Effects of Shallow Reflectors on Amplitude vs. Offset (Seismic Lithology) Analysis

ABSTRACT Reflection amplitude .versus CDP gather offset analysis has become a major new tool in the exploration seismologist's arsenal. During the last few years several articles have been published on this topic, i.e. W. Ostrander (1982), G. Gassaway and H. J. Richgels (1~83-84), Graul et al (1983), and G. Gassaway (1984). These methods would be easy to implement if the observed amplitudes were solely related to the reflection coefficients at the elastic boundary. This is because the seismic (P-wave) reflection amplitude at an elastic boundary is a function of seven parameters. These seven parameters are three elastic constants in each media and the angle of incidence (Richter 19~8). These elastic constants are functions of mineralogy, lithology, and pore fluids, (see Simon &Wang, 1971; Toksoz et aI, 1976; and Garmichael, 1982). In fact, several methods have been developed to invert the observed amplitude variations in a GDP gather to the elastic constants and lithologic estimations, (Gassaway &Richgels, 1~83; and Hilterman in Graul et al, 1983). However, mental reflection upon the problem leads one to a whole list of items that ultimately affect the observed amplitudes. This list can be subdivided into those items that are easy and those that are difficult to correct. Some of the items that are easy to correct are:spherical spreading of the wave front,geophone and shot arrays,free surface effects,geophone sensor not parallel to the emerging ray path, (particle motion), anddata processing programs. Among those items that are more difficult to correct are:inelastic attenuation,anisotropic effects,transmission losses at overlying reflectors, andthin bed tuning effects. It has been our experience that transmission effects are an order of magnitude greater than the others. This paper addresses transmission losses and comes to the conclusion that changes in transmission losses in the shallow reflectors can totally mask the true reflection coefficient versus angle of incidence Changes at the objective horizons. Therefore, it is concluded that correct "amplitude versus offset" interpretations must include a correction for the transmission losses at the shallow reflector boundaries. INTRODUCTION The analysis of amplitude versus offset within a CDP gather has created practical ways of estimating elastic coefficients. From the elastic coefficients, further estimates of lithologies and pore fluids are made. The resultant practice of "seismic lithology" has been developed. Ostrander (191:$2) proposed that gas bearing sands should show a strong increase in amplitude with offset on a CDP gather. He showed many examples where this was the case. Figures #1 and #2 show a shallow gas zone which conforms to Ostrander's model. The amplitudes show a strong increase with increasing offset. These two figures also show a second gas zone Which does not show the expected amplitude increase with offset.