Overview of geophysical methods used in geophysical ... 2006/0301LudvikGeorgsson02.pdf · Overview...

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



The role of the The role of the geophysicistgeophysicist



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.



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)



MethodsMethods

Direct• Thermal methods • Electrical methods• SP

Structural / indirect• Magnetics• Gravity• Seismic methods• Seismicity



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.



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



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



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



ÁÁsgardursgardur –– thermal mapthermal map



HvalfjHvalfjöördurrdurgradient mapgradient map



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.



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



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.



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



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



DC methods DC methods ––Schlumberger Schlumberger



ReykjahverfiReykjahverfiResistivity at 500 m Resistivity at 500 m b.s.lb.s.l..



ÖÖxarfjxarfjöördurrdurSchlumberger resistivity crossSchlumberger resistivity cross--section section



HeadHead--on profilingon profiling



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.



TEM TEM configurationconfiguration

Transmitted current

Measured voltage



Hengill Hengill --TEM resistivity TEM resistivity

map at map at 600 m 600 m b.s.lb.s.l..



HengillHengillTEM resistivity crossTEM resistivity cross--sectionsection



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



SPSPDC-component of earth’s nat.

electrical potentials

Significant anomalies

associated w. geothermal

activity



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



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



ÁÁsgardursgardur –– Magnetic mapMagnetic map



ÁÁsgardursgardur -- 3D magnetic map3D magnetic map



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



ÖÖxarfjxarfjöördur rdur ––BougerBouger gravity gravity

mapmap



Svartsengi Svartsengi --Mean gravity Mean gravity

change change 19751975--19991999in in µµgal/yeargal/year



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



ÖÖxarfjxarfjöördurrdurBougerBouger gravity & seismic crossgravity & seismic cross--sectionsection



Reykjanes Reykjanes Peninsula Peninsula Seismic Seismic

zonezone



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



Integrated results Integrated results --Key to Key to

understanding understanding ––Theistareykir Theistareykir

aeromagnetic map & aeromagnetic map & resistivity mapresistivity map



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.



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