How does seismic inversion work?

Seismic inversion is the process of estimating the electrical resistivity of the subsurface at each receiver location from incoming seismic survey data. To do this, you need data gathered by an array of geophones or hydrophones, which are typically deployed on the surface of the Earth or under the ocean’s surface. And once you have that data, it can be inverted to produce models of what lies beneath the surface. But how does seismic inversion work?

An introduction to seismic inversion

Seismic inversion is a complex field of geophysics that most people don’t really understand. You may know that seismic data provides an image of subsurface layers and structures, but you might not know how it works. This post is going to give you an introduction to seismic inversion and explain how it gives us a view of subsurface earth. In general, seismic data comes from high-quality reflection seismology, which refers to measuring reflected waves from rocks or other subsurface materials. 

What is inverting data all about?

Seismic inversion is a way of trying to figure out what’s down there under your feet. By processing seismic data, geoscientists are able to generate subsurface images that help give us an idea of what our planet looks like beneath our feet. Seismic inversion works by finding patterns in waves that have been reflected from geological boundaries underground, but one question people often ask is how does it work? To understand how we obtain these images we must look at some theoretical basics and explore all we know about geological boundaries and how they reflect waves. The rest is just math!

Types of seismic data

Seismic data is collected by means of an explosion, which shakes down geophones (receivers) that are spread out over a wide area. The geophones detect vibrations caused by the shock wave that travels through Earth’s crust, and measure both their travel time and their amplitude. Seismic data can be grouped into two categories: refraction and reflection data. Refraction data measures reflected waves between layers beneath Earth’s surface, whereas reflection data directly measures primary waves at interfaces with different material properties.

Common problems that can be solved using seismic inversion

When a reflection seismogram is recorded at a number of stations along a profile, with a known noise level and signal-to-noise ratio at each station, then inverting for source properties is a standard procedure. The resulting inverse problem can be solved using deconvolution or least squares. When there are many sources that emit into different parts of an Earth structure (e.g., piercement events), then it may be possible to solve for multiple source properties simultaneously by creating subfolders within subfolders for each event, and analyzing them independently. This method is called block diagonalization, and provides solutions where neither deconvolution nor least squares could do so separately.

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