Gradients and phase velocities of ULF magnetic disturbances (F = 0.1 - 0.4 Hz) before and during strong earthquakes in 2003 year at Bosso Peninsula (Japan)
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Abstract:
Measurements of ULF EM variations (T = 1-1000 s) were carried out in Japan before and during a seismically active period in 2003. Three of the same type 3-component high-sensitivity torsion magnetometers, MVC-2DS, were located at Bosso peninsula at the vertices of a triangle at a distance 4-5 km from each other (a magnetic gradiometer). The magnetic gradiometer gives an opportunity to construct gradient and phase velocity vectors of the ULF magnetic disturbances. The gradient vectors are usually directed to the source of the magnetic disturbances. The phase velocity vectors usually have the opposite direction. Seismic activity was observed in this region during the whole year. We observed anomalous behavior of the gradients and phase velocities before strong earthquakes.Keywords:
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Abstract The detection of induced and remanent magnetization of subsurface features provides a rapid survey technique. Rejection of the unwanted geomagnetic field and its diurnal variations is accomplished by adopting a gradient magnetometer arrangement in which only the difference signal from two magnetometers, separated by a short distance, is recorded. Nuclear resonance scalar magnetometers offer high sensitivity but have higher power requirements and therefore suffer a weight penalty relative to fluxgate types. Fluxgate magnetometers are vector sensing and their sensitivity is limited, mainly by thermally induced mechanical and electronic drift. This paper describes the development of a 1‐m separation, vertical component, fluxgate magnetometer that alleviates these problems. Single and multiple array gradiometer configurations are presented, together with a description of data logging software developed to enhance survey efficiency. Survey data acquired during trials and testing are presented. The ergonomics of portable instrument surveying is also discussed. Copyright © 2003 John Wiley & Sons, Ltd.
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