oa First Experience of DAS and DTS Surveys in Shallow Boreholes on the Arctic Shelf

Vertical seismic profiling (VSP) using Fiber optic distributed acoustic sensors (DAS) is an excellent replacement for conventional geophones array. Especially in the case of measurements from a board of drilling research vessel, where the risk of losing equipment in an open hole is quite high. One of the most important DAS parameters is the gauge length, the choice of which is always a balance between bandwidth of seismic signal and signal-to-noise ratio. The noise conditions on board of research vessel are extreme for DAS measurement. The main purpose of the DAS site surveys was a well tie the ultra-high resolution (UHR) seismic reflectors in the upper part of the geological section. That aim required wide seismic bandwidth (up to 500 Hz) and consequently low gauge length parameter (1m and 2.25 m) which provides to low signal-to noise ratio. But even in case like that, it is possible to collect the DAS VSP data that can be matched with UHR common middle point stack gather. The second part of the research consisted in assessing the temperature characteristics of the near wellbore space, which was performed using distributed temperature sensing (DTS). The surveys included at least 2 temperature measurements: first one right after drilling, when near-borehole sediment is still heat, and the second one up to 3–5 days after drilling, when the sediment has cooled down. In result the DTS data correlates with DAS-VSP and UHR section, and colling rate of the rock became additional information to the geologists.