## 4 Application of Fiber Optic Cables in Geophysics Exploration
### Subsurface Imaging of Combined SMF and MMF
Further processing and resulting imaging discrepancies in the VSP data for extremely closely spaced images were analyzed to estimate the time-lapse noise from field recordings. During the data imaging process, the reduced SNR of the DAS data was mitigated by a noise attenuation algorithm as measured by NRMS values. Nevertheless, it is clear that the SNR of the DAS data was lowered by the NRMS attenuation of noise. The DAS measurements used fiber optic cables consisting of two MMFs for DTS and two SMFs for DAS measurement. The cable was deployed inside stainless-steel production tubing clamped to the outside of the well casing and cemented in place, as illustrated on Figure 8. The DAS data were recorded using a 10 m gauge length and 2.036 m sensor spacing and measured as strain rate. The obtained DAS VSP data were further processed and converted to high time-lapse NRMS values and an image of the horizon of the reservoirs. The processing and imaging results indicated that the reservoir baseline exhibited fluctuating SNR characteristics. However, it still achieved an acceptable level of repeatability to identify time-lapse noise. The research outcome highlights that DAS VSP is considered a useful technique for time-lapse monitoring of CO\({}_{2}\) reservoirs. For future enhancements in DAS VSP results, more attempts should be made with respect to the processing step and the development of its framework. All relevant literature discussed in this section is summarized in Table 6 below. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Authors** & **Scope of Work** & **Cable Deployment** & **Outcome** \\ \hline [PERSON] et al. [28] & Simultaneous DAS VSP & Cemented outside of the well casing & No significant difference in the comparison of SMF and MMF recording results, and SNRs were improved by weighted stacking pf DAS VSP data \\ \hline [PERSON] et al. [85] & DAS VSP on Store Glacier & Installed in a borehole enclosed in a gel-filled, stainless-steel capillary tube & DAS VSP was able to detect the sediment layer and interpret temperate ice and seismic properties in transitions of ice-crystal fabric and temperature regime \\ \hline [PERSON] et al. [86] & Seismic exploration in geothermal areas & Surface layout (x and y directions) and deployed behind casing inside the well (z direction) & DAS with data from the seismic network deployed at the surface increased the resolution of the seismic data and proved to be suitable for geothermal environments \\ \hline [PERSON] et al. [87] & DAS VSP configuration test & Cemented behind well casing and buried in a shallow trench (surface) & Surface DAS fiber configurations increased the sensitivity to steep-angle P-waves of seismic reflections \\ \hline [PERSON] et al. [88] & Aquistore reservoir imaging & Inside stainless-steel tubing clamped to the outside of the well casing and cemented in place & DAS VSP produced time-lapse monitoring and accurately imaged NRMS values and CO\({}_{2}\)-based anomalies at specific reservoir depths \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Summary of subsurface imaging applications of combined SMF and MMF cable used in relevant studies. Figure 8: Schematic illustration of SMF and MMF along a wellbore.