6.1. Geophysical Inversion Basics

What is geophyiscal inversion?

Geophysical inversion is a tool which we can use to recover the subsurface distribution of a physical property from field-collected data. Each type of geophysical data may be inverted using one or more inversion algorithms.

What does geophysical inversion recover?

Geophysical inversion recovers a physical property model which:

  • fits the data (field observations), and

  • is geologically reasonable (contains plausible structures)

Geophysical inversion does not recover the exact distribution of a physical property; i.e. the true model. There may be a number of physical property models which satisfy the aforementioned requirements. Therefore, it is up to the user to set appropriate starting parameters for the inversion.

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Inversion of 2D DC resistivity data.

Example:

The benefits of geophysical inversion are illustrated in the figure on the right.

  • In (a), we see several conductors and resistors buried below the Earth; i.e. the true model.

  • In (b), we see the apparent resistivity data collected by a 2D DC resistivity survey plotted in pseudo-section.

  • In (c), we see the physical property model recovered through inversion. If the geology is complex, the pseudo-section data can be very difficult to interpret. In this case, the inversion results are much easier to interpret.

When is inversion able to recover a subsurface structure?

Inversion is successful in recovering geological structures when:

  • There is a sufficient physical property contrast between the target structure and the surrounding geology

  • The data are sensitive to the target structure; i.e. the geophysical response from that structure is observed in the data

  • Parameters relevant to the inversion are set appropriately