That could open up the possibility of the testee later claiming (on a false-positive): "They didn't dig down deep enough!"
I think that objection could equally apply to any dowsing test but it is controlled by having the dowser pretest the site.
By testing the the holes prior to filling the dowser determines if there is any pre-existing problem with the site.
I haven't described all my ideas of how to run a fair and honest test. I was hoping that John would reply and help in designing the test.
John, hello, John, come in John, I have bottles.