This cannot be done on the assumption that the vibration is sinusoidal because generally it is not. The most useful method of interpretation is to locate the regions of maximum slope on the record and use these slopes to determine the velocities. This amounts to a graphical differentiation of the interesting parts of the record. Another common type of instrument is a portable accelerograph, similar to a seismograph except that the moving elements are resonant at a frequency above the region of interest. Unpredictable break-throughs sometimes occur, however, in cracked or inhomogeneous material, and it is prudent, if there are buildings, people or equipment nearby to use a safety mat.
Most municipal regulations require it in any case.
Air blast results when the explosive is inadequately covered. Generally there is no difficulty with the explosive proper if the drill holes are back-filled. Sometimes, however, the charges in an array are connected by lengths of a detonating material, which itself may produce substantial air blast if the surface sections are left exposed. The problem can be solved by covering the detonating material with a few inches of sand or other convenient fill. Even snow provides-good protection.