تعیین بسامد طبیعی نوسان خاک در محدوده معدن مس سرچشمه با استفاده از نگاشت‌های مایکروترمور

نویسندگان

چکیده

در این بررسی داده‌های مایکروترمور جمع‌آوری شده،‌ در محدوده معدن مس سرچشمه به منظور محاسبه بسامد طبیعی خاک، مورد تحلیل و بررسی قرار گرفته است. به منظور پردازش داده‌ها از روش نسبت طیفی مؤلفه افقی به عمودی، معروف به رهیافت ناکامورا (H/V) استفاده شده است. روش نسبت‌های طیفی امکان کاربرد روی داده‌های زلزله و شتاب را دارد ولی براساس بسیاری از بررسی‌ها، روشن شده است که استفاده از مایکروترمورها برای محاسبه بسامد طبیعی کاملاً قابل اطمینان است. داده‌های موجود در 12 جایگاه اندازه‌گیری در محدوده معدن مس سرچشمه و برخی نقاط اطراف به منظور مطالعات نوفه سنجی برداشت شده‌اند. در برخی جایگاه‌های اندازه‌گیری تعداد دفعات برداشت داده 3 و در برخی جایگاه‌ها 4 بار بوده است. داده‌ها بر اساس استانداردهای گروه طرح SESAME (Site EffectS assessment using AMbient Excitations) مورد پردازش قرار گرفته است. در هر جایگاه محاسبات روی همه برداشت‌ها صورت گرفته و بسامد طبیعی برای آن جایگاه محاسبه شده است. براساس بسامد طبیعی به‌دست آمده، دسته‌بندی رده خاک و متوسط سرعت موج برشی برآورد شده است. در نهایت نوع و رده خاک با مشاهدات میدانی موجود مقایسه شده است.

کلیدواژه‌ها


عنوان مقاله [English]

Determination of natural frequency of soil vibration in Sarcheshmeh copper mine, using microtremor records

چکیده [English]

It is well known that ground-shaking site effect caused by an earthquake can vary significantly within a small distance. This is because at sites having soft soil and/or topographic and basement undulations, seismic energy gets trapped, leading to amplification of vibration that may cause considerable damage to man-made structures. Theoretical analysis and observational data have shown that each site has a specific resonance frequency at which ground motion gets amplified. Man-made structures having resonance frequency matching that of the site have the maximum likelihood of getting damaged. Hence, in order to construct seismically-safe structures, it is important to know the site response.
Various methods are available for the estimation of site response. The best method is to record strong ground motion caused by a large local earthquake. However, fortunately such events are not very frequent in many areas. Hence, for site response analysis, this method is not very practical. Another method is to carry out extensive seismic reflection and/or refraction surveys and geotechnical surveys; this method is extremely expensive and time-consuming. Recently microtremor data have also been widely used for estimation of site response. The advantage of this method is that it takes very little time for data collection. One does not have to wait for an earthquake to occur. Very few instruments are required; the data collection can be handled even with a single instrument.
The method involves recording microtremor data from the site to be investigated. It is assumed that signals from a hard rock site are carried equally well at all frequencies. On the other hand, a soft-soil site amplifies the signal at its resonance frequency, which depends on factors such as the soil type, basement configuration, etc. Hence, if the source and the path effects were removed from the spectra of the signal, then we should get a flat spectra at a hard-rock site and spectra showing peaks at resonance frequency at a soft-soil site.
In this study microtremor measurements were carried out in the Sarcheshmeh copper mine area at about 12 sites and the natural frequency at each site was estimated considering the main peak in the spectral ratio between the horizontal and the vertical component, the method called the NAKAMURA technique (H/V). Many experimental and theoretical studies have shown the reliability of microtremor measurements in site predominant frequency estimation. At each site traces have been collected 3 or 4 times and for all of them natural frequency based on SESAME project group standards have been analyzed. Then on the basis of natural frequency, site soil types have been determined and average Shear wave velocity for each site has been predicted. All results compared with available field observations.

کلیدواژه‌ها [English]

  • Microtremor
  • Sarcheshmeh copper mine
  • Nakamura technique
  • natural frequency