Depth conversion by the use of forward and inverse 2-D raypath modelling and compensation for the heterogeneity of the earth

Abstract

The technique presented in this article is an alternative approach to the established depth-conversion processes and is suitable for use in areas which lack well control, such as the dipping flanks of structures, where the reflectors are assumed to be of any arbitrary dip angles, but the change in dip is not very severe arid abrupt. Consideration is given to the difficulties in predicting, away from well control, interval velocities, conversion factors compensating for the heterogeneity of the ground and for the conversion of normal incidence (Nl) raypath rms velocities (Vnir) to 'true Vrms', and a method is presented to check on their validity. The checking procedure relies heavily on the preparation of a smooth, geologically representative, nmo velocity (Vnmo) map of each horizon by applying a suitable method of computer gridding and smoothing to the stacking velocities. This is then followed by the preparation of depth maps from which information on interval velocity, thickness and dip can be extracted to produce depth-interval velocity models. The application of forward raypath tracing to these models will generate mathematically computed Vnmo and To values. The deviation between the model and original Vnmo and To attributes is related to inaccuracies in the estimation of the conversion factors which can be adjusted, and new models generated. The process is repeated until convergence is reached. The paper discusses the nature of the two most important parameters related to the non-homogeneity of the ground, the Bias and the Heterogeneity Factor, Al-Chalabi (1973, 1974, 1979), which are utilised in the estimation of the conversion factors away from well control. The results of applying the present technique to the mapping of the Snapper gasfield (Megallaa and Ashworth, 1987) are discussed. Fortran code for the forward and inverse raypath tracing and for converting Vnir velocities to average velocities for isovelocity plane-dipping models is provided.