Determination of anisotropic parameters,in situ, using a multi‐offset vertical seismic profile

Abstract

Seismic anisotropy is an issue when time recorded seismic data is converted to depth, especially in complexly structured regimes. The velocities used in the conversion are generally isotropic and do not account for any velocity anisotropy that may be present in the geologic formations. Refraction surveys have been used successfully in areas of outcropping, steeply dipping strata, to measure the velocity variations in the bedding perpendicular, bedding parallel and 45 to bedding directions. These velocities have also been used to calculate the Thomsen anisotropic parameters, to be used in depth migration routines. An experiment was designed and executed in the Rocky Mountain Foothills in Alberta, Canada to determine whether seismic velocity anisotropy could be measured, using first arrival traveltime analysis from a multi-offset vertical seismic profile (VSP). Requirements for the VSP well are that it should penetrate a uniform panel of strata with intermediate dips, ideally 45. The experiment was successful in measuring the anisotropy of Cretaceous age shales in the well. These results are consistent with previous in situ results obtained from surface refraction seismic survey measurements.