploited to distinguish between different types of rocks with different types of mineralization. Unavoidably, the field data measured using either a four electrode array or an inductive type setup employing ungrounded loops will not directly provide the induced polarization properties of the geological medium to the exclusion of the ‘electromagnetic coupling’. In fact one can not measure one effect without the other. Inference of the medium constitutive parameters from field data is often masked and confused by the purely electromagnetic effects that would be present even when the medium is non-dispersive.
In this paper we present a framework by which one can separate the electrochemical effects from the electromagnetic effects which are usually intertwined in a complicated manner in the measured field data. We formulate the problem in a fairly general way without introducing the customary quasi-static approximation. The model we use is that of a layered earth with one of the layers representing a polarizable medium and the rest of the layers are non-polarizable. The effective complex resistivity of the polarizable medium, which is described in a companion paper, is given by a generalized Cole-Cole type resistive network.
Tortuosity: a guide through the maze
Authors: Clennell M. Ben 
Published: September 2007

Abstract
Despite its widespread use in petrophysics, tortuosity remains a poorly understood concept. Tortuosity can have various meanings when used by physicists, engineers or geologists to describe different transport processes taking place in a porous material. Values for geometrical, electrical, diffusional and hydraulic tortuosity are in general different from one another. Electrical tortuosity is defined in terms of conductivity whereas hydraulic tortuosity is usually defined geometrically, and diffusional tortuosity is typically computed from temporal changes in concentration. A better approach may be to define tortuosity in terms of the underlying flux of material or electrical current with respect to the forces which drive this flow.
Unsteady transport processes, including diffusion, can be described only by a population of tortuosities corresponding to the different flow paths taken by particles traversing the medium. In measurements of steady flow (e.g., those normally used to obtain resistivity or permeability), information about particle travel times is lost, and so the multiple values of tortuosity are homogenised. It can be shown that the maximum amount of information about pore structure is embedded in transport processes that combine advective and diffusive elements.
Most existing formulations of tortuosity are model-dependent, and cannot be correlated with independently measurable pore-structure properties. Nevertheless, tortuosity underpins the rigorous relationships between transport processes in rocks, and ties them with the underlying geometry and topology of their pore spaces. Tortuosity can be redefined in terms of the energetic efficiency of a flow process. The efficiency is related to the rate of entropy dissipation (or isothermally, energy dissipation) with respect to a simple, non-tortuous model medium using the postulates of non-equilibrium thermodynamics. Through Onsager’s reciprocity relation for coupled flows it is possible to inter-relate efficiency for pairs of transport processes, and so go some way towards unifying tortuosity measures. In this way we can approach the goal of predicting the value of one transport parameter from measurements of another.
Scattering attenuation as a function of depth in the Upper Oceanic Crust
Authors: Goldberg D. , Sun Y. F. 
Published: September 2007

Abstract

The
in situ
attenuation is computed through 2.1 km of the upper oceanic crust in the vicinity of ODP Hole 504B on the southflank of the Costa Rica Rift in the eastern equatorial Pacific. The results strongly tie crustal properties to seismic measurables and observed geological structures: we find that the attenuation can be used to define seismic layer boundaries and is closely related to vertical heterogeneity. The
in situ
attenuation
Q
−1
consists of both intrinsic and scattering contributions, but is dominated by the scattering attenuation unless porosities are near zero, when it approaches typical estimates from seismic refraction studies. The attenuation is analytically modeled by multiple backscattering from heterogeneities observed in a sonic
V
p
log and is found to decrease step-wise from
Q
= 25 to
Q
> 300 between the top of seismic layer 2A and a sharp discontinuity at 1.3 km depth. These changes correspond with heterogeneities at 1.0–1.3 m and at 10.0 m wavelengths that are associated with fracturing and the structure of pillow basalts and lava flows in seismic layers 2A and 2B. Although seismic velocity studies suggest that the layer 2–3 boundary also occurs at about 1.3 km, the large variation in
Q
(140 to 460) below this depth indicates that a seismically homogeneous layer 3 has not been reached in Hole 504B. From the observed results, we derive an empirical relationship between attenuation and porosity
Q
−1
=
Q
0
−1
e
βφ
, where
Q
0
−1
= 0.004, β= 25, that may be applicable at other oceanic crust locations and useful for constraining seismic inversion models.
The PCP theorem by gap amplification
Authors: Dinur Irit 
Published: September 2007

The PCP theorem [Arora and Safra 1998; Arora et. al. 1998] says that every language in NP has a witness format that can be checked probabilistically by reading only a constant number of bits from the proof. The celebrated equivalence of this theorem and inapproximability of certain optimization problems, due to Feige et al. [1996], has placed the PCP theorem at the heart of the area of inapproximability.
In this work, we present a new proof of the PCP theorem that draws on this equivalence. We give a combinatorial proof for the NP-hardness of approximating a certain constraint satisfaction problem, which can then be reinterpreted to yield the PCP theorem.

