Field trial of DAS VSP with a single-use bare fibre optic probe during the ICDP-DIVE Phase 1 project

The ICDP Drilling the Ivrea-Verbano zonE (DIVE) project is focused on studying the lower continental crust (LCC) at key sites in the Ivrea-Verbano Zone of the Italian Alps. The stratigraphically placed research borehole 5071_1_A is drilled in the settlement of Megolo within the municipality of Pieve Vergonte, Val d’Ossola. It is deviated at an angle of c.a. 18 degrees from vertical and penetrates to a depth of 910m. The borehole was diamond drilled with 100% core recovery allowing for a very comprehensive rock characterisation program of the LCC lithologies penetrated.

As part of the wide-ranging geophysical research program, a field trial of a novel single-use borehole fibre optic sensing array, Fibre Line Intervention (FLI) tool, was conducted. The FLI tool is designed to rapidly deploy a continuous fibre optic sensor for distributed acoustic sensing (DAS) and temperature (DTS) measurements into open or cased boreholes. The FLI deploys a single-mode bare optical fibre via a single-use 50 cm long aluminium spool housing (shuttle). The bare fibre unspools from the shuttle as it falls into the borehole. The FLI shuttle tested fits in a borehole with a minimum diameter of 10 cm and a maximum length of 1000 m. After data acquisition, the bare fibre is cut and left to sink to the bottom of the borehole, where it remains along with the shuttle. Acoustic and thermo coupling of the fibre with the formations is achieved via the borehole fluid (water, mud, etc.). Several tests were conducted within the experiment’s program, including passive seismic recording and a multiple offset vertical seismic profile (VSP) survey. A 26,000 lb seismic source – EnviroVibe seismic truck – was used to acquire zero-offset and offset VSP data. The DAS data were collected using an iDAS-MG (Silixa) interrogator. 

The trial successfully demonstrates the applicability of the Fibre Line Intervention (FLI) tool for carrying out borehole seismic surveys in scientific drilling projects. The experiment was the first test deployment of such a shuttle to collect downhole seismic data in ICDP projects. The trial has shown that such an approach for deploying distributed acoustic sensing is fast and can be handled by a minimum number of people. The probe is light (less than 5 kg and 1.5 m long).

Passive data revealed the nature of the seismic environment in the area within a wide frequency range and, if deployed for an extended period of time, would allow for the recording of teleseismic events such as earthquakes. Additionally, active water flow zones are identified, whilst the VSP data highlighted the locations of fracture zones intersected by the borehole. The results will be demonstrated in the presentation.