Kirchhoff diffraction mapping in media with large velocity contrasts

Finite-difference methods for calculating traveltimes are superior to ray-t racing methods in inhomogeneous media. However, when these techniques are a pplied to Kirchhoff migration, a severe problem occurs in the presence of l arge velocity contrasts. If finite-difference traveltime methods are used t o calculate first arrivals, an incomplete image is created because substant ial subsurface information is often carried by direct body waves. We propos e a solution to this problem by developing a new method of calculating late r arrival times and applying both first and later arrival times to a Kirchh off diffraction mapping algorithm. A comparison shows that the implementati on of both first arrivals and later arrivals in Kirchhoff migration can sub stantially improve the images in media with large velocity contrasts.