Understanding hydrothermal circulation patterns at a low-enthalpy thermal spring
using audio-magnetotelluric data: a case study from Ireland
Blake, S., T. Henry, M.R. Muller, A.G. Jones, J.P. Moore, J. Murray, J. Campanya,
J. Vozar, J. Walsh, and V. Rath
Kilbrook spring is a thermal spring in east-central Ireland. The temperatures in the spring are
the highest recorded for any thermal spring in Ireland (maximum of 25 C). The temperature
is elevated with respect to average Irish groundwater temperatures (9.5 – 10.5 C), and
represents a geothermal energy potential, which is currently under evaluation.
A multi-disciplinary investigation based upon an audio-magnetotelluric (AMT) survey, and
hydrochemical analysis including time-lapse temperature and chemistry measurements, has
been undertaken with the aims of investigating the provenance of the thermal groundwater
and characterising the geological structures facilitating groundwater circulation in the
The three-dimensional (3-D) electrical 24 resistivity model of the subsurface at Kilbrook spring
was obtained by the inversion of AMT impedances and vertical magnetic transfer functions.
The model is interpreted alongside high resolution temperature and electrical conductivity
measurements, and a previous hydrochemical analysis.
The hydrochemical analysis and time-lapse measurements suggest that the thermal waters
have a relatively stable temperature and major ion hydrochemistry, and flow within the
limestones of the Carboniferous Dublin Basin at all times. The 3-D resistivity model of the
subsurface reveals a prominent NNW aligned structure within a highly resistive limestone
lithology that is interpreted as a dissolutionally enhanced strike-slip fault, of Cenozoic age.
The karstification of this structure, which extends to depths of at least 500 m directly beneath
the spring, has provided conduits that facilitate the operation of a relatively deep
hydrothermal circulation pattern (likely estimated depths between 560 and 1,000 m) within
the limestone succession of the Dublin Basin. The results of this study support the hypothesis
that the winter thermal maximum and simultaneous increased discharge at Kilbrook spring is
the result of rapid infiltration, heating and re-circulation of meteoric waters within this
structurally controlled hydrothermal circulation system.
This paper illustrates how AMT may be useful in a multi-disciplinary investigation of an
intermediate-depth (100 – 1,000 m), low-enthalpy, geothermal target, and shows how the
different strands of inquiry from a multi-disciplinary investigation may be woven together to
gain a deeper understanding of a complex hydrothermal system.
Journal of Applied Geophysics, , 132, 1-16.
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Alan G Jones / 11 September 2016 /
alan.jones.geophysics -at- gmail.com