Soğurulma Tomografisi
Özet
Sismik dalgalar yerkürenin iç yapısı ve elastik olmayan özellikleri hakkında bilgi verirken, ortamın heterojenliği ve anelastik yapısı nedeniyle genlik azalımına uğrayarak soğurulmaktadır. Sismik tomografi yöntemleri arasında önemli bir yere sahip olan soğurulma tomografisi, yer yapısının iki ve üç boyutlu olarak yüksek çözünürlükte görüntülenmesine imkân sağlar. Bu kapsamda, yer içindeki süreksizliklerden ve saçılımlardan kaynaklanan, S dalgalarının kuyruk kısmını oluşturan koda dalgaları, kabuk yapısındaki düzensizlikleri ve tektonik aktiviteyi açıklamada kritik bir rol oynar. Koda dalga analizlerinde Aki ve Sato gibi araştırmacılar tarafından geliştirilen tekil ve çoklu saçılma modelleri kullanılarak ortamın kalite faktörü (Q) ile frekans bağımlılık derecesi (n) hesaplanmaktadır. Soğurulma parametrelerinin belirlenmesinde sıklıkla başvurulan Koda Normalizasyon (CN) yöntemi; kaynak, zemin ve aletsel transfer etkilerini elimine ederek P ve S dalgalarının üç boyutlu soğurulma yapısını güvenilir bir şekilde ortaya koyar. MuRATv2 gibi gelişmiş yazılımlar aracılığıyla yürütülen bu süreçlerde, dama tahtası ve sentetik anomali testleri uygulanarak tomografik çözünürlük doğrulanmaktadır. Elde edilen soğurulma değerleri (ΔQp⁻¹, ΔQs⁻¹) ve frekans bağımlılığı katsayıları; fay zonları, volkanik aktivite alanları, sedimanter havzalar ve süreksizliklerin dağılımını göstererek sismik aktivitenin, zemin davranışlarının ve yer altı kaynaklarının belirlenmesine önemli katkılar sunmaktadır.
While seismic waves provide information about the internal structure and non-elastic properties of the Earth, they undergo amplitude reduction and attenuation due to the heterogeneity and anelastic structure of the medium. Attenuation tomography, which holds an important place among seismic tomography methods, enables high-resolution two- and three-dimensional imaging of the Earth's structure. In this context, coda waves—which originate from discontinuities and scattering within the Earth and constitute the tail portion of S-waves—play a critical role in explaining crustal irregularities and tectonic activity. In coda wave analyses, the quality factor (Q) and the degree of frequency dependence (n) of the medium are calculated using single and multiple scattering models developed by researchers such as Aki and Sato. The Coda Normalization (CN) method, frequently utilized in determining attenuation parameters, reliably reveals the three-dimensional attenuation structure of P- and S-waves by eliminating source, site, and instrumental transfer effects. In these processes conducted via advanced software such as MuRATv2, tomographic resolution is verified by applying checkerboard and synthetic anomaly tests. The obtained attenuation values (ΔQp⁻¹, ΔQs⁻¹) and frequency dependence coefficients indicate the distribution of fault zones, volcanic activity areas, sedimentary basins, and discontinuities, offering significant contributions to the determination of seismic activity, ground behavior, and underground resources.
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