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Magnetic Signal (magnetic + signal)
Selected AbstractsSea-surface observations of the magnetic signals of ocean swellsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2004F. E. M. Lilley SUMMARY Ocean swells have a magnetic signal, caused by the motional induction of sea water moving in the steady main magnetic field of Earth. To check the character of such signals at the sea surface, a magnetometer has been set free from a ship to float unrestricted on the surface of the ocean for periods of several days. The path of the floating magnetometer was tracked by satellite; this procedure enabled also the eventual recovery of the magnetometer by the ship. Superimposed upon a background of slow change of magnetic field, as the magnetometer drifted across different patterns of crustal magnetization, are high-frequency signals generated by the strong ocean swell present at the time. These wave-generated signals are typically up to 5 nT trough-to-peak, consistent with theory for their generation by ocean swells several metres trough-to-peak in height. The power spectra of the magnetic signals show a consistent peak at period 13 s, appropriate for the known characteristics of ocean swell in the area. The power spectra then exhibit a strong (,7 power) fall-off as period decreases below 13 s. This strong fall-off is consistent with oceanographic observations of the spectra of surface swell, combined with motional induction theory. [source] Ferromagnetic Spots in Graphite Produced by Proton Irradiation,ADVANCED MATERIALS, Issue 20 2003K.-h. Han A proton microbeam produces ferromagnetic spots on a highly oriented graphite surface. The Figure shows the topography and the magnetic force microscopy (MFM) images obtained after irradiating the surface with a beam size of 1,,m,×,1 ,m and a dose of 0.098 nC,,m,2. The strength of the magnetic signal is only a factor of ,,10 smaller than that obtained on Fe particles. [source] Magnetic characterization of U/Co multilayersPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2003M. A. Rosa Abstract With the aim of expanding the studies on 2D systems containing uranium, U/Co multilayers with layer thickness ranging from 50 to 200 Ĺ were recently prepared by dc magnetron sputtering onto glass. The multilayers were characterized by Grazing-Incidence X-Ray Diffraction (GIXRD) and Rutherford Backscattering Spectrometry (RBS). Magnetization measurements performed with a squid magnetometer showed that the multilayers have a ferromagnetic behaviour, with the magnetic signal increasing with the thickness of the layers. The analysis of magnetic anisotropy evidenced an easy magnetic direction in the film plane with large anisotropy fields, which increase with the thickness of the layers and suggests a positive contribution of surface anisotropy to the effective anisotropy Keff. [source] Complex attributes of the magnetic signal for multiple sources: Application to signals from buried ditchesARCHAEOLOGICAL PROSPECTION, Issue 2 2010Christian M. Milea Abstract Complex attributes of the magnetic signal are computed using a multiple source approach. Polygonal bodies are considered and the attributes of each corner are summed to produce the overall response; that is a decomposition of the polygonal body into magnetized steps to simulate the vertices. The method is tested on synthetic examples of buried ditches, as well as on real magnetic data. This type of target was selected because it comprises a common objective in ,archaeological geophysics'. The resolution of the method in detecting the edges of the buried ditches depends on their lateral dimensions with respect to the sampling interval used. In general, the signal of the shallowest edges of the buried targets obscures the signal from the deeper corners, thus prohibiting their detection. In some extreme cases, the signals from the deeper corners posed a recognizable signature. The analytic signal may be seen as an anomaly rectification technique. In this context, it is suitable for the presentation of ,archaeological prospection' data since it provides an image that resembles the plane view of the buried antiquities better than the total field anomaly itself. Copyright © 2010 John Wiley & Sons, Ltd. [source] Theoretical evaluation of magnetoreception of power-frequency fieldsBIOELECTROMAGNETICS, Issue 5 2010Jacques Vanderstraeten Abstract Several effects of power-frequency (50/60,Hz) magnetic fields (PF-MF) of weak intensity have been hypothesized in animals and humans. No valid mechanism, however, has been proposed for an interaction between PF-MF and biological tissues and living beings at intensities relevant to animal and human exposure. Here we proposed to consider PF-MF as disrupters of the natural magnetic signal. Under exposure to these fields, an oscillating field exists that results from the vectorial summation of both the PF-MF and the geomagnetic field. At a PF-MF intensity (rms) of 0.5,µT, the peak-to-peak amplitude of the axis and/or intensity variations of this resulting field exceeds the related discrimination threshold of magnetoreception (MR) in migrating animals. From our evaluation of the 50/60,Hz responsiveness of the putative mechanisms of MR, single domain particles (Kirschvink's model) appear unable to transduce that oscillating signal. On the contrary, radical pair reactions are able to, as well as interacting multidomain iron,mineral platelets and clusters of superparamagnetic particles (Fleissner/Solov'yov's model). It is, however, not yet known whether the reception of 50/60,Hz oscillations of the natural magnetic signal might be of consequence or not. Bioelectromagnetics 31:371,379, 2010. © 2010 Wiley-Liss, Inc. [source] Magnetic signal prospecting using multiparameter measurements: the case study of the Gallic Site of LevrouxARCHAEOLOGICAL PROSPECTION, Issue 3 2010M. Pétronille Abstract The ,magnetic signal' that combines both the induced (Ji) and the remanent (Jr) magnetization is widely used in archaeological and pedological prospecting. Magnetic prospecting recording the lateral variations of the total magnetization is the most frequently used measurement before in-phase magnetic susceptibility (Kph) and magnetic viscosity (Kqu) mapping. The work presented here brings together three types of prospecting technique: magnetic field survey and electromagnetic measurements with both frequency and time domain devices that measure magnetic susceptibility and viscosity respectively. The site studied, the Gallic town of Levroux (Indre, France), is particularly interesting because it includes features such as pits and ditches dug into the calcareous substratum partly filled with topsoil and with residues of different metallurgical and fire activities. The field results indicated anomalies with different types of characterization: (i) many compact features filled with magnetic, electrically conductive and minimally viscous materials; and (ii) elongated anomalies characterized by lower magnetic properties and electric conductivity but relatively higher magnetic viscosity than those of the compact features. In addition to the location of the features, the combination of the information brought by the different types of measurements allows us to evaluate the possible erosion of their upper parts by ploughing, to assess their depth (never deeper than 1.30,m) and to precise the nature of the feature's fill. Copyright © 2010 John Wiley & Sons, Ltd. [source] Sea-surface observations of the magnetic signals of ocean swellsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2004F. E. M. Lilley SUMMARY Ocean swells have a magnetic signal, caused by the motional induction of sea water moving in the steady main magnetic field of Earth. To check the character of such signals at the sea surface, a magnetometer has been set free from a ship to float unrestricted on the surface of the ocean for periods of several days. The path of the floating magnetometer was tracked by satellite; this procedure enabled also the eventual recovery of the magnetometer by the ship. Superimposed upon a background of slow change of magnetic field, as the magnetometer drifted across different patterns of crustal magnetization, are high-frequency signals generated by the strong ocean swell present at the time. These wave-generated signals are typically up to 5 nT trough-to-peak, consistent with theory for their generation by ocean swells several metres trough-to-peak in height. The power spectra of the magnetic signals show a consistent peak at period 13 s, appropriate for the known characteristics of ocean swell in the area. The power spectra then exhibit a strong (,7 power) fall-off as period decreases below 13 s. This strong fall-off is consistent with oceanographic observations of the spectra of surface swell, combined with motional induction theory. [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] | ||||||||||