Detection of layer with abnormally low velocity using converted-wave equation

In the hydrocarbon exploration is common to find reservoirs underlying complex formations, particularly in offshore conditions. From many possible complex characteristics, one of them is when a geological formation presents an abnormally low velocity. This kind of characteristic is so different, because the increasing overburden of rocks with the depth usually results in an increase of propagation velocity of seismic waves, for both compressional (P) and shearing (S) waves. An important step in the seismic processing is the velocity analysis, what helps to characterize the formation geologically and structurally. In complex situations, the travel-time curves associated to waves reflected in interfaces between layers are extremely distorted and present a nonhyperbolic behaviour. Thus, it is necessary to use an equation able to control the effects of the nonhyperbolicity of the reflection event. However, for the nonhyperbolic equations, it is assumed that the velocity always increases with the depth, and that is the reason for these equations present some difficulty to fit the calculated curve to the recorded curve. Despite this, the limitation of the capacity of fit in this kind of condition results in a good way to detect the layers with abnormally low velocity, once the quality of the curve fitting decreases abruptly. So, it was proposed in this work the utilization of this limitation in the curve fitting to identify this characteristic, testing in different models what allows to analyse in what kind of situation the proposed method works in a reliable way.