Seasonal and diurnal variations in long-period noise at SPREE stations: The influence of soil characteristics on shallow stations’ performance

The Superior Province Rifting Earthscope Experiment (SPREE) recorded continuous seismic data over the Midcontinent Rift from April 2011 through October 2013. Analysis of power spectral density (PSD) estimates shows that horizontal noise levels at periods > 20 s vary seasonally and diurnally. During winter, horizontal noise power at many SPREE stations is within 5 dB of nearby Transportable Array (TA) stations. As the ground thaws, SPREE stations in fine-grained material such as silt or clay become noisier due to changes in the mechanical properties of the soil. During summer, the daily mean PSD value of stations in fine-grained material is approximately 10–20 dB higher than in the winter, and daytime noise levels are 20–30 dB higher than nights. Stations in sandy material also show diurnal variations of 20–30 dB during summer, but the daily mean PSD value varies no more than 5–10 dB during the year. Most neighboring TA stations have relatively constant daily mean PSDs, and their horizontal components show summer diurnal variations of 10–15 dB. Some very quiet TA stations, such as SPMN, show a 5–10 dB increase in horizontal noise power during winter. The timing and amplitude of horizontal noise power variations between 20 and 800 s correlate with variations in atmospheric pressure PSDs. We propose that the grain size and pore water content of the material surrounding a shallow seismic station influences the local response to atmospheric pressure. Stations that must be placed in soft sediments should be installed in sandy, well-drained material to minimize long-period noise generated by atmospheric pressure variations.