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8/2/2019 _Overview of Geophysical Methods
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Overview of geophysical methodsused in geophysical exploration
Ldvk S. GeorgssonUnited Nations University
Geothermal Training Programme
Orkustofnun Reykjavk ICELAND
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
The role of thegeophysicist
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Measuring physical
properties of earth Geophysical exploration of geothermal resources
deals with measurements on the physical propertiesof the earth.
Emphasis on parameters sensitive to temperatureand fluid content of the rocks.
Aim is to delineate a geothermal resources, outline aproduction field, locate aquifers and site wells orestimate properties of the system.
Ultimately the information is used for economicexploitation of the resource.
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
ParametersReservoir
Temperature Porosity
Permeability Chemical content
of fluid (salinity) (Pressure)
Measured Temperature (C) Electrical resistivity (m)
Magnetisation (Vs/m2) Density (kg/m3) Seismic velocity (km/s)
Seismic activity Thermal conductivity (W/mK) Streaming potential (V)
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
MethodsDirect Thermal methods Electrical methods SP
Structural / indirect Magnetics Gravity Seismic methods Seismicity
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
ApproachCombine methods No method universally applicable Different for low-temperature and high-temperature Choose carefully Usually two or more give most reliable results Different approach in different countries Important to be ready to improvise or try new methodsIntegrated surveys Geophysical exploration does not stand alone, needs to be
integrated with geology and geochemistry?Success of a survey Success is best measured by time, effort and money the
survey has saved.
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Thermal methods Direct measurements
of temperature and heat. No methods correlate better with theproperties of the geothermal system.
Heat exchangeConduction - atomic vibrations, important for transfer of heat inthe earth's crust.Convection - transfers heat with motion of mass, natural
circulation of hot water.Radiation - not in geothermal
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Thermal conduction Heat flow Convection
The simplified geothermal relationship is (conductive heattransfer only):Qcond-z = - k T/z
where k is the thermal conductivity (W/mC) andT/z the thermal gradient
Anomalous values, above 80-100 mW/m2, may indicategeothermal conditions in the subsurfaceThermal conductivity of rocks ranges between 1 and 5 W/mCConvection
Free, driven by density gradients
Forced, driven by an external pressure gradient, likehydrostatic headGeothermal systems are of mixed type
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Application
Thermal distribution at the surface Detailed mapping (GPS) Soil temperature measurement Airborne IR survey
Temperature in 20-100 m gradient wells Used to delineate regional or local gradient anomalies
Heat flow surveys for regional assessment Thermal conductivity measurements, gradient survey with
possible terrain corrections
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
sgardur thermal map
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Hvalfjrdurgradient map
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Electrical methods Most important in geothermal exploration Electrical current is induced into the earth - signals that are
generated are monitored at the surface - many varying methods DC methods, current injected into earth through electrodes at the
surface - the signal measured is the electrical field generated atthe surface. MT, current is induced by the time variations in earth's magnetic
field - the signal measured is the electromagnetic field at thesurface.
TEM, current induced by a time varying magnetic field from acontrolled source - the monitored signal is the decaying magneticfield at surface.
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
ResistivityOhms law
= jE is electrical field strength (V/m)
j is current density (A/m2
)is electrical resistivity ( m) - material constantFor a unit cube/bar, resistivity is defined as
= V / IThe reciprocal of resistivity is conductivity
Most rocks are resistive, conduction is through water in pores andat water-rock contact.
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Resistivity of water bearing rocksControlled by: Porosity and pore structure
Intergranular sedimentsJoints-fissures - tension, cooling - igneous rocks
Vugular dissolved material, gas - volcanics, limestone Alteration (water-rock interaction) Salinity of the water Temperature Amount of water - saturation - steam content
PressureElectric conduction is mainly through
interconnected water-filled pores.
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
ALTERATION RESISTIVITY TEMPERATURE
Rel. unaltered
Smectite- zeolite zone
Mixed layer clay zoneChlorite zone
Chlorite-epidote zone
Pore fluidconduction
Mineralconduction
Boiling
curve
Amb.temp
Freshwater
Salinewater
Resistivity Structure summarised
50-100C
230-250C
250-300C
OO
Resistivity structure summarized
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
DC methods
Sounding - ProfilingSounding - centre fixed, electrode spacing varied used for
mapping resistivity changes with depthProfiling - electrode distances fixed, whole array moved in profile
line - for mapping lateral changesMany methods - different electrode arrays Schlumberger sounding, widely used Dipole sounding or profiling, various arrays Wenner, not much used today Head-on profiling, for locating fractures
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
DC methods
Schlumberger
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Reykjahverfi
Resistivity at 500 m b.s.l.
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
xarfjrdur
Schlumberger resistivity cross-section
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Head-on profiling
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Electromagnetic methodsNatural-source electromagnetics - MT, AMT
Natural EM field used as an energy source.Low frequencies, 0.0001 - 10 Hz are used for deep crustalinvestigations, higher freq., 10 - 1000 Hz, for the upper crust.
Contro.-source electromag. TEM, LOTEM, CSMTIn TEM, constant magnetic field is built up by transmittingcurrent I through a big loop (grounded dipole), and then I isabruptly turned off. A secondary field is induced, decaying withtime. This decay rate is monitored by measuring the voltage
induced in a receiver coil in the centre of the loop. Currentdistribution and decay rate recorded as a function of timedepend on the resistivity structure.
