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Total Field (total + field)
Selected AbstractsRemoval of DC power-line magnetic-field effects from airborne total magnetic-field measurementsGEOPHYSICAL PROSPECTING, Issue 3 2000Mehran Gharibi Power lines carrying DC current can strongly affect total magnetic-field measurements. A simple algorithm using Biot,Savart's law was made to remove magnetic-field components due to a DC power line from airborne total magnetic-field measurements in the Gävle area, Sweden. The power-line location was estimated from observed data and then split into short line segments. The magnetic-field components due to each segment were calculated and summed together to give the total magnetic effect due to the power line at each observation point. The corrected total magnetic field was calculated by subtracting the power-line magnetic-field vector, projected on to the direction of the main field, from the measured total field. The results show a successful removal of the power-line magnetic effect from the total magnetic-field measurements. However, an error in the estimation of the power-line location can result in a magnetic-field residual after correction. A non-linear median filtering was used to remove this residual when needed. [source] A spectral-element method for modelling cavitation in transient fluid,structure interactionINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 15 2004M. A. Sprague Abstract In an underwater-shock environment, cavitation (boiling) occurs as a result of reflection of the shock wave from the free surface and/or wetted structure causing the pressure in the water to fall below its vapour pressure. If the explosion is sufficiently distant from the structure, the motion of the fluid surrounding the structure may be assumed small, which allows linearization of the governing fluid equations. In 1984, Felippa and DeRuntz developed the cavitating acoustic finite-element (CAFE) method for modelling this phenomenon. While their approach is robust, it is too expensive for realistic 3D simulations. In the work reported here, the efficiency and flexibility of the CAFE approach has been substantially improved by: (i) separating the total field into equilibrium, incident, and scattered components, (ii) replacing the bilinear CAFE basis functions with high-order Legendre-polynomial basis functions, which produces a cavitating acoustic spectral element (CASE) formulation, (iii) employing a simple, non-conformal coupling method for the structure and fluid finite-element models, and (iv) introducing structure,fluid time-step subcycling. Field separation provides flexibility, as it admits non-acoustic incident fields that propagate without numerical dispersion. The use of CASE affords a significant reduction in the number of fluid degrees of freedom required to reach a given level of accuracy. The combined use of subcycling and non-conformal coupling affords order-of-magnitude savings in computational effort. These benefits are illustrated with 1D and 3D canonical underwatershock problems. Copyright © 2004 John Wiley & Sons, Ltd. [source] An extended Huygens' principle for modelling scattering from general discontinuities within hollow waveguidesINTERNATIONAL JOURNAL OF NUMERICAL MODELLING: ELECTRONIC NETWORKS, DEVICES AND FIELDS, Issue 5 2001Ronald L. Ferrari Abstract The modal fields, generalized scattering matrix (GSM) theory and dyadic Green's functions relating to a general uniform hollow waveguide are briefly reviewed in a mutually consistent normalization. By means of an analysis linking these three concepts, an extended version of the mathematical expression of Huygens' principle is derived, applying to scattering from an arbitrary object within a hollow waveguide. The integral-equation result expresses the total field in terms of the incident waveguide modal fields, the dyadic Green's functions and the tangential electromagnetic field on the surface of the object. It is shown how the extended principle may be applied in turn to perfect conductor, uniform material and inhomogeneous material objects using a quasi method of moments (MM) approach, coupled in the last case with the finite element method. The work reported, which indicates how the GSM of the object may be recovered, is entirely theoretical but displays a close similarity with established MM procedures. Copyright © 2001 John Wiley & Sons, Ltd. [source] Sliding multislice MRI for abdominal staging of patients with pelvic malignancies: A pilot studyJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 3 2008Gregor Sommer MS Abstract Purpose To integrate SMS (sliding multislice imaging technique for acquiring axial images during continuous table motion) into a high-resolution pelvic MRI protocol for additional staging of the entire abdomen within one examination. Materials and Methods Axial two-dimensional images were acquired during continuous table motion using a fat-saturated contrast-enhanced T1-weighted gradient echo sequence. Patients held their breath during the first 20 s of the examination and breathed normally afterward while data acquisition continued. Measurement parameters were adjusted to optimize image quality throughout the total field of view. The method was investigated in 22 patients with pelvic malignancies. Two readers independently compared SMS image quality to conventional abdominal MR images, generated by a stationary multi-breath-hold gradient echo sequence. Results Qualitative evaluation yielded high diagnostic value of SMS data in body regions with no or minor breathing motion, and in those acquired during the initial breath-hold. Image quality in the upper abdomen, retroperitoneum, and pelvis is reproducible and equivalent to stationary MRI. Interfering artifacts are related to the intestine in the mid-abdomen. Conclusion SMS is a promising technique that may have the potential for a first-line abdominal staging tool in patients with pelvic malignancies. J. Magn. Reson. Imaging 2008;27:666,672. © 2008 Wiley-Liss, Inc. [source] Integral equation methods for scattering by infinite rough surfacesMATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 6 2003Bo Zhang Abstract In this paper, we consider the Dirichlet and impedance boundary value problems for the Helmholtz equation in a non-locally perturbed half-plane. These boundary value problems arise in a study of time-harmonic acoustic scattering of an incident field by a sound-soft, infinite rough surface where the total field vanishes (the Dirichlet problem) or by an infinite, impedance rough surface where the total field satisfies a homogeneous impedance condition (the impedance problem). We propose a new boundary integral equation formulation for the Dirichlet problem, utilizing a combined double- and single-layer potential and a Dirichlet half-plane Green's function. For the impedance problem we propose two boundary integral equation formulations, both using a half-plane impedance Green's function, the first derived from Green's representation theorem, and the second arising from seeking the solution as a single-layer potential. We show that all the integral equations proposed are uniquely solvable in the space of bounded and continuous functions for all wavenumbers. As an important corollary we prove that, for a variety of incident fields including an incident plane wave, the impedance boundary value problem for the scattered field has a unique solution under certain constraints on the boundary impedance. Copyright © 2003 John Wiley & Sons, Ltd. [source] Earth's magnetic field: ocean current contributions to vertical profiles in deep oceansGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2001F. E. M. (Ted) Lilley SUMMARY The Earth's main magnetic field, arising in the core, should, in the ocean, have a well-defined uniform gradient with depth. Superimposed upon this uniform gradient may be magnetic signals due to a variety of sources. These include crustal magnetization, the transient fluctuations arising external to the Earth and causing secondary induced fields within it; and, the focus of the present paper, magnetic signals arising from the motional induction of seawater moving in the steady main magnetic field of Earth. There are circumstances where theory predicts such motionally-induced magnetic fields to be of order 102 nT, and to vary with depth in a way which is directly related to the velocity profile. Exploratory soundings of the magnetic field with depth have been made in the oceans around Australia, both to test these predictions, and to investigate the practicability of measuring such profiles. The magnetic field parameter observed has been that of the ,total field', which should sense the component of the ocean velocity which lies in the magnetic meridian. The magnetometer has been lowered by cable from a ship and also operated free-fall to the seafloor (and return). The observations appear both to confirm the theoretical gradient of the main field where there is no ocean current and, where ocean currents exist, to give evidence of their profiles resolved in the direction of magnetic north. In particular, observations taken in an eddy of the East Australian Current show the correct contrast in sign for north and south flowing streams. [source] | |||||||||||||