IRANIAN JOURNAL OF GEOPHYSICS
Year:2008 | Volume:2 | Issue:1|Papers Count:7

Seismic waves suffer from absorption while propagating in anisotropic medium. Due to the frequency dependency of attenuation, it is desirable to use frequency based methods to study attenuation. Some of the methods use frequency dependent seismic attributes to determine absorption and quality factor. In this study continuous wavelet transfonn and extracted timescale map are used so introduce a new seismic attribute called centroid of scale. A relationship between quality factor of propagation medium and centroid of scale is derived. Usually absorption related anomalies on seismic data are very important and are attributed to the presence of hydrocarbon reservoir and/or fault zones. It is expected that the deriving the behavior of, attribute of centroid of scale can detect directly hydrocarbon reservoir or the fault zones. In this study all required computer codes are written in MATLAB environment. To evaluate the efficiency of the introduced seismic attribute in locating attenuation related anomalies, we applied the method on both real and synthetic seismic data. It is found that the low cost run time and high resolution are advantages of the method compare to others.

Inversion of amplitude versus offset (AVO)can be used to determine elastic properties of rocks, which can be used for identifying lithology and fluid content of reservoirs. AVO inversion may be used to derive parameters proportional to Lame's constants, Lambda and Mu. More specifically, the actual rock properties that can be derived from pre-stack data are Lambda Rho and MuRho. Lambda Mu Rho (LMR) analysis is one example of how interpreters are using advanced AVO analysis to identify hydrocarbon and reservoir rocks. LMR analysis uses multiple volumes of data to interpret correctly litho logy and fluid content. This paper describes LMR analysis, carried out in a part of 3-D pre-stack seismic data of an oil field in Persian Gulf, and compared the results with common AVO attributes such as Poisson ratio changes. Seismic data were calibrated by well log data of a borehole in this area. P-wave and S-wave reflectivity volumes were generated via AVO analysis, then proceeded to invert these volumes for P-wave and S-wave impedances. These impedance volumes were combined to generate Lambda Rho and MuRho sections. From this analysis and correlation with well log data hydrocarbon bearing zones and different lithologies were identified.

According to different properties of P and S-waves and their different effects from elastic properties of rocks, valuable information including rock matrix and saturating fluid can be extracted when both P- and S-wave velocity data are available. Recording both P- and S-waves together is expensive and difficult. In this paper, it is shown that using seismic compressional wave, shear wave information can be estimated. Then, both of these data can be used to extract seismic attributes in order to detect variations of lithology and fluid information. In this paper, by applying inverse method both P- and S-wave reflection coefficients are estimated. Also, P- and S-wave impedances are estimated by applying the band limited seismic inversion method. Finally, we considered the application of this method on a carbonate reservoir in S.W. Iran. This study showed that the compressional and shear impedances are sensitive to reservoir properties and are able to distinguish fluid changing in carbonate reservoirs.modification factor is defined to take in to account the effect of rupture directivity in empirical velocity attenuation relations. The modifications are based on modeling March-31st-2006 Darbeastane- Silakhor earthquake, using finite element method, applying ANSYS software. The ground motion parameters that are modified, include ratio of strike -normal (Vn) to strike parallel (Vp) component of horizontal velocity and strike- normal component to average horizontal velocity (V). The ratio of strike- normal to strike- parallel velocity is large in both the forward directivity direction, where velocity is larger, and in the backward directivity direction, where velocity is smaller. We therefore e expect the strike- normal to strike parallel ratio to be mainly controlled by directivity angle. Also the variation of fault normal velocity to average horizontal velocity ratio by directivity angle (b) is defined from earthquake modeling. It shows Vn to V ratio is controlled by directivity angle, distance between the site and epicenter and rupture' length. This ratio has the same trend in 2006 Darbeastane- Silakhor earthquake strong ground velocity data. In this research the equation for Vn to Vp variations by directivity angle is p recommended. We used Somerville et al. (1997) directivity model parameters as (R/ L) cos2 b to define directivity effect on Vn to V ratio. Therefore, directivity factor is the determined to account in empirical strong ground velocity attenuation relations.

The quality factor (Q) is an important factor in reservoir characterization, in which low quality factor shows high attenuation and also may indicate existence of hydrocarbons in the reservoir zone. Determination of Q at different depths can predict Q values in the reservoir. In this paper, VSP data from two exploration wells in the south part of Iran were used. Quality factors were calculated using the spectral ratio method. For comparison, quality factors for different depths were determined using VI' Ns and Poisson's ratio. Low Q factors (lower than 25) may be an evidence for existence of hydrocarbons in the reservoir zones. The determination of hydrocarbons in the reservoir zones from relative changes in Q were verified by using well logs.