بررسی پدیده همبوسی مدها در روش‌‌های MASW و MALW

نوع مقاله : پژوهشی

نویسندگان

1 دانش‌آموخته کارشناسی ارشد، گروه فیزیک زمین، مؤسسه ژئوفیزیک، دانشگاه تهران، تهران، ایران

2 استاد، گروه فیزیک زمین، مؤسسه ژئوفیزیک، دانشگاه تهران، تهران، ایران

چکیده

تخمین پروفیل سرعت موج برشی لایه‌‌های سطحی در پروژه‌‌های مهندسی از اهمیت بالایی برخوردار است. این پارامتر به‌عنوان مثال در پروژه‌های ژئوتکنیکی به‌منظور دسته‌بندی نوع خاک و در پروژه‌های مهندسی زلزله برای تعیین پاسخ ساختگاه مورد استفاده قرار می‌گیرد. در حال حاضر روش‌‌های مختلفی برای تخمین پروفیل سرعت موج برشی وجود دارد که از میان آنها روش‌‌های MASW و MALW به‌واسطه سرعت اجرای بالا، هزینه کم و در مواردی غیرمخرب بودن، بسیار پرکاربرد هستند. یکی از مشکلات مهم پیش روی روش‌‌های امواج سطحی پدیده همبوسی مدهاست که در حضور تباین شدید سرعتی (شرایط مرسوم نهشته شدن رسوبات جوان روی سنگ‌بستر) رخ می‌دهد. این پدیده می‌تواند منجر به تفسیر غلط ازمنحنی پاشش و حصول مدل سرعتی اشتباه شود. این موضوع برای تصمیم‌سازی در پروژه‌‌های مهندسی می‌تواند بسیار خطرناک باشد. در این مطالعه ضمن بررسی شرایط ایجاد همبوسی مدها، عملکرد دو روش تحلیل امواج سطحی فوق در حضور این مشکل مقایسه شده است. نتایج نشان می‌دهد برخلاف روش MASW که به وجود تباین‌‌های سرعتی بالا حساسیت زیادی دارد، روش MALW در مقابل این شرایط پیچیده عملکرد مطلوبی از خود نشان می‌دهد. همچنین در این مطالعه نشان داده شده است که با استفاده از روش HVSR در کنار روش‌‌های امواج سطحی می‌توان فرکانس وقوع همبوسی را نیز پیش‌بینی کرد و از منحنی H/V حاصل در وارون‌سازی همزمان با داده‌‌های امواج سطحی بهره برد. روشن است که این موضوع از عدم‌قطعیت ذاتی موجود در روش‌‌های ژئوفیزیکی کاسته و بر دقت مدل سرعتی نهایی می‌افزاید.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

Investigation of mode osculation phenomenon in MASW and MALW methods

نویسندگان [English]

  • Hossein Kazemnezhadi 1
  • Hamid Reza Siahkoohi 2
1 M.Sc. Graduated, Department of Earth Physics, Institute of Geophysics, University of Tehran, Tehran, Iran
2 Professor, Department of Earth Physics, Institute of Geophysics, University of Tehran, Tehran, Iran
چکیده [English]

There are two types of seismic waves: those that can propagate inside a medium (body waves) and those traveling along the Earth’s surface (surface waves). In the last decades, a number of papers dealing with surface waves have been published but it must be recalled that their theoretical description and first applications date back to almost a century ago. Surface waves have been in fact used for a number of applications since 1920s: nondestructive testing (even for medical applications, geotechnical studies and crustal seismology). Recently the interest toward their applications has increased both for the increasing demand for efficient methodologies to apply in engineering projects and because the recent regulations addressing the assessment of the seismic hazard (for instance the Eurocode8) that are giving the necessary emphasis to the determination of shear-wave velocity vertical profile. This parameter is commonly used in geotechnical studies for classifying soil types.
Among various methods for estimating shear-wave velocity profile, MASW and MALW methods are most popular because of their fast performance, low cost and their nondestructive nature. These methods are based on analyzing dispersive properties of Rayleigh and Love waves. In surface wave methods a correct identification of the modes is essential to avoid serious errors in building near surface shear wave velocity model. Here we consider the case of higher-mode misidentification known as “osculation” where the energy peak shifts at low frequencies from the fundamental to the first higher mode. This jump occurs around a well-defined frequency where the two modes get very close to each other. This problem is known to take place in complex subsurface situations, for example in inversely dispersive sites or in presence of a strong impedance contrast, such as a soil layer resting on top of the bedrock. This phenomenon can cause a misleading interpretation of dispersion curve by the operator, which is completely hazardous for engineering projects.
In this paper we investigated mode osculation phenomenon for both MASW and MALW methods using synthetic and real datasets. We showed that MALW has a far better performance facing this problem, while it is a main drawback for the MASW method. Generally, when we encounter a low-velocity layer in the subsurface, the identification of Rayleigh wave’s fundamental mode (MASW method) becomes almost impossible, while at the same time dispersion modes of Love waves (MALW method) are well separated, even in extreme conditions. In addition, we showed that performing single-station microtremor ellipticity analysis can also be quite useful. It can warn against the presence of a strong impedance contrast, it indicates the critical frequency at which mode osculation takes place, and also the HVSR data can be used as a constraint in the inversion process of surface wave data. So performing HVSR method alongside MASW and MALW methods not only can predict mode osculation frequency and strong impedance contrasts presence, but also can help us with joint-inversion of the surface wave data, resulting in a more solid Vs profile. We evaluated the performances of the proposed methods on real and synthetic seismic data and results were satisfying.

