Horizon-based semiautomated nonhyperbolic velocity analysis

Abstract

With higher capacity recording systems, long-offset surveys are becoming common in seismic exploration plays. Long offsets provide leverage against multiples, have greater sensitivity to anisotropy, and are key to accurate inversion for shear impedance and density. There are two main issues associated with preserving the data fidelity contained in the large offsets: (1) nonhyperbolic velocity analysis and (2) mitigating the migration/NMO stretch. Current nonhyperbolic velocity analysis workflows first estimate moveout velocity V nmo based on the offset-limited gathers, then pick an effective anellipticity η eff using the full-offset gathers. Unfortunately, estimating V nmo at small aperture may be inaccurate, with picking errors in V nmo introducing errors in the subsequent analysis of effective anellipticity. We have developed an automated algorithm to simultaneously estimate the nonhyperbolic parameters. Instead of directly seeking an effective stacking model, the algorithm finds an interval model that gives the most powerful stack. The searching procedure for the best interval model was conducted using a direct, global optimization algorithm called differential evolutionary. Next, we applied an antistretch workflow to minimize stretch at a far offset after obtaining the optimal effective model. The automated velocity analysis and antistretch workflow were tested on the data volume acquired over the Fort Worth Basin, USA. The results provided noticeable improvement on the prestack gathers and on the stacked data volume.

Brittleness evaluation of resource plays by integrating petrophysical and seismic data analysis

“…We apply a workflow to mitigate these two phenomena at a far offset (Zhang et al, 2014a;Zhang et al, 2015) beginning by mitigating the "hockey stick" using the automatic nonhyperbolic velocity followed by a wavelet-based correction to minimize the stretch at a far offset. Zhang et al (2014b) find that inverted results from conditioned gathers have better resolution and higher correlation coefficients with well logs.…”

“…The workflow begins by performing reverse NMO on the time-migrated gathers using the initial migration velocity. Then, we obtain the optimal velocity and effective anisotropy model using our automatic algorithm (Zhang et al, 2014b). Next, we apply nonstretch NMO correction (Zhang et al, 2013) to the time-migrated gathers using a new velocity and anellipticity model resulting in flattened nonstretched prestack gathers.…”

Multiazimuth coherence

“…These data were migrated using an isotropic velocity model, such that anisotropy gives rise to lateral shifts (indicated by yellow arrows) in the coherence anomalies. Zhang et al (2014Zhang et al ( , 2015 and Verma et al (2016) apply a prestack structure-oriented filter to suppress coherent noise, processing, and migration artifacts. Perez and Marfurt (2008) apply a spatial crosscorrelation technique to the coherence slices to measure lateral shifts of discontinuities and then correct them using a data warping algorithm.…”

Joint Anisotropic Parameter Inversion Based on Nonhyperbolic Moveout and Azimuthal Amplitude Variance

“…The interaction of velocity alters the RMS velocity estimate that is determined just from the near offsets. There have been numerous papers on the topic of VTI and estimation of its effects from the non-hyperbolic moveout apparent on seismic gathers (Tsvankin and Thomsen, 1994) and semblance scan results (Zhang et al, 2014). In a horizontally transverse isotropic layer, the usual source of the anisotropy is regional stress or vertical parallel cracks, causing the axis of symmetry to be horizontal.…”