_Overview of Geophysical Methods

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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The role of thegeophysicist


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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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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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MethodsDirect Thermal methods Electrical methods SP

Structural / indirect Magnetics Gravity Seismic methods Seismicity


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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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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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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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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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sgardur thermal map


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Hvalfjrdurgradient map


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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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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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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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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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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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DC methods

Schlumberger


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Reykjahverfi

Resistivity at 500 m b.s.l.


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xarfjrdur

Schlumberger resistivity cross-section


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Head-on profiling


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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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TEMconfiguration

Transmitted current

Measured voltage


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Hengill -TEM resistivity

map at600 m b.s.l.


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Hengill

TEM resistivity cross-section


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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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SPDC-componentof earths nat.

electrical

potentials

Significantanomalies

associated w.

geothermalactivity


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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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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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sgardur Magnetic map


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sgardur - 3D magnetic map


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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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xarfjrdurBouguer gravity

map


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Svartsengi -Mean gravity

change1975-1999in gal/year


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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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xarfjrdurBouguer gravity & seismic cross-section

UNU G h l T i i P

KM78000

[a]


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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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ReykjanesPeninsula

Seismiczone

UNU G th l T i i P


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Integrated results -Key tounderstanding

- sgardur

geothermal modelbased on soiltemperature

measurements,magnetic mapping

and regional survey



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Integrated results -Key to

understanding

Theistareykiraeromagnetic map &resistivity map



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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.



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Thank you

for theattention

Vellir geyser

_Overview of Geophysical Methods

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