عنوان مقاله [English]
Shear wave information plays a valuable role in the interpretation of seismic data and reservoir characterization studies. Due to the high cost and technical difficulties in data recording of full elastic seismic data (3D-9C), using converted waves can be helpful to extract valuable S-wave information. Analysis of the amplitude and phase variations of converted waves with offset would be useful in better identification of converted reflections in the seismic sections and uncertainty reduction in the processing and interpretation of these data.
In this regard, by using Zoeppritz equations, the energy distribution of different seismic modes (P-P, S-S, P-Sv, and Sv-P) on the interface of the layers with different acoustic impedances at various incidence angles has been studied. Then, by using forward modeling and ray tracing methods, it should be obtained that how converted waves propagate in isotropic and anisotropic media. Besides, the effect of the Vp/Vs variations in the travel time and amplitude of the converted modes are inspected.
For data recorded by receivers oriented in the same direction as the seismic source (both vertical or both horizontal), the polarity of converted waves is the same in the positive and negative offsets. However, if the direction of receivers is perpendicular to the seismic source, the converted waves will have opposite polarities on different sides of the source.
Moreover, for primary P-Sv reflections, the largest P-to-S conversion occurs beyond the critical angle that is related to large source-receiver offsets. In contrast, S-to-P conversion significantly appears before the critical angle. However, it should be noted that for the same takeoff angle, the Sv-P and P-Sv waves will be recorded in the same offset, because the P-Sv incidence angle equals the Sv-P reflection angle and vice versa.
For example, the synthetic models show that in seismic layers with Vp/Vs = and horizontal reflector, maximum P-Sv conversion occurs at an incidence angle of about 64° and reflection angle of about 31°, while maximum Sv-P conversion occurs around 31° angle of incidence and around 64° angle of reflection. This means that from the viewpoint of source-receiver distance, depending on the intended depth, mid and far offsets data are necessary for good recording of the conversion mode reflections. Moreover, according to Snell law and Zoeppritz equations, in the case of converting Sv to P, the transmitting converted P-wave would have a small amplitude. Hence, in Sv-P studies, only the converted waves with purely Sv incoming and purely P reflection are considered.