Seasonal and Diurnal Variations in Long‐Period Noise at SPREE Stations: The Influence of Soil Characteristics on Shallow Stations’ Performance

Abstract

The Superior Province Rifting Earthscope Experiment (SPREE) recorded
\ncontinuous seismic data over the Midcontinent Rift from April 2011 through October
\n2013. Analysis of power spectral density (PSD) estimates shows that horizontal noise
\nlevels at periods >20 s vary seasonally and diurnally. During winter, horizontal noise
\npower at many SPREE stations is within 5 dB of nearby Transportable Array (TA)
\nstations. As the ground thaws, SPREE stations in fine-grained material such as silt or
\nclay become noisier due to changes in the mechanical properties of the soil. During
\nsummer, the daily mean PSD value of stations in fine-grained material is approximately
\n10–20 dB higher than in the winter, and daytime noise levels are 20–30 dB
\nhigher than nights. Stations in sandy material also show diurnal variations of
\n20–30 dB during summer, but the daily mean PSD value varies no more than 5–10 dB
\nduring the year. Most neighboring TA stations have relatively constant daily mean
\nPSDs, and their horizontal components show summer diurnal variations of 10–15 dB.
\nSome very quiet TA stations, such as SPMN, show a 5–10 dB increase in horizontal
\nnoise power during winter. The timing and amplitude of horizontal noise power variations
\nbetween 20 and 800 s correlate with variations in atmospheric pressure PSDs.
\nWe propose that the grain size and pore water content of the material surrounding a
\nshallow seismic station influences the local response to atmospheric pressure. Stations
\nthat must be placed in soft sediments should be installed in sandy, well-drained
\nmaterial to minimize long-period noise generated by atmospheric pressure variations.