Eliminating time statics from depth imaging

Dennis K. Ellison, Greg Cameron

ABSTRACT

Refraction and reflection statics are calculated because often the lack of detailed nearsurface information leads to inaccuracies. A normal moveout (NMO) velocity field is picked and applied to stack the data in preparation for the reflection statics calculations. NMO is a correction based on the assumption that the moveout can be approximated by a hyperbola. The accuracy of this assumption is valid when the moveout on data is nearhyperbolic and symmetric, and deviates when the moveout is more complicated due to complex geology. Scenarios of non-hyperbolic non-symmetric moveout are when high velocities are near the surface and when there are variations in the seismic weathering thickness and velocities.

This paper is a continuation on the “Improved resolution in depth imaging through reflection static corrections derived from model-based moveout” report done last year at the 2016 CREWES sponsors meeting. That report focused on synthetic data this is about a field dataset from the Canadian foothills. The focus this paper will be about how using reflection static corrections coupled to depth migration and merging the near-surface tomographic model with the depth velocity model will improve the final image.

Three methods are used on a foothills field dataset. First, was the conventional approach to depth imaging. The input traces to depth imaging had the same refraction and reflections statics applied that were calculated for the time stack. Second, the input traces to depth imaging had the refraction statics calculated from the tomographic model but reflection statics that were derived from model-based moveout. Third, the input traces to depth imaging had the refraction model merged with the velocity model and reflection statics that were derived from model-based moveout

Raytracing in depth migration has overcome many of the issues with the assumptions in time migration. Foothills datasets and other geologically complex environments compel us to look for ways to overcome these assumptions as they are violated. By merging the near-surface tomographic with the depth velocity model and calculating a model-based moveout correction for reflection statics, depth imaging can be enhanced.

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