On the Diffusivity of Noise Data Recorded (2-8 Hz) at Stations Located in North-Western Iran

نوع مقاله : مقاله تحقیقی‌ (پژوهشی‌)

نویسندگان

1 Ph.D Student, Department of Seismology, Institute of Geophysics, University of Tehran, Tehran, Iran

2 Associate Professor, Department of Seismology, Institute of Geophysics, University of Tehran, Tehran, Iran

چکیده

Since the advent of seismic interferometry, the cross-correlation of received random noises has been very frequently used for the approximate assessment of the empirical Green’s function between the station pairs. Theoretically, the diffusivity of noise wavefield isthe key factor that contributes to the success of this idea in real applications. Diffusivity itself requires the fulfillment of two conditions of energy equipartitioning as well as a homogeneous distribution of the incoming noise. To meet these requirements, the attempts are made to select a longer study period to homogenize the incoming azimuths. This solution is mostly logical in the range of 0.1-0.3 Hz, because it is believed that in this frequency range, microseismic noise waves are received continuously by the stations. But at higher frequencies, seismic sources are mostly changing both spatially and temporally, and their spectral content is not stationary over time. Therefore, in high-frequency seismic interferometry, prolonging the study interval will not help much in improving the signal-to-noise ratio of the retrieved Green’s functions. For this reason, the main focus in such studies is on highly heterogeneous regions because multiple scattering in these regions may be able to azimuthally homogenize the wave field to some extent. According to the scientific studies, however, the homogeneity of the azimuthal distribution of noise is very unlikely to happen, even in the most heterogeneous areas. So to know which claim is more correct, we turned our attention to the northwestern region of Iran, where the severe heterogeneity of the medium for frequencies above 1 Hz has already been confirmed by recent studies. With the help of a Frequency-Dependent Polarization Analysis (FDPA), we first extracted those elliptically polarized waves whose degree of polarization (IDOP) was greater than 0.8. This constraint assures us that the received waves have maintained their coherency for a relatively long time before reaching the station. Our analysis on the azimuths of these polarized data fail to pass Kuaper and  tests, indicating that the azimuth distribution of the received signals at the stations located in north-western Iran is azimuthally inhomogeneous. At the stations near the topography, even strong directivity in coherent seismic noises was observed.  

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

On the Diffusivity of Noise Data Recorded (2-8 Hz) at Stations Located in North-Western Iran

نویسندگان [English]

  • Amir Ali Hamed 1
  • Ali Moradi 2
1 Ph.D Student, Department of Seismology, Institute of Geophysics, University of Tehran, Tehran, Iran
2 Associate Professor, Department of Seismology, Institute of Geophysics, University of Tehran, Tehran, Iran
چکیده [English]

Since the advent of seismic interferometry, the cross-correlation of received random noises has been very frequently used for the approximate assessment of the empirical Green’s function between the station pairs. Theoretically, the diffusivity of noise wavefield isthe key factor that contributes to the success of this idea in real applications. Diffusivity itself requires the fulfillment of two conditions of energy equipartitioning as well as a homogeneous distribution of the incoming noise. To meet these requirements, the attempts are made to select a longer study period to homogenize the incoming azimuths. This solution is mostly logical in the range of 0.1-0.3 Hz, because it is believed that in this frequency range, microseismic noise waves are received continuously by the stations. But at higher frequencies, seismic sources are mostly changing both spatially and temporally, and their spectral content is not stationary over time. Therefore, in high-frequency seismic interferometry, prolonging the study interval will not help much in improving the signal-to-noise ratio of the retrieved Green’s functions. For this reason, the main focus in such studies is on highly heterogeneous regions because multiple scattering in these regions may be able to azimuthally homogenize the wave field to some extent. According to the scientific studies, however, the homogeneity of the azimuthal distribution of noise is very unlikely to happen, even in the most heterogeneous areas. So to know which claim is more correct, we turned our attention to the northwestern region of Iran, where the severe heterogeneity of the medium for frequencies above 1 Hz has already been confirmed by recent studies. With the help of a Frequency-Dependent Polarization Analysis (FDPA), we first extracted those elliptically polarized waves whose degree of polarization (IDOP) was greater than 0.8. This constraint assures us that the received waves have maintained their coherency for a relatively long time before reaching the station. Our analysis on the azimuths of these polarized data fail to pass Kuaper and  tests, indicating that the azimuth distribution of the received signals at the stations located in north-western Iran is azimuthally inhomogeneous. At the stations near the topography, even strong directivity in coherent seismic noises was observed.  

