Optimal receiver depth for seismic monitoring in shallow boreholes

Long-term reservoir monitoring often uses seismic receivers placed in shallow boreholes to increase the Signal-to-Noise Ratio (SNR) of recorded seismic waves, either for passive or active seismicity. This study demonstrates that both seismic signals and noise at frequencies above 1 Hz decrease with depth in shallow boreholes. This is the first study to show that the seismic noise decays exponentially in shallow depths and nearly linearly for greater depths. We explain this observation using the seismic noise model formed by surface and body waves. This model explains why seismic noise levels at depths smaller than one wavelength of surface waves decay exponentially. We conclude that the optimal receiver depth to obtain the most significant improvement in the SNR (either microseismic events or reflections from interfaces) can be achieved at a depth greater than one wavelength of surface waves.