This data set is a subset of the data used to conduct the research as presented in a manuscript entitled "Simultaneous Tracking of Multiple Whales using two Fibre-Optic Cables in the Arctic" by Rørstadbotnen et al. in 2023. It contains processed strain data in strain unit measured by two OptoDAS interrogators, for two separate days, through a dark fiber in a submarine telecommunication cable, which was trenched into soft sediments at 0--2 m below the seafloor from Longyearbyen to Ny-Ålesund in Svalbard, Norway. The raw data was acquired from 2022-08-22 and 2022-09-06 and converted to strain data. To reduce file sizes, we resampled the raw data with an antialiasing filter from 625 Hz to 78.125 Hz and 125 Hz for the respective dates. 2-hour segments of the 5.3 and 3.3 hr data are archived as the total size of the raw data is more than 2.5 TB. The archived versions have a total size of 268 GB. To get access to the entire dataset, contact the corresponding author. A Tukey taper with a taper rate of 0.02 was applied before resampling to avoid artefacts. The distance range was also resampled to cover the studied data, for 2022-08-22 the range is channel 9803 to channel 24509, while for 2022-09-06 it is 2450 to 9191. The processed data are stored as structured arrays in MATLAB file format (.mat). This data set has no exact positioning data of the fiber optic cable because they are proprietary to SIKT.