Overview of Geophysical MethodsUsed in Geothermal Exploration
Lúdvík S. Georgsson
Director UNU-GTP
SDG Short Course IV on Exploration and Development of Geothermal ResourcesLake Bogoria and Lake Naivasha, November 13 – December 3, 2019
Surface Exploration
The objective is to obtain information about the properties of the geothermal system prior to drilling
Successful surface exploration reduces the cost at later stages in the development and thus saves a lot of money in the end
Geothermal surface exploration is a multidisciplinary task
Exploration strategy depends on many factors, including geological settings and temperature in the system
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Surface ExplorationCan Provide Information on
Temperature in the geothermal reservoir
Permeability (fractures) of the reservoir
Areal extent of the thermal anomaly
Depth to useful temperatures
Location of the up-flow zone
Chemical composition of fluid
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
Components of Geothermal Surface Exploration
• Geological mapping
• Geophysical exploration• Geochemical exploration• Environmental impact
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Measuring Earth´s Physical Properties
• Geophysical exploration of geothermal resources deals with measurements on the physical properties of the earth.
• Emphasis is on parameters sensitive to temperature and fluid content of the rocks.
• Aim is to delineate a geothermal resource, outline a production field, locate aquifers and site wells or estimate the properties of the system.
• Ultimately the information is intended to be used for economic exploitation of the resource.
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
Parameters
Reservoir
• Temperature
• Porosity
• Permeability
• Chemical content of fluid (salinity)
• (Pressure)
Measured
• Temperature (°C)
• Electrical resistivity (Ωm)
• Magnetization (Vs/m2)
• Density (kg/m3)
• Seismic velocity (km/s)
• Seismic activity
• Thermal conductivity (W/mK)
• Streaming potential (V)
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Methods
Direct
• Thermal methods
• Electrical methods
• SP
Structural / indirect
• Magnetics
• Gravity
• Seismic methods
• Seismicity
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
ApproachCombine methods
• No method universally applicable
• Different for low-temperature and high-temperature systems
• Choose carefully
• Usually two or more methods give better results
• Different approach may suit different countries
• Important to be ready to improvise or try new methods
Integrated 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 may have saved
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
Thermal Methods
• Direct measurements
of temperature and heat. No method correlates 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
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
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, possibly with terrain corrections
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
Reykjanes Field - 2004Soil Temperature at 15 cm depth
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
Reykjanes Field - 2007Soil Temperature at 15 cm Depth
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
Ásgardur – Thermal Map
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StykkishólmurGradient Map
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Electrical / Resistivity 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.
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
ResistivityOhm’s law
E = ρ j
E is electrical field strength (V/m)j is current density (A/m2)ρ is electrical resistivity (Ωm - ohmm) - material constant
For a unit cube/bar, resistivity is defined as
ρ = V / I
The reciprocal of resistivity is conductivity - σ
Most rocks are resistive, conduction is through water in pores and at water-rock contacts.
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
Resistivity of Water Saturated Rocks
Controlled by:
• Porosity and pore structure
Intergranular – sediments
Joints-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
• Pressure
Electric conduction is mainly throughinterconnected water-filled pores
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
DC Methods: Sounding – Profiling
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 and a key method for decades
• Dipole sounding or profiling, various arrays
• Wenner, not much used today
• Head-on profiling, for locating fractures
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Regional Mapping of Resistivity
Resistivity at 500 m b.s.l.
Borgarfjördur & Hvalfjördur
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
Faxaflói AreaGeothermal systems and
flow channels
Regional Mapping of Resistivity
Borgarfjördur& Hvalfjördur
Flow Patternof geothermal
water
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
Resistivity and Alteration
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m a.s.l.400 -
200 -
- 200 -
- 400 -
- 600 -
0 -
0 500 1000 1500 2000 m
ResistivityTemperature°C
Alteration
Unaltered rocks
Smectite - zeolite zone
Mixed layered clay zone
Chlorite zone
Chlorite-epidote zone
> 25 m
10 - 25 m
2 - 10 m low resistivity cap
High resistivity core
250
200
200
150
100
50
NJ-
11
NG
-7
NG
-10
Nesjavellir
Electromagnetic Methods - TEM
Controlled-source electromagnetics– TEM
In TEM, constant magnetic field is built up by transmitting current I through a big loop (grounded dipole), 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 below.
