Continuous DAS measurements in a deep borehole provides new insights into the generation of the coastal microseisms

Abstract:

Ocean-generated seismic signals are the main component of the ambient seismic wavefield, which may be both, noise in the earthquake seismology or signal for subsurface imaging. The mechanisms that couple motion of ocean into the seismic signals are well-studied using kinematic and polarisation analysis of the global seismological networks. However, transition of the seismic waves into continents is still poorly understood, because conventional seismological arrays lack coverage and density to image the rapid changes of the wavefield. We present an experimental evidence that a single borehole monitoring using modern distributed acoustic sensing may decipher the coastal ambient seismic wavefield.

We analyse 80 days of continuous acquisition in a 1600m deep borehole at the CO2CRC Otway Project site, which is located 5 km away from the coast of Victoria (Australia). The data provides excellent signal-to-noise ratio from 0.03Hz to over 100Hz. Interferometric analysis of the vertical seismograms and comparison with the wave climate clearly identified the origin of the seismic energy: (1) noise notch < 50mHz; primary microseisms and teleseismic earthquakes at 50mHz and 120mHz; the double frequency microseisms from austral storms peaked at 150mHz; the local microseisms dominating the records have frequency between 0.3Hz and 2Hz; the range 2Hz to 20Hz contains frequently repeated body waves from large surf break located around 8 km from the borehole. The spectral peak around 0.6Hz violates a commonly-accepted hypothesis that the microseisms obey a world-wide constant Holu spectrum, which rapidly decreases after 0.3Hz. However, the seismic response is in perfect agreement with theoretical predictions using directional spectra from the wave buoys in the area.

Availability of the vertical seismograms enables normal mode analysis of the leaky Rayleigh waves that carry the microseisms energy. The estimated energy partition between the fundamental and higher Rayleigh modes agrees with the kinematic analysis of the dense seismological arrays in the vicinity of the monitoring borehole. In an upshot, the link between the amplitudes variation with depth and ocean wave is so robust that the passive seismic records in our borehole may be used to monitor both, surrounding rocks and remote parts of the ocean.