Chi

The construction of seismic chi projections from intercept and gradient data has become a widely used method in the industry to aid fluid and lithology prediction. Chi angles are typically obtained empirically by correlating extended elastic impedance (EEI) curves for a range of chi angles with a target log such as a gamma ray.

The recent development of relative rock physics allows for the calculation of theoretical chi values that optimise correlation with a number of elastic properties. The chi values depend only on the value of k; the background ratio (Vs/Vp)2. This approach also demonstrates why correlations between EEI and elastic properties are so good.

An analysis of the errors inherent in the measurement of the seismic gradient indicate however that it is generally not possible to create a seismic chi projection at a specified angle; the actual chi value will differ from the apparent chi. As a mitigation, if gradient errors are reasonably stable, the seismic can be scanned through a range of chi values and an angle selected that best enhances the desired aspects of the geology. However, a complex velocity field will also certainly mean that even this option will not provide reliable results.