A B S T R A C TThe presence of triplications (caustics) can be a serious problem in seismic data processing and analysis. The traveltime curve becomes multi-valued and the geometrical spreading correction factor tends to zero due to energy focusing.We analyse the conditions for the qSV-wave triplications in a homogeneous transversely isotropic medium with vertical symmetry axis. The proposed technique can easily be extended to the case of horizontally layered vertical symmetry axis medium. We show that the triplications of the qSV-wave in a multilayered medium imply certain algebra. We illustrate this algebra on a two-layer vertical symmetry axis model.
I N T R O D U C T I O NThe qSV-wave triplications in a homogeneous transversely isotropic medium with vertical symmetry axis (VTI medium) have been discussed by many authors (Dellinger ). The condition for incipient triplication was given in Dellinger (1991) and Thomsen and Dellinger (2003). According to Musgrave (1970), we consider axial (on-axis vertical), basal (on-axis horizontal) and oblique (off-axis) triplications. He also provided the conditions for the generation of all the types of triplications. The approximate condition for off-axis triplication was derived in Schoenberg and Daley (2003) and Vavryčuk (2003). The condition for on-axis triplication in multilayered VTI medium was shown in Tygel et al. (2007).We use the parametrization of a VTI medium proposed by Schoenberg and Daley (2003). In our paper we define the conditions and the range of qSV-wave triplications in a multilayered VTI medium based on the ray theory and consider * two special cases: two-layer VTI medium and converted wave in a homogeneous VTI medium.
q S V -W A V E I N A H O M O G E N E O U S V T I M E D I U MThe parametric offset-traveltime equations in a homogeneous VTI medium of thickness z are given bywhere p and q are respectively the horizontal and vertical components of the slowness vector, q = dq/dp is the derivative of vertical slowness. The condition for triplication (caustic in the group space or concavity region on the slowness curve) is given by setting the curvature of the vertical slowness to zero, q = 0 , or by setting the first derivative of offset to zero, x = 0. These points at slowness surface have the curvature equal to zero. If the triplication region is degenerated to a single point, it is called incipient triplication. At that point we have q = 0 and q = 0. For the incipient horizontal on-axis triplication, this is not the case, because at that point q is equal to infinity and we have to take the corresponding limit.