1,334 research outputs found
Recommended from our members
An Integrated Approach to Seismic Event Location: 1. Evaluating How Method of Location Affects the Volume of Groups of Hypocenters
When seismic events occur in spatially compact clusters, the volume and geometric characteristics of these clusters often provides information about the relative effectiveness of different location methods, or about physical processes occurring within the hypocentral region. This report defines and explains how to determine the convex polyhedron of minimum volume (CPMV) surrounding a set of points. We evaluate both single-event and joint hypocenter determination (JHD) relocations for three rather different clusters of seismic events: 1) nuclear explosions from Mururoa relocated using P and PKP phases reported by the ISC, 2) intermediate depth earthquakes near Bucaramanga, Colombia, relocated using P and PKP phases reported by the ISC, and 3) shallow earthquakes near Vanuatu (formerly, the New Hebrides), relocated using P and S phases from a local station network. This analysis demonstrates that different location methods markedly affect the volume of the CPMV, however, volumes for JHD relations are �not always smaller than volumes for single-event relocations.Phillips Laboratory, Directorate of Geophysics, Air Force Material Command, Hanscom Air Force Base, MassachusettsInstitute for Geophysic
Template-Cut: A Pattern-Based Segmentation Paradigm
We present a scale-invariant, template-based segmentation paradigm that sets
up a graph and performs a graph cut to separate an object from the background.
Typically graph-based schemes distribute the nodes of the graph uniformly and
equidistantly on the image, and use a regularizer to bias the cut towards a
particular shape. The strategy of uniform and equidistant nodes does not allow
the cut to prefer more complex structures, especially when areas of the object
are indistinguishable from the background. We propose a solution by introducing
the concept of a "template shape" of the target object in which the nodes are
sampled non-uniformly and non-equidistantly on the image. We evaluate it on
2D-images where the object's textures and backgrounds are similar, and large
areas of the object have the same gray level appearance as the background. We
also evaluate it in 3D on 60 brain tumor datasets for neurosurgical planning
purposes.Comment: 8 pages, 6 figures, 3 tables, 6 equations, 51 reference
The dynamical environment of asteroid 21 Lutetia according to different internal models
One of the most accurate models currently used to represent the gravity field
of irregular bodies is the polyhedral approach. In this model, the mass of the
body is assumed to be homogeneous, which may not be true for a real object. The
main goal of the present paper is to study the dynamical effects induced by
three different internal structures (uniform, three- and four-layers) of
asteroid (21) Lutetia, an object that recent results from space probe suggest
being at least partially differentiated. The Mascon gravity approach used in
the present work, consists of dividing each tetrahedron into eight parts to
calculate the gravitational field around the asteroid. The zero-velocity curves
show that the greatest displacement of the equilibrium points occurs in the
position of the E4 point for the four-layers structure and the smallest one
occurs in the position of the E3 point for the three-layers structure.
Moreover, stability against impact shows that the planar limit gets slightly
closer to the body with the four-layered structure.
We then investigated the stability of orbital motion in the equatorial plane
of (21) Lutetia and propose numerical stability criteria to map the region of
stable motions. Layered structures could stabilize orbits that were unstable in
the homogeneous model.Comment: 10 pages, 7 figures, and 4 Tables. Accepted for publication in MNRA
Extracting curve-skeletons from digital shapes using occluding contours
Curve-skeletons are compact and semantically relevant shape descriptors, able to summarize both topology and pose of a wide range of digital objects. Most of the state-of-the-art algorithms for their computation rely on the type of geometric primitives used and sampling frequency. In this paper we introduce a formally sound and intuitive definition of curve-skeleton, then we propose a novel method for skeleton extraction that rely on the visual appearance of the shapes. To achieve this result we inspect the properties of occluding contours, showing how information about the symmetry axes of a 3D shape can be inferred by a small set of its planar projections. The proposed method is fast, insensitive to noise, capable of working with different shape representations, resolution insensitive and easy to implement
Tropical Geometry of Phylogenetic Tree Space: A Statistical Perspective
Phylogenetic trees are the fundamental mathematical representation of
evolutionary processes in biology. As data objects, they are characterized by
the challenges associated with "big data," as well as the complication that
their discrete geometric structure results in a non-Euclidean phylogenetic tree
space, which poses computational and statistical limitations. We propose and
study a novel framework to study sets of phylogenetic trees based on tropical
geometry. In particular, we focus on characterizing our framework for
statistical analyses of evolutionary biological processes represented by
phylogenetic trees. Our setting exhibits analytic, geometric, and topological
properties that are desirable for theoretical studies in probability and
statistics, as well as increased computational efficiency over the current
state-of-the-art. We demonstrate our approach on seasonal influenza data.Comment: 28 pages, 5 figures, 1 tabl
Polytopes, Feasible Regions and Occlusions in the n-view Reconstruction Problem
This paper assesses the question, given a arbitrary point in P3, can it be reconstructed by a given camera orbit? We show that a solution to this problem can be found by intersecting the frustrums of the cameras in the sequence creating a polyhedron that bounds the area in P3 observed by all cameras. For a projective set of cameras this can be considered as an expansion of the chetral inequalities. We also show an exception to this basic principle is encounted when the point in P3 is occluded. Thus giving a weak condition for occlusion of an arbitrary point in P3
- …