Sedimentary basin exploration with receiver functions: Seismic structure and anisotropy of the Dublin Basin (Ireland)

Teleseismic receiver functions (RFs) were used to investigate the seismic structure of the southern margin of the Dublin Basin, a potential geothermal site. Through an inversion-based approach, the elastic properties and seismic anisotropy of sedimentary basin units were examined, using data from a linear array of closely spaced seismic stations. Our results were compared with sonic logs and lithostratigraphies from two nearby boreholes, NGE1 and NGE2 and colocated active seismic data. Including a high-frequency RF (up to 8 Hz) allowed us to compute S-wave velocity models with a vertical resolution <500 m. The results indicated the presence of a subvertical lateral discontinuity in VS, in correspondence with the main basin-bounding fault (Blackrock-Newcastle Fault [BNF]). North of this discontinuity, a shallow low-velocity layer thickens (from 0.7 to 1.0 km thick) toward the inner basin, in agreement with the geometry of the shallowest reflector found by active seismics. A good correlation was also found between the sonic log at NGE1 and our velocity model. Station DB02 showed an increase in VS at a depth of approximately 0.7 km and a decrease in VS at approximately 1.4 km in depth. Two velocity jumps with matching polarities were also observed in the NGE1 sonic log at the contact between the Upper and Lower Calp formations (positive jump, 688 m deep), and between a calcarenite and a sandstone layers (negative jump, 1337 m deep). Moreover, the main velocity contrasts in our model agree with the major lithostratigraphic boundaries inferred from borehole-drilled samples. Two juxtaposed anisotropic layers are identified close to the BNF. Directions of the slow axis of anisotropy are consistent with the borehole structural data. From these observations, the presence of aligned open cracks within the sandstones, possibly fluid-filled, was inferred up to a depth of 2.3 km in the vicinity of the BNF.