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 windshields 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 couplingrelated 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.
The results of an experiment aimed at identifying the nature of major noise sources within an urban area are described. We found the strongest noise source to be an irrigation pump located adjacent to the geophones. The noise from the pump had a narrow bandwidth centered at 75 Hz with a duration of 5 minutes every 17 and 34 minutes during the day and night, respectively. Traffic noise was mainly restricted to between 10 and 25 Hz, with its strength decreasing between 9 p.m. and 6 a.m. Passing aircraft resulted in noise between 30 and 200 Hz lasting about 1 minute. Electrical noise was observed at the supply frequency of 50 Hz, although additional noise at 45 Hz also was observed. Given these results we recommended that acquisition within the area should be restricted to late evening or early morning, receiver locations should be selected to avoid strong localized sources of electrical and/or mechanical noise, and any cables associated with the recording system should be as short as possible (although nodal systems are preferable). If nodal systems are deployed for logistical reasons, real-time noise monitoring should be deployed to identify and avoid bursts of high-amplitude, short-duration noise.
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