In exploration seismology, the acquisition, processing and inversion of P-wave data is a routine. However, in orthorhombic anisotropic media, the governing equations that describe the P-wave propagation are coupled with two S waves that are considered as redundant noise. The main approach to free the P-wave signal from the S-wave noise is the acoustic assumption on the wave propagation. The conventional acoustic assumption for orthorhombic media zeros out the S-wave velocities along three orthogonal axes, but leaves significant S-wave artefacts in all other directions. The new acoustic assumption that we propose mitigates the S-wave artefacts by zeroing out their velocities along the three orthogonal symmetry planes of orthorhombic media. Similar to the conventional approach, our method reduces the number of required model parameters from nine to six. As numerical experiments on multiple orthorhombic models show, the accuracy of the new acoustic assumption also compares well to the conventional approach. On the other hand, while the conventional acoustic assumption simplifies the governing equations, the new acoustic assumption further complicates them—an issue that emphasizes the necessity of simple approximate equations. Accordingly, we also propose simpler rational approximate phase-velocity and eikonal equations for the new acoustic orthorhombic media. We show a simple ray tracing example and find out that the proposed approximate equations are still highly accurate.

中文翻译：

---

斜方介质的新声学假设

在勘探地震学中，P波数据的采集，处理和反演是很常规的。然而，在正交各向异性介质中，描述P波传播的控制方程式与两个S波耦合，这两个S波被视为冗余噪声。使P波信号免于S波噪声的主要方法是对波传播的声学假设。斜方介质的常规声学假设将沿三个正交轴的S波速度归零，但在所有其他方向上留下了明显的S波伪像。我们提出的新的声学假设可以减轻S通过将它们沿正交正交介质的三个正交对称平面的速度归零，可以将它们放置在波浪中。与传统方法类似，我们的方法将所需模型参数的数量从9个减少到6个。正如在多个正交正交模型上进行的数值实验所示，新的声学假设的准确性也与传统方法相当。另一方面，尽管传统的声学假设简化了控制方程，但新的声学假设使它们变得更加复杂，这个问题强调了简单近似方程的必要性。因此，我们还为新的正交各向异性介质提出了更简单的有理近似相速度和推导方程。我们展示了一个简单的光线跟踪示例，发现所提出的近似方程仍然非常准确。