## 4 Application of Fiber Optic Cables in Geophysics Exploration
### Subsurface Imaging of Combined SMF and MMF
Further data preprocessing steps such as shift and stack correlation were performed to generate estimated SNR and processed along with data recorded by a seismic station network. Measured DAS and geophone vertical components were compared in terms of SNR at certain depth intervals, indicating that significant result in terms of quantitative SNR values. The measured signals of surface optical fiber and a 3C geophone were compared to detect the signal generated by the seismic vibrator with an appropriate SNR value. Combining the DAS data with data from a seismic network deployed on the surface increased the resolution of the seismic survey imaging. These research findings also highlight the use of DAS as an option to overcome the operating temperature limitation of conventional geophones, as well as the specifications within a magmatic geothermal environment required for fiber optic cable material to be able to tolerate the geothermal reservoir temperature. When used to image a subsurface geothermal reservoir, the result proved that DAS is well-suited to be used for seismic applications in high-temperature wells. The research reported in [87] discusses the sensing sensitivity improvement of a surface-deployed DAS fiber configuration to detect steep-angle P-wave reflections. The aim of this DAS measurement was to enhance the sensitivity of the existing surface DAS tests conducted at the site. The sensitivity was improved by augmenting the total vertical alignment of the fiber inside experimental setups consisting of both helical and straight fiber segments. The test was conducted using fiber optic cables that consisted of two SMFs and two MMFs placed inside a stainless-steel tube and wrapped with 0.61 mm stainless-steel armor wires and a polyethylene jacket. For the horizontal configuration, the cable was placed along a shallow trench. The specified DAS VSP settings of the acquisition parameters were a gauge length of 10 m, spatial sampling of 1.02 m, and a fiber cable length up to 4288 m. The DAS recording was measured as strain rate. During the 3D seismic survey conducted at the designated site for the DAS test configurations, data were collected simultaneously. A total of 401 dynamite shots used as active sources were captured by the DAS test cable. The raw data for single dynamite shots and NMO-corrected receiver gathers were compared to assess their similarity to geophone data. The variability in the responses of the various configurations along the test cable was observed to proceed with significant data processing, NMO correction, and trace stacking. The DAS configurations of these measurements, the buried geophone, and a receiver from the permanent DAS fiber were compared, indicating surface coupling within amplitude variation offset (AVO) plots referring to the data. Subsequently, numerical modeling was applied to assess the DAS response of the asymmetric helical fiber. This modeling result indicated that the recorded field response was mostly influenced by the fiber shape rather than the borehole coupling. The results of the sensitivity analysis showed that the asymmetric helix and vertical straight fiber configurations had the maximum sensitivity increases with steep-angle P-wave reflections, which were attained by decreasing the surface-wave response and increasing the signal response. However, the research result indicated that it was still associated with uncertainties in locating channels along the fiber and gauge length within the shot configuration. For future DAS testing, increasing the configuration cable length to the gauge length is necessary to associate the seismic measurement observed on multiple channels and reduce the uncertainties of channel authorization using tap tests to obtain the best cable coupling of the DAS fiber. The research reported in [88] involved a time lapse of DAS VSP imaging in the Aquistore CO\({}_{2}\) reservoir. The Aquistore project utilized 4D VSP to perform volumetric reservoir assessment and locate the injected CO\({}_{2}\) plume over time.