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ORKUSTOFNUN
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
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
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
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
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
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
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
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
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
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
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
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
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
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
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
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
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
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
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.
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
The The ÖxarfjörduÖxarfjördu
r deltar delta
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
Öxarfjördur - SchlumbergerÖxarfjördur - Schlumberger2D resistivity cross-section A-A’2D resistivity cross-section A-A’
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme Öxarfjördur - SchlumbergerÖxarfjördur - Schlumberger
2D resistivity cross-section B-B’2D resistivity cross-section B-B’
textbox
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme Öxarfjördur – Öxarfjördur –
Schlumberger resistivity map at 500 m Schlumberger resistivity map at 500 m b.s.l. b.s.l.
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
Öxarfjördur – Öxarfjördur – Old model of Old model of
the the Bakkahlaup Bakkahlaup geothermal geothermal
systemsystem
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
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
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
BA-02BA-04
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
Resistivity cross-section B-B’
BA-03
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
Öxarfjördur – Öxarfjördur – New model of New model of
the the Bakkahlaup Bakkahlaup geothermal geothermal
systemsystem
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
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
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
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
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
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
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
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
ORKUSTOFNUN
Ethiopia – ARGeo C-1 – LSG / 29.11.2006
UNU Geothermal Training Programme
Thank youThank you for the for the
attentionattention