Subsurface characterization using passive seismic in the urban area of Dublin City, Ireland

We investigate the shallow geological features and previously unknown depths of the main geological interfaces in the highly urbanized Dublin City area by using a set of complementary ambient noise seismic methods. A sparse seismic array, composed of 19 broadband sensors, was installed in a 5 x 5 km area across the city centre. The dataset was acquired over a 5-month-long deployment and used to perform horizontal/vertical spectral ratios and frequency-wave number analysis cross-correlation interferometry. From the horizontal/vertical high-frequency resonance peaks, we estimate the depth to bedrock. Then we use a subset of six seismic stations to obtain the frequency-wavenumber Rayleigh wave fundamental mode in the 0.8-3 Hz frequency band. Next, the ambient noise dataset is cross-correlated in order to extract the empirical Green's functions before measuring surface-wave phase velocities by undertaking a dispersion analysis in the 0.5-9 Hz frequency band. Then, a Monte Carlo global optimization algorithm is used to invert the phase velocity dispersion measurements. We generate a reference one-dimensional depth S velocity profile along with a set of localized one-dimensional S velocity profiles. Finally, a smooth three-dimensional shear wave velocity model is derived for the top 1000 m in the sedimentary Dublin Basin. From the one-dimensional velocity profiles and three-dimensional shear velocity model, with some sensitivity down to approximately 1.2 km, we observe velocity changes with depth that highlight the presence of three consistent interfaces. We discuss interpretative scenarios that correlate the velocity features to potential stratigraphic boundaries occurring within the sedimentary Dublin Basin, suggesting that basement rocks could be located at a depth significantly greater than the top kilometre. The results of this study may indicate that future geothermal studies should be directed at structures deeper than 1 km, towards the bottom of the sedimentary basin beneath the city.

Automated summary

In this study, shallow geological features and previously unknown depths of main geological interfaces in the highly urbanized Dublin City area are investigated using complementary ambient noise seismic methods. The results indicate the presence of three consistent interfaces and suggest further research on deeper structures for potential geothermal studies.