Wave-equation based seismic AVO inversion: A pragmatic approach to use the full elastic wave-field for exploration and production

Abstract

The introduction of reflection seismic data acquisition and processing has been a major step forward in identifying and producing various hydro-carbon resources from the subsurface. For many decades, this statement applied primarily to the exploitation of hydrocarbons, but today 2D and 3D seismic data is also utilized for alternative applications like geothermal energy, carbon capture and underground storage as well as the development of large- scale offshore wind parks. In this context not only seismic, but geophysics in general is expected to play an important role in the energy transition. Originally seismic data was used for structural interpretation of geological structures and for stratigraphic interpretation based on migrated seismic amplitudes. For more than three decades now seismic inversion techniques also allow the derivation of elastic property models from seismic data. Since the first appearance of seismic inversion in the late 1980s, in principle two classes of techniques have developed: localized 1.5-D seismic AVO inversion of prestack migrated seismic data, and global 3-D Full-Waveform Inversion (FWI) of seismic surface data directly. AVO inversion is computationally significantly cheaper than Full-Waveform Inversion, mainly for two reasons: the inversion can be applied in a localized, e.g., target oriented, setup while FWI aims to solve for a global subsurface model while treating all seismic data simultaneously. It should be mentioned that global in this context refers to the entire part of the subsurface under investigation rather than the entire earth. The second aspect is that linear AVO inversion computes the 1.5-D primary response of the subsurface only which is significantly less expensive than computation of full elastic 3-D wave-fields. In this paper a hybrid approach of the two techniques will be presented, called Wave-Equation Based AVO inversion. WEB-AVO inversion solves the full elastic 1.5-D waveequation, thereby combining the benefits of the localization of the inversion problem while properly modelling elastic wave-effects like interbed multiples, mode conversions and transmission effects. The purpose of this paper is not to discuss the theoretical and technical aspects of WEB-AVO inversion in detail. It is more to revisit the historic development and current use of AVO inversion and FWI and how full elastic wave-equation based seismic AVO inversion strikes a balance between what is desirable and what is achievable.