We present remote measurements from a large-scale interferometric optical sensor system, using a 500km optical transmission link between interrogator and sensor array, 3 times longer than the longest reported so far to our knowledge. A phase noise floor of -80dB re 1 radÃÂ·Hz-0.5 peak was achieved (equivalent to 1 mPaÃÂ·Hz-0.5). 256 sensors may be interrogated via the link using a single fibre pair, making the system highly suitable for remote interrogation of large scale sensor arrays for applications such as seismic and acoustic sensing. Eight amplified 125km spans using standard Corning SMF-28 single mode fibre form the transmission link.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.