Measurement of Ambient Magnetic Field Gradients Using a Superconducting Magnetic Gradiometer.

摘要

This report presents the results of investigations involving the relationship of the motion of ocean waves to measurement of fluctuating magnetic fields that result from a wave progressing horizontally in a stratified ocean. This report (1) describes design features of a superconducting magnetic gradiometer that will be employed to measure gradients of magnetic fields generated at a fixed point above the ocean surface due to waves passing the oceanographic tower operated by the U.S. Naval Undersea Center near San Diego,CA. In addition,the report gives a formulation describing instrument response to ambient magnetic gradients that are sensibly constant over the distance separating centers of the pickup loops in the gradiometer. This report also presents the first measurements of spectra that characterize noise in the frequency range 5 x 10 (-4) to 20Hz of a superconducting magnetic gradiometer operating in a magnetically quiet environment. Two techniques that provide means of suppressing noise from nearby magnetic objects are examined. A procedure for operating at the Oceanographic tower that both uses the techniques to suppress noise from magnetization currents in the tower's steel structure,and gives a maximum response to gradients from internal waves is presented. (Author)