Kinematics of shot-geophone migration

In contrast to prestack migration methods based on data binning, common image gathers produced by shot-geophone migration exhibit the appropriate semblance property in either offset domain (focussing at zero offset) or angle domain (focussing at zero slope), when the migration velocity is kinematically correct and when events to be migrated arrive in the data along non-turning rays. The latter condition is required for successful implementation via wavefield depth extrapolation (“survey sinking”). Thus shot-geophone migration may be a particularly appropriate tool for migration velocity analysis of data exhibiting structural complexity. INTRODUCTION The basis of migration velocity analysis is the semblance principle: prestack migrated data volumes contain flat image gathers, i.e. are at least kinematically independent of the bin or stacking parameter, when the velocity is correct (Kleyn, 1983; Yilmaz, 1987). Migration velocity analysis (as opposed to standard NMO-based velocity analysis) is most urgently needed in areas of strong lateral velocity variation, i.e. “complex” structure such as salt flanks, chalk tectonics, and overthrust geology. However strong refraction implies multiple raypaths connecting source and receiver locations with reflection points, and multiple raypaths in turn imply that the semblance principle is not valid: that is, image gathers are not in general flat, even when the migration velocity closely approximates the true propagation velocity (Stolk and Symes, 2004). The failure of the semblance principle in complex structure afflicts all prestack migration techniques based on data binning, i.e. for which each data bin creates an independent Department of Applied Mathematics, University of Twente, Drienerlolaan 5, 7522 NB Enschede, The Netherlands, email [email protected] Center for Wave Phenomena, Colorado School of Mines, Golden, CO 80110 USA, email [email protected] The Rice Inversion Project, Department of Computational and Applied Mathematics, Rice University, Houston TX 77251-1892 USA, email [email protected]