Signal and Noise on Buried and Surface Geophones for Microseismic Monitoring

Surface or near-surface monitoring on microseismic events overcomes the low signal by stacking many geophones. To reduce the number of geophones it is essential to find the optimal placement of geophones, to obtain the highest possible signal to noise ratio. The noise level decreases with depth, but the signal is enhanced in near surface layers, due to high impedance contrasts in the near surface layers and the free surface boundary condition. For better understanding of this phenomenon we study unique dataset of large microseismic events recorded on the surface and at 100 m depth. We investigate the frequency spectra of signal and noise, and the frequency dependency of SNR and the ratio of borehole and surface SNRs. We show that signal of the events are limited below 40 Hz (P-waves) and 20 Hz (S-waves), and the noise in the shallow borehole is dominated by low frequencies (below 10 Hz). Burying the geophones to 100 m depth can improve the SNR by a factor of 2-3, hence one buried geophone can replace 4-9 surface geophones.