Estimation of Migration Velocity by Forward Modelling of Old Seismic Stacks

Abstract

This research is a case study of southern basin in Iran where velocity modelling has been done on Asmari and Gachsaran formations. Geologically complex formations, it is important to determine the optimum velocity to achieve the best migratd seismic section. We introduced a method that the seismic processor can utilise to better migrate a post-stack seismic section. Methodology We have created a model by combining interpretation of vintage seismic, surface geology and well data (Fig. 1). Ray tracing was performed on the model with various velocity inputs. After each step, results analysed and the deviation from original horizons is estimated. The optimum velocity is the one that has the highest match with the input data. The results of ray tracing will be sent back for re-interpretation. The difference between the new interpretation and the older one will hint on the corrections that need to be applied on the ray tracing model. Several methods are available for ray tracing. They vary in application based on the input data. For this particular case, we have dealt with vintage stack seismic; hence vertical ray tracing has been used to achieve the optimum velocity of data. Where there is mismatch between the model and the input data, the amount and location of deviation will determine the appropriate adjustments for the next run. This iterative process will continue until the error falls below the defined threshold. Figure 2 illustrate deviation amount for two horizons. Conclusion This method will create a velocity file for post stack migration that can help checking processing steps as well as introducing some control on the interpretation. Upon establishing an accurate velocity for each formation, analysis of rock properties and effects of changing in this parameter on wave propagation will be available with higher accuracy. With exported velocity file it will be possible to check other migration algorithm on data and make comparison between methods to achieve an acceptable migration of reflectors.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015