ExxonMobil

About: ExxonMobil is a company organization based out in Irving, Texas, United States. It is known for research contribution in the topics: Catalysis & Polymer. The organization has 16969 authors who have published 23758 publications receiving 535713 citations. The organization is also known as: Exxon Mobil Corporation & Exxon Mobil Corp..

On the implementation of perfectly matched layers in a three‐dimensional fourth‐order velocity‐stress finite difference scheme

Abstract: [1] Robust absorbing boundary conditions are central to the utility and advancement of 3-D numerical wave propagation methods. It is in general preferred that an absorbing boundary method be capable of broadband absorption, be efficient in terms of memory and computation time, and be widely stable in connection with sophisticated numerical schemes. Here we discuss these issues for a promising absorbing boundary method, perfectly matched layers (PML), as implemented in the widely used fourth-order accurate three-dimensional (3-D) staggered-grid velocity-stress finite difference (FD) scheme. Numerical results for point (explosive and double couple) and extended sources, velocity structures (homogeneous, 1-D and 3-D), and different thickness PML zones are excellent, in general, leaving no observable reflections in PML seismograms compared to the amplitudes of the primary phases. For both homogeneous half-space and 1-D models, typical amplitude reduction factors (with respect to the maximum trace amplitude) range between 1/100 and 1/625 for PML thicknesses of 5–20 nodes. A PML region of thickness 5 outperforms a simple exponential damping region of thickness 20 in a homogeneous half-space model by a factor of 3. We find that PML is effective across the simulation bandwidth. For example, permanent offset artifacts due to particularly poor absorption of long-period energy by the simple exponential damping are effectively absent when PML is used. The computational efficiency and storage requirements of PML, compared to the simple exponential damping, are reduced due to the need for only narrow absorbing regions. We also discuss stability and present the complete PML model for the 3-D velocity-stress system.

Random Surface Model for the L3-Phase of Dilute Surfactant Solutions

Abstract: We present a simple model for the anomalous (flow-birefringent) isotropic phase, known as L3, that is seen in certain surfactant solutions at volume fractions of a few percent. The proposed structure consists of locally sheetlike sections of semi-flexible surfactant bilayer, connected up at larger distances into a multiply connected random surface, having a preferred structural length scale of order the persistence length of the bilayer. A first-order transition between this isotropic sheetlike phase and the nearby swollen lamellar phase is described.

Determination of 4-connected framework crystal structures by simulated annealing

Abstract: THE initial derivation of atomic-scale models of the 4-connected framework crystal structures of zeolites and related materials is generally difficult. Interpretation of powder X-ray diffraction data to yield a unit cell and some symmetry information is often straightforward. Chemical analyses and sorption experiments indicate the number of framework tetrahedra, nT, present in the unit cell (T represents the framework tetrahedral species, such as Si or Al) and the approximate pore dimensions. However, because synthetic zeolites are almost invariably microcrystalline, the possibilities for structure solution by conventional diffraction methods are limited, and framework structure determinations have traditionally relied heavily on the building of physical models. We describe here an alternative approach, in which approximate T-atom coordinates are derived from the unit-cell size and symmetry, and the value of nT from computer modelling. An initially arbitrary T-atom configuration is optimized with respect to a 'cost function' based on the T–T distances, T–T–T angles and number of first-neighbour T-atoms, by simulated annealing using Monte Carlo methods. The potential of the method for structural determinations in both two dimensions (for structural projections) and three dimensions are illustrated by results obtained for a hexagonal cell of space group P6/mmm and dimensions a = 18.4 A and c = 7.5 A, and by determination of the previously unknown structure of lithium gallosilicate.