The new heterodyne distributed vibration sensing (hDVS) distributed acoustic sensing (DAS) system is a technological breakthrough for borehole seismic applications, reducing acquisition time from the hours required for conventional seismic operations and vertical seismic profiles (VSPs) to just minutes. The system’s optical interrogator unit at surface is connected to any optical-fiber cable deployed in a well, from hybrid wireline logging cable to production tubing with fiber installed or optical fiber permanently cemented behind casing. Single-mode fibers deliver the best-quality signal, but multimode fibers can also be used.
Because the hDVS DAS system makes every logging run or fiber installation an opportunity for seismic data acquisition, it significantly improves the efficiency of borehole seismic operations while lowering the cost. No longer is time required for rigging up and down and deploying conventional borehole seismic tools because the system is simply connected to the fiber at surface and records seismic data in conjunction with other stationary logging services or on its own. Despite the speed of acquisition, the hDVS system maintains data quality by employing a proprietary multifrequency method that enhances the signal-to-noise ratio to maintain data quality.
Overall operational risk is also decreased because there’s only minimal equipment at surface and no requirement to run tools in the well, which removes the likelihood of tool sticking or fishing.
A full well profile along the length of the cable is recorded by the hDVS DVS system for each shot. For checkshots and velocity model calibration, the massive density of traces supports very fine resolution of the time-to-depth relationship.