Title: SPREE stations sense soil-dependent response to atmosphere

NSF award number: EAR-0952345 ("SPREE") 2010-2014

Collaborators:

Emily Wolin1, Suzan van der Lee1,

Trevor A. Bollmann1, Douglas A. Wiens3, Justin Revenaugh2, Fiona A. Darbyshire5, Andrew W. Frederiksen4, Seth Stein1, and Michael E. Wysession3

1. Northwestern University

2. University of Minnesota

3. Washington University

4. University of Manitoba

5. University of Quebec at Montreal

Summary:

Seismic stations of the Superior Province Rifting Earthscope Experiment (SPREE) were installed in shallow hand-dug vaults in Minnesota, Wisconsin, and Ontario around the Mid-continent Rift. Because of their shallow burial in soft soil, SPREE stations were able to sense the tilting of the Earth's surface caused by fluctuations in atmospheric pressure. Fascinating changes occur in the strength of this low-frequency "noise" on daily and seasonal cycles.

During winter, noise strength was comparable to nearby Earthscope Transportable Array stations installed in much deeper vaults. But in summer, noise strengthened by up to 10 dB for SPREE stations installed in sandy soil and up to 20 dB for stations installed in clayey soil. These differences in noise are related to the amount and phase of water trapped in the surrounding soil. Clayey soil holds more water than sandy soil, so it responds more strongly to atmospheric pressure changes when its pore water is liquid, but also freezes more solidly in winter.

From nights to days, low-frequency noise at SPREE stations strengthened by up to 30 dB, due to the daily presence of strong atmospheric convective systems. The more deeply buried Transportable Array stations in the area showed half those variations.

Understanding sources of noise in SPREE seismograms improved the quality of our data analyses (see other SPREE nuggets). This analysis also showed how seismometer data can be analyzed to turn seismometers into virtual barometers, thermometers (to distinguish frozen from thawed conditions), and measurers of soil type and moisture.

Location tags: Minnesota, Wisconsin, Ontario

Figure 1: Temperature from two seismic stations and a weather station (top), and daily ratios of seismic noise and atmospheric pressure strengths for two SPREE stations (SN56, SN55) (middle) and one Transportable Array station (F38A) (bottom). The ratio at F38A and SN55 remains relatively steady year-round. All stations have significantly weaker noise levels when the temperature is below freezing. SN56, which was buried in clayey soil, shows large ratios in the wetter seasons of spring and fall, potentially illustrating changing soil properties (like moisture content) from season to season.

Figure 2: Summary sketch of seasonal changes in soil response.

Photo: PI Suzan van der Lee with Wisconsin soil in which a SPREE station was buried.

Reference:

Wolin, E., et al., Seasonal and Diurnal Variations in Long-Period Noise at SPREE Stations: The Influence of Soil Characteristics on Shallow Stations' Performance, Bulletin of the Seismological Society of America, Vol. 105, No. 5, pp. –, October 2015, doi: 10.1785/0120150046 .