Geophysical reservoir characterisation in a complex geologic environment remains a challenge. In particular, conventional amplitude inversion assumes that the seismic amplitudes are correctly located and can be inverted to derive a true representation of the rock properties (often referred as “true amplitude” images).

This case study is located in the Gulf of Mexico in an area with a complex salt structure. The target is subsalt and poorly illuminated due to the complexity of the salt overburden (Figure 1a). In order to obtain an image of the subsurface, large offset full azimuth data has been acquired. This enables the interpretation of key subsalt horizons on a reverse time migration (RTM) image (Figure 1b). However, the inadequate subsurface illumination due to complex geology or the acquisition geometry has detrimental effects on the amplitudes and phase of the migrated image.

Analysis of the amplitudes draped over the key horizons shows that there is a clear correlation with variable illumination (Figure 2). Areas that are very well illuminated by a large range of offsets and azimuths seem to correspond to the higher amplitudes, whereas other areas illuminated by a restricted range of offsets and azimuths seem to correspond to low amplitude areas along the key horizon. Conventional amplitude inversion techniques do not compensate for these amplitude and phase variations. Consequently, any attributes derived from the image amplitudes will not accurately represent the properties of the corresponding lithology.

In order to address this simplification, we propose a technique to perform amplitude inversion directly in the depth domain, correcting for the dip dependent illumination effects caused by the acquisition geometry and complex geology, and thus creating consistent and more reliable imaging products and seismic inversion attributes from depth migrated data.