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Muntele Rosu Seismic Station Conclusions ! ! ! 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) ! ! ! 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) References Acknowledgments ANALYSIS OF THE BACKGROUND NOISE AT THE AUXILIARY SEISMIC STATION MUNTELE ROSU 1 1 1 1 Daniela Ghica , Bogdan Grecu , Constantin Ionescu and Mihaela Popa 1 National Institute for Earth Physics, 12 Calugareni St., PO Box MG-2, 077125 Magurele, Romania, Tel.: +4021 405 60 65, e-mail: [email protected] CTBT: Science and Technology 2011 T3 - P18 ! ! ! ! ! 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 o o The MLR station (45.490 N, 25.945 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 2 MLR station - General view with landscape Figure 3 MLR station - Topography Figure 1 MLR station - Geographical postion Data Analysis ! ! ! ! ! ! ! ! ! ! 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) Figure 4 Overall Characteristics Figure 5 Diurnal Variation Automatic Detections from MLR data 0 200 400 600 800 1000 1200 1400 1600 Dec-06 Jan-07 Feb-07 Mar-07 Apr-07 May-07 Jun-07 Jul-07 Aug-07 Sep-07 Oct-07 Nov-07 No. of phases Regional Teleseismic Noise Figure 7 Figure 6 Seasonal Variation a) b) ! ! ! ! ! ! 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
