مرکز منطقه ای اطلاع رساني علوم و فناوري - شناسايي مستقيم منابع هيدروكربني به روش تعيين فاكتور كيفيت با استفاده از محاسبه چگالي انرژي در حوزه زمان- مقياس

چكيده لاتين :

In exploration seismology quality factor is widely used as a seismic attribute to identify anomalies related to attenuation, especially those caused by hydrocarbon. Previous studies have indicated that seismic energy loss known as attenuation is greater for high frequency components of seismic data compared to the low frequency components. Here the continuous wavelet transform is used to study the attenuation of seismic data and to calculate the energy density at different scales. The results show that the energy loss at low scales is more than that of the high scales. The method is also used for determination of the anomalies related to energy attenuation due to the presence of hydrocarbon. The results indicated that using modified complex Morlet wavelet needs fewer computationsthan the regular complex Morlet wavelet. We investigated the efficiency of the method on both synthetic and real seismic data and the results are compared to the results obtained from inversion of seismic data to acoustic impedance using the Hampson-Russell software. The results showed an acceptable correlation. We also found that regular complex Morlet wavelet is more sensitive to the presence of noise than the modified complex Morlet wavelet. Continuous Wavelet Transform: The time domain continuous wavelet transform (CWT) of a signal / (t) can be defined as: where, * denotes the complex conjugate, a is scale, b is time shift and y/ (t) is the mother wavelet. Shifted and scaled version of the mother wavelet can be computed as >Ja \ a J We can define the frequency domain CWT as: Wf (a,b) = F(o))ejby * (aco) (3) where, co is angular frequency, F(co) and y/(co)are the Fourier transform of / (t) and mother wavelet, respectively (Poularikas, 2000). Since the Morletʹs wavelet is similar to the seismic source wavelet, we used the complex Morlet wavelet and a modified version of it as the mother wavelet in our study (Li et. al., 2006). Energy Attenuation Density Equation: Considering a plane wave U (co,z) propagating in the anelastic medium, assuming that the quality factor Q is constant, its propagating equation is defined as (Aki and Richard, 1980): }(OZ —coz U(co,z) = U(co,0)e^e^ (4) where, co is angular frequency, z is propagating distance and c(co) is phase velocity. The energy density E (co,z) at any angular frequency co is by definition: E(a>,z) = U(a>9z)V(a>9z) (5) By introducing Eq. (4) to Eq. (5) and calculating the frequency domain CWT, assuming that |U (&>, 0)|2 =1 and Q »lthe wavelet domain energy density of signal can be obtained as: -tco^ Ea=eQa (6) Equation (6) shows that the energy of a signal in the wavelet domain is a function of the quality factor Q and scale factor a as well as travel time t. The larger Q is, the more slowly the energy attenuates; The smaller Q is, the faster the energy attenuates. The smaller the scale, the less the energy involved in the signal, because high scales correspond to low frequencies, and low scales correspond to high frequencies. V? Discussion: This paper derives an energy attenuation formula for seismic waves in the wavelet- scale domain from wavelet theory and the seismic propagation equation in the anelastic medium. To investigate the efficiency of this method, we tested the method on both synthetic and real