Testing Kirchhoff-based Prestack-depth Migration Methods for Imaging the Asse Salt Structure in Northern Germany

Abstract

Kirchhoff prestack-depth migration is a fast method to image geological structures. However, its ability to image arbitrarily dipping reflectors is limited when no apriori information about the dip of the structures to image is available. The limitations of realistic acquisition geometries cause typical artefacts such as migration smiles due to the smearing of the image along the whole two-way-traveltime isochrones. This migration noise limits the resolution of the resulting depth image and makes it difficult to detect reflector truncations or faults. Fresnel volume migration and coherency migration overcome this problem by introducing additional weighting functions into the Kirchhoff integral formula. Both methods represent a data-driven approach to confine the migration operator to that part of the two-way-traveltime isochrone where diffractions or reflections occur, i.e. to the immediate vicinity of the reflection point. All three methods are applied to a synthetic data set, which accounts for the complex geology of the Asse salt structure and its overburden in the North German basin with steeply dipping reflectors and strong velocity variations. The comparision of the migration results with the model reveals that the two modified migration methods yield significant improvements in the quality of the depth images compared to Kirchhoff prestack-depth migration.