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We used the seismic physical modelling approach to study the effect of fracture thickness or aperture on P-wave attenuation, using a laboratory scale model of two horizontal layers. The first layer is isotropic while the second layer has six fractured blocks, each consisting of thin penny-shaped chips of 3 mm fixed diameter and same thickness to simulate a set of aligned vertical fractures. The thickness of the chips varies according to the blocks while the fracture density remains the same in each block. 2D reflection data were acquired with the physical model submerged in a water tank in a direction perpendicular to the fracture strikes using the pulse and transmission method. The induced attenuation was estimated from the preprocessed CMP gathers using the QVO method, which is an extension of the classical spectral ratio method of attenuation measurement from seismic data. The results of our analysis show a direct relationship between attenuation and the fracture thickness or aperture. The induced attenuation increases systematically with fracture thickness, implying more scattering of the wave energy in the direction of increasing aperture. This information may be useful to differentiate the effect caused by thin microcracks from that of large open fractures. [ABSTRACT FROM AUTHOR]

We consider a large-scale of wireless ad hoc networks whose nodes are distributed randomly in a two-dimensional region Ω (more specifically, a unit square). Given n wireless nodes V, each with transmission range rn, the wireless networks are often modeled by graph G (V, rn) in which two nodes are connected if and only if their Euclidean distance is no more than rn. We first consider how to relate the transmission range with the number of nodes in a fixed area such that the resulted network can sustain k fault nodes in its neighborhood with high probability when all nodes have the same transmission range. We show that, for a unit-area square region Ω, the probability that the network G (V, rn) is k-connected is at least ${\rm e}^{-{\rm e}^{-\alpha}}$ when the transmission radius rn satisfies $n \pi r_n^2 \ge {\rm ln}\ n \,+ (2k - 3) {\rm ln}\, {\rm ln}\, n - 2 \,{\rm ln}(k - 1)! + 2 \alpha \ {\rm for} \, k >\,1$ and n sufficiently large. This result also applies to mobile networks when the moving of wireless nodes always generates randomly distributed positions. We also conduct extensive simulations to study the practical transmission range to achieve certain probability the network being k-connectivity, when the number of nodes n is not large enough. The relation between the minimum node degree and the connectivity of graph G (V, r) is also studied. Setting the transmission range of all nodes to rn guarantees the k-connectivity with high probability, but some nodes may have excessive number of neighbours in the graph G (V, rn). We then present a localized method to construct a subgraph of the network topology G (V, rn) such that the resulting subgraph is still k-connected but with much fewer communication links maintained. We show that the constructed topology has only O (k · n) links and is a length spanner. Here a graph H ⊆ G is spanner for graph G, if for any two nodes, the length of the shortest path connecting them in H is no more than a small constant factor of the length of the shortest path connecting them in G. Finally, we conduct some simulations to study the practical transmission range to achieve certain probability of k-connected when n is not large enough. Copyright © 2004 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We present an overview of the recent progress of applying computational geometry techniques to solve some questions, such as topology construction and broadcasting, in wireless ad hoc networks. Treating each wireless device as a node in a two-dimensional plane, we model the wireless networks by unit disk graphs in which two nodes are connected if their Euclidean distance is no more than one. We first summarize the current status of constructing sparse spanners for unit disk graphs with various combinations of the following properties: bounded stretch factor, bounded node degree, planar, and bounded total edges weight (compared with the minimum spanning tree). Instead of constructing subgraphs by removing links, we then review the algorithms for constructing a sparse backbone (connected dominating set), that is, subgraph from the subset of nodes. We then review some efficient methods for broadcasting and multicasting with theoretic guaranteed performance. Copyright © 2003 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

An introduction is presented in which editors discuss various papers within the issued focused on mobile sensing covering several topics including, multisensor architecture, routing algorithm, and data collection protocol meant for wireless sensor networks.