The Poorman’s Guide to Depth-calibrating Seismic Data in the Presence of Allochthonous Salt  

It is a well known problem that depths from isotropic PSDM data generally do not match well depths. This often leads to problems using such data for well planning and resource estimation. In this talk we demonstrate the fundamental limitations of isotropic velocity analysis and how it relates to depth errors. We also demonstrate methods to correct these types of errors.

While the difference between optimal imaging velocity and vertical velocity is not apparent when imaging in time, it is clearly manifest on data imaged in depth. Not only are the depths of seismic events misrepresented on isotropic PSDM images, but the magnitude and direction of dip can be misrepresented!

In the Gulf of Mexico and West Africa apparent dip errors are most pronounced in the vicinity of salt overhangs where isotropic strata (salt bodies) are juxtaposed with anisotropic strata (shale and inter-bedded silt). Distinguishing between isotropic and anisotropic strata is the keys to characterizing the correction function required to “verticalize” the velocity field post-stack. Post-stack depth calibration examples will be presented from West Africa and the Gulf of Mexico where seismic-scale anisotropy can be approximated by a second or third order polynomial.