Near-wellbore VSP imaging without overburden

Abstract

In seismic exploration a complex overburden can cause defocused and distorted seismic images of deeper target areas. While in surface seismic imaging it is difficult to overcome this problem, vertical seismic profile (VSP) borehole seismic imaging with its well-oriented recording geometry has proven able to circumvent the overburden’s detrimental impact. Recently, VSP near-wellbore salt flank imaging without knowledge of the overburden has shown encouraging results. In this paper, we introduce another method for a more common imaging purpose: imaging near-wellbore horizons without knowledge of the overburden. A key component of our method is a reverse ray-tracing technique that uses actual VSP direct-wave traveltime picks and extrapolates through a local velocity model to obtain the direct-wave traveltimes in the vicinity of the well. The extrapolated traveltimes then serve as references for migration of the VSP reflected data from near-wellbore horizons. The merits of this method are that all source-side effects such as shot position and triggering time statics are avoided, and it can efficiently and robustly image the near-wellbore horizons because it does not need an estimated overburden velocity model prone to errors. However, the image is limited to a diamond-shaped area around the well whose size is proportional to the VSP array. A synthetic VSP experiment on the SEG/EAGE 2D overthrust model shows that this method, with a local (even an estimated 1D) velocity model, effectively images structural features, such as dip direction and discontinuities of the near-wellbore horizons.