## 4 Application of Fiber Optic Cables in Geophysics Exploration
### Vsp of Smf
The DAS data were recorded under a strain rate with a 5 m depth interval and correlated with vibroseis source sweep data, enabling first-breaks picking. Further DAS VSP data processing included vertical stacking until 3D migration and plotting with 3D seismic cross sections. This helped in clearly imaging the key stratigraphy horizon and CO2 injection target within the SMF cable. This resulted in a good correspondence in the seismic reflectors in the datasets while comparing a 3D DAS VSP image with surface 3D seismic imaging. The 3D DAS VSP shows higher image resolution and provides enhanced precision in determining the depth of reflectors within the well area. Therefore, the implementation of multiwell DAS VSP emerges as a viable approach for continuous monitoring systems. The results of multiwell DAS VSP could have been improved by taking more action to tackle missing seismic volumes from different typical wells. In 2016, the authors of [54] conducted research to validate DAS VSP data integrity with a conventional geophone at the Aquistore site for subsurface CO2 storage. This data acquisition used an SMF cable with a total fiber length of 5.4 km, and the configuration involved the embedding of the initial portion of the cable near the surface, while the subsequent section was permanently cemented behind casing and vertically positioned along the cable deployment. The acquisition also involved active sources, such as dynamite and vibroseis, to cover entire boreholes. Each near offset and far offset shot were applied to dynamite shot gather to indicate direct arrival, reflection events, and ground roll in shallow sections. These three DAS events were compared with geophones in terms of noise characterization and identified the noise as optical system noise, optical fading, common mode noise, or checkerboard noise. Initially, the raw DAS data on strain rate were also converted to geophone-equivalent units to achieve a good comparison of the two raw data acquisitions for further assessment. Both up- and down-going events managed to match the polarity between geophone and DAS data. The up- and down-going wave separation might have been enhanced by a suitable denoising filter between the two input trace gathers. The result indicated that converting DAS VSP data to equivalent geophone data can help with the up- and down-going separation step without using deep filters. With the mitigation of depth calibration in seismic DAS, the data processing workflow can be improved. The authors of [55] carried out a DAS VSP field survey to evaluate the effectiveness of fiber optic DAS by comparing four cable deployment approaches, namely with cables behind the well casing, behind an inflatable liner, clamped to production tubing, and deployed with wireline logging. The SMF cable was lowered along the well with a different approach and connected to a DAS interrogator. To generate seismic waves, active sources were employed using P-wave and an Envivoibe S-Wave Vibrator. During the final imaging of VSP for Common Depth Point (CDP) transform, DAS data attenuation accrued for all cable deployment methods. Noise attenuation reduced the SNR, influencing the quality of the seismic image. The results indicated that the deployment method with the fiber cable cemented behind casing was considered the most effective method for cable coupling with the subsurface layer, with a high-quality generated image in the pre-defined velocity model. Such an improvement can be generated by a depth-calibrated SMF cable against a reference to account for source statics resulting from variations in elevation. This correction was essential in achieving optimal resolution in the subsurface imaging. Further DAS measurement research was carried out by the authors of [56] to conduct real-time DAS VSP acquisition and processing using an SMF cable with seismic-source synchronization and real-time DAS data processing to provide contemporary work. The SMF cables were deployed outside the casing in vertical and lateral sections along the wellbore.