In a project that I'm working in, there are 5 equal soil humidity sensors displayed in the exact same point but at different depths (20,40,60,80 and 100cm from the surface, respectively). Therefore, each sensor measures the humidity level at these different depths providing info that is later used into a prediction model (including some other variables though). As the differences of conditions between each sensor vary so little, there's little difference and a high linear correlation between each sensor.

How can I select a subset of this sensors (without having a response variable) or demonstrate that among those variables there is redundant information (if it is redundant) and so I can dispense with using some of them, considering that the fewer sensors I use, the cheaper the system gets?