We can now see that all three tubes in the sand section contain water.



We can measure the level of the water in each tube to the level of the sand next to that tube.

From this angle we cannot know the undulations of the sand.

We cannot know if the tubes are connected or if the water levels are equal as we have no common level to measure against.

This is exactly the same as measuring the voltages that are known a pipe-to-soil potentials in cathodic protection monitoring.