Illumination properties and imaging promises of blended, multiple-scattering seismic data: a tutorial

In traditional seismic surveys the firing time between shots is such that the shot records do not interfere in time. In the concept of blended acquisition, however, shot records do overlap, allowing denser source sampling and wider azimuths in an economic way. A denser shot sampling and wider azimuths make that each subsurface gridpoint is illuminated from a larger number of angles and will therefore improve the image quality in terms of signal-to-noise ratio and spatial resolution. In this tutorial, we show that, even with very simple blending parameters like time delays, the incident wavefield at a specific subsurface gridpoint represents a dispersed time series with a complex code. For shot record migration purposes, this time series should be designed such that it facilitates a stable inversion process. In a next step, we explain how the illumination property of the incident wavefield can be further improved by utilizing the surface-related multiples. This means that these multiples can be exploited to improve the incident wavefield by filling angle gaps in the illumination and/or by extending the range of angles (natural blending). In this way the energy contained in the multiples now contributes to the image, rather than decreasing its quality. One remarkable consequence of this property is that the benefits to be obtained from the improved illumination depend on the detector locations in acquisition geometries as well. We explain how to quantify the contribution of the blended surface multiples to the incident wavefield for a blended source configuration. In addition, we explain how blended measurements can be directly used in an angle-dependent migration process with a bifocal, least-squares imaging condition. The result is a densely sampled reflectivity function in the ray parameter domain, also improving the capability of velocity estimation. We will show examples to illustrate the theory.