Non invasive imaging provides us with information at a specific time. While this is of value, in many cases one is interested in how things change. This leads to the intuitive simple concept of timelapse imaging. This is also known as time-lapse, 4D or four dimensional imaging (where the four dimensions are the three spatial dimensions and time).
In timelapse imaging one collects images at different times. This is ideally done using the same instrument. acquisition and processing parameters to minimize the potential from artifacts resulting from different hardware (something which is actually a large challenge for the exploration seismic industry).
By investigating the changes between the different images (typically in conjunction with auxiliary data and information) one can now get insights on subsurface processes of interest.
An example of such changes is shown on the right. These images are from a Department of Defense (DoD) ESTCP funded demonstration/validation effort of electrical geophysical monitoring at the Brandywine site in Brandywine, MD. These images show the changes in bulk electrical properties resulting from an amendment injected at the Brandywine site.
The precise interpretation of these changes can be very challenging. However, these changes are related to subsurface processes which change bulk electrical conductivity. Thus, as endusers will often have a good understanding of the processes it turns out that even looking at a sequence of these images (either in stills or as an animation) can often be extremely enlightening.