Long-term self-noise estimates of seismic sensors from high-noise vault installations
We report on results from a self-noise test experiment of Güralp seismic instruments (3T, 3ESPC broadband seismometers, Fortis strong motion accelerometer) that were installed in the sub-surface vault of the Eskdalemuir Seismic Observatory in Scotland, UK over the period October 2016–August 2017. Due to vault’s proximity to the ocean, secondary microseism noise is strong, so we efficiently compute the angle of misalignment that maximises waveform coherence with a reference sensor. Self-noise was calculated using the 3-sensor correlation technique and we compute probability density functions of self-noise to assess its spread over time.
We find that not correcting for misalignments as low as 0.1° can cause self-noise to be artificially higher by up to ~15 dB at frequencies of 0.1-1 Hz. Our method thus efficiently removes the effect of microseism contamination on self-noise; for example, it restores the minimum noise floor for a 360s – 50 Hz 3T to -195 dB at 0.2 Hz. Furthermore, based on the analysis of our calculated probability density functions, we find at long-periods (> 30 s) the average self-noise can be up to 5 dB higher than the minimum noise floor. We discuss the validity of these results in terms of making direct comparisons with self-noise results from much quieter installations.