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| Titel |
Joint Interpretation of Magnetotellurics and Airborne Electromagnetics in the Rathlin Basin, Northern Ireland |
| VerfasserIn |
Robert Delhaye, Volker Rath, Alan G. Jones, Derek Reay, IRETHERM Team |
| 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 |
250103860
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| Publikation (Nr.) |
 EGU/EGU2015-3279.pdf |
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| Zusammenfassung |
| In this study we present results from geophysical investigation of the sedimentary Rathlin
Basin in Northern Ireland in order to assess the potential for low-to-medium enthalpy
geothermal aquifers within the porous Permian and Triassic sandstone groups. The area and
groups were identified as a potential geothermal resource due to the presence of both an
elevated geothermal gradient (observed in two deep boreholes onshore) and favourable
hydraulic properties (measured on core samples in the offshore part of the basin). Previous
seismic experiments were not able to fully characterise the sediments beneath the
overlying flood basalt. Complementing these earlier results, magnetotelluric data
were acquired on a grid of 56 sites across the north-eastern portion of the onshore
Rathlin Basin, and an additional 12 sites on the nearby Rathlin Island, in order to
image the thickness, depth, and lateral continuity of the target sediments. Analysis
and 3D modelling, including the effects of the highly conducting ocean, has been
successful in deriving a resistivity model that maps the variation in the top of the
sediments (base of the basalts) and the truncation of the basin sediments against the
Tow Valley Fault, and gives a reasonable estimate of the thickness of the sediment
fill.
However, the resulting models show significant effects from distortion caused by
near-surface inhomogeneities in the responses that cannot be resolved using the given frequency
range and site density. Fortunately, for the area of Rathlin Basin, airborne electromagnetic
data from the TELLUS project (http://www.bgs.ac.uk/gsni/tellus/contact/index.html) are
available. These data were measured at four frequencies between 0.9ÂkHz and 25ÂkHz in a
verical-coplanar loop configuration, with the dipole axis in flight direction. The spatial
sampling distance was less than 25Âm, with about 200Âm distance between flight
lines. Survey altitudes vary between 56Âm and 244Âm. Thus, for the top ≈100Âm
penetrated by these frequencies an additional data set of more than 300 000 sites were
available. To use these data with the existing MT array, the data were interpreted
by a pointwise 1D inversion using the AirBeo code from the AMIRA collection
(http://www.amirainternational.com/). Subsequently these results were upscaled to the
resolution of the 3D MT model, and used for static shift correction for the MT soundings.
This procedure produced very promising results, which will be presented in this
contribution.
Whilst the simple approach used up till now only solves one of the aforementioned
problems, namely the static shift at the MT sites, joint interpretation of the two methods
shows improved model formation continuity between MT sites, even at depth. Regional-scale
features, however, are mostly retained. Currently more sophisticated procedures are being
developed with the aim of using the full amount of information from the airborne
electromagnetics to improve spatial coverage at the high-frequency range of the MT data, and
thus improving model realism near the surface. |
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