Spatial Coherence Measurements and Evaluation of a Noise Reduction Technique for Ambient Noise from 0.3 to 40 Hz

摘要

Measurements of the ambient magnetic field in the frequency range from 0.3 Hz to 40 Hz were made at a mid-latitude location in winter (China Lake, California) and an auroral location in summer (Tromso, Norway). At each location two sensitive triaxial SQUID magnetometers were installed at sites whose separation was varied from 0.5 km to 15 km (in California) and from 0.5 km to 9.3 km (in Norway). Approximately two weeks of nearly continuous data were recorded at each location. Schumann resonances with a particularly high ratio of signal-to-instrument noise were observed, with six orders of the resonances typically visible. Peaks attributed to wind-induced motion noise were observed in the spectra, particularly at the exposed China Lake sites. Prominent spikes were present in the spectra above about 10 Hz; except for several of these spikes that were traced to the tape recorder playback electronics, they are believed to represent genuine man-made ambient signals. Probable sources of the interference include 4-pole synchronous motor emissions, powerline subharmonics, and electrified railway or telephone ringing signal emissions. In California the coherence between the nominally parallel sensors at the two sites was relatively high (greater than 0.75) at all separations and showed definite peaks at the frequencies of the Schumann resonances. The coherence was highest at frequencies from about 5 Hz to 20 Hz, usually 0.9 or more. No structure in the coherence attributable to the effects of geological inhomogeneities was seen in the California data. The coherence between orthogonal sensors in both California and Norway was usually 0.25 or less and was highly variable.