Low frequencies for better continuity beneath complex media

Abstract

A geologic formation is composed of several sub-elements, such as minerals, lithologic fragments, inclusions, small faults, fractures, cavities etc. A seismically complex formation can be defined as being formed by sub-elements with spatial dimensions equal or comparable to the seismic wavelengths. A seismic wavelet travelling through a complex formation, passes through a particular sequence of sub-elements. In conventional common mid-point recording geometry, all the traces that form a CMP gather, are simply the superpositions of individual wavelets which have travelled through different sequences of sub-elements of the complex formation. This fact introduces irregular time shifts among the wavelets reflected from the same horizon beneath the complex medium, that is, a problem of phase which cannot be solved by conventional dynamic and static correction methods, not even by cross-correlation based residual static correction algorithms. We can overcome this problem by shifting the spectrum of the wavelet to lower frequencies and so making the time shifts correspond to smaller phase differences, resulting in a better stacking output. To achieve this goal, envelopes of traces of a CMP gather can be taken and after being debiased and further filtered if necessary, can be input to the stacking process. The problem and the method of solution will be described by a model and the application of the algorithm to real data examples will be presented.