کلیدواژه‌ها [English]

  • MASW
  • MALW
  • Shear wave velocity profile
  • HVSR
  • Joint-inversion
Aki, K. and Richards, P. G., 2002, Quantitative seismology. Sausalito, California, University Science Books.
Ampuero, J. P., 2008, SEM2DPACK a spectral element method tool for 2Dwave propagationand earthquake source dynamics. https://geodynamics.org/cig/software/specfem2d/.
Antipov, V. V. and Ofrikhter,V. G., 2019, Field estimation of deformation modulus of the soils by multichannel analysis of surface waves.Data in Brief, 24, 2352-3409.
Boaga, J., Cassiani, G., Strobbia, C. L. and Vignoli, G., 2013, Mode misidentification inRayleigh waves. Geophysics,78, 4, EN17–EN28.
Dalmoro,G., 2015, Surface Wave Analysis for Near Surface Applications. Elsevier books.
Jakka, R. S. and Roy, N., 2018, Uncertainties in Site Characterization Using Surface Wave Techniques and Their Effects on Seismic Ground Response, In: Krishna A., Dey A., Sreedeep S. (eds), Geotechnics for Natural and Engineered Sustainable Technologies. Developments inGeotechnicalEngineering. Springer, Singapore.
Krishna, A, Jakka, R. S. and Roy, N., 2018, Uncertainties in Site Characterization Using Surface Wave Techniques and Their Effects on Seismic Ground Response, In:. Dey A., Sreedeep S. (eds), Geotechnics for Natural and Engineered Sustainable Technologies. Developments inGeotechnicalEngineering. Springer, Singapore.
Luo, Y., Xia, J., Liu, J., Liu, Q. and Xu, S., 2007, Joint inversion of high-frequency surface waves with Fundamental and higher modes. J. Appl. Geophysics. 62, 375–384.
Luo, Y., Xia, J., Xu, Y., Zeng, C. and Liu, J., 2010, Finite-difference modeling and dispersion analysis of high frequency Love waves for near-surface applications. Pure Appl Geophys, 167(12), 1525–1536.
Mi, B., Xia, J., Bradford, J. H. and Shen, C., 2020, Estimating Near-Surface Shear-Wave-Velocity Structures Via Multichannel Analysis of Rayleigh and Love Waves: An Experiment at the Boise Hydrogeophysical Research Site. Surv Geophys, 41, 323–341. https://doi.org/10.1007/s10712-019-09582-4.
Nazarian, S., Prajwol, T., Azari, H. and Yuan, D., 2017, Implementation of spectral analysis of surface waves approach for characterization of railway track substructure., Transportation Geotechnics, 12, 101-111.
Nogoshi, M. and Igarashi, T., 1971, On the amplitude characteristics of Microtremor (part 2). Jour. Seismol. Soc. Jpn, 24, 26–40, (Japanese with English abstract).
Yudi, P., Xia, J., Xu, Y. and Gao, L., 2016, Multichannel analysis of Love waves in a 3D seismic acquisition system, GEOPHYSICS 81, EN67-EN74.
Park, C. B. and Miller, R. D., 1999, Multichannel analysis of surface waves. Geophysics, 64, 800–808.
Prodehl, C., Kennett, B. and Artemieva, I.andThybo, H., 2013, 100 years of seismic research on the Moho. Tectonophysics, 609, 9-44.
Safani, J., O’Neill, A., Matsuoka, T. and Sanada, Y., 2005, Applications of Love wave dispersion for improved shear-wave velocity imaging. J Environ Eng Geophys, 10(2), 135–150.
Safani, J., O’Neill, A. and Matsuoka, T., 2006, Love wave modeling and inversion for low velocity layer cases. In: Proceedings of the symposium on the application of geophysics to engineering and environmental problems (SAGEEP). Annual meeting of the environmental and engineering geophysical society (EEGS) Seattle, WA, 1181–1190.
Socco, L. V., Foti, S. and Boiero, D., 2010, Surface-wave analysis for building near-surface velocity models — Established approaches and new perspectives. Geophysics, 75, 75A83-75A102.
Socco, L. V, Boiero, D., Maraschini, M., Vanneste, M., Madshus, C., Westerdahl, H., Duffaut, K. and Skomedal, E., 2011, On the use of NGI’s prototype seabed-coupled shear wave vibrator for 1 shallow soil characterization— Part II: Joint Inversion of multi-modal Love and Scholte surface waves. Geophys J Int, 185, 237–252.
Socco, L. V., Comina, C. and KhosroAnjom, F., 2017, Time-average velocity estimation through surface-wave analysis: Part 1 — S-wave velocity. Geophysics, 82, U49-U59.
Socco, L. V., Foti, S., Hollender, F. and Garofalo, F., 2018, Guidelines for the good practice of surface wave analysis: a product of the InterPACIFIC project. Bull. Earthquake Eng, 16, 2367–2420.
Strobbia, C., Foti, S., Rix, G. J. and Lai, C. G., 2015, Surface Wave Methods for Near-Surface Site Characterization. CRC Press, Taylor & Francis Group.
Tavasoli, O. and Ghazavi, M., 2018, Wave propagation and ground vibrations due to non-uniform cross-sections piles driving. Computers and Geotechnics, 104, 13-21.
Xia, J., Xu, Y., Luo, Y. and Miller, D., 2012, Advantages of Using Multichannel Analysis of Love Waves (MALW) to Estimate Near-Surface Shear-Wave Velocity. Surv Geophys, 33, 841–860. https://doi.org/10.1007/s10712-012-9174-2.