Introduction

Amplitudes of reflected seismic waves have concerned geophysicists since the beginnings of the profession Gutenberg (1936). However, until the advent of digital recording in the late sixties, efforts in this direction were mainly theoretical Bortfeld (1961). ``Bright-spot'' technology started a first wave of applied amplitudes research in the early seventies Craft (1973), and the emergence of Amplitude Variation with Offset (AVO) techniques Shuey (1985) assured amplitudes a solid place in the geophysicist's toolbox. The line of research to which this paper belongs was started by Kjartansson (1979).

Kjartansson observed that zones with amplitudes too large to be explained by lithological contrasts at the reflector or by tuning were correlated in a predictable way in the prestack data volume, and he provided a conceptual explanation for it. I will call these phenomena Focusing-Effect AVO/AVA (FEAVO/FEAVA). The section that follows after the Introduction will describe them in detail. The important thing to note, and the motivation for this work, is that: (1) those anomalous amplitudes are caused by wavefield focusing through velocity or absorption lenses, (2) that they impede proper AVO/AVA analysis or any other amplitude analysis techniques, and that therefore (3) they should be removed from the seismic image. Work on this subject has been published sporadically, especially at SEP, ever since Kjartansson's paper. The most recent publications related to this topic are Vlad and Biondi (2002), Vlad et al. (2003a), and Vlad . While these articles dealt with highly specific details, the material that follows will provide an overview of the current state of knowledge about FEAVO and its removal, with assessments of the problems that remain to be solved, in highlighted paragraphs.