I pump septic tanks and know most of the methods of locating tanks. Some times often times I get a call where they don't know where the tank is and hasn't been pumped in several years. Most larger septic pumping co. send out two people. I have always run mine by myself. In dry weather you wont have much luck in probing for a tank. I know that the way I do it any more is just simply call a plumber and they can use a flushable sonde locator or a camera with sonde and go through the vent pipe. At my age I cant justify the cost of 3 to 4 K for a good set up. These guys charge $100. Well worth spent when you don't know where your tank is. Then if it is very deep, you then have to get a backhoe to comer in and dig it up. I use to dig up a few with my backhoe. But no more. The last one I did, I had a good understanding with the owner. I would only dig where he wanted me to dig . He had only a vague idea from when it was installed 20 years ago. He said the top of the tank was over 8' down. When I finally got done, his yard looked like the mine field. Here is my reasoning why I quit digging them up. Law requires that all septic tanks have a riser to top of ground. I can show up with my pumper truck and pump a tank in about 30min and be on my way to the next customer. If I show up with my back hoe it is almost a all day job and longer as I have found elec. wires running right across the tank. Law requires you to wait 72 hours after notifying the "call before you dig". And I have had a couple cave in while hand digging. It doesn't make sense to have a $40,000 boat in your drive way and you don't know where your septic tank is and never had it pumped. Just pumped one the other 2 months ago, hadn't been pumped in 20 years. 18' to top of tank with riser. On a very small lot, no sewer. I told the home owner it had failed due to not pumping and would fill back up. Sad situation. Older woman on a fixed income and no money. It filled back up in a week and she called the health dept. and said I didn't pump it. Water from rain saturated ground and it came back in and flooded basement. I got a call from a contractor fri. to come to a certain address. They had a track hoe in there and had a humongous hole. Need tank pumped out so they could crush it. Health dept. said she had to install a mound system, it is going to take up the whole back yard. In the mean time I guess I will just continue using my "Wiley Coyote" witching coat hangers to try and locate a tank.