Wide-angle incidence and P-wave transmission

P波广角入射的透射P波特征研究（英文）

Polarity reversals may occur to transmitted P waves if the incidence angle is greater than the critical incidence angle. We analyze the characteristics of reflection and transmission coefficients under the condition of wide incidence angle based on Zoeppritz equations. We find that for specific conditions, as the incidence angle increases, the characteristic curve of the transmitted P-wave coefficient enters the third quadrant from the first quadrant through the origin, which produces a transition in the transmitted P wave and the corresponding coefficient experiences polarity reversal. We derive the incidence angle when the transmitted P-wave coefficient is zero and verify that it equals zero by using finite-difference forward modeling for a single-interface model. We replace the water in the model reservoir by gas and see that the reservoir P-wave velocity and density decrease dramatically. By analyzing the synthetic seismogram of the transmitted P wave in the single-interface model, we show that the gas-saturated reservoir is responsible for polarity reversal.

P波入射到地下反射界面时,如果入射角超过临界角,透射P波可能会出现极性反转。本文首先从Zoeppritz方程出发,研究了广角入射条件下的反射系数和透射系数特征,发现满足一定条件时,随着入射角的增加,透射P波系数的特征曲线从第一象限经过原点进入第三象限,透射P波系数发生极性反转,它的辐角发生突变。文中给出了透射P波系数为零时入射角的表达式及其近似公式,通过对单界面模型进行有限差分正演,证明了透射P波系数为零的存在性。储层从饱含水到饱含气变化时,储层的P波速度和密度明显降低,通过单界面储层模型的透射P波的合成记录,说明了储层饱含气时透射P波系数可能发生极性反转。经理论分析和数值模拟得到了一些结论,这些结论为井间地震勘探的应用奠定了理论基础。