ORKUSTOFNUN Ethiopia – ARGeo C-1 – LSG / 29.11.2006 UNU Geothermal Training Programme COMPARISON OF SCHLUMBERGER MEASUREMENTS AND TEM MEASUREMENTS WITH

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Ethiopia – ARGeo C-1 – LSG / 29.11.2006

UNU Geothermal Training Programme COMPARISON OF SCHLUMBERGER COMPARISON OF SCHLUMBERGER MEASUREMENTS AND TEM MEASUREMENTS AND TEM

MEASUREMENTSMEASUREMENTSWITH FIELD EXAMPLES FROM THEWITH FIELD EXAMPLES FROM THE

ÖXARFJÖRDUR GEOTHERMAL FIELD, NE-ÖXARFJÖRDUR GEOTHERMAL FIELD, NE-ICELANDICELAND

Lúdvík S. GeorgssonLúdvík S. Georgsson11 & Ragna & Ragna KarlsdóttirKarlsdóttir22

11United Nations University United Nations University Geothermal Training ProgrammeGeothermal Training Programme

Orkustofnun – Reykjavik – ICELANDOrkustofnun – Reykjavik – ICELAND

22 ÍSOR - Iceland GeoSurvey ÍSOR - Iceland GeoSurveyReykjavik – ICELANDReykjavik – ICELAND

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UNU Geothermal Training Programme

Resistivity methodsResistivity methods• Resistivity methods have for a long time been the most

important method in outlining geothermal systems• DC resistivity methods, particularly using the

Schlumberger sounding configuration, was the common approach used from the 1960s into the 1990s, when prospecting for resistivity in the uppermost kilometre, and Schlumberger soundings are still widely used

• TEM, or the “Central loop Transient ElectroMagnetic sounding method, where current induced in the earth is by a time varying magnetic field from a controlled source came onto the geothermal scene around 1990 and has since replaced Schlumberger soundings as the routine resistivity method in many countries

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DC – SchlumbergerDC – Schlumbergerconfiguration configuration

a = ∆V/I (S2–P2)π/2P

with S = AB/2P = MN/2

a as a function of AB/2 is plotted on a bilogarithmic paper with increasing electrode separation

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Appar

ent r

esis

tivity

[m

]

Appar

ent r

esis

tivity

[m

]

Ö ´ t 1000AB/2 [m]

Schlumberger soundingTEM sounding

Schlumberger resistivity Schlumberger resistivity sounding Presentation & sounding Presentation &

interpretationinterpretation

The sounding reflects the true image of the resistivity distribution in the earth below

1D manual interpretation quite easy but generally done by 1D inversion programs

2D/3D interpretation often necessary in areas of complicated resistivity structures

Terrain correction also advisable in complicated terrain

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TEM configurationTEM configuration

Transmitted current

Measured voltage

ρa a function of several variables:Measured voltage; time elapsed from

turn off; area of loops/coils; number of windings in loops/coils; magnetic permeability

For homogeneous half-space, apparent resistivity ρa, is expressed in terms of induced voltage at late times after the source current is turned off is given by

For a layered earth the expression is more complicated

32

25

000

,5

2

4

rtVt

InAnA ssrra

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UNU Geothermal Training Programme TEM soundingTEM sounding

Presentation & Presentation & interpretationinterpretation

Apparent resistivity is plotted as a function of time since current was turned of

Resembles the Schlumberger sounding graphically

However, it does not reflect the image of the true resistivity compared to Schlumberger sounding

1D interpretation by 1D inversion programs, manual interpretation not possible

3D interpretation possible but very complicated

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Interpretation using Occam Inversion improves resolution even further

“Continuously” changing resistivity instead of few specific layers

Together with improved instrumentation in recent years this has given additional details in the geothermal interpretation

TEM soundingTEM soundingPresentation & Presentation & interpretationinterpretation

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General advantages of TEMGeneral advantages of TEM• In TEM no current has to be injected into the earth and the wires

are shorter - important in deserts, lava fields and cold areas – thus data collection is even possible on snow and ice

• In TEM distortions due to local inhomogeneities are small – TEM is much more focussed

• Similarly, TEM is much less sensitive to lateral resistivity variations than DC methods. Thus, 1-D interpretation is much better justified

• In DC-soundings, the monitored signal is low when surveying over low-resistivity structures like in geothermal areas, but strong in TEM-soundings, increasing depth penetration in target areas

• TEM needs much less manpower, both in the field and for interpretation, and measurements are faster – thus very importantly it is more cost effective, or allows much higher data density

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Öxarfjördur - Öxarfjördur - LocationLocation

• Öxarfjördur is chiefly a N-S trending, 25 km wide, downfaulted trough filled by sediments

• Region dominated by the delta of Jökulsá river and three active N-S trending fissure swarms

• Geothermal activity and active volcanic systems in sedimentary stratigraphy is unique for Iceland

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Öxarfjördur – Öxarfjördur – Geology & Geology &

geothermalgeothermalKrafla central volcano ~50

km from the coast - its fissure swarm stretches into the Öxarfjördur bay.

