Seismic inversion (or seismic imaging) is the process of using seismic waves to create a two-dimensional map of the Earth’s subsurface. Seismic inversion can be performed either by stacking together seismic data from multiple shots at different times and depths, or by using real-time seismic to measure changes in the velocity of the wave as it propagates through the Earth’s crust and mantle. Both approaches have strengths and weaknesses, but this article will focus on the stacking method. For more information on real-time seismic, see How Does Real-Time Seismic Work?
Most geophysical imaging methods use data from waves generated by earthquakes, explosions, or by other seismic sources. These methods are known as active or passive. Active methods involve introducing energy into a subsurface area and then measuring data that is reflected back to receivers placed around an area of interest. Passive techniques use naturally occurring seismic signals that propagate through an earth region and detect those signals with sensors outside of the study area.
Seismic inversion is a geophysical technique for finding subsurface structures by inverting travel times of seismic waves. It is used in oil exploration, earthquake seismology and for regional studies to find reservoirs. Seismic inversion provides detailed information about various features on Earth’s surface, such as salt domes, sedimentary basins and others. This method has been applied to exploration of a variety of geological features, including petroleum reservoirs and other hydrocarbon deposits (oil sands), as well as geothermal reservoirs and natural hazards such as volcanoes. The most important aspect of seismic inversion is that it can be applied not only to passive seismic data but also to acquired data collected by active sources like air guns or explosions.
Before we can explore how seismic inversion works, we need to first review what an earthquake does and how its energy is generated. An earthquake is caused by a sudden slip on a fault within Earth’s crust. The displacement can be small, or it may even take place on a fault that was already stuck (slip failure). When slip occurs, enormous amounts of energy are released as waves radiate away from the point of rupture. How large an earthquake is depends on two things: 1) The amount of friction between tectonic plates and 2) How close they are to each other. Earthquakes only occur where these two things come together — which means earthquakes don’t happen very often.
Seismic inversion is a complex process to understand, particularly as it relates to oil and gas exploration. The basic concept is relatively simple: use seismic data to get an idea of what a subsurface looks like and how much oil or gas might be present. Still, there are some challenges involved that can make seismic inversion much more difficult than it sounds. First, for seismic data to be useful for inversion purposes, it needs to have a specific set of characteristics that allow for various algorithms to be applied. Second, because you’re trying to determine what’s underground based on sound waves sent through Earth’s surface, variations in terrain can drastically affect your results even if conditions are ideal above ground.