Downhole Distributed Acoustic Sensing Provides Insights Into the Structure of Short‐Period Ocean‐Generated Seismic Wavefield

Abstract

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.

The seismic wavefield as seen by distributed acoustic sensing arrays: local, regional and teleseismic sources

“…At a number of sites, cables have been deployed in vertical boreholes, and such recordings can have significant value for analysing many different types of seismic wave propagation phenomena in a lower noise environment (e.g. [ 46 ]). The analysis in terms of the slowness–frequency domain can be extended to downhole studies by modification of modelling codes for VSP simulation with differencing in the vertical direction, and orientation factors relative to the vertical rather than the horizontal.…”

A Small CO2 Leakage May Induce Seismicity on a Sub‐Seismic Fault in a Good‐Porosity Clastic Saline Aquifer

“…The spectra of stacked wavelets were compensated for the geophone sensitivity (https://www.sercel.com/products/Lists/ProductSpecification/Geophones_specifications_Sercel_EN.pdf) and are converted from particle velocity to displacement. The latter operation amplified the ocean‐generated noise (Glubokovskikh et al., 2021) below the natural frequency of the geophones (8 Hz), hence we filtered these signals out. Finally, we compensated for seismic attenuation using the quality factor Q = 90, an estimate from numerous VSP surveys at the site (Pirogova et al., 2019).…”

Monitoring Injected CO2 Using Earthquake Waves Measured by Downhole Fibre-Optic Sensors: CO2CRC Otway Stage 3 Case Study

“…Composition of the ambient seismic energy recorded by DAS in the downhole environment was studied previously in a number of Australian wells instrumented with fibre-optic cables, including CRC-2 and CRC-3 [ 10 , 27 ], a training well at the Curtin University campus [ 8 ], and South West Hub [ 9 ] in Western Australia. These studies show strong surface waves caused by the interaction between ocean and land (oceanic microseisms), and surface and body waves generated by earthquakes, mine site blasts, and other events related to human activity.…”

“…The data were pre-processed with a bandpass filter (2–5–20–30 Hz). The lower cut-off frequency was selected as frequencies below 2 Hz are usually dominated by ocean microseisms (ocean surface waves converted into surface or body waves in the crust; see, e.g., Glubokovskikh, et al [ 10 ]). Frequencies higher than 30 Hz were filtered out as usual inspection showed no coherent signal from the selected events above that frequency (due to seismic attenuation).…”

“…In addition to signals from controlled sources, permanent receivers such as DAS record an abundance of seismic energy from various natural sources and human activities (earthquakes, mine blasts, ocean waves, machinery, and vehicular traffic) [ 8 , 9 , 10 ]. Harvesting some of this energy can potentially complement or even replace for certain tasks active seismic sources, reducing the cost of monitoring even further.…”