High frequency (1–100 HZ) noise and signal recorded at different depths in a mine, northwest Adirondacks, NY

Abstract

Seismograms of noise recorded simultaneously at the surface and at two subsurface stations (335m and 945m below surface level) show that high frequency (1–100 Hz) background noise is reduced at the subsurface stations. Seismometers at all three stations are well coupled to hard crystalline bedrock. Most of the noise reduction occurs between the surface and 335m depth. Between 1 and 3 Hz, seismic noise correlates well with wind speed at all levels. Above 3 Hz, wind speed up to 8 m/sec does not influence high frequency noise levels at 335m or 945m depths. At the surface, however, high frequency noise is associated with wind speeds in excess of 2 m/s. A site resonance is characteristic of the surface spectra, but is not characteristic of the subsurface sites. Spectral signal‐to‐noise (S/N) ratios, measured for S‐waves from a regional earthquake (Δ = 530 km, mb = 4.1), are roughly equal from 1–10 Hz at all depths. Above 10 Hz, however, S/N is greater at the subsurface stations with S/N greater than 1 up to 30 Hz for the surface station, ≈ 50 Hz for the 335m station, and ≈ 70 Hz for the 945m station. Results indicate the advantage of employing high frequency sensors sufficiently below the surface to reduce the level of noise and to improve the S/N ratio in the high frequency range. Such an advantage could be critically important for nuclear test detection and discrimination.