AVO and the nature of light

Abstract

Most geophysicists working in the hydrocarbon exploration industry will have come across the famous Zoeppritz (1919) equations that govern the AVO (amplitude variation with offset) behavior of reflected seismic waves. That is how the refection coefficient changes with angle of incidence at the boundary of two horizontal layers with different VP/VS ratios. This set of equations was published many years before the advent of exploration reflection seismology and it might be helpful to shed some light, in more than a proverbial sense, on the background of their derivation. The era of AVO as a DHI for gas sands was ushered in by Ostrander (1982) who popularized the method as a hydrocarbon exploration tool. Since then, the interest in this technique has continued with multiple sessions at the SEG Annual Meeting, special issues of The Leading Edge, dedicated seminars, and specialized processing/interpretation packages. Koefoed(1955) and Bortfeld (1961) often are cited as pioneers who introduced the principles of AVO to applied geophysics. Less known, however, is an early application by Blut (1932) who calculated AVA (amplitude variation with angle) responses for various lithologies, including ice, overburden (clastic sediments), basalt, and granite by using the relationships derived by Zoeppritz. Practical background of Blut's work was the interpretation of recordings from quarry blasts. Because of the instrumental limitations of his time, these remained rather theoretical considerations since calculating the AVA response curves in the days of logarithm tables and slide rules was quite an achievement given the rather cumbersome algebra of the Zoeppritz equations. Despite these computational challenges, apart from a different sign convention, Blut's AVA curves fit quite well with curves derived by modern Zoeppritz solvers. Zoeppritz's original publication “On the reflection and transmission of seismic waves at discontinuites” focuses on earthquake seismology. It is one …