Harmonic reverse time migration

Abstract

In subsurface regions containing gas or hydrocarbons, porous rocks, fluid-saturated zones, and fracture networks, seismic waves may propagate nonlinearly, resulting in harmonic generation, waveform distortion, and even the formation of shocks. To address these acoustic nonlinear effects, we present a novel approach based on solving the Westervelt equation for seismic imaging, which achieves approximately twice the resolution of conventional reverse-time migration (RTM). The Westervelt equation incorporates an intensity term proportional to the square of the seismic wave amplitude or the product of two interacting waves. As waves travel through a nonlinear medium undergoing compression and rarefaction, this term acts as a virtual secondary source, generating harmonic high-frequency waves and broadening the seismic bandwidth. The high-frequency aspects of subsurface imaging are investigated using this technique, referred to as harmonic reverse time migration (hRTM). Numerical simulations demonstrate that hRTM effectively combines the fundamental and harmonic components to achieve superior structural delineation and significantly enhanced imaging resolution compared with conventional RTM.