general comments, thoughts, etc . . .
recip compressors generate a pulse wave that is transmitted through the piping system (can be upstream and downstream of compressor). the pulse wave will continue to travel through the piping system until the wave is dampened or attentuated. generally, the pulsations are dampened/attentuated at or near the compressor.
in your case, you state the higher levels of vibration are at the pipeline. think of this, a pulsation wave is generated at the compressor (presumably the discharge) and travels through equipment and pipe of varying diamters(?) and gets to the pipeline. as the pulse wave strikes piping bends (elbows, tees, etc), this creates movement of the pipe. also, large diameter pipelines are generally not anchored as frequently as smaller diameter pipelines.
as TD2K recommends, an accoustical analysis/pulsation study may be an option.
hope this helps!
good luck!
-pmover