ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
Overview of geophysical methods Overview of geophysical methods used in geophysical explorationused in geophysical exploration
LLúúdvdvíík S. Georgssonk S. GeorgssonUnited Nations University United Nations University
Geothermal Training ProgrammeGeothermal Training ProgrammeOrkustofnun Orkustofnun –– ReykjavReykjavíík k –– ICELANDICELAND
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
The role of the The role of the geophysicistgeophysicist
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
Measuring physicalMeasuring physicalproperties of earthproperties of earth
• Geophysical exploration of geothermal resources deals with measurements on the physical properties of the earth.
• Emphasis on parameters sensitive to temperature and fluid content of the rocks.
• Aim is to delineate geothermal resources, outline drilling fields, locate aquifers and site wells or estimate properties of the system
• Thus providing information on which economic exploitation of the resource can be based.
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
ParametersParametersActual
• Temperature• Porosity• Permeability• Chemical content
of fluid (salinity)• (Pressure)
Measured• Temperature (°C)• Electrical resistivity (Ωm)• Magnetisation (Vs/m2)• Density (kg/m3)• Seismic velocity (km/s)• Seismic activity• Thermal conductivity (W/mK)• Streaming potential (V)
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
MethodsMethods
Direct• Thermal methods • Electrical methods• SP
Structural / indirect• Magnetics• Gravity• Seismic methods• Seismicity
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
ApproachApproachCombine 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, what about
geology and geochemistry?Success of a survey• Success is best measured by time, effort and money the
survey has saved.
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
Thermal methodsThermal methods• Direct measurements of temperature and heat. No methods correlate better with
the properties of the geothermal system.
• Heat exchange Conduction - atomic vibrations, important for transfer of heat
in the earth's crust. Convection - transfers heat with motion of mass, natural
circulation of hot water. Radiation - not in geothermal
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
Thermal conduction Thermal conduction –– Heat flow Heat flow –– ConvectionConvectionThe simplified geothermal relationship is (conductive heat
transfer only):Qcond-z = - k )T/ )z
where k is the thermal conductivity (W/m°C) and )T/ )z the thermal gradient
Anomalous values, above 80-100 mW/m2, may indicate geothermal conditions in the subsurface
Thermal conductivity of rocks ranges between 1 and 5 W/mKConvection
Free, driven by density gradientsForced, driven by an external pressure gradient, like hydrostatic head
Geothermal systems are of mixed type
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
ApplicationApplication
Thermal distribution at the surface• Detailed mapping• 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
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
ÁÁsgardursgardur –– thermal mapthermal map
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
HvalfjHvalfjöördurrdurgradient mapgradient map
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
Electrical methodsElectrical 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 at the surface.
• MT, current is induced by the time variations in earth's magnetic field - the signal measured is the electromagnetic field at the surface.
• TEM, current induced by a time varying magnetic field from a controlled source - the monitored signal is the decaying magnetic field at surface.
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
ResistivityResistivityOhm’s law
Ē = D jE is electrical field strength (V/m)j is current density (A/m2)D is electrical resistivity (Sm) - material constant
For a unit cube/bar, resistivity is defined asD = V / I
The reciprocal of resistivity is conductivityMost rocks are resistive, conduction is through water in pores
and at water-rock contact
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
Resistivity of water bearing rocksResistivity of water bearing rocksControlled by:• Porosity and pore structure
Intergranular – sedimentsJoints-fissures - tension, cooling - igneous rocksVugular – dissolved material, gas - volcanics, limestone
• Alteration (water-rock interaction)• Salinity of the water• Temperature• Amount of water - saturation - steam content• Pressure
Electric conduction is mainly throughinterconnected water-filled pores.
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
ALTERATION RESISTIVITY TEMPERATURE
Rel. unalteredSmectite- zeolite zoneMixed layer clay zoneChlorite zoneChlorite-epidote zone
Pore fluidconduction
Mineralconduction
Boilingcurve
Amb.temp
Freshwater
Salinewater
Resistivity Structure summarised
50-100°C
230-250°C
250-300°C
OOResistvityResistvity structure summarizedstructure summarized
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
DC methods DC methods Sounding Sounding -- ProfilingProfiling
Sounding - centre fixed, electrode spacing varied used for mapping resistivity changes with depth
Profiling - electrode distances fixed, whole array moved in profile line - for mapping lateral changes
Many 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
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
DC methods DC methods ––Schlumberger Schlumberger
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
ReykjahverfiReykjahverfiResistivity at 500 m Resistivity at 500 m b.s.lb.s.l..