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
TEMconfiguration
Transmitted current
Measured voltage
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Hengill -TEM resistivity
map at600 m b.s.l.
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Hengill
TEM resistivity cross-section
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Menengai - MT cross-section
4000 6000 8000 10000 12000 14000 16000 18000
Horizontal distance (m)
-6000
-4000
-2000
0
2000
Elevation(m)
MT01 MT13 MT58 MT57MT55 MT53 MT59
MT51MT60
1
5
1
1
2
2
2
3
3
4
4
4
5
5
6
78
1
1
1
1
1
Menengai Crater
m
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
SPDC-componentof earths nat.
electrical
potentials
Significantanomalies
associated w.
geothermalactivity
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Structural methodsMagnetic methods are widely used in geothermal exploration
often together with gravity and refraction in mapping geologicalstructures - based on varying magnetisation in rocks
Gravity surveys are used in geothermal exploration to detectgeological formations with different densities, are as such a typicalstructural method
Active seismic methods detect sound velocity distribution and
anomalies in the earth and attenuationPassive seismic methods detect seismic activity in the earth
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Magnetic method Two kinds of magnetisation
Induced magnetisation Mi - same direction as the ambient earth's field; Permanent magnetisation Mp, in igneous rocks it often predominates; it
depends upon their properties and history
Magnetic anomaly is a local disturbance caused by local change inmagnetisation; characterised by the direction and magnitude of the effectivemagnetisation and the shape, position, properties and history of the anomalousbody
Measurements aim mainly at finding location and depth estimate of hiddenintrusives or tracing buried dykes and faults, or areas of reduced magnetization
due to thermal activity Procedures - On ground, regular measurements in profiles or grid, In
aeromagnetic surveys, e.g. 100 m a.g. and with 100 m between lines
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
sgardur Magnetic map
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
sgardur - 3D magnetic map
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Gravity & density -
Gravity measurements Gravity measurements are based on density contrasts of rocksin the earth which lead to different gravitational force usuallymeasured in mgal or 10-3 m2/s
Gravity usually shown as Bouguer anomaly (after corrections):
gB = gM + CFA - CB + CT - gN Density depends on rock composition & porosity, ~2-3 g/cm3
Important applicationsBasement depth variation (sedimentary area)Intrusive rocks (possible heat source)
Fault or dyke systems etc.Alteration, cementation due to thermal effectsMonitoring mass extraction with production
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
xarfjrdurBouguer gravity
map
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Svartsengi -Mean gravity
change1975-1999in gal/year
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Elastic waves seismic methods
Elastic waves - different velocity in different rock types Refracted and reflected at discontinuities in formation Two types of elastic body waves:
P-waves, wave movement in the travel directionS-waves material movement perpendicular to wave direction
Seismic methods use this for info. on the geothermal system Two types of measurements Active methods not used routinely in geothermal expensive.
Info. on density, porosity and texture; fluid-filled zones & temp.Include seismic refraction and seismic reflection
Passive methods - seismic activity. Info. on active faults andpermeable zones (shear wave splitting), S-wave shadow canindicate partial melt
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
xarfjrdurBouguer gravity & seismic cross-section
UNU G h l T i i P
KM78000
[a]
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Olkaria
Eventdistribution
188000 192000 196000 200000 204000 208000
-8
-7
-6
-5
-4
-3
-2
-1
0
Depth[km]
OWF OCF NEF-DOMES
0 2000 4000 M
[b]
DM1KM5 OS1
KR6DM4
WE2 DM5
KM6
EP4
NE2
KM8 CE1NE3 DM2
KM2KR2
DM3
188000 192000 196000 200000 204000 208000
9
896000
9900000
9904000
9908
GridNorthin
gs[m]
0 2000 4000M
UNU G th l T i i P
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
ReykjanesPeninsula
Seismiczone
UNU G th l T i i P
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Integrated results -Key tounderstanding
- sgardur
geothermal modelbased on soiltemperature
measurements,magnetic mapping
and regional survey
UNU Geothermal Training Programme
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Integrated results -Key to
understanding
Theistareykiraeromagnetic map &resistivity map
UNU Geothermal Training Programme
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ORKUSTOFNUN Kenya Short Course II - LSG 10.11.2007
UNU Geothermal Training Programme
Selected referencesrnason, K., and Flvenz, .G., 1992: Evaluation of physical methods in
geothermal exploration of rifted volcanic crust. Geoth. Res. Council,Transactions, 16, 207-214.
rnason, K., Karlsdttir, R., Eysteinsson, H., Flvenz, .G., and Gudlaugsson,S.Th., 2000: The resistivity structure of high-temperature geothermal systemsin Iceland. Proceedings of the World Geothermal Congress 2000, Kyushu-Tohoku, Japan, 923-928.
Bjrnsson, A., and Hersir, G.P., 1991: Geophysical exploration for geothermalresources, principles and applications. UNU G.T.P., Iceland, report 15, 94 pp.
Flvenz, .G., and Saemundsson, K., 1993: Heat flow and geothermal processesin Iceland. Tectonophysics, 225, 123-138.
Keary, P., and Brooks, M., 1992: An introduction to geophysical exploration.Blackwell Scientific Publications, Oxford, 254 pp.
UNU Geothermal Training Programme
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UNU Geothermal Training Programme
Thank you
for theattention
Vellir geyser