AKTİF ÇOK KANALLI YÜZEY DALGASI ANALİZİ (A-ÇKYDA)YÖNTEMİNDE VERİ TOPLAMA VE TABAKA PARAMETRELERİNİN TEMEL MOD DİSPERSİYON EĞRİSİ ÜZERİNDEKİ ETKİLERİ

Öz

Aktif ÇKYDA yönteminde, sığ yeraltının güvenilir bir S-dalgası hız-derinlik profilini elde etmek için en önemli aşamalarından biri, geniş frekans aralığında sürekli ve yüksek ayrımlı bir dispersiyon eğrisinin elde edilmesidir. Bu makalede, tabaka (S- ve P-dalga hızları, yoğunluk ve kalınlık) ve veri toplama (kaynak ofseti-X0, alıcı aralığı-dx ve sayısı-N, serim boyu-L=(N-1)*dx) parametrelerinin, yüzey dalga alanının temel mod dispersiyon eğrisi üzerindeki etkileri, yapay atış verilerinin analizi ile incelenmiştir. Yapay veriler, tabaka yansıma/iletim katsayısı tekniği ile hesaplanan dispersiyon eğrisi kullanılarak, her bir alıcıda harmonik mod toplama tekniği ile oluşturulmuştur. Buna göre, düşük S-dalga hızlı ara tabakalar dispersiyon eğrisinin düşük frekans bölgesinde düşük hızlara kaymasına, yüksek hızlı ara tabakalar ise, dispersiyon eğrisinin yüksek frekanslarda sürekliliğinin bozulmasına (zig-zag etkisi) ve yüksek mod etkisi yanılgısına neden olabileceği gözlenmiştir. Kaynak ofseti yüzey dalgalarının tam dalga formlarının oluşmasında etkin iken, ilk ve son alıcı arası uzaklık olan serim uzunluğunun ise, dispersiyon eğrisinin frekans bandını ve çözünürlüğünü etkilediği görülmüştür. Ayrıca, geniş alıcı aralıklarının, dispersiyon eğrisinin düşük frekans bölgesindeki güvenirliğinin arttırmasının yanında, yüksek frekans bölgesinde dalga sayısı katlanması nedeniyle dispersiyon eğrisinin sürekliliğini bozduğu görülmüştür. Sonuç olarak, farklı dalga hızlarına sahip yer modelleri için üretilen yapay veriler üzerinde yapılan analizler N<24 4.0m≤dx≤2m, N≥24 0.5m≤dx≤2m ve X0≥4*dx veya X0≥L/5 kombinasyonlarının, farklı yer altı koşullarının kestirimi amacıyla toplanacak aktif ÇKYD verileri için güvenilir olacağını göstermiştir.

Anahtar Kelimeler

• Veri Toplama
• Tabaka Parametreleri
• Dispersiyon Eğrisi
• A-ÇKYDA
• Modelleme

INFLUENCES OF DATA ACQUISITION AND LAYER PARAMETERS ON FUNDAMENTAL DISPERSION CURVE IN ACTIVE MULTICHANNEL ANALYSIS OF SURFACE WAVE (A-MASW) METHOD

Öz

Obtaining a continuous and high resolution dispersion curve in a wide frequency range is one of the most important steps to determine a reliable shear (S) wave velocity-depth profile of the shallow subsurface in active MASW method. In this article, the effects of layer (or physical) (S- and P-wave velocities, density and thickness) and data acquisition (source offset-X0, receiver spacing-dx and number-N, spread length-L=(N-1)*dx) parameters on the fundamental mode dispersion curve of the surface wave field were investigated by analyzing the synthetic shot gathers. The synthetic data were generated by harmonic mode summation technique at each receiver with using of the dispersion curve calculated by the layer reflection/transmission coefficient technique. Accordingly, it was observed that low S-wave velocity interlayers may cause shifting the dispersion curve to low velocities in the low frequency band, while high velocity interlayers may cause both discontinuity on the dispersion curve at high frequencies (zig-zag effect) and high-mode effect error. It was seen that while the source offset is effective in the formation of full waveforms of surface waves, the distance between the first and last receiver affects the frequency band and resolution of the dispersion curve. In addition, it is observed that wide receiver sampling increases the reliability of the dispersion curve in the low frequency region, though it disrupts the continuity of the dispersion curve at high frequencies due to wavenumber aliasing. As a result, the analyses made on the synthetic data generated for various subsurface layer models with different wave velocities demonstrate that the combinations of N<24 4.0m≤dx≤2m, N≥24 0.5m≤dx≤2m and X0≥4*dx or X0≥L/5 would provide practical convenience for the selection of acquisition parameters for active MASW data in the field.

Anahtar Kelimeler

• Data Acquisition
• Layer Parameters
• Dispersion Curve
• MASW
• Modelling

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