Study On The Calculation And Stability Of Quality Factor Q

The earth belongs to the viscoelastic inhomogeneous media. When the seismic wave propagates in the underground, because of the attenuation of stratum and dispersion caused by absorption, the energy of the seismic wave will decrease with the increase of the propagation distance, the amplitude will decrease and the phase will be distorted. It is important to study the absorption and attenuation of seismic wave by stratum media, and it has important significance for improving seismic data resolution, high accuracy tomography, geophysical properties inversion and analyzing the distribution of hydrocarbon reservoir. Stratigraphic absorption and attenuation is a big problem for high resolution seismic exploration. The quality factor Q is one of the most important parameters to describe the elastic properties of rocks, and it also an important symbol of oil-bearing formation. Therefore, the study of quality factor Q estimation is very important to analysis absorption and attenuation effect of underground medium, and it also has theoretical and practical guiding significance for the prediction and evaluation of oil and gas reservoirs. According to the issues related to the estimation of quality factor Q, this paper introduced spectral ratio method(SR),central frequency shift method(CFS), peak frequency shift method(PFS), these three methods have their respective limitations in dealing field seismic data. The lack of a reliable method for estimating Q from reflection seismic data is an issue when utilizing Q value for hydrocarbon detection. Through the comparative analysis of the above three methods, study the advantages and disadvantages and comprehensive improvement to improve the prediction accuracy of Q estimation are the main research contents in the following three aspects:1) Spectral ratio method is the most widely used method for Q estimation. This paper describes the basic principle of spectral ratio method. Using Ricker wave as seismic wavelet, and combined with the least square principle to calculate the quality factor Q from the geological model. And analyzed the layer thickness, noise, time window length, frequency and other factors on the application effect of the spectral ratio method. Multi-layer model analysis results show that the spectral ratio method can effectively estimate the change of Q value, but the method used in the actual data processing is less effective. In view of a series of shortcomings of the spectral ratio method, the generalized S transform is used to replace the FT in this paper.Therefore,the choice of time window is avoided. And in the data processing, we add filter processing which can effectively improve the extraction accuracy of Q value.2) The peak frequency method makes use of the relationship between changes in the quality factor Q and peak frequency of the seismic wave. The peak frequency is the frequency corresponding to the maximum of the amplitude, and it is easy to beaffected by noise when identifying the peak frequency, which cannot be ignored.Compared with the spectral ratio method, the centroid frequency shift method(CFS)is a milestone in the frequency shift method, and its anti-noise ability is better than the spectral ratio method. However, we note that the seismic wavelet satisfied the Gauss distribution and the variance of the wavelet spectrum remains unchanged are the prerequisite for using the peak frequency method. Although we can consider using rectangular and triangular distribution to replace the Gauss distribution, the calculated Q value and the theoretical value have a big gap. When dealing with the seismic data,the seismic wave spectrum is usually non Gauss distribution and the attenuation of formation absorption will change the variance of the frequency spectrum, which will bring errors to the centroid frequency shift method.3) In this paper, we derive an approximate equation and propose a dominant and central frequency shift(DCFS) method by combining the quality factor Q, the travel time, dominant and central frequencies of two successive seismic signals, along the wave propagating direction. A test on synthetic data and statistical experiments show the proposed method can achieve higher accuracy and robustness compared with existing methods. Application of field data also shows its potential and effectiveness to estimate seismic attenuation. So, dominant and central frequency shift is a kind of Q value estimation method with strong anti-noise ability, high efficiency and stable calculation results, and has good universality and effectiveness.