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
Hengill
TEM Resistivity Mapat 600 m b.s.l.
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TEM – 3D interpretation
• Good and regular coverage is needed and all soundings are interpreted together
• Resistivity can vary in all three directions
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Electromagnetic Methods - MT
Natural-source ElectromagneticsMT
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.
Frequency range 10-4 Hz to 10 kHz.
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
MT - Strengths & Weaknesses
• MT is a powerful method to probe deep resistivity structures
• Data collection is simple and the equipment portable• Sensitive to cultural noise (power lines etc.)• Probes a large volume of rocks and is therefore sensitive to
3D resistivity variations• Consequently detailed interpretation is difficult (2D or even
3D interpretation may be needed)• Good to use in combination with TEM to enhance general
depth penetration• MT combined with TEM is now State of Art for resistivity
measurements in high-temperature geothermal fields
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
Menengai, Kenya - MT Cross-section
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Structural Methods
Reflect the structure of the rock – not the temperature
Magnetic methods are used in geothermal exploration often together with gravity and refraction in mapping geological structures - based on varying magnetisation in rocks
Gravity surveys are widely used in geothermal exploration to detect geological formations with different densities, and 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
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
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 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 above ground and with 100 m between lines
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
Ásgardur – Magnetic MapMeasured at 2.5 m elevation
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Ásgardur - 3D Magnetic Map
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Chinameca, El Salvador
Magnetic Map&
Structures
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Gravity & DensityGravity 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 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
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
MiravallesCosta Rica
BouguerGravity Map
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Svartsengi
Mean gravity change 1975-1999
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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 and permeable zones (shear wave splitting), S-wave shadow can indicate partial melt, brittle – ductile transition in the rocks informs on temp.
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
Olkaria
Event distribution
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG188000 192000 196000 200000 204000 208000-8
-7
-6
-5
-4
-3
-2
-1
0
Dep
th [
km
]
OWF OCF NEF-DOMES
Horizontal error
0 2000 4000 M
[b]
Vertical error
DM1KM5 OS1
KR6DM4
WE2 DM5
KM6EP4
NE2
KM8 CE1NE3 DM2
KM2KR2
KM7
DM3
188000 192000 196000 200000 204000 208000
989
60
00
99
00
00
09
90
40
00
99
08
00
0
Gri
d N
ort
hin
gs
[m
]
0 2000 4000M
[a]
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
Miravalles, Costa RicaEvent Distribution
Reykjanes Peninsula – Seismic Zone
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Integrated Results –the Key to
Understanding
Ásgardur geothermal model
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Conceptual Model of Olkaria, Kenya
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Selected References
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.
Flóvenz, Ó.G., Hersir, G.P., Saemundsson, K., Ármannsson, H., and Fridriksson Th., 2012: Geothermal energy exploration techniques. In: Syigh, A., (ed.), Comprehensive renewable energy, vol. 7. Elsevier, Oxford, 51-95.
Georgsson, L.S., 2013: Geophysical methods used in geothermal exploration. Paper written for Short Course VIII on Exploration for Geothermal Resources, organized by UNU-GTP, GDC and KenGen, at Lake Bogoria and Lake Naivasha, Kenya, 16 pp.
Hersir, G.P., and Björnsson, A., 1991: Geophysical exploration for geothermal resources, principles and applications. UNU-GTP, Iceland, report 15, 94 pp.
Keary, P., and Brooks, M., 1992: An introduction to geophysical exploration. Blackwell Scientific Publications, Oxford, 254 pp.
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
Thank youfor the
attention!
Vellir geyser
SDG Short Course IV on Exploration and Development of Geothermal Resources Nov. 21, 2019 / LSG
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