Sonic check LA area

Yes sonic checked,I want to bore my block .104 over,I have been told to get block sonic checked first,from what I know its kinda like a x ray that checks block for uniform thickness thru out.Ireally cant explain it better because I dont fully understand,But have told by SEVERAL people in the know to have it sonic checked.I try to listen and obey.Hopefully someone will chime in and really explain it.Guzzi Mark