Time domain reflectometry

I am working on a project involving TDR. I have to simply observe that the pulse is being reflected and from the time difference have to calculate the length of the coaxial cable. (the cable is open at the other end).
While doing the experiment I am also getting the reflected pulse on the oscilloscope but the time difference is coming out to be very short. (since i know the length, the time should be around 70 ns but i am getting only 15 ns).

Can anyone help me out please? Does anyone have any links for study materials?