analysis of the background noise at the auxiliary seismic

ANALYSIS OF THE BACKGROUND NOISE AT THE AUXILIARY SEISMIC
STATION MUNTELE ROSU
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Daniela Ghica , Bogdan Grecu , Constantin Ionescu and Mihaela Popa
CTBT: Science and Technology 2011
T3 - P18
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National Institute for Earth Physics, 12 Calugareni St., PO Box MG-2, 077125 Magurele, Romania, Tel.: +4021 405 60 65, e-mail: [email protected]
Data Analysis
Muntele Rosu Seismic Station
! The auxiliary seismic station Muntele Rosu (AS081, MLR), is part of International Monitoring System
(IMS), being operated by the National Institute for Earth Physics (NIEP, Bucharest) in support of the
verification regime of the Comprehensive Nuclear-Test-Ban Treaty
! The MLR station (45.490o N, 25.945o E, 1,360 m altitude) (figure 1) has been running since 1970; in
2001, a new seismic monitoring system was installed at MLR: three-component (3C) broad band
seismic sensor STS-2 and Quanterra datalogger with GPS antenna
! Seismic data are recorded locally and forwarded directly to the IDC upon request at any time through
on-line connections and VSAT transmission
! Relatively quiet background noise conditions, with very few noise sources (except of the natural
environmental one) are characteristic for the MLR station (figure 2 and 3)
! Site geology consists of limestone and conglomerate, Cretaceous flysch
Figure 1 MLR station - Geographical postion
! The analysis of the background noise at MLR station was carried out for one year: between December 2006 and November 2007
! The characteristic of the seismic noise recorded at MLR is analyzed using Power Spectral Density (PSD) estimates and their corresponding Probability Density Functions (PDFs) (McNamara and Buland, 2004)
! Probability Density Function (PDF) for EW, NS and Z components of 3C station MLR were constructed using more than 14000 PSDs to show the overall characteristics (figure 4); the color bar represents the probability of
occurrence, the dashed line represents the PDF median and the two grey lines represent the Petersons's (1993) NHNM and NLNM models
! For frequencies f > 1 Hz, noise level lays 20 dB above NLNM; for f < 1 Hz, this difference varies between 10 and 30 dB
! Diurnal and seasonal variations are observed in the MLR background noise
! The diurnal variation for the MLR vertical component is represented by the variation in the PDF median as a function of hour of the day at station (figure 5):
- for f = 0.5 - 30 Hz throughout the day: daytime noise level is 30dB above nighttime level
- clearly increase of the noise level during the daylight hours
! The seasonal variation of the MLR noise level implies frequency dependence
! Seasonal variation for vertical component of MLR station is depicted in figure 6a):
- for the secondary microseismic band (1 - 10 sec) the seasonal variability of noise is evident, with the noise power increasing during the colder months
- for periods T > 30 sec, the noise level is lower from April to July
! Median of the PSD was computed for 3 months, from December 2006 to November 2007 (figure 6b))
- the noise power levels are lower during summer and highest during winter, while during spring and fall, the noise level is very similar and in-between the firsts two
- the maximum of the double-frequency peak shifts from smaller periods (T ~ 4.5 sec) in the summer to higher periods (T ~6.5 sec) in the winter
- at lower periods (T < 1 sec), the noise level is slightly higher during summer
! Noise analysis results are consistent with the MLR automatic detection performance reported by IDC for regional, teleseismic and noise phases (figure 7):
- the number of noise phases detected at MLR is visibly higher in the winter season
- the MLR detection capability for regional and seismic phases is lower during the summer; this behavior could be associated with the specific seasonal human activity and atmospheric conditions (thunderstorms)
Overall Characteristics
Diurnal Variation
Figure 4
Figure 5
Figure 2 MLR station - General view with landscape
Seasonal Variation
Automatic Detections from MLR data
1600
Regional
Teleseismic
Noise
1400
Figure 3 MLR station - Topography
1200
Conclusions
No. of phases
1000
! The analysis of the background noise at MLR station for one year showed that in terms
of overall characteristics, noise level lays 20 dB above Peterson’s NLNM for f above 1
Hz, whilst for f below 1 Hz, this difference varies between 10 and 30 dB
! The noise diurnal variation of the MLR noise is represented by an increasing of the
daytime noise level with 30dB above nighttime level, for a frequency band between 0.5
and 30 Hz
! The noise seasonal variation at MLR implies frequency dependence, i.e. the noise level
increases during the winter for T = 1 - 10 sec., whilst from April to July, this level is lower
! For T > 1 sec, the noise power levels are lower during summer and highest during
winter, while during spring and fall, the noise level is very similar and in-between the
firsts two
! At lower periods (T < 1 sec), the noise level is slightly higher during summer
! Noise analysis results are consistent with the MLR automatic detection performance
reported by IDC for regional, teleseismic and noise phases
800
600
400
200
b)
a)
0
Dec-06
Figure 6
References
! McNamara, D. E., and R. P. Buland (2004). Ambient Noise Levels in the Continental United States, Bulletin of Seismological Society of America, 94,
no. 4, 15717-152
! Peterson, J. (1993). Observations and Modeling of Background Seismic Noise, in U.S. Geological Survey Open-File Report 93-322, Albuquerque,
New Mexico
! Wessel P., Smith W .H .F. (1995). New version of the Generic Mapping Tool released. EOS Trans. AGU, p 329 (see also URL: gmt.soest.hawaii.edu)
Jan-07
Feb-07
Mar-07
Apr-07
May-07
Jun-07
Jul-07
Aug-07
Sep-07
Oct-07
Nov-07
Figure 7
Acknowledgments
! This work was partly supported by CNCSIS-UEFISCDI, project number PN II-RU 120/2010
! This paper exploited some of the data reported in the International Data Centre (IDC)
Monthly Performance Reports for the period analyzed
! Maps were created using GMT software (Wessel & Smith 1995)