## 4 Application of Fiber Optic Cables in Geophysics Exploration  
### Vsp of Smf  
Single-mode researchers such as the authors of [23] have introduced DAS for borehole seismic applications. DAS has the potential to expand beyond its initial use and encompass other surveillance domains, including production and completion reservoir monitoring. This extension is made feasible by leveraging existing fiber optic networks for enhanced data acquisition and monitoring capabilities. However, applying a suitable DAS data acquisition and processing workflow is also crucial in obtaining the best images; hence, many papers have utilized diverse DAS outputs that are applicable to exploration activity. VSP data are used to complement seismic data through the process of separating the signals from multiple sources that overlap in time and were acquired by deploying seismic sensors in a wellbore and recording the response of the subsurface to a controlled seismic source [50]. Figure 5 shows a schematic deployment for land VSP acquisition. DAS offers the distinct advantage of remarkable spatial sampling, spanning the complete length of a well. Furthermore, as the underlying technology, SMF optic cable presents enhanced robustness compared to conventional geophones. This robustness mitigates the potential hazards of deploying mechanical and electrical components within a well [51]. This makes SMF an excellent tool for replacing geophones, providing subsurface properties, in addition to having been used widely in various subsurface applications. In 2015, the authors of [52] located an indication of a reservoir of the Aquistore CO\({}_{2}\) geological storage project through elementary imaging using DAS in Paleozoic carbonates and Mesozoic sandstones and shales. SMF optical fiber cables were permanently installed inside steel tubing, strapped behind casing, and cemented in placed in observation wells during completion, even though it was only the fiber optic cable deployed in the observation well that eventually operated well. SMF within the observation well was employed to conduct comparisons with geophone VSP measurements. A processing framework was executed for each shot obtained through both DAS and conventional geophones. The general processing framework encompassed trace balancing; correction for spherical divergence, notches, and low-pass filtering; and a frequency-wavenumber filtering algorithm of the down-going wavefield. To compare it with geophones, DAS trace gathering was superimposed with the geophone traces within a specific depth range. SMF DAS and  
Figure 5: Schematic layout of a typical land DAS VSP acquisition system modified in [28]. vertical-component geophone traces were also imaged for analysis. The results indicated that geophone data distinctly showcase a higher SNR than DAS data. However, it is worth noting that the DAS for SMF SNR has significant room for enhancement through the application of noise attenuation techniques and the averaging of data over cable lengths, as it is used in geophone results. In contrast, numerous equivalent reflections are obscured by noise within the DAS data for the same depth range. However, DAS still has the advantage of potentially replacing many geophones within one SMF. This may potentially generate subsurface images by notably enhancing spatial resolution and coverage when compared to traditional geophones. Moreover, this improved performance can be achieved at a relatively lower cost. DAS VSP has also proven to be useful in acquiring VSP data in carbon capture and geological storage (CCS) projects [53]. By applying multiwell 3D DAS VSP data acquired at the CO2 CRC Otway, DAS data were processed and analyzed for their applicability for continuous lime-lapse monitoring of CO2 injection. The acquisition process was performed by deploying four SMF cables cemented along the well, where two SMF cables were connected with constellation fiber (CF) and each SMF cable was connected to an iDAS v3 interrogator unit.