پردازش نوفه محيطي ابزاري قوي براي تعيين ساختار سرعتي زمين

Ambient Noise Processing as a Functional Tool forSeismic Velocity Earth’s Structure Determination

در ايـن مقاله نوفه محيطـي ثبـت شده در ايستگاههاي لـرزه نگـاري از منظر محتواي فـركانسي، دامنه و منشا آنها بررسي شده است. در مرحله بعد بررسي مباني نظري محاسبه تابع همبستگي متقابل نوفه محيطي بين دو ايستگاه لرزه نگاري بر اساس تيوري ميدان موج انجام شده است. سپس نوفه محيطي ثبت شده در ايستگاههاي لرزه نگاري باند پهن بندرعباس، زاهدان، شارخت (قاين)، طبس، نايين، كرمان و چابهار وابسته به پژوهشگاه بين المللي زلزله شناسي و مهندسي زلزله براي محاسبه تابع همبستگي متقابل دو ايستگاهي استخراج شده است. بررسي توابع همبستگي متقابل محاسبه شده نشان مي دهد كه اين توابع متشابه با شكل موجهاي سطحي از نوع ريلي است كه در مولفه قايم لرزه نگاشتهاي باند پهن مشاهده مي-شود. با استفاده از تبديل فوريه گسسته و به روش فركانس- زمان، آناليز طيفي امواج سطحي بر روي توابع همبستگي متقابل ناشي از نوفه محيطي انجام شده است. نتايج اين فرآيند منحني هاي پاشيدگي موجهاي سطحي ريلي است كه تفسير كيفي آن لايه بندي زمين در مسير دو ايستگاه متوالي را نشان مي دهد. مقايسه منحني هاي پاشيدگي ناحيه جنوب شرق كشور با منحني هاي پاشيدگي مرجع جهاني تا حدود زيادي تطابق دارد. اين موضوع، اهميت و كارايي نوفه محيطي ثبت شده در ايستگاههاي لرزه نگاري باند پهن براي محاسبه ساختار سرعتي پوسته زمين بر اساس آناليز طيفي موجهاي سطحي را تاييد مي نمايد.

In this article, we try to investigate the ambient noise characteristics based on amplitude, frequency content and its origin that were recorded on the broad-band seismic stations. To do this, we reviewed the technical literature from middle of 20th century noise studies until recent years. Moreover, cross correlation function of noise that is equivalent with the experimental green function is explained as the background study. For the first time, Campillo and Paul (2003) have calculated group velocity of Rayleigh and Love surface waves from waveforms of 101 teleseismic earthquakes recorded in the national Mexican seismic network. After that, the investigation into ambient noise for Green function analysis have been continued by Shapiro and Campillo (2004, 2005), Schuster et al. (2004), Snieder (2004), Bensen et al. (2007) and Wapenaar et al. (2013). They showed that it is possible to get the Green function between stations through calculating Cross Correlation Function of recorded ambient noise. Ambient noise is important since it does not dependent on occurred an earthquake. That’s why ambient noise is used widely in order to study inner structure of the ground between two seismic stations. It should be noted that the dynamic range and frequency content of the seismometers has important effect on the wide band of group velocity dispersion curves. In other words, the ambient noise recorded by means of short period sensors is useful to derive low period dispersion group velocity curves only. Therefore, we strongly suggest using broadband seismic stations to study surface wave analysis based on ambient noise. The next key factor to derive sharp experimental green function is optimum distance between two seismic stations. The shorter distance generates a high frequency green function which is due to shallow depth penetration of the energy. Therefore, the depth of target will define the distance between two seismic stations. However, the type of sensor can be important. Based on seismic wave propagation theory, the Rayleigh wave can be seen on vertical component and the Love wave can be seen on the horizontal components of a seismic station. Therefore, if one like to study dispersion curves of Rayleigh wave need to process vertical component of seismograms. Of course, in practice, using rotated horizontal component can generate sharp Love wave. The seven seismic broad-band stations located in the south-southeastern part of the country are selected for this research. The cross-correlation method was used to calculate the experimental Green’s functions for pairs of seismic stations (BNDS-CHBR, BNDS-ZHSF, BNDS-KRBR, BNDS-NASN, BNDS-TABS, and BNDS-SHRT). Dispersion curves of group velocities belonging to fundamental mode Rayleigh waves determined by frequency - time analysis. The general form of calculated dispersion curves is very similar to the earthquake dispersion curves. Besides, results of dispersion curves are comparable with the average global model. Our study, show the ability of ambient noise analysis for determining the earthʹs velocity structure. The results may prepare the basic curves for the seismic structure modeling like surface wave velocity structure and/or seismic surface wave tomography.