Does anyone know if there is a way to determine the founding depth (bottom) of an existing 24" diameter plain concrete pier that supports an existing reinforced concrete slab on grade that spans between the piers? We could drill down thru it I suppose, but that would be messy. The length of the pier is known to be between 4 and 8 feet. GPR will not determine the length and we are told that ultrasound and impact-echo are also not likely to work. We want to know the bottom elevation of the pier so that we do not undermine it when digging a 5 foot square hole, within about 2 feet of the pier face, to an underground drain line that requires repair of a sagging portion. The drain line is about 7 feet below the floor, and was originally installed about 50 years ago into a relatively narrow trench.