High-frequency seismic noise characteristics at the four United Kingdom-type arrays

The noise spectra are described from 0.3 to 8 Hz for the United Kingdom (UK)-type medium aperture arrays EKA, YKA, GBA, and WRA. These arrays have been operating in the same configuration since the mid-1960's and therefore provide a unique source of high-quality data for frequencies in this range. The array noise spectra are compared to spectra recorded at sites (Lajitas, NORSAR, and RSTN) specifically selected for high-frequency signal recording. At nearly all of these stations and arrays (borehole or near-surface), the characteristic noise spectrum follows a linear trend above 2 Hz, with displacement amplitude slope varying from f −1.7 to f −2.5. The GBA spectrum is an exception in that it flattens above 4 Hz, apparently due to locally generated high-frequency noise trapped near the surface. This shows that spectral flattening is not an infallible indicator of system noise contamination. System noise imposes no significant limitation on the ability to recover seismic signal or noise spectra to at least 8 Hz for EKA, GBA, and YKA. The WRA seismic noise spectra are contaminated by system noise above 3 to 4 Hz, apparently due to transmission link problems. It seems likely that borehole seismometers at YKA, GBA, and WRA would achieve noise levels between those recorded at NORSAR and the very quiet Lajitas site; i.e., similar to those at the quieter RSTN sites RSNT, RSON, and RSSD.