Fluctuating magnetic gradients over oceans come from electric currents flowing in seawater arising from its motions across the earth's magnetic field. Gradients of 0.3 to 0.6 picoteslas per meter for each meter of internal wave displacement have been measured at frequencies of 2 to 5 millihertz with a superconductive magnetic gradiometer supported 7 meters above the surface of water 18 meters deep about 1.5 kilometers offshore from San Diego, California.
First measurements of the performance of a superconductive microprobe, made for eddy current evaluation of subsurface flaws, show its depth of field and resolution are far superior to conventional eddy current probes. The prototype detects millimeter size flaws through a few millimeters of aluminum. Later microprobes promise to detect submillimeter size flaws through several millimeters of aluminum.
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