کلیدواژه‌ها [English]

  • Seismic noise interferometry
  • diffusivity
  • Azimuthal homogeneity
Afonin, N., Kozlovskaya, E., Nevalainen, J., Narkilahti, J., 2019. Improving the quality of empirical Green's functions, obtained by cross-correlation of high-frequency ambient seismic noise. Solid Earth, 10, 1621-1634.
Burjánek, J., Edwards, B., Fäh, D., 2014. Empirical evidence of local seismic effects at sites with pronounced topography: a systematic approach. Geophysical Journal International 197, 608-619.
Burjánek, J., Gassner-Stamm, G., Poggi, V., Moore, J.R., Fäh, D., 2010. Ambient vibration analysis of an unstable mountain slope. Geophysical Journal International 180, 820-828.
Campillo, M., Roux, P., Romanowicz, B., Dziewonski, A., 2014. Seismic imaging and monitoring with ambient noise correlations. Treatise on geophysics 1, 256-271.
Choulakian, V., Lockhart, R.A., Stephens, M.A., 1994. Cramér-von Mises statistics for discrete distributions. The Canadian Journal of Statistics/La Revue Canadienne de Statistique, 125-137.
Gorbatikov, A., Stepanova, M.Y., 2008. Statistical characteristics and stationarity properties of low-frequency seismic signals. Izvestiya, Physics of the Solid Earth 44, 50-59.
Gribler, G., Liberty, L.M., Mikesell, T.D., Michaels, P., 2016. Isolating retrograde and prograde Rayleigh-wave modes using a polarity mute. Geophysics 81, V379-V385.
Hodgson, M., 1996. When is diffuse-field theory applicable? Applied Acoustics 49, 197-207.
Koper, K.D., Hawley, V.L., 2010. Frequency dependent polarization analysis of ambient seismic noise recorded at a broadband seismometer in the central United States. Earthquake Science 23, 439-447.
Koper, K.D., Seats, K., Benz, H., 2010. On the composition of Earth’s short-period seismic noise field. Bulletin of the Seismological Society of America 100, 606-617.
Liu, X., Ben-Zion, Y., 2018. Analysis of non-diffuse characteristics of the seismic noise field in southern California based on correlations of neighbouring frequencies. Geophysical Journal International 212, 798-806.
Mardia, K.V., Jupp, P.E., 2009. Directional statistics. John Wiley & Sons.
Margerin, L., Campillo, M., Van Tiggelen, B., Hennino, R., 2009. Energy partition of seismic coda waves in layered media: Theory and application to Pinyon Flats Observatory. Geophysical Journal International 177, 571-585.
Massa, M., Barani, S., Lovati, S., 2014. Overview of topographic effects based on experimental observations: meaning, causes and possible interpretations. Geophysical Journal International 197, 1537-1550.
Mulargia, F., 2012. The seismic noise wavefield is not diffuse. The Journal of the Acoustical Society of America 131, 2853-2858.
Mulargia, F., Castellaro, S., 2010. Nondiffuse elastic and anelastic passive imaging. The Journal of the Acoustical Society of America 127, 1391-1396.
Naghavi, M., Rahimi, H., Moradi, A., Mukhopadhyay, S., 2017. Spatial variations of seismic attenuation in the North West of Iranian plateau from analysis of coda waves. Tectonophysics 708, 70-80.
Nakata, N., Gualtieri, L., Fichtner, A., 2019. Seismic ambient noise. Cambridge University Press.
Pilz, M., Parolai, S., 2014. Statistical properties of the seismic noise field: influence of soil heterogeneities. Geophysical Journal International 199, 430-440.
Ping, P., Chu, R., Zhang, Y., Xie, J., 2020. Enhancing Signal‐to‐Noise Ratios of High‐Frequency Rayleigh Waves Extracted from Ambient Seismic Noises in Topographic Region. Bulletin of the Seismological Society of America 110, 793-802.
Richards, P.G., Aki, K., 1980. Quantitative Seismology: Theory and Methods. Freeman.
Sánchez-Sesma, F.J., Pérez-Ruiz, J.A., Luzon, F., Campillo, M., Rodríguez-Castellanos, A., 2008. Diffuse fields in dynamic elasticity. Wave motion 45, 641-654.
Schimmel, M., 1999. Phase cross-correlations: Design, comparisons, and applications. Bulletin of the Seismological Society of America 89,
    1366-1378.
Schimmel, M., Stutzmann, E., Ardhuin, F., Gallart, J., 2011. Polarized Earth's ambient microseismic noise. Geochemistry, Geophysics, Geosystems, 12(7), 1-14.
Schimmel, M., Stutzmann, E., Gallart, J., 2011. Using instantaneous phase coherence for signal extraction from ambient noise data at a local to a global scale. Geophysical Journal International 184, 494-506.
Schimmel, M., Stutzmann, E., Ventosa, S., 2018. Low‐frequency ambient noise autocorrelations: Waveforms and normal modes. Seismological Research Letters 89, 1488-1496.
Stephens, M.A., 1970. Use of the
 
     Kolmogorov–Smirnov, Cramer–Von Mises and related statistics without extensive tables. Journal of the Royal Statistical Society: Series B (Methodological) 32, 115-122.
Wapenaar, K., Draganov, D., Snieder, R., Campman, X., Verdel, A., 2010. Tutorial on seismic interferometry: Part 1—Basic principles and applications. Geophysics 75, 75A195-175A209.
Wapenaar, K., Slob, E., Snieder, R., Curtis, A., 2010. Tutorial on seismic interferometry: Part 2—Underlying theory and new advances. Geophysics 75, 75A211-275A227.
Weaver, R.L., Lobkis, O.I., 2006. Diffuse fields in ultrasonics and seismology. Geophysics 71, SI5-SI9.