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 wellplanning 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.
The simple reason for these errors is the fundamental difficulty of determining the vertical component of the subsurface velocity field from data shot and recorded at the surface. There are numerous mechanisms that may lead to inaccurate estimation of vertical velocity from seismic velocities using standard isotropic processing techniques.
However, in many cases seismic-scale anisotropy is the first-order cause for this problem. Thus isotropic seismic velocities which flatten gathers and optimize the seismic image need to be “verticalzed” for depth conversion and before computation of other velocity-related geophysical attributes.
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.
Adam Niven Shumaker is a geophysicist for Noble Energy Inc. in Houston where he is currently working on a development project in the Bohai Bay, China. Niven has had experience working the in the Deepwater Gulf of Mexico and West Africa Business units as part of a new hire rotation program and is developing expertise in fluid pressure prediction. Prior to working at Noble Energy, Niven began his career working in the New Ventures group at Vintage Petroleum in Tulsa, OK.