Main Article Content
Zoeppritz equations are used to determine the reflection coefficient against the angle curves, which are often valid only for small seismic parameter changes across reflectors, but generally inaccurate close to the critical angle. These inaccuracies affect the quality of amplitude variations with offset (AVO) analysis, which might results in systematic errors when estimating relative seismic parameter variations at the reflectors. Thus modifying the Zoeppritz equations at the given angles allows for more accurate estimation of the usual AVO attributes, such as intercept, the gradient, and a possible third coefficient, which often leads to a better estimation of seismic-parameter contrasts at reflecting interfaces. The modification of Zoeppritz equations was analyzed using well data from oil fields in a sedimentary basin, onshore of Niger Delta area. This paper analyzed the modification of Zoeppritz equations and using them in AVO analysis to collect information on how seismic amplitudes vary with incident angles, which when combined with the P-P reflectivity (RPP) or the P-S Reflectivity (RPS) expressions, is used to obtain information on the properties of the earth layers, with the emphasis only on interface reflectivity, while thin-bed effects, attenuation and other propagation factors well known to influence AVO measurements are not considered. The modified equations are subsequently used to generate the AVO reflectivity curves, the results obtained shows that the modified Zoeppritz predicted the AVO effects correctly for the different zones of interest in the basin. The results show that the Shuey’s approximation gives better accurate results up to angle of 300 compared with others approximations, while the 3-term approximation shows that the modified Zoeppritz equations predicted AVO response accurately to about 500 of angle of incident. The result obtained can also be used to classify the different sand base types and their fluid contents, either oil, gas or brine sand base.
Aki KI, Richards PG. Quantitative seismology - Theory and methods, volume I, sec. 5.2: W. H. Freeman and Company, San Francisco; 1980.
Yong Xu. Rock physics and seismic methods for characterizing the heterogeneity of oil sands reservoirs in the Western Canadian Sedimentary Basin. PhD Thesis; 2012.
Balogun A, Ebeniro JO. Evaluation of seismic attributes generated from extended elastic impedance for lithology and fluid discrimination, International Journal of Science and Research (IJSR). 2017;6(9):776–779.
Shuey RT. A simplification of the Zoeppritz Equations: Geophysics. 1985;50:609- 614.
Balogun A, Ehirim CN. Lithology and fluid discrimination using bulk modulus and mu-rho attributes generated from extended elastic impedance, International Journal of Science and Research (IJSR). 2017;6(10): 639 – 643.
Bortfeld R. Approximations to the reflection and transmission coefficients of plane longitudinal and transverse waves: Geophysical Prospecting. 1961;9:485- 502.
Gelfands VP, Nguyen H, Larner K. Seismic lithology modeling of amplitude verses offset data, 56th Annual meeting and international exposition, Society Exploration. Geophysics, Expanded abstracts. 1986;334-337.
Ramos ACB, Castagna JP. Useful approximations for converted-wave AVO: Geophysics. 2001;66:1721-1734.
Awosemo OO. ‘Evaluation of Elastic Impedance Attributes in Offshore High Island, Gulf of Mexico’, (MSc. Thesis), Department of Earth and Atmospheric Sciences, University of Houston. USA; 2012.
Young RA, LoPiccolo RD. A Comprehensive AVO Classification: The Leading Edge. 2003;11-18.