EGU2020-481, updated on 12 Jun 2020
https://doi.org/10.5194/egusphere-egu2020-481
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Simultaneous body and surface wave retrieval from the seismic ambient field and discrimination from unavoidably arising spurious artifacts

Ali Riahi1, Zaher-Hossein Shomali2,3, Anne Obermann4, and Ahmad Kamayestani5
Ali Riahi et al.
  • 1Institute of Geophysics of University of Tehran (IGUT), University of Tehran , Iran (ali.riahi@ut.ac.ir)
  • 2Institute of Geophysics of University of Tehran (IGUT), University of Tehran , Iran (shomali@ut.ac.ir)
  • 3Department of Earth Sciences, Uppsala University, Sweden (Hossein.Shomali@geo.uu.se)
  • 4Swiss Seismological Service (SED), ETH Zurich, Switzerland (anne.obermann@sed.ethz.ch)
  • 5Department of Geophysics, National Iranian Oil Company- Exploration Directorate, Iran (a.kamayestani@niocexp.ir)

We simultaneously extract both, direct P-waves and Rayleigh waves, from the seismic ambient noise field recorded by a dense seismic network in Iran. With synthetics, we show that the simultaneous retrieval of body and surface waves from seismic ambient noise leads to the unavoidable appearance of spurious arrivals that could lead to misinterpretations.

We work with 2 months of seismic ambient noise records from a dense deployment of 119 sensors with interstation distances of 2 km in Iran. To retrieve body and surface waves, we calculate the cross-coherency in low-frequency ranges, i.e. frequencies up to 1.2 Hz, to provide the empirical Green’s functions between each pair of stations. To separate the P and Rayleigh waves, we use the polarization method that also enhances the small amplitude body waves.

We observe both P and Rayleigh waves with an apparent velocity of 4.9±0.3 and 1.8±0.1 km/s in the studied area, respectively, as well as S or higher mode of Rayleigh waves, with an apparent velocity of 4.1±0.1 km/s. Besides these physical arrivals, we also observe two spurious arrivals with similar amplitudes before/after the P and/or Rayleigh waves that render the discrimination challenging.

To better understanding these arrivals, we perform synthetic tests. We show that simultaneously retrieving the body and surface waves from seismic ambient noise sources will unavoidably lead to the appearance of superior arrivals in the calculation of empirical Green’s functions.

How to cite: Riahi, A., Shomali, Z.-H., Obermann, A., and Kamayestani, A.: Simultaneous body and surface wave retrieval from the seismic ambient field and discrimination from unavoidably arising spurious artifacts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-481, https://doi.org/10.5194/egusphere-egu2020-481, 2019

Comments on the presentation

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Presentation version 1 – uploaded on 04 May 2020
  • CC1: Comment on EGU2020-481, Taghi Shirzad Iraj, 05 May 2020

    Dear A. Riahi

    As you know, not only extraction body waves but also retrieving Rayleigh waves directly depend on stationary and non-stationary sources. If the emitted energy from these non-stationary sources a bit stronger than the stationary sources, a large bias would be in retrieving EGF. Hence the extracted signal is not EGF if the distribution of energy flow is non-uniform/inhomogeneous. Because the theory of ambient seismic noise was established for an isotropic medium with uniform distribution of sources and energy flow (see many publications of R. Snieder etc.). You can test these effects by using 1.5 times of non-stationary and one time of stationary sources in your stacking procedure. In brief, extracting body wave without locating noise sources is impossible. I would be appreciated if you could give some information about removing the effect of non-stationary sources as well as uncertainties of your signals?  
    Moreover, anisotropy anomalies could play a major role in the polarization of extracted signals, you can test it by adding a bit ( e.g., 2%) radial and/or azimuthal anisotropy in your synthetic test medium (page 13 your presentation). Then, after stacking, you can extract a Rayleigh wave signal (retrieved for anisotropic medium) that equivalent to the P-wave signal extracted in the isotropic medium. 
    I would be grateful if we have some information about decreasing the effect of radial and azimuthal anisotropy in your processing. Please note that the extracted P-wave signals without investigation of these two points would be Rayleigh wave signal which is caused by stationary and non-stationary sources. So, by multiplying this P-wave with cos(phi) operator, the reliable EGF can be retrieved. 
    Finally, in order to extract P- Rayleigh-waves, different combinations of ZR and RZ were applied in the presentation. This condition just can be true for an isotropic medium with uniform distribution of sources and energy flow. 

    Best Regards

    T.Shirzad, Ph.D.,

    Assistant Professor,

    Institute of Geophysics, Polish Academy of Sciences,

    Tel: +48 12 292 38 04

    • AC1: Reply to CC1, Ali Riahi, 05 May 2020

      Dear Dr. Shirzad

      Many thanks for commenting on our research. Your points about stationary sources are very helpful. Please let me keep in touch, so that I can send you some further information about the distribution of the ambient noise sources, in the future.

      Regarding the separation of P and Rayleigh waves, please see section 5 of Takagi et al., (2014). That research proved that polarization method and summation and difference of ZR and RZ are valued even in anisotropic conditions.

      Best regards

      Ali Riahi

  • CC2: Comment on EGU2020-481, Alemayehu Jemberie, 05 May 2020

    Surface can also be used for attenuation modeling.