Our firm performs surveying in a local salt mine on a regular basis. Our work typically consists of extending alignment control in the mine as salt is excavated. Our client has contacted our firm to check the alignment of the shaft going down to the mine which has a depth of 650 feet from the surface. I have read in the past about methods used and was wanting any additional information that might be available.
Is the shaft vertical? Thirty years ago surveyinng the Washington DC metro system we would set control at the top (four points) and crisscross a plumb line between them. Then from the bottom of the shaft using a right angle eyepiece on the transit, wiggle in directly below the cross of the strings. It leaves you with a short backsight situation, but we only used the data to check the underground route surveys.
There are a variety of valid ways to do this. It really depends on the equipment you have available. The previous suggestion of crossed plumb lines is a good start but mines tend to have a lot of air moving in and out so that may be a factor. Things might be jumping around a bit.
A salt mine is another factor. Some equipment will only work in a salt mine for a brief period and then fail. My personal experience is based on a major salt mine in Cleveland in the 1990's.
What equipment do you have available? What results are you looking for at 650'?
I am not too sure of the accuracy requirements our client is after as we are in the preliminary stages. I have been using a NIKON total station for our work in the mine. I am checking in with an instrument dealer that specializes in equipment for this type of work and they might make some suggestions also.
I read about a laser scanner that was lowered down the shaft with a gyroscope attached to keep the orientation. This type of equipment may be out of our clients budget and I have not had any experience with scanning and would probably have to sub this work out.
this thread reminds of the "mine shaft gap" from Dr. Strangelove.
A low-tech method once used in the western PA bituminous coal mines was to hang a 10 pound plumb bob on a piano wire, into a bucket of heavy oil.