SEISMIC NOISE ANALYSIS AT RECENT BROADBAND STATIONS OF EGYPTIAN NATIONAL SEISMOLOGICAL NETWORK

Abd El-Aziz Khairy Abd El-AalNational Research Institute of Astronomy and

Geophysics, Helwan, Cairo, Egypt

S&T2011 conference in Vienna, Austria from 8 to 10 June 20011

Aim of the study

Study the nature and sources of seismic noise in the sites of recent installed very broadband stations.Measuring the noise stability and the efficiency of station to record regional and teleseismic events.The results of this study could be used to evaluate station quality, improve those processes that require background noise values, such as automatic association and to improve the estimation of station and network detection and location thresholds.The period of this experiment is completely one year to measure the difference of noise level in summer, autumn, winter, and spring also to determine difference of noise values between day and night.First step to develop new noise model for north Africa, Meddle East, Arab world, not included in Peterson 1993

Seismic networks working in Egypt

1- Egyptian National seismological network (ENSN)- One Main center and five sub-centers.- 64 stations distributed all over Egypt (10 very broadband

+ 7 broadband stations + one borehole station + 46short period stations).

- Seismometer type: Trillium (240, 120 ,40), SS1, STS, L4C).

- Software used for seismic analysis: Hydra, Atlas and Dan.

2- Egyptian National strong motion network (ENSN)- 20 stations distributed all over Egypt.

3- The entrance of Gulf of Suez network (Japanese network)

4- Aswan network (around Naser lake)

Geographic distribution of Egyptian National seismological network (ENSN)

64 stations:

10 very broadband7 broadband 1 borehole 46 short period

Communication:- Satellite- Radio

Sensor type:Trillium (240, 120

,40) SS1, STS, L4C, and

CMG3T).

Location map of recent installed very broadband stations in ENSN

Sensor type:TRILLIUM 240

STS2 (KOT station)

Digitizer Type:TRIDENT

Communication:SATELLITE

Stations:SWA, SLM, KOT, FRF, NKL,

HRG, ADB

Components of one recent permanent installed very broadband station in ENSN

- depth of vault is 8 meters consists of three rooms, the lower room for very broadband seismometer, the middle for isolation, the upper for battery other instruments

- Very well isolation, - Transmitter and

digitizer - satellite communication

Data acquisition and analysis

Continues recording of noise data for all studied stations.Recording sampling is 100 s/s.Time of experiment is complete one year.Take data samples continually every 4 days for one complete year for each station. Length of data samples is 20 minutes, 2 hours and 96 hours ( more than 4 days).Two developed codes (Matlab code and Fortran code) were used to obtain power spectral density (PSD).Re-sampling or dissemination of noise data at VBB, LP, ULP periods.

Noise Models at SWA and SLM stations during complete year (2009)

Noise Models at NKL and KOT stations during complete year (2009)

Noise Models at HRG and FRF stations during complete year (2009)

Noise Models at NADB station during first months of operation different periods

An overlay of spectra (SP, VBB, ULP) at SWA, SLM, NKL, and Kot stations.

An overlay of spectra (SP, VBB, ULP) at

HRG and Kot stations.

Noise level at different periods during 2009 at SWA and SLM stations

Noise level at different periods during 2009 at NKL and KOT stations

Noise level at different periods during 2009 at HRG and FRF stations

Map shows worldwide stations used in Noise Models of Peterson 1993

• 75 stations• No station in North

Africa, Meddle East red rectangle.

• Peterson 1993 said in pages 1 and 31 “ one might hope for a better distribution of sations in southern hemisphere, especially Africa, and south America in order to look for regional variations in seismic noise.

• “The new low-noise model NlNM may also need revision if data records from mid-continent sits are found”

Two curves of New Noise Model

1- Low Noise Model (NHNM) need modify than Peterson 1993NLNMacc =A+Blog10 (P) dB ref to 1 (m/sec²)²/HzNLNMvel = NLNMacc + 20log10 (P/2π) dB ref to 1 (m/sec²)/HzNLNMdisp = NLNMacc + 20log10 (P²/4π²) dB ref to 1 (m²)/Hz

2- High Noise Model (NHNM) same as Peterson 1993NHNMacc =A+Blog10 (P) dB ref to 1 (m/sec²)²/HzNHNMvel = NHNMacc + 20log10 (P/2π) dB ref to 1 (m/sec²)/HzNHNMdisp = NHNMacc + 20log10 (P²/4π²) dB ref to 1 (m²)/Hz

Where P is the period, A is power spectral density at that period, B is constant for each segment depending on graphically fitting of lower and upper envelopes of spectral overlay.

Conclusion

This study is the first step to develop new noise model for north Africa, Middle East, Arab world.The obtained results give us good pictures about the station efficiency, quality and also about good site selection in all stations.Data missing in some days or months due to data transfer or satellite communication.The results indicate that the noise level in all stations are below High Noise Model (NHNM) of Peterson 1993.The results also indicate that the noise level in some months specially in VBB and ULP periods are below Low Noise Model (NLNM) of Peterson 1993.According to this study The below Low Noise Model (NLNM) of Peterson 1993 may be need to modify.Noise data from Saudi Arabia, Oman, Libya, Algeria are required also for better estimation of new Noise Model.

THE END