Methods for reducing unwanted noise (and increasing signal) in passive seismic surveys

Abstract

Passive seismic surveys are becoming of increasing interest for characterising the near surface, in particular the depth of cover. For passive seismic acquisition ambient noise is both signal and noise. The ‘signal’ component is generally considered to be energy resulting from distant sources (earthquakes, storms, tides, etc.) while the ‘noise’ component is a result of near sources (vehicles, vegetation movement, etc.). To record low-amplitude passive seismic signals a combination of a low natural-frequency highly sensitive geophone with a low noise-floor acquisition system is essential to record low-amplitude passive seismic signals. To avoid wind noise the sensor should be well coupled to the surface and ideally buried. We found that placing wind-shields over the sensor made no difference to the noise level but increased noise if the sensor was poorly coupled. Our results show that good coupling is crucial for recording good quality data. If the geophone is not coupled adequately then the sensor rocks, resulting in the coupling-related resonance peak occurring at frequencies well within the bandwidth of interest. If the HVSR method is being employed then anomalous peaks can occur. The best coupling is obtained by burying the sensor, failing that then long, tapered spikes should be used. If the surface is too hard to use spikes then sandbags should be placed underneath, and on top of, the sensor.