Abstract Seismic instruments are commonly installed at depth to reduce ambient noise, yet lower noise levels do not necessarily translate into improved event detectability if the signal amplitudes also vary with depth. Evaluating borehole performance, therefore, requires considering both background noise and signal behavior. We analyze a vertical borehole array at the Glasgow Observatory, Scotland, consisting of five identical broadband accelerometers deployed between 29 and 198 m depth within a single borehole, allowing depth-dependent effects to be isolated from site and instrumentation differences. We combine long-term power spectral density (PSD) analysis with event-based short-term average/long-term average and root mean square measures to assess how instrument depth influences seismic signal quality. PSD results show a reduction in background noise with increasing depth at frequencies above 1 Hz, with the deepest instrument exhibiting a noise reduction on the order of 10 dB in the 1–10 Hz band. Analysis of 39 regional earthquakes shows that seismic signal amplitudes also vary with depth and can partially offset noise reductions at intermediate depths, such that improvements in detection are observed only where noise attenuation exceeds depth-dependent changes in signal amplitude. These results demonstrate that deeper instrument installation does not guarantee improved detection performance. Instead, detectability reflects the balance between noise attenuation, depth-dependent signal behavior, and site-specific structure.
Golriz et al. (Wed,) studied this question.