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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Seismic migration images the subsurface by propagating a wave into the Earth and listening for the reflections that return to the surface. Like any waves, the major properties of seismic waves are speed, frequency and wavelength. Seismic wave speed is largely determined by the type of rock it is traveling through, as well as how much pressure that rock is under. Wave speeds in rock typically vary from 3000 m/s to 6000 m/s, which is up to 20 times the speed of sound in air. The frequency of the wave is determined by a few factors, but mostly by the type of energy that is injected at the surface and how well that energy travels through the rock (the dispersive nature of the rock). At the end of the day, usable seismic frequencies recorded at the surface are usually between 20Hz and 60Hz.

The seismic wavelength is determined by the other two properties: wavelength = speed / frequency. And it is wavelength that determines the resolution possible to achieve in seismic imaging. The smallest subsurface Earth feature that is visible to the seismic wave is approximately 1/4 of the wavelength. Typical seismic wavelengths therefore vary from

to

Putting this into perspective, a large mountain face is on the order of 1000m high and is made up of features from 100s of meters in size down to centimeter-sized details. The imaging resolution determines which features are visible and which are lost. Of course, the complex layering and folding of the visible geology above ground are similar to the complex structures that seismic imaging is meant to reveal in the subsurface.