Intense volcanic and rifting activity in 1975-1984.

Rifting episodes associated with magmatic intrusions - 3-4 in the Öxarfjördur area.

Meagre surface geothermal activity.

Geothermal activity increased during Krafla fires, but is now declining.

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The The ÖxarfjörduÖxarfjördu

r deltar delta

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Öxarfjördur - SchlumbergerÖxarfjördur - Schlumberger2D resistivity cross-section A-A’2D resistivity cross-section A-A’

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UNU Geothermal Training Programme Öxarfjördur - SchlumbergerÖxarfjördur - Schlumberger

2D resistivity cross-section B-B’2D resistivity cross-section B-B’

textbox

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UNU Geothermal Training Programme Öxarfjördur – Öxarfjördur –

Schlumberger resistivity map at 500 m Schlumberger resistivity map at 500 m b.s.l. b.s.l.

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Öxarfjördur – Öxarfjördur – Old model of Old model of

the the Bakkahlaup Bakkahlaup geothermal geothermal

systemsystem

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A

B

CD

E

F

Öxarfjördur – TEM Öxarfjördur – TEM anomaliesanomalies

TEM shows 4 main low-resistivity anomalies, and 2 smaller ones

The Bakkahlaup anomaly, A, is comparatively small and a high-resistivity body is not found below it, nor elsewhere

Other anomalies are typical for low-temperature surroundings with saline ground water, some correlating quite well with surface geothermal activity

The upflow is probably mainly associated with N-S trending fissures

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BA-02BA-04

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Resistivity cross-section B-B’

BA-03

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Öxarfjördur – Öxarfjördur – New model of New model of

the the Bakkahlaup Bakkahlaup geothermal geothermal

systemsystem

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Öxarfjördur – Comparison DC-Öxarfjördur – Comparison DC-TEMTEM

• Schlumberger soundings outlined one large low-resistivity anomaly with a high-resistivity body below which is typical for Icelandic high-temperature fields; they also outlined some smaller low-resistivity anomalies

• Drilling of two deep exploration wells in the main anomaly at Bakkahlaup did not confirm the existence of a large high-temperature area, and indicated that the resource was more limited and the temperatures probably not much exceeding 200°C

• Drilling also confirmed the existence of the thick sediments reaching about 500 m at Bakkahlaup and probably up to 1000 m at the coast within the Krafla fissure swarm

• The combination of sediments and varying salinity of the ground water may have influenced the results

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UNU Geothermal Training Programme Öxarfjördur – Comparison DC-Öxarfjördur – Comparison DC-

TEMTEM• TEM measurements show a much more detailed picture• At deeper levels the resistivity distribution is complicated and

quite different from the one based on Schlumberger soundings• The main low-resistivity area or upflow zone, at Bakkahlaup,

appears to be quite limited, as indicated by the drilling, and no confirmed high-resistivity body can be seen at deeper levels

• Several other low-resistivity areas are seen, some agreeing quite well with the older maps

• The upflow is probably mainly through fissures, trending N-S• With no high-resistivity body, temperatures ought to be lower

than in conventional high-temperature fields, as seen from wells• Drilling of the third exploration well in 2005 has further

strengthened the model based on the results of the TEM• The central part of the Bakkahlaup area is below the glacial river

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Main conclusionsMain conclusions• TEM has many advantages over Schlumberger soundings

- in requiring less manpower and thus in cost effectiveness- in its low sensitivity to local inhomogeneities- in no current transmission into ground- in strong signal associated with low resistivity (geothermal)- in easy computer interpretation - usually 1D is enough- in improved resolution of the resistivity distribution in the uppermost kilometre, thus giving improved information on the geothermal system

• Schlumberger soundings have also some advantages- in their simpler and more robust equipment- in the transparency of the data giving confidence in results- showing better resolution for near-surface layers

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Main conclusionsMain conclusions• For a good picture of the resistivity in the

uppermost kilometre in geothermal systems TEM soundings are definitely recommended, if a detailed information on the uppermost 200-300 m is necessary, short Schlumberger soundings could compliment them

• For information at deeper levels, down to 2-4 km depth, the conventional production part in high-temperature fields, MT compliments TEM in being able to outline the resistivity distribution in this depth interval

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Thank youThank you for the for the

attentionattention

Documents

ORKUSTOFNUN Ethiopia – ARGeo C-1 – LSG / 29.11.2006 UNU Geothermal Training Programme COMPARISON OF SCHLUMBERGER MEASUREMENTS AND TEM MEASUREMENTS WITH