ABSTRACT
A mechanical vibration is a periodic (or quasi-periodic) displacement of the medium in which it takes place. A specific motivation for using optical distributed vibration sensor (DVS) techniques in borehole seismic applications is the elimination of downhole electronics. A comparison of the Helically wound cable (HWC) and straight fibre results demonstrated, once again, that the broadside insensitivity problem must be addressed for the technology to be applicable because in surface seismic acquisition, the signal broadly arrives at the surface at an angle close to normal incidence. The ratio of pulse duration to gauge length is typical of that used in actual field use; the pulse width imposes a further wavenumber limitation on the system response, but its effect is minor for conditions illustrated. In conventional Vertical seismic profiling (VSP), tools are lowered on a wireline cable into the well whilst the survey is shot; modern tools consist of a number of levels that allow parallel acquisition at multiple depths.
