Distributed fibre-optic sensing is being actively used in various exploration and monitoring applications. Distributed temperature sensing (DTS) is used for measurements and monitoring of the temperature and Distributed acoustic sensing (DAS) for recording of the seismic wavefield. However, DAS measurements are also sensitive to temperature changes. Understanding how exactly DAS measurements are affected by temperature variations is important in order to remove this effect from DAS seismic data. This can be particularly critical for the DAS time-lapse seismic and DAS passive monitoring focused on low-frequency signals. On the other hand, such understanding can help to adapt DAS as a temperature sensor, which can be used to track temperature changes in the absence of DTS.In order to analyze low-frequency DAS data recorded during borehole water injections in Otway site and to estimate temperature change response on DAS measurements, we conducted a series of separate experiments at the Curtin/NGL research facility and Curtin Rock-Physics Laboratory. We examined three various DAS cables. Two fibres were tested in the laboratory and one cable, which is installed behind the casing in the NGL well, was examined in the well. After that, we analysed passive DAS and DTS field data recorded in four different wells during water injections within the Otway project. The results demonstrate that DAS is sensitive to long-period temperature changes and its response is proportional to the time derivative of temperature. Our study shows that there is a linear dependency between strain and slow temperature change and this dependency can be estimated for a particular cable. Proportionality constants in these relationships show some dependency on the cable type/design and acquisition setup, but are all of the same order of magnitude. DAS measurements can also be affected with low-frequency noise possibly associated with the effect of temperature on the DAS acquisition unit itself.

Ocean‐generated seismic waves are omnipresent in passive seismic records around the world and present both a challenge for earthquake observations and an input signal for interferometric methods for characterization of the Earth's interior. Understanding of these waves requires the knowledge of the depth dependence of the oceanic noise at the transition into the continent. To this end, we examine 80 days of continuous acquisition with distributed acoustic sensor (DAS) system deployed in two deep boreholes near the south‐eastern coast of Australia. The iDASv3™ system deployed in a deep borehole at the CO2CRC Otway Project site provides sufficiently high sensitivity and low instrumentation noise for frequencies from 100 mHz to 20 Hz. Analysis of the seismograms and correlation with wave climate allows decomposing the DAS response into microseisms generated by swell from remote storms (∼ $\sim $0.15 Hz) and local winds (between 0.2 and 2 Hz), and strong body wave energy from large surf breaks at the coast (from 2 to 20 Hz). The depth dependence of the microseisms provides useful insights into the energy partition between the Rayleigh wave modes and may augment conventional kinematic analysis of the sparse surface seismological arrays. Overall, ocean‐generated signals at each channel along the borehole are strongly related to the wave climate, so that—with sufficient amount of training data—the passive seismic records on several downhole DAS sensors has a potential for high‐precision monitoring of formations surrounding the borehole as well as remote storms in the ocean.

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