Seismic attenuation analysis of vertical seismic profiles for gas-bearing sedimentary reservoirs

• Main Author: Talukder, Waheduzzaman Md.
• Other Authors: Memorial University of Newfoundland. Dept. of Earth Sciences
• Format: Thesis
• Language: English
• Published: 2008
• Subjects: Seismic prospecting > Mathematical models; Seismic waves > Mathematical models; Seismometry; Newfoundland studies; University of Newfoundland
• Online Access: http://collections.mun.ca/cdm/ref/collection/theses4/id/80661

Thesis (M.Sc.)--Memorial University of Newfoundland, 2009. Earth Sciences Includes bibliographical references (leaves 88-89) Quality factor (Q) estimation of vertical seismic profiles requires precise computation of seismic energy or amplitude over a certain frequency band width of a propagating wave field through the earth media. It is a big challenge to have the correct amplitude spectra over a frequency band for reliable estimation of anelastic attenuation factor in seismic data. Q is a very useful petrophysical parameter in the characterization of different layered earth media. Estimation of Q in this study is for both numerically generated zero-offset vertical seismic profiles and field VSP. A finite difference technique has been applied to model wave propagation in elastic isotropic layered media. In finite difference modeling a staggered grid scheme is implemented in second-order in time and second-order in space that provides a convenient way to define model boundaries. For forward 2D Q analysis of model VSP and field VSP data, spectral ratio (SR) and centroid frequency down shift (CFD) methods have been used in this study. Validity and sensitivity studies of SR and CFD methods, has been done on synthetically generated model VSP data prior to attenuation analysis of field VSP data. The use of two different methods (SR and CFD methods) provides an opportunity to validate attenuation measurements. The results of the synthetic modeling studies with and without intrinsic attenuation in different models shows very close agreement with model values and validates these methods. The Q analysis of total field VSP over the 30m depth interval, illustrates that the Q estimation is satisfactory, however a few shows a physically unrealizable phenomenon of negative values. The formation wise average Q analysis result of the field VSP indicates that the Bhuban formation is more attenuative than Tipam and Bokabil formations and the gas bearing zone (2420-2460m) has a low Q value (SR=28 and CFD=23). This highly attenuative thin gas bearing layer alters the seismic signal both in shape and size. The spectral analysis and estimated Q values of the total VSP section indicates that amplitude spectra drastically losses energy when the signal passes through the gas bearing zone.