Vgg13_geophysics For Geothermal Exploration

Geophysics for geothermal exploration SCGF603522 – Vulkanologi & Geologi Geothermal Geoscience, Faculty of Mathematics & Natural Sciences Universitas Indonesia 2018 E-mail: [email protected]

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Vulkanologi & Geologi Geothermal

References • Mussett, A.E. & Khan, M.A. 2000. Looking into the earth: an introduction to geological geophysics, Cambridge University Press: United Kingdom.

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Vulkanologi & Geologi Geothermal

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Vulkanologi & Geologi Geothermal

Implementing different types of waves

Lithologies with different properties

GEOLOGY

GEOPHYSICS

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GEOCHEMISTRY

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GEOTHERMAL CONCEPTUAL MODEL

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Vulkanologi & Geologi Geothermal

Geophysical investigations • Describe the subsurface in physical terms such as density, electrical resistivity, magnetism etc. • Geophysical investigations complