High Accuracy Prestack Q-determination Based On Raypath Theory

When seismic wave propagating in the underground media, there must have a loss of the energy and the energy’s loss and velocity dispersion are the main characteristics of the propagating process. There are a lot of factors that cause the attenuation. Generally, there are two main causes:one is related with the characteristic of the seismic wave’s propagation, for example, the spherical scattering, the interbed multiples, and the attenuation caused by the underground media’s anisotropy, another is related with the nature of the underground medium, one of which is the stratigraphic absorption. The absorption effect is usually measured with quality factor QThe accurate determination of seismic attenuation, or the precise calculation of Q, is useful for signal enhancement and reservoir characterization. This paper discusses seismic attenuation’s mechanism first, then discusses kinds of methods to calculate Q value based on the development of domestic and foreign.The most common method in picking up the Q value under actual production is the spectral ratio method. Conventional spectral ratio method faced up with the deviation coming from the wrong moveout, although later improved it, but the precision doesn’t improve evidently. Meantime, the conventional method have a low precision in the presence of scattering and tuning came from the choice to the time window. We introduce a modified spectral ratio method from prestack seismic data. We all know the the prestack contain more abundant information than the poststacks. We made use of the τ-p transform to operate on traces of constant horizontal slowness. Then the error in the calculation of the traveling time difference to be eliminated. Then I compare fixed and variable-window time-frequency transforms to test their ability to obtain robust spectral estimates in the presence of scattering and tuning. Variable-window time-frequency transforms, such as generalized S-transform, reduce spectral interference, producing more robust attenuation measurements. we use the variable-window of the generalized S transform to analyze the instantaneous frequency spectrum between the stratum, which overcomes the effect came from the choice in the length and width of time window in the traditional way and enhanced the evaluation of the Q value as well. As the exist of attenuation of anisotropy, I introduce a anisotropy parameter to improve the precision of Q calculation.