Exploration and drilling practices in a complex tectonic environment have always been a challenge. Look-ahead VSP has been introduced to help identifying the potential hazards below the bit prior to drilling. The procedure is to invert the VSP data for acoustic impedance while accounting for the structural effects. The technique used is based on Monte Carlo sampling of reflection data while incorporating known impedance and structural dip constraints (model) at different times through Bayes’ rule. The technique has been successfully employed for look-ahead purpose in complex geological structures of North Pakistan. The borehole deviation, however, remains a critical challenge for the quantitative applicability of the technique since the direct rays traveling down to the bottom-most receivers follow highly slant trajectories in this case. In such cases, the inverted acoustic impedance gives a reasonable quantitative guess and a good qualitative look-ahead prediction.


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