Accurate Estimation of Sediment Conductivities

in the Rathlin Basin Using Magnetotellurics

R. Delhaye (rdelhaye@cp.dias.ie), V. Rath, A. G. Jones, M. R. Muller, D. Reay, and the IRETHERM Team

Hot Sedimentary Aquifers - Concept

If we can find a reservoir with:

Good hydraulic properties:

Porosity (how much free space there is

within the rock).

Permeability (how well connected the

porosity is).

Good thermal properties:

Elevated temperatures within the

reservoir.

Then, depending on the reservoir properties, it

can be exploited for heat or energy,.

IRETHERM Workshop

1st April, 2016

Hot Sedimentary Aquifers – Ireland?

How applicable is the HSA

concept to Ireland?

• Several notable onshore

sedimentary basins,

especially in Northern

Ireland.

• Two of these have previous

geophysical studies.

• Larne Basin, S Sihelnik.

• Lough Neagh Basin,

• M Loewer.

• The Rathlin Basin less

studied…

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Permian/Triassic Basins as Reservoirs

Permian & Triassic sandstones known from

offshore hydrocarbon prospects as reservoirs.

Core measurements from five onshore

boreholes in the three PT basins show promise!

Porosities from 10 to 25%.

Permeabilities from 1 to 1000 mD.

IRETHERM Workshop

1st April, 2016

Permian/Triassic Basins as Reservoirs

Permian & Triassic sandstones known from

offshore hydrocarbon prospects as reservoirs.

Core measurements from five onshore

boreholes in the three PT basins show promise!

Porosities from 10 to 25%.

Permeabilities from 1 to 1000 mD.

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Permian/Triassic Basins as Warm Spots

Thermal understanding has been

re-assessed, with slightly less

exciting results...

Circles on map indicate deep

borehole locations and

measurements.

Lough Neagh, Larne not exciting.

But, Rathlin still hot! (relatively).

Temperature measurements

taken three times, in two

different deep wells

All three support geothermal

gradient of 37 – 42 oC/km!

Heat Flow Density

Estimate (mW/m2)

Estimated heat flow density map (Jones et al., 2014).

IRETHERM Workshop

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Previous Work in Lough Neagh Basin

Top left is Bouguer gravity anomaly, showing decrease in mass towards SE.

Left is density model, with constraining borehole information. PT sandstones shown in

oranges and yellows.

Right is resistivity model from 2D magnetotelluric inversion, with density boundaries

overlain. Conductive zone of < 5 Ωm matches gravity modelled PT sandstones. Same lithologies expected in contemporaneous Rathlin Basin?

Work taken from M. Loewer MSc thesis, 2010.

Ωm

1

10

100

Depth

(km)

0

2

4

S

E

N

W

S

E

N

W

IRETHERM Workshop

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MT Survey of the Rathlin Basin

MT/AMT station.

Deep borehole.

Antrim Lava Group.

Paleogenic basalt, tholeiitic

basalt.

Dalradian Metasediment.

Pre-Cambrian to Cambrian.

Carboniferous sediments.

56 sites in seven magnetotelluric profiles planned, aligned across the

basin.

Equal site and profile spacing in order to facilitate three dimensional

modelling.

IRETHERM Workshop

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Rathlin Basin – the Port More Borehole

The Port More Borehole:

Drilled in 1965 – 1967.

TD of 1896.7m, in Permian

Sandstones.

Limited wireline logging.

Thick, unexpected dolerite sill,

unknown lateral extent across basin.

IRETHERM Workshop

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The Static Shift Problem

A decent portion of the MT data show a vertical offset, or static shift, between the two data

modes

At depth, the two modes diverging indicates larger scale resistivity contrasts, but the near-

surface is often near-one dimensional.

What's going on? We appear to have massive electrical anisotropy near the surface!

Typically caused by sub-grid resistivity variations below our resolution.

IRETHERM Workshop

1st April, 2016

Airborne FDEM – A Solution?

The resistivity of the Earth itself should be invariant of geophysical measure (to first order).

So, model the Tellus airborne FDEM data to obtain near-surface resistivity distribution.

Calculate corrected MT forward responses from these resistivities, and shift observed MT data.

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How are Data Affected?

MT data (ρxy

) at 10 kHz, corresponding to uppermost

~100 m.

Improved regional consistency of data, less blobby

nature.

Note that scaling factor presented in order of

magnitudes – scale from -0.4 to 0.4 indicates range of

~x/3 to ~3x.

Improved Data

logRatio

Original Data

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How is Model Affected?

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Mc

In Profile

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• Static shift correction clearly enhances resistivity contrasts at

the upper boundary of the target conductive sediments.

Preferred Model

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10 Ωm isosurface of Mc, extending to ~1800 m depth. Resistivity distribution

of Mc with depth.

Implications for Geothermal Modelling

IRETHERM Workshop

1st April, 2016

The classic tool for estimating sandstone porosities as functions of

resistivity is Archie’s Law, relating total resistivity Rt to the porosity φ

(depending on the cementation, tortuosity and saturation, and the fluid

resistivity itself).

Take-home message

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If we want to have confidence in our estimates of

resistivity at depth, we must also be confident in our

estimates of shallow resistivity!

Acknowledgements

IRETHERM Workshop

1st April, 2016

The Tellus Project, for AEM data

www.tellus.ie

AMIRA International Ltd, for AEM codes

www.amirainternational.com

Irish Centre for High End Computing, for computations

www.ichec.ie

Gary Egbert, Naser Meqbel, Jan Vozar, for provision and

implementation of the ModEM code

blogs.oregonstate.edu/modem3dmt/

Questions?

IRETHERM Workshop

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References

IRETHERM Workshop

1st April, 2016

Berdichevsky, M. N., et al.,(1980), Geoelectrical model of the Baikal region, Phys. Earth Plan. Int.

Dentith, M., and Mudge, S. (2014). Geophysics for the Mineral Exploration Geoscientist. Cambridge University Press.

Goodman, R., et al., (2004). Geothermal Energy Resource Map of Ireland. SEI report, July 2004.

Jones, A.G., et al., (2014). The lithosphere-asthenosphere system beneath Ireland from integrated geophysical-

petrological modeling – 1: Observations, 1D and 2D, Lithos.

Kelbert, A., Meqbel, N., Egbert, G. D., and Tandon, K., (2014), ModEM: A modular system for inversion of

electromagnetic geophysical data, Computers and Geosciences.

Loewer, M., (2010). "Investigation of the geothermal energy potential of the Lough Neagh Basin, Northern Ireland, using

magnetotelluric and gravity modelling." MSc thesis.

Mitchell, W. I. (ed.), (2004). The Geology of Northern Ireland - Our Natural Foundation (Second Edition). Geological

Survey of Northern Ireland, Belfast.

Raiche, A., (1999). A flow-through Hankel transform technique for rapid, accurate Green's function computation. Radio

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