The prevalence of cocaine-associated chest pain in a London hospital

Objectives: To determine the prevalence of cocaine misuse in patients presenting to an Accident and Emergency department with chest pain. 4 of 12 FAEM abstracts www.emjonline.co

Secondary Shear Waves From Source Boreholes

The purpose of this paper is to synthesize the most important results of the thesis work of Meredith (1990) concerning radiation from seismic sources in boreholes. Previous studies of radiation from sources in boreholes have been far-field studies and have neglected the explicit contribution of the borehole. In general, this is fine for P-wave radiation and for S-wave radiation into high velocity rocks. However, tube waves "leak" shear conical waves (Mach waves) which propagate when the tube wave velocity is greater than the shear wave velocity of the surrounding medium. These Mach waves are of high amplitude because of the dominance of the tube wave and radiate away from the borehole in a fixed conical shape. The shape of the cone is dependent on the shear wave velocity of the medium and the tube wave velocity. This paper defines the conditions under which these Mach waves exist and thoroughly describes them in a physical sense and less so in a mathematical sense. Finally, the relationship of Mach waves to data sets is examined and how Mach waves may be confused with receiver borehole tube waves. To keep the presentation simple, radiation from axial or torsional sources or radiation from empty boreholes is omitted in this paper but fully addressed in Meredith (1990).Massachusetts Institute of Technology. Full Waveform Acoustic Logging Consortiu

Modelling Of Downhole Seismic Sources II: An Analysis Of The Heelan/Brekhovskikh Results And Comparison Of Point Source Radiation To Radiation From Boreholes

The work of Heelan (1952, 1953a,b) was one of the first studies of wave propagation from a cylindrical boundary. Heelan attempted to model the radiation emanating from a cylindrical shot hole filled with dynamite. To do so he applied a constant stress to a finite length of an empty infinite cylindrical cavity embedded in an infinite elastic, homogeneous medium. The stresses he considered were axial, torsional, and radial stresses. The radial and axial stresses were required to be proportional to each other and of the same duration. To date Heelan's work has been referenced in over 100 articles and 15 different journals including recent works (Paulsson, 1988) . His results have also been compared with results from the reciprocity theorem (White, 1953, 1960) and played an integral part of important books including those by Brekhovskikh (1960, 1980) and White (1965, 1983). His fundamental contributions were the description of shear wave lobes, the famous four-leaved rose, generated from a radial source in a borehole and that the radiation patterns for an axial source and a torsional source in a borehole have the same geometries as the point axial and torsional sources in infinite media. Despite the importance of this work, Heelan's results have been criticized by Jordan (1962) who dismissed the work as mathematically unsound and Abo-Zena (1977) who devoted an appendix of his 1977 paper to criticizing Heelan's results. The main point of contention has been the use of contour analysis in his first paper (Heelan, 1953a). Although Heelan's work did not include a fluid-filled borehole which is a crucial omission for our purposes, his work may nonetheless be seen as a starting point for the modelling of downhole seismic sources. For instance, Lee and Balch (1982) developed radiation patterns for fluid boreholes which were simple extensions of Heelan's results. Additionally, one particular application of Heelan's theory is in the preliminary development of downhole seismic Sources that often require dry holes until the electronics can be properly shielded. For that reason, an exhaustive examination of the mathematics and physics that went into Heelan's first paper was undertaken to determine if his formulation was correct. The fundamental basis of Heelan's work was a variant of the Sommerfeld integral, an integral of cylindrical waves, in which he unfortunately did not specify the contour. To overcome this obstacle of an unknown contour a parallel method suggested by Brekhovskikh (1960, 1980) was implemented. Brekhovskikh used the Weyl integral, an integral over plane waves, to duplicate Heelan's results for the radial and torsional stresses. However he does no justification of the extensive algebra or analysis involved and does not include the effects of axial stress. Thus in this paper, we have completed and elucidated the work that Brekhovskikh initiated and moreover indirectly verified that Heelan's results were correct. Additionally, we found that Abo-Zena's and Heelan's initial formulations were equivalent. The only difference was in a reversal of the separation of variables procedure necessary to replicate this work and also in Abo-Zena's USe of the Laplace transform where Heelan used the Fourier transform. However, Abo-Zena's results do extend Heelan's by allowing the source function to vary over the distance in which it is applied. The far field results of Abo-Zena and Heelan are equivalent (White, 1983) only if a 1/μ correction is applied to Abo-Zena's results. The first half of this paper is very involved mathematically but much of the algebra is relegated to Appendix A. Having verified that Heelan's results were correct we then proceed to compare Heelan's results with well established point source representations known in the literature (White, 1983) and also with radiation patterns from point sources and stress sources in a fluid-filled borehole (Lee and Balch, 1982). These comparisons will help us isolate the propagation effects of the fluid and the geometrical effect of the borehole. One unique aspect to our approach will be the consideration of radiation from boreholes surrounded by varying lithologies instead of just the Poisson solid as is commonly done. The lithologies to be considered include a soft sediment (Pierre shale) and two more indurated sediments, Berea sandstone and Solenhofen limestone. By following this approach we show that the effect on the radiation magnitude can be substantial due to changes in lithology in addition to isolating the relative effects of the borehole and the fluid.Massachusetts Institute of Technology. Full Waveform Acoustic Logging Consortiu