Our approach is to consider the
unsat value
of a constraint system, which is the smallest fraction of unsatisfied constraints, ranging over all possible assignments for the underlying variables. We describe a new combinatorial amplification transformation that doubles the unsat-value of a constraint-system, with only a linear blowup in the size of the system. The amplification step causes an increase in alphabet-size that is corrected by a (standard) PCP composition step. Iterative application of these two steps yields a proof for the PCP theorem.

The amplification lemma relies on a new notion of “graph powering” that can be applied to systems of binary constraints. This powering amplifies the unsat-value of a constraint system provided that the underlying graph structure is an expander.

We also extend our amplification lemma towards construction of assignment testers (alternatively, PCPs of Proximity) which are slightly stronger objects than PCPs. We then construct PCPs and locally-testable codes whose length is linear up to a
polylog
factor, and whose correctness can be probabilistically verified by making a
constant
number of queries. Namely, we prove
SAT
∈
PCP
1/2,1
[log
2
(
n
⋅poly log
n
),
O
(1)].
Coverage breach problems in bandwidth-constrained sensor networks
Authors: Cheng Maggie X. , Ruan Lu , Wu Weili 
Published: September 2007

Recent research in sensor networks highlights the low-power mode operation of sensor networks. In wireless sensor networks, network lifetime can be extended by organizing sensors into mutually exclusive subsets and alternatively activating each subset. Coverage breach occurs when a subset fails to cover all the targets. In bandwidth-constrained sensor networks, coverage breach is more likely to happen because when active sensors periodically send data to the base station, contention for channel access must be considered. Channel bandwidth imposes a limit on the cardinality of each subset. To make efficient use of both energy and bandwidth with minimum coverage breach requires optimal arrangement of sensor nodes.
This article addresses three coverage breach problems related to the low-power operation of wireless sensor networks where channel bandwidth is limited. The three coverage breach problems are formulated using integer linear programming models. A greedy approximation algorithm and a heuristic based on the LP-relaxation method are proposed. Effects of changing different network resources on sensor network coverage are studied through simulations. One consistent result is that when the number of sensors increases, network lifetime can be improved without loss of network coverage only if there is no bandwidth constraint; with bandwidth constraints, network lifetime may be improved further at the cost of coverage breach.
Lighting with paint
Authors: Pellacini Fabio , Battaglia Frank , Morley R. Keith , Finkelstein Adam 
Published: September 2007

Lighting is a fundamental aspect of computer cinematography that involves the placement and configuration of lights to establish mood and enhance storytelling. This process is labor intensive as artists repeatedly adjust the parameters of a large set of complex lights to achieve a desired effect. Typical lighting controls affect the final image indirectly, requiring a large number of trials to obtain a suitable result.
We present an interactive system wherein an artist paints desired lighting effects directly into the scene, and the computer solves for parameters that achieve the desired look. The artist can paint color, light shape, shadows, highlights, and reflections using a suite of tools designed for painting light. Our system matches these effects using a nonlinear optimizer made robust by a combination of initial estimates, system design, and user-guided optimization. In contrast, previous work on painting light has not permitted the lights to move, allowing for linear optimization but preventing its use in computer cinematography.
To demonstrate our approach we lit several scenes, mainly using a direct illumination renderer designed for computer animation, but also including two other rendering styles. We show that painting interfaces can quickly produce high quality lighting setups, easing the lighting artist's workflow.
Adaptive sampling of reflectance fields
Authors: Fuchs Martin , Blanz Volker , Lensch Hendrik P.A. , Seidel Hans-Peter 
Published: September 2007

