The results of seismic surveys are used in many ways, from finding new sources of oil and gas to studying how fault lines form and even the structure of the Earth’s interior, but one type of analysis stands out among the rest: seismic inversion. Seismic inversion uses seismic data to generate high-resolution images of subsurface structures like hydrocarbon reservoirs, faults, and even the mantle rock that lies deep within the Earth’s crust. But what makes this type of analysis so powerful? How does it work? And can you use it on your own seismic data?
What is seismic inversion?
Seismic inversion is a technique that can be used to estimate properties of subsurface geologic formations from seismic data. A seismic survey involves setting off an explosion or recording vibrations generated by air guns on ships or aircraft. These vibrations travel through Earth and reflect off different subsurface layers of rocks at varying speeds depending on rock type, density, and elasticity. After arriving back at the surface, these reflected waves are recorded as echoes in long strips of paper.
How does it work?
The most common form of seismic inversion is called a velocity model. It uses survey data to produce subsurface maps that indicate where rock layers are fast, slow, or stationary—often hundreds of meters deep. The process requires extensive field work, from identifying potential sites for conducting surveys to recording geophysical signals using specialized seismometers. All told, data analysis can take several weeks after fieldwork is completed. Velocity models can be used for things like determining how much oil or natural gas lies beneath specific regions (i.e., prospecting), tracking fluid flow within subsurface formations (i.e., water supply), and determining how tectonic plates have moved over time (i.e., earthquake prediction).
When might you use seismic inversion?
You might use seismic inversion if you’re trying to map out a large area (i.e., multiple acres or hectares) and need to know what’s below your feet, but aren’t able to get a satellite image or any other type of aerial photograph for that area. In that case, you can look at surface maps and perform ground surveys, but these won’t be as detailed as an aerial photograph will be. For example, if you’re looking at an open field used for agriculture and want to know what’s below your feet without disturbing anything on top of it, then seismic inversion might be a helpful tool for you. There are many more reasons why someone might need aerial photographs; however, not everyone has access to them.
What are some limitations of seismic inversion?
While seismic inversion provides useful information about subsurface geology, it does have its limitations. The first is that it doesn’t work well when there are changes in geological conditions over a short distance; seismic waves will refract around them, or even bounce back to their source, making it difficult to get accurate data on what lies underneath. These changes could be due to faults (horizontal breaks in rock) or different types of rock material with different acoustic properties. For example, a shale layer will absorb and reflect sound differently than limestone would. This can cause problems for seismic imaging as you may be looking at reflections from one layer rather than another.
