As discussed in the previous section there are two types of approaches to find out what is below the surface. These are
Invasive methods are methods in which we go to the location we want to have information on, and directly obtain this information. If the area we want to know about is shallow enough we can use a shovel, but in most cases one would drill a hole and take a measurement or use a direct push method to push a sensor into the ground and take a measurement.
Invasive methods have many advantages. They are conceptually straightforward and we can often measure the property we want with very high precision. However, invasive methods have also disadvantages. They are expensive (with cost increasing with depth of the location of interest) and they can impact the subsurface in unintended ways (drilling a hole can damage utilities or create pathways for contaminant migration). However, the biggest disadvantage is that in many cases the questions what is down there and what is happening down there is not asked about a single location, but about multiple locations - in fact, we would like to have knowledge about what is down there for all locations.
This motivates the use for subsurface investigations of non-invasive approaches, which can provide information on the subsurface without the need to dig or drill a hole (hence the name non-invasive).
Non-invasive approaches to obtain information are well known to most everybody from the medical field (where they provide information on the body). In the medical field these methods are collectively referred to as medical imaging methods. The name for non invasive imaging methods in the geosciences are geophysical imaging methods.