Image-based relighting achieves high quality in rendering, but it requires a large number of measurements of the reflectance field. This article discusses sampling techniques that improve on the trade-offs between measurement effort and reconstruction quality.
Specifically, we (i) demonstrate that sampling with point lights and from a sparse set of incoming light directions creates artifacts which can be reduced significantly by employing extended light sources for sampling, (ii) propose a sampling algorithm which incrementally chooses light directions adapted to the properties of the reflectance field being measured, thus capturing significant features faster than fixed-pattern sampling, and (iii) combine reflectance fields from two different light domain resolutions.
We present an automated measurement setup for well-defined angular distributions of the incident, indirect illumination. It is based on programmable spotlights with controlled aperture that illuminate the walls around the scene.
Visualizing tags over time
Authors: Dubinko Micah , Kumar Ravi , Magnani Joseph , Novak Jasmine , Raghavan Prabhakar , Tomkins Andrew 
Published: September 2007

We consider the problem of visualizing the evolution of tags within the Flickr (flickr.com) online image sharing community. Any user of the Flickr service may append a tag to any photo in the system. Over the past year, users have on average added over a million tags each week. Understanding the evolution of these tags over time is therefore a challenging task. We present a new approach based on a characterization of the most interesting tags associated with a sliding interval of time. An animation provided via Flash in a Web browser allows the user to observe and interact with the interesting tags as they evolve over time.
New algorithms and data structures are required to support the efficient generation of this visualization. We combine a novel solution to an interval covering problem with extensions to previous work on score aggregation in order to create an efficient backend system capable of producing visualizations at arbitrary scales on this large dataset in real time.
Design, implementation, and evaluation of a compilation server
Authors: Lee Han B. , Diwan Amer , Moss J. Eliot B. 
Published: September 2007

Modern JVM implementations interleave execution with compilation of “hot” methods to achieve reasonable performance. Since compilation overhead impacts the execution time of the application and induces run-time pauses, we explore offloading compilation onto a compilation server. In this article, we present the design, implementation, and evaluation of a compilation server that compiles and optimizes Java bytecodes on behalf of its clients.
We show that the compilation server provides the following benefits for our benchmark programs: (i) lower execution time by reducing the compilation overhead and by enabling more aggressive optimizations; (ii) lower memory allocation by eliminating allocations due to optimizing compilation and the footprint of the optimizing compiler; (iii) lower execution time of the application due to sharing of profile information across different runs of the same application and runs of different applications.
We implemented the compilation server in Jikes RVM, and our results indicate that it can reduce running time by an average of 20.5%, interruptions due to compilation by an average of 81.0%, and dynamic memory allocation by 8.6% for our benchmark programs. Simulation results indicate that our current implementation of the compilation server can handle more than 50 concurrent clients while still allowing them to outperform the best performing adaptive configuration.
Algorithm 867
Authors: Beebe Nelson H. F. , Ball James S. 
Published: September 2007

A collection of subroutines and examples of their uses are described for the quadrature method developed in the companion article. These allow the exact evaluation (up to computer truncation and rounding errors) of integrals of polynomials with two general types of logarithmic weights, and also with the corresponding nonlogarithmic weights. The recurrence coefficients for the related nonclassical orthogonal polynomials with logarithmic weight functions can also be obtained. Tests of accuracy on various platforms are presented.
The routines are usable from Fortran, C, and C++ programs conforming to any of at least six international programming-language standards.
deal.II—A general-purpose object-oriented finite element library
Authors: Bangerth W. , Hartmann R. , Kanschat G. 
Published: September 2007

An overview of the software design and data abstraction decisions chosen for deal.II, a general purpose finite element library written in C++, is given. The library uses advanced object-oriented and data encapsulation techniques to break finite element implementations into smaller blocks that can be arranged to fit users requirements. Through this approach, deal.II supports a large number of different applications covering a wide range of scientific areas, programming methodologies, and application-specific algorithms, without imposing a rigid framework into which they have to fit. A judicious use of programming techniques allows us to avoid the computational costs frequently associated with abstract object-oriented class libraries.
The paper presents a detailed description of the abstractions chosen for defining geometric information of meshes and the handling of degrees of freedom associated with finite element spaces, as well as of linear algebra, input/output capabilities and of interfaces to other software, such as visualization tools. Finally, some results obtained with applications built atop deal.II are shown to demonstrate the powerful capabilities of this toolbox.