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
ÖÖxarfjxarfjöördurrdurSchlumberger resistivity crossSchlumberger resistivity cross--section section
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
HeadHead--on profilingon profiling
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
Electromagnetic methodsElectromagnetic methodsNatural-source electromagnetics - MT, AMT
Natural EM field used as an energy source. Low frequencies, 0.0001 - 10 Hz are used for deep crustal investigations, higher freq., 10 - 1000 Hz, for the upper crust.
Controlled-source electromagnetics - TEMConstant magnetic field is built up by transmitting current I through a big loop, and then I is abruptly turned off. A secondary field is induced, decaying with time. This decay rate is monitored by measuring the voltage induced in a receiver coil in the centre of the loop. Current distribution and decay rate recorded as a function of time depend on the resistivity structure.
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
TEM TEM configurationconfiguration
Transmitted current
Measured voltage
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
Hengill Hengill --TEM resistivity TEM resistivity
map at map at 600 m 600 m b.s.lb.s.l..
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
HengillHengillTEM resistivity crossTEM resistivity cross--sectionsection
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
MenengaiMenengai -- MT crossMT cross--sectionsection
4000 6000 8000 10000 12000 14000 16000 18000
Horizontal distance (m)
-6000
-4000
-2000
0
2000
Elev
atio
n (m
)
MT01 MT13 MT58 MT57 MT55 MT53 MT59 MT51MT60
15101820232731364145495359647186110122140159184
Menengai Crater
m
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
SPSPDC-component of earth’s nat.
electrical potentials
Significant anomalies
associated w. geothermal
activity
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
Structural methodsStructural 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 detect geological formations with different densities, are as such a typical structural method
Active seismic methods detect sound velocity distribution and anomalies in the earth and attenuation
Passive seismic methods detect seismic activity in the earth
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
Magnetic methodMagnetic 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 in magnetisation; characterised by the direction and magnitude of the effective magnetisation and the shape, position, properties and history of the anomalous body
• Measurements aim mainly at finding location and depth estimate of hidden intrusives 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
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
ÁÁsgardursgardur –– Magnetic mapMagnetic map
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
ÁÁsgardursgardur -- 3D magnetic map3D magnetic map
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
Gravity & density Gravity & density --Gravity measurementsGravity measurements
• Gravity measurements are based on density contrasts of rocks in the earth which lead to different gravitational force – usually measured in mgal or 10-3 m2/s
• Gravity usually shown as Bouger 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
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
ÖÖxarfjxarfjöördur rdur ––BougerBouger gravity gravity
mapmap
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
Svartsengi Svartsengi --Mean gravity Mean gravity
change change 19751975--19991999in in µµgal/yeargal/year
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
Elastic waves Elastic waves –– seismic methodsseismic 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 and permeable zones (shear wave splitting), S-wave shadow can indicate partial melt
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
ÖÖxarfjxarfjöördurrdurBougerBouger gravity & seismic crossgravity & seismic cross--sectionsection
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
Reykjanes Reykjanes Peninsula Peninsula Seismic Seismic
zonezone
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
Integrated results Integrated results --Key to Key to
understandingunderstanding-- ÁÁsgardursgardur
geothermal model geothermal model based on soil based on soil temperature temperature
measurements and measurements and magnetic mappingmagnetic mapping
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
Integrated results Integrated results --Key to Key to
understanding understanding ––Theistareykir Theistareykir
aeromagnetic map & aeromagnetic map & resistivity mapresistivity map
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
Selected referencesSelected referencesÁrnason, K., and Flóvenz, Ó.G., 1992: Evaluation of physical methods in
geothermal exploration of rifted volcanic crust. Geoth. Res. Council, Transactions, 16, 207-214.
Árnason, K., Karlsdóttir, R., Eysteinsson, H., Flóvenz, Ó.G., and Gudlaugsson, S.Th., 2000: The resistivity structure of high-temperature geothermal systems in Iceland. Proceedings of the World Geothermal Congress 2000,Kyushu-Tohoku, Japan, 923-928.
Björnsson, A., and Hersir, G.P., 1991: Geophysical exploration for geothermal resources, principles and applications. UNU G.T.P., Iceland, report 15, 94 pp.
Flóvenz, Ó.G., and Saemundsson, K., 1993: Heat flow and geothermal processes in Iceland. Tectonophysics, 225, 123-138.
Keary, P., and Brooks, M., 1992: An introduction to geophysical exploration. Blackwell Scientific Publications, Oxford, 254 pp.
ORKUSTOFNUN Kenya Short Course - LSG 14.11.2006
UNU Geothermal Training Programme
Thank youThank youfor the for the
attentionattention
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