Signal-to-Noise Ratio Models for Shallow Borehole Seismic Monitoring

Seismic receivers are placed in shallow boreholes to increase recorded seismic waves’ signal-to-noise ratio, especially for detecting small seismicity or microseismicity. This study demonstrates that both the seismic signal and seismic noise at frequencies greater than 1 Hz decay with depth in shallow boreholes. Furthermore, we observe that the seismic noise consists of body and surface waves. The body-wave noise can be mod-eled as a wave originating from sources at the surface that penetrate depths exceeding one wavelength of the surface waves. We show that seismic noise at the surface and its immediate vicinity decays exponentially because it is dominated by surface waves for depths smaller than one wavelength of surface waves. The specific sources of seismic noise significantly vary between two studied datasets (anthropogenic in Groningen and wind in FORGE), the observed shallow borehole noise levels can be characterized with the same conceptual model.