## 4 Application of Fiber Optic Cables in Geophysics Exploration
### Vsp of Mmf
This implies that MMF measurement managed to have sufficient capacity for the DAS interrogator and acquired DAS VSP data in the MMF cable with 8 dB loss. However, a limitation of the researchers findings is that the MMF installation has shallow image due to the increase in attenuation with significant hydrogen darkening. For future work, there should be a sufficient SM-MM converter for DAS sensing of MMF deployed in legacy DTS installations to improve SNR and signal loss. Improving reservoir monitoring relies on many factors like real-time data analysis and detailed subsurface imaging, leading to a better utilization of fiber optic technology. Accurate imaging of these fiber optic measurement data is essential in lateral andvertical spatial distributions in reservoir monitoring. In their advanced research on 4D reservoir monitoring, the authors of [76] applied DAS 4D VSP to MMF cable in a deep-water environment to reveal injected water-sweep dynamics. MMF was deployed in active wells to perform detailed monitoring of the water flood injection in two deep hydrocarbon reservoirs in a field in the Gulf of Mexico. In the technology trials, the research compared DAS VSP and Ocean Bottom Node (OBN) to obtain the 4D imaging performance of the two data sources. The DAS data were continuously measured over the years to understand the influence of small active sources and active well noises in 3D and 4D DAS VSP on MMF imaging resolution. The progression began with a DAS VSP recording in 2015 within an active well while water flooding injection operations were ongoing. Subsequently, DAS fibers captured in a P-cable trial featuring 300 events from three sources in early 2017 served as benchmarks for additional iterations. Furthermore, in late 2017, a standalone DAS VSP survey was conducted, distinct from the simultaneous OBN deployment, encompassing three active injector wells. The measurements encountered additional challenging conditions for DAS, including notable well deviation that resulted in diminished sensitivity to desired reflections, as well as the presence of extended MMF lengths that contributed to heightened levels of optical noise. However, this measurement endeavor yielded crucial insights into acquisition and processing methodologies. It was discovered that DAS 4D images exhibited qualitative similarity to OBN 4D outcomes. Furthermore, the repeatability of DAS images proved to be exceptional, as demonstrated by a well-controlled Normalized Root Mean Square (NRMS). The reservoirs are sandstone reservoirs with turbidite deposits with high-quality sands composed of different lobes with varying thicknesses across the field located on deep-water acreage. The results of DAS MMF and OBN were compared in two reservoirs within the same vertical conformity of the time-lapse signals in both datasets. These results show that frequent 4D DAS VSP can assist in characterizing all phases of a water flooding injection; the data were also used for sweep monitoring of the evolution of the injector. On the other hand, the survey result demonstrated that the associated costs remained reasonable, while the resultant seismic images proved to be well-suited for monitoring the immediate surroundings of wells. These images facilitate the assessment of injector performance, the evolution of sweep processes, the progression of water towards producers, and the containment dynamics within the reservoir. However, the study highlighted several limitations of DAS in MMF VSP regarding flow- and injection-related well noise. Injection noise predominantly presents itself through the propagation of robust tube waves along the wellbore, characterized by comparatively lower velocities in DAS data. A data processing framework should be developed to incorporate insights from 4D DAS VSP data modeling, adding well log data to extract physical properties of reservoirs and consider the understanding of reservoir dynamics behavior.