Image mispositioning due to dipping TI media: A physical seismic modeling study

Abstract

A scaled physical model was constructed to investigate the magnitudes of imaging errors incurred by the use of isotropic processing code when there is seismic velocity anisotropy present in the dipping overburden. The model consists of a block of transversely isotropic (TI) phenolic material with the TI axis of symmetry dipping at an angle of 45°. Its scaled thickness is 1500 m, and it is intended to simulate the dipping clastic sequences found in many fold-thrust belts. A piece of isotropic Plexiglas, affixed to the underside of the anisotropic block, has a step function in it to simulate a target reef edge or fault. The anisotropy parameters of the material are 8 = 0.1 and e = 0.24.On zero-offset data the imaged position of the target is shifted laterally 320 m in the updip direction of the beds, whereas on time-and depth-migrated multichannel sections the shift is 300 m. The lateral shift is offset dependent, with the amount of shift in any commonmidpoint gather decreasing from 320 m on the near offsets to 280 m on the far offsets. Prestack depth-migration velocity analysis based upon obtaining consistent depth images in the common-offset domain results in the base of the anisotropic section being imaged 50 m (about 3%) too deep.

Travel times and reflection points for reflected P-waves in transversely isotropic media

“…The shot gather shows that the reflection NMO of the P-waves reflected from a horizontal anisotropic layer with θ = 45° is hyperbolic within the largest offset/depth = 1.4 (Isaac and Lawton 1999). In our study, the ratio of the largest offset/depth for the models where θ = 0°, 30°, 45°, 60° and 90° is respectively 1.75, 1.75, 1.38, 1.25, and 1.63.…”

“…Our results suggest that the measurable nonhyperbolic reflection NMO can only be detected in highly anisotropic media and at the far offset. Uren et al (1991), and Isaac and Lawton (1999) showed that the near offset trace has a greater deviation of the reflection position from the CMP than does the far offset trace. In the present study, this is only the case for the models where θ = 45°, and 60°.…”

“…In the case of seismic migration which repositions reflected energy from its apparent position to its true subsurface location, care must be taken when treating the reflection points for waves reflected from anisotropic strata (Larner and Cohen 1993;Alkhalifah and Larner 1994;Uzcategui 1995;Alkhalifah 1995;Ball 1995). Uren et al (1991), Isaac and Lawton (1999) and Vestrum et al (1999) showed the dramatic shifts in the positioning of reflection points below TI media with a tilted symmetry axis. Vestrum (2002) found that anisotropic depth migration can reduce the depth error on a depth migrated section, so he was able to image sharper reflector terminations and a larger range of reflector angles.…”

High Resolution/High Fidelity Seismic Imaging and Parameter Estimation for Geological Structure and Material Characterization

“…It is well known that isotropic algorithms applied to modeling and imaging in transversely isotropic (TI) media produce mispositioning of structures (Martin et al, 1992;Larner and Cohen, 1993;Isaac and Lawton, 1999). In response to these shortcomings many imaging methods for TI media have been developed (Meadows et al, 1987;Uren et al, 1990;Gonzalez et al, 1991;Kitchenside, 1991;Sena and Toksoz, 1993;Meadows and Abriel, 1994;Uzcategui, 1995;Le Rousseau, 1997;Le Rousseau and de Hoop, 2001;Ferguson and Margrave, 2002).…”

Elastic wave mode separation for TTI media

“…Using the VTI assumption to image structures characterized by TTI anisotropy introduces both kinematic and dynamical errors in migrated images. For example, Vestrum et al (1999) and Isaac and Lawyer (1999) show that seismic structures can be mispositioned if isotropy, or even VTI anisotropy, is assumed when the medium above the imaging targets is TTI. To carry out elastic wave-equation migration for TTI models and apply the imaging condition that crosscorrelates the separated wave-modes, the wave-mode separation algorithm needs to be adapted to TTI media.…”