Using LIGO and IRIS to Monitor Seismic Noise Produced by Hydroelectric Dams
Interpreting Power Density Function Plots
08/04/2011
Abstract
Hydroelectric dams are a significant source of seismic noise in the 1-3 Hz frequency band at LIGO Hanford Observatory (LHO). This seismic noise shows up at LHO during the months of May and June when the dams are discharging spring runoff over their spillways. This study uses Power Density Function plots (PDF's) from the Incorporated Research Institutes for Seismology (IRIS) and LIGO Bluestone data to determine the contribution of the dams to the seismic signal detected at LHO seismometers.
Introduction
2011 was the second largest flow year for the Snake and Colombia River in over 30 years. Over several years, operators at LIGO Hanford have observed significant increases in 1-3 Hz seismic noise that corresponds with water flowing over dam spillways. LHO is surrounded by several hydroelectric dams. The closest dam is Priest Rapids located 43.6 km to the northwest of the LVEA on the Columbia River. Wanapum dam is located farther northwest on the Columbia River at a distance of 57 km from the LVEA. The largest dam near LIGO is McNary located 63 km due South on the Columbia, below the confluence of the Snake and Columbia Rivers. LIGO operators have noted in the LIGO ilog that the observed seismic noise correlates better with McNary dam than with the other dams despite the fact that it is situated 25% farther from LIGO than Priest Rapids. McNary is a much larger dam than either Wanapum or Priest Rapids because it receives water from both the Snake and Columbia rivers. The purpose of this investigation was to determine which dam is the most significant contributor to the LIGO seismic noise in the 1-3 hz frequency range.
IRIS maintains and monitors a network of seismometers throughout North America. In order to monitor data quality from these seismometers, and observe the ambient seismic noise of the environment, IRIS produces PDF plots that can be accessed through the Quality Analysis Control Kit (QUACK). These plots can be used to determine the power of seismic signals at specific frequencies measured in decibels. This study looks at several seismometers between the dams and LIGO in order to find when the seismic noise from the dams begins to attenuate.
Procedures
Seismic data from LIGO Bluestone EX sies X 1-3 Hz channel was selected because the data stream was continuous during the period of time of the study. The Following IRIS seismometers were chosen for the study due to their location and data quality: BVW, MDW, GBB, GBL, RSW, H2O, and YPT.
Monthly PDF plots were generated through QUACK and analyzed via the "explore time dependencies" option. This option allows users to see the time of day dependence of a specific frequency of seismic waves at a specific power. The IRIS PDF plots were generated for spring from April-July and compared to spill data from the dams in order to identify time dependent relationships. The strongest relationships were found in the 1-3 Hz range, consistent with observations from seismometers at LIGO. Spill data for the dams was obtained via the Fish Passage Center at http://www.fpc.org/river/flowspill/FlowSpill_Query.html.
Results
The PDF plots of ambient siesmic noise seem to point to McNary Dam as the major contributor of seismic noise at LIGO Hanford. As you follow the line of seismic stations from Priest Rapids and Wanapum dams in toward LIGO, the seismic signal becomes weak quickly. Seismic stations on the other side of the observatory, closer to McNary dam show a very clear time-dependent relationship between seismic noise and McNary Dam spillway output. IRIS seismic data obtained from seismic station BVW located on Saddle Mountain between Priest Rapids and Wanapum dam shows a strong time dependant relationship to both priest rapids and wanapum dams. The next closes seismic station is MDW which did not show a clear time dependant relationship other than there was noise spread across the power spectra at 1-3 Hz. Seismic station GBB shows a clear relationship to the timing of the spillway discharge from Priest Rapids. Seismic Station GBL did not show any time-dependant relationship to the dams, but Hanford construction diurnal trends were very clear. RSW and H20 are clearly more closely correlated to McNary Dam. Seismic Station H2O is located due east of the EY station of LIGO, just off the intersection of Highway 240 and Rout 10. Seismic station YPT near Wallula gap is strongly correlated to the spill coming from McNary Dam
Figures
Figure 1. Google Map of seismic Stations
Figure 2. LHO EX Seis X 1-3 HZ
Figure 3. Dam spill output and LIGO seismic data
Figure 4. Seismic station BVW
Figure 5. Seismic Station GBB
Figure 6. Seismic Station H2O
Figure 7. Seismic Station YPT
Discussion and Conclusions
The seismic disturbance in the 1-3 Hz band can be seen in IRIS seismometers BVW and GBB that are to the northwest of the observatory. These disturbances are associated with the spillway discharge from Priest Rapids and Wanapum dams in 2011. These disturbances are not seen in IRIS seismometers closer in to the observatory. Seismic disturbances in the 1-3 Hz band that correlate to McNary Dam spillway discharge can be seen in IRIS seismometers RSW and H2O. These seismometers are located to the south of the observatory and are both found within 10 km of the EY station. This indicates that the majority of the seismic disturbance observed on LIGO seismometers is due the spillway discharge at McNary dam
Bibliography
Spillway data for McNary Dam obtained from Army Corps of Engineers Data Query site: http://www.nwd-wc.usace.army.mil/perl/dataquery.pl
Spillway data for Priest Rapids and Wanapum dams obtained through the Fish Pasage Center at http://www.fpc.org/river/flowspill/FlowSpill_Query.html.
PDF Plots Obtained from http://www.iris.edu/dms/products/pdfpsd/