I would think Pile Integrity Testing (PIT) should work. This is the type of testing referred to by DRC1. It measures the time the impulse wave takes to travel to the bottom of the pile and then reflect back to the top. Using the wave speed in steel, you can calculate the length. However, it is usually used for discrete piles. If the sheet pile interlocks have "frozen" then the pile may appear to be one long piece of metal and not indvidual sheets. This may make some of the assumptions in the testing method invalid. I would give GRL a call to find out though.