Occasionally, scientific images can be both enlightening and beautiful

During a seismic survey, each shot sends a wave propagating through the Earth, while receivers on the surface listen for reflections as that wave bounces off of geologic layers. Real-world geology can be extremely complex, and because of the different wave propagation velocities the of the different layers, the wave never expands in simple circles, like ripples in a pond. Instead it is scattered off of high-velocity contrasts, refracts around slower regions, is focused into beams.

The purpose of Reverse Time Migration (RTM) is to take those incredibly complex wavefields, as recorded at the surface, and form an image of the underlying geological structure.

The images shown here are Illumination Maps, which show how much energy from a single shot reached each point in the subsurface.

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The bright white blob at the top of each image indicates the large amount of energy immediately below the shot location. High velocity contrasts can produce sharp illumination contrast, such as below-right of the shot in the above map.

In the above image, from a simulated marine seismic survey, the bottom of the ocean is clearly seen as a white line running horizontally across the top of the image.

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Like a room lit by a single bulb, the light doesn't reach everywhere, and the better a region is illuminated, the clearer the image of that region can be. It can be difficult to image in "shadow zones" that lie beneath complex, high velocity structures, such as salt bodies, for example.

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Illumination maps are therefore used to help design seismic surveys - specifying the relative locations and densities of shots and receivers in order to provide even illumination of the region of interest.

In Reverse Time Migration, Illumination Maps are used to normalize the migration image in order to correct for the uneven illumination of the subsurface, and are an almost-free output of RTM processing.