Anisotropic ambiguities in TI media

Introduction In recent years we have seen processing and imaging algorithms re-written to handle anisotropic effects. The most common type of anisotropy that one deals with in seismic data is polar anisotropy (transverse isotropy). Media with vertical (VTI), tilted (TTI), and horizontal (HTI) axes have been shown to exist as a result of sedimentary deposition or fracturing. The consequences of ignoring polar anisotropy vary depending on the degree and type of anisotropy. Typically, using isotropic imaging in an anisotropic medium results in mis-ties between the preSDM and the well depths. Such mis-ties, in some extreme cases, can exceed 10% of the true depth (for example, in the Franklin-Elgin field operated by TFE in the North Sea there is a 600m mis-tie at a depth of 5km). In addition to the vertical depth error, there is also a lateral shift, most pronounced for the steepest dips, and noticeable in fault surface reflections. Experience has shown that one cannot image simultaneously flat and steep dips with an isotropic velocity field. The difficulty in addressing anisotropy lies in the estimation of reliable parameters to be used with the processing or imaging algorithms. In this work, we assess the effects of errors in anisotropic parameter estimation for the case of polar anisotropy, and attempt to quantify the consequences of these errors with some specific synthetic examples.