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| Titel |
IRETHERM: Multidimensional geophysical modeling of the southern margin of the Dublin Basin |
| VerfasserIn |
Jan Vozar, Alan G. Jones, Volker Rath, Joan Campanyà, Riccardo Pasquali |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250108027
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| Publikation (Nr.) |
 EGU/EGU2015-7756.pdf |
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| Zusammenfassung |
| Multi-dimensional magnetotelluric (MT) modelling of data from the Newcastle area west of
Dublin, acquired as part of the geothermal potential of Ireland (IRETHERM) project, is
presented. The Newcastle area, situated on the southern margin of the Carboniferous Dublin
Basin, exhibits elevated geothermal gradient (>30 Ë C/km) in the exploratory boreholes
drilled by GT Energy.
The MT soundings were carried out in the highly urbanized Dublin suburb
and are heavily noise-contaminated and distorted due to EM noise from nearby
industry and the DC tram system (LUAS). We obtained reliable and interpretable
MT impedance and geomagnetic transfer functions at most sites by processing the
“quietest” 4-hour night time subsets of data using several robust codes and the ELICIT
method.
Tensor decomposition was applied at each site to ascertain if the data are suitable for 2-D
modelling and to determine the appropriate geoelectric strike direction. The obtained 2-D
models underwent examination using a new stability technique, and the final two 2-D profiles
with reliability estimations, expressed through conductance and resistivity, were
derived.
3-D models, including all usable MT data in the Newcastle area, have also been
determined with and without resistivity constrains for shallow structures from resistivity
measurements in one of the boreholes (borehole NGE1). The 3-D models exhibit structures
with higher conductivity in comparison to the 2-D models, with similarly resistive
background rocks. The shallow conductive structures, to a depth of 1 km, have north-south
elongations correlated with the surface traces of faults that are perpendicular to the regional
Blackrock to Newcastle Fault (BNF). Deeper structures become more oriented to a regional
geoelectric strike similar to 2-D regional strike.
To obtain superior characterization of the thermal transport properties of the investigated
area, we used porosity and resistivity data from borehole NGE1 to estimate relation between
porosity/permeability and electrical conductivity. The formulae are based on a generalized
Archie’s law for multiple phases. Due to the poor quality of porosity data from the borehole,
we used these values as a maximum boundary in the input porosity data for estimation of the
relation.
The 2-D and 3-D modeling reveal that the BNF is imaged as a conductive zone to depths
of 4 km, and is likely highly fractured. Generally, the area south of the BNF is more resistive
and compact, with a horizontal conductive layer at approximately 1 km depth and with a very
thin surficial sedimentary layer. In contrast, the structures north of the BNF are more
heterogeneous, with deeper conductive layers (2-3 km depth) and thicker (several hundred
meters) sedimentary layers above.
The deeper conductive layers are interpreted as water bearing or geothermal fluids and
estimated porosity and permeability indicates potential to deliver warm water to the surface. |
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