Modelling Of Downhole Seismic Sources I: Literature Review, Review Of Fundamentals, Impulsive Point Source In A Borehole

This paper represents the first of a two paper sequence comprising a multi-faceted introduction to the numerical and analytical modelling of seismic sources in a borehole. Part one will present a literature review and a review of the fundamental mathematical descriptions of wave propagation outside a borehole. By listing the mathematical descriptions here we can show the equivalence and interrelationships of many treatments presented in the literature. Part one will conclude with an outline of the familiar discrete wavenumber technique as applied to modelling radiation outside a borehole from a point source inside a borehole. Part two will attempt to isolate the effects of the fluid-filled borehole on the radiation of a borehole source by comparing radiation patterns for three cases: a point source in an infinite medium, a stress applied to the wall of an empty borehole (Heelan's (1953) result) and a point source in a fluid-filled borehole (Lee and Balch, 1982). Heelan's results will also be analyzed and defended against criticism of them brought by Jordan (1962) and Abo-Zena (1978). The literature review will be thorough and will include the work done directly on modelling downhole seismic sources and the comparatively larger amount of work done on modelling sources for acoustic logging purposes which is directly applicable. Different authors publishing work on seismic sources have made widely different symmetry assumptions ranging from no symmetry to axisymmetry to symmetry along the z axis. These differences hamper the ability to directly compare results between the different treatments. Compounding the differences in symmetry are the use of different time dependencies ([superscript -iwl], [superscript iwl]) and the use of different Hankel function or modified Bessel function strategies. Therefore, the mathematical fundamentals of wave propagation in a borehole from the different symmetry systems are presented here in a consistent notation and are related to each other and treatments in the literature to help eliminate duplication of effort. Finally, wave propagation outside a borehole is examined using synthetic seismograms. For the synthetic seismograms, a point source inside the borehole is used as a source and the synthetics are calculated using the discrete wavenumber method. The synthetic seismograms are measured along vertical arrays of point receivers placed outside the borehole and for lithologies of Pierre shale, Solenhofen limestone, and Berea sandstone. The method and the resulting seismograms are outlined in this paper along with a brief description of the discrete wavenumber technique.Massachusetts Institute of Technology. Full Waveform Acoustic Logging Consortiu

Stress Distribution in Mandible Regulated by Bone and Dental Implant Parameters: Part I - Methodology

The complicated inter-relationships between mandibular bone components and dental implants have attracted the attention of many a structural mechanics researcher as well as many a dental practitioner. This paper describes the methodology and analysis techniques employed to enable accurate evaluation of a vast range of the implant and bone parameters. The complex material and geometric properties of the bone and implant are modelled using two-dimensional (2D) triangular and quadrilateral plane strain elements. Assumptions made in the analysis include: (a) 50% osseointegration between bone and implant; (b) linear relationships exist between the stress value and the Young’s moduli of the cancellous and cortical bone at any specific point. In the companion paper (Part II) various bone, implant and loading parameters are evaluated for their influence on the stress distribution within the bone, in particular in the mandible

Detecting and understanding non-compliance with conservation rules

AbstractThis paper establishes the context for the special issue, “Detecting and Understanding Violations of Conservation Rules”. Illicit or non-compliant human behaviors may occur in all ecosystems and range from subsistence illegal resource collection to poaching by organized criminal syndicates. Such acts have an enormous impact on social–ecological systems, but monitoring non-compliance is challenging, primarily because the topic is sensitive and victims are voiceless. The future of many conservation areas depends upon compliance with conservation rules. However, with a growing human population, consumptive societies, and rapid expansion of business opportunities fueled by new technology, there is little doubt that demand will remain steady or increase for many of our natural resources. We outline major conservation compliance issues and impacts, and review models and methods used to monitor and respond to the problem for both subsistence and commercial non-compliance

How Lightning Tortuosity Affects the Electromagnetic Fields by Augmenting Their Effective Distance

A novel approach for developing the electromagnetic fields from a lightning return stroke which follows a tortuous path will be presented. The proposed model is unique in that it recognizes that the symmetrical tortuosity of lightning directly impacts the observable distance r, which in turn, alters the resulting electromagnetic fields. In the literature, lightning return stroke models typically employ the assumption that the cloud-to-ground path is straight. Although this assumption yields fairly consistent results across an array of varying approaches, it does not account for lightning\u27s natural physical appearance. Furthermore, straight-line models only account for the cloud-to-ground discharges and do not address branching and/or cloud-to-cloud discharges which are far more common. In reality, the steps which make up the lightning channel\u27s initial descent are staggered or tortuous with respect to each other. Given this fact, the upward traveling current wavefront which follows this prescribed path will exhibit the same characteristics. In doing so, each current segment, which forms along its respective step, induces electromagnetic fields with angular aggregates that propagate outward from their origin. This, in turn, will generate spatial points where there are fields of higher and lower intensities. The results presented in this paper will show how the effective observable distance due to symmetrical tortuosity alters the resulting electromagnetic fields. Furthermore, it will be shown that as the observable distance r is increased, results from the proposed model closely resemble the straight-line model which strongly suggests that symmetrical tortuosity is only influential at relatively close distances