## 4 Application of Fiber Optic Cables in Geophysics Exploration
### Vsp of Mmf  
The subjectivity of VSP usually depends on the interpretation and calibration of the downhole seismic arrays that can be addressed by applying fiber optic installations. Field research in a hybrid wireline using MMF cable was conducted to assess the results of a borehole seismic survey [75]. Two MMF cables with 62.5 \(\upmu\)m core diameters were deployed within a 5 km hybrid optical-electric wireline cable and connected via splicing at the cable head. This fusion created a length of 10 km up-going and down-going DAS. A real-time \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Authors** & **Scope of Work** & **SMF Deployment** & **Outcome** \\ \hline [PERSON] et al. [66] & Microseismic event detection & Installed in a horizontal well being casing in a deviated drilled well & Automatic detection of microseismic events with high accuracy in an unconventional reservoir \\ \hline [PERSON] [72] & Machine learning algorithm & Laid out on a surface in a suburban area & Machine learning and deep learning applied to DAS data could detect various intrusion events along the pipeline, including manual digging and tapping \\ \hline [PERSON] et al. [74] & Machine learning-assisted & Deployed on the surface in an ice stream & The supervised ML method was able to denoise DAS data automatically with a higher SNR and faster processing speed \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of microseismic and machine learning applications of SMF cable used in relevant studies. active system source controller with a 6 kg dynamite shot triggered the DAS interrogator. The recording configuration was established to capture measurements at intervals of 0.25 m, with a sampling rate of 1 ms and encompassing 8 km along the MMF cable. The MMF cable was set to acquire the strain rate (rate change) within the fiber as the physical property. Raw DAS VSP data in the preprocessing step showed that there was a high coherence in MMF cable slapping and ringing noise; this noise was further processed in the following VSP sequence: seismic trace editing, denoising filter, spherical divergence correction, up-down separation, and deconvolution. DAS walkaway VSP imaging data were processed to build a velocity model. This resulted in a wider lateral image from a single well with a larger vertical aperture than VSP with a geophone. The processing of DAS data yielded improved outcomes, addressing certain practical challenges associated with the hybrid wireline in borehole deployment. The result indicated that DAS with MMF cable walkaway VSP provided an excellent result with vertical and lateral imaging resolution and managed to detail the structure in the objective area. The application of MMF in the exploration field can help minimize the operation costs of VSP in vertical wells, owing to its utilization of a DAS system and its expanded applicability and reliability. The authors of [56] used fiber optic installations with MMF cable for sensing according to the principle of Raman DTS applied to real-time DAS VSP data acquisition and processing. MMF cables were synchronized with seismic sources by directly encoding auxiliary seismic signals onto the optical data stream. This was applied through the incorporation of piezoelectric fiber optic stretchers aligned with the sensing fiber cables. Inside the schematic interrogator unit, MMF cables were connected to SMF cables using an SM-MM converter. MMF cables were deployed through the circulator and sensing fiber along a far offset of the measurement. MMF was deployed as a scrambler to transfer energy from the fundamental mode to all the (low-loss) modes. This is essential to increase the amount of light power emitted. The DAS VSP shot record for MMF was recorded at a 16 kHz pulse rate and a gauge length of 20 m. The MMF result, in terms of system performance, indicated that MMF yielded a 2 dB reduction in SNR for DAS VSP, while the optical settings of DAS interrogators were placed at 5 dB.