Imaging Angle-Dependent Reflectivity using the Marchenko Method

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Abstract

When reflection images are studied, often only the zero-offset reflectivity is considered, however, taking into account the angle-dependent reflectivity can add additional information about the Earth's subsurface. This additional information can be used to extract the properties of the subsurface using the amplitude variation with offset (AVO) analysis techniques. However, the presence of a complex overburden can significantly deteriorate the AVO response, especially for deep targets. To overcome this problem, the overburden effects can be removed by redatuming the reflection response at a depth level below the overburden. The Marchenko method has the potential to correctly retrieve the angle-dependent reflectivity in acoustic media without distortions due to multiple scattering caused by the overburden. The method estimates the downgoing and upgoing Green's functions of a virtual source located in the subsurface from surface reflection data and an estimate of the direct arrival from the location of the virtual source. The estimated Green's functions represent accurate upgoing and downgoing wavefields as they contain all orders of internal multiple reflections of the subsurface. These internal multiple reflections contribute to retrieving the reflectivity accurately in the redatumed reflection response. By deconvolving the retrieved upgoing Green’s function with the downgoing Green’s function, a new reflection response is obtained, with virtual sources and virtual receivers in the subsurface. The resultant reflection response is free of spurious events related to internal multiples in the overburden and contains the correct amplitudes. The angle-dependent reflectivity of the redatumed response can be obtained by summing the reflection coefficients along lines of constant ray parameter or angle. Potentially, the retrieved angle-dependent reflection coefficients obtained by this method can be used as input in a subsequent inversion process to obtain the velocity and density of the subsurface.