## 4 Application of Fiber Optic Cables in Geophysics Exploration
### Subsurface Imaging of Combined SMF and MMF
The result of DAS VSP migrated image volume in this field acquisition at the Aquistore site for continuous monitoring can be expanded to 3D imaging in future work. In the study reported in [85], DAS measurements of seismic properties were conducted on the Store Glacier ice sheets, recording englacial and subglacial seismic properties. DAS measurements were recorded along seismic shots as an active survey to obtain P-waves and S-waves at a depth of 1043 m in the formation, and VSP and data were sampled at 10 m with vertical resolution. The fiber optic cables were installed enclosed in a gel-filled, stainless-steel capillary tube in a borehole on Store Glacier. This cable consisted of two SMFs for DAS measurements and four MMFs for DTS measurements. The interrogator unit was set to a 4000 Hz sampling frequency, with a sample length every 1 m along the cable and a 10 m gauge length for approximately half the expected seismic wavelength. A seismic source was generated using a sledgehammer along the shot 1 m away from the well surface and 500 m offset from the wellbore. The fiber optic cables were recorded continuously without any source. The DAS VSP measurements were able to image 0 m offset and 300 m offset VSP and detect direct SV-waves and P-waves. The utilization of the DAS recording technique facilitated the quantification of changes in the properties of P- and SV-waves. This enabled the identification of transitions associated with ice-crystal fabric and environmental temperature, which influenced the seismic signal. Additionally, the presence of a subglacial layer composed of consolidated silt was successfully identified. The measurement result indicated that DAS offers evident advantages in the characterization of seismic properties of ice masses with a high image resolution. It is also worth noting that DAS cables possess the capability to be monitored for DTS, enabling the combined analysis of seismic and thermal data. Nevertheless, a limitation of DAS is the disadvantage of cables in terms of applicability as a surfaced receiver in P-wave seismic surveys; however, the ability of DAS to image distinct seismic responses at roughly 10 m intervals throughout the ice column can compensate for these drawbacks. Subsequently, the deployment of DAS interrogators incurs significant expenses, but these costs can be mitigated by the resulting improvement in survey effectiveness. Once permanently deployed, the DAS cable consists of SMF and MMF, enabling continuous field monitoring to focus solely on source deployment. In comparison, traditional seismic surveys necessitate ongoing efforts for the relocation and operation of both source and receiver systems. In future work, MMF cables for DTS measurement should be observed in the same vertical borehole to verify the depth in DAS VSP. The research reported in [86] applied DAS technology for seismic exploration to image subsurface geothermal reservoirs in magmatic environments. DAS technology was used to mitigate the deficiency of downhole seismic survey results for subsurface geothermal exploration. The fiber optic cables were equipped with an SMF for DAS and MMF for DTS measurement, operating at temperatures above 400 \({}^{\circ}\)C; hence, the cable was coated with copper. The conceptual acquisition design of the DAS cable installation was based on a surface layout (buried in a trench) of optical cable deployed in the x and y directions and along the wellbore installation in the z direction to achieve a three-dimensional cable array. The cable installation result indicated that the main effect on the SNR of the measurement was caused by the coupling between the wellbore and fiber optics. DAS data were recorded along fiber optic cables with a 5.9 km cable length and settings of spatial resolution of 1 m and measured as strain rate. A seismic vibrator truck was used for DAS measurement as its active source. Subsequently, conventional VSP used a swept-impact seismic technique as an active source.