In order to evaluate operational and geophysical feasibility of vertical seismic profile (VSP) survey using distributed acoustic sensing (DAS) for future time-lapse monitoring, we conducted a field experiment of VSP with DAS measurement (DAS-VSP) at onshore Japan. The main purposes of this field experiment are 1) to quantitatively evaluate data acquired in a deep and highly deviated well and with fiber optics deployed inside coiled tubing (CT-DAS-VSP),2) to investigate the possibility of subsurface imaging by DAS-VSP, 3) to acquire DAS-VSP data under flowing condition for future evaluation of the possibility of subsurface monitoring during fluid injection. Among several methods to install fiber optic cables, Inside CT method was adopted that we deployed a coiled tubing in which a fiber optic cable had already been inserted. VSP shot gathers with more than 800 channels with 5 m spacing were acquired at 70 source locations as a 2D walk-away VSP survey. Although data quality was of our biggest challenge, it is confirmed that not only P-wave direct arrival, but also P-P reflections and P-S converted wave were recorded with acceptable signal to noise ratio even with poor coupling condition inevitable by the optical-fiber deploying method, and even under flowing condition. The effect of gauge length, which is one of key parameters for DAS measurement, on the quality of DAS-VSP shot gather were examined with the real data during post-field data optimization. It is realized that the important parameter must be determined carefully, and that, if possible, post-field optimization using raw optical backscatter information is recommended.
