The shallow subsurface characterization in relation to geothermal resources in the Mallow area, Ireland, using passive seismic and gravity data inversion

Abstract

One key aim of the DIG (De-risking Ireland’s Geothermal Energy Potential) project is to determine and evaluate the potential low-enthalpy geothermal resources at reservoir scale in the Mallow warm springs area (MWSA), by performing a joint interpretation of new and existing geophysical, geochemical and petrophysical datasets together with structural geology and hydrochemistry results.As a first step, based on the ambient detected noise sources in the study area, about 100 seismic stations (5Hz nodes) were deployed for two weeks along the railway, straddling fault structures that are thought to control hot spring fluid flow in the Mallow area. We performed seismic interferometry imaging on the recorded train-induced vibrations to map shallow subsurface (top ~2 km) structures and to extract the physical properties (e.g. seismic velocity and density) of these structures. Preliminary result shows a good correlation between S-wave velocity variation and the near-surface lateral changes of lithology, especially across the Killarney-Mallow Fault Zone (KMFZ).In the next step, 2D interactive modeling of the gravity data was performed, using physical properties determined from the previous step to constrain shallow structures. Additionally, we used the result from the receiver function method that is applied to the data recorded by four broadband stations in the study area, in order to better constrain the deeper interfaces. The 2D inversion of gravity data reveals an anomalous zone in the vicinity of the KMFZ that could be related to the possible fault conduit, associated with the Mallow warm springs area.The project is funded by the Sustainable Energy Authority of Ireland under the SEAI Research, Development & Demonstration Funding Programme 2019 (grant number 19/RDD/522) and by the Geological Survey Ireland.