## 4 Application of Fiber Optic Cables in Geophysics Exploration
### Subsurface Imaging of Combined SMF and MMF  
The development of subsurface imaging by utilizing DAS technology rely on many elements, like fiber optic cables, which enable compatibility with an interrogator and data acquisition. The selection of optical SMF and MMF for DAS measurement is contingent upon the specific application and desired aim. Combined SMF and MMF have not been widely employed for the purpose of DAS measurement as a prevalent practice. DAS commonly depends on SMF cables, owing to its capacity to preserve signal integrity at extended distances and deliver measurements with high resolution [79]. However, it is conceivable that advancements have taken place in the scope of fiber optics and distributed sensing, considering the potential evolution of technology and practices in subsurface imaging. The combination of the two types of fiber optics is usually contained inside a single cable equipped with optical SMF and MMF, where MMF is commonly used for DTS measurements and SMF for DAS measurements. The authors of [28] carried out simultaneous acquisition of DAS VSP with SFM and MMF cables at the Aquistore CO\({}_{2}\) storage facility. This field acquisition also utilized a (3C) wireline geophone array for quantitative comparison with DAS measurement results. The fiber optic cables comprised one MMF cable loop utilized for DTS measurements, alongside one SMF cable employed for telemetry to downhole gauges and to conduct DAS measurement. During the first well completion process, the cables were installed outside of the well casing using cement. The simultaneous DAS VSP acquisitions of SMF were used as data for the full 3D surface explosive survey as an active source, and MMF was used for a significant fraction of the survey. All active sources shot data were measured with a geophone array, and a 3C VSP geophone and DAS fiber optics were used for recording. The obtained raw measurement data were recorded as strain rate, with further data processing converting them to particle velocity for equivalence with geophones for comparison. The SMF and MMF measurement results were compared within the same source shot map. The comparison result indicated that that there was no quantitatively significant distinction in the quality of the two recordings. However, it was observed that the SMF data exhibited \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Authors** & **Scope of Work** & **MMF Deployment** & **Outcome** \\ \hline [PERSON] et al. [66] & Microsseismic events detection & Installed in a horizontal well behind a casing in a deviated drilled well & Automatically detected microseismic events with high accuracy in an unconventional reservoir \\ \hline [PERSON] et al. [84] & Machine learning algorithm & Deployed on multiwell DAS datasets acquired during hydraulic fracturing well completions & Machine learning algorithm and CNN automatically detected microseismic events (hypocenter location) recorded in low-SNR DAS data \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of microseismic and machine learning applications of MMF cables in relevant studies. a slightly higher level of noise. This discrepancy is likely attributed to a little variation in the used interrogator units rather than being influenced by the type of fiber deployed. Other findings include that MMF was quite efficient because of the numerous wellbores in which MMF was deployed for DTS measurement. The comparison of DAS VSP data with geophone response was consistent, showing similar trace gather within each shot point. Further DAS VSP data processing in weighted stacking revealed in improvement in SNR and 3D migrated DAS VSP imaging. Data acquired from SMF and MMF with identical coupling in the fiber optic demonstrated comparable SNRs and sensitivity. The comparable SNR and sensitivity of MMF installed in a borehole for DTS can be considered for VSP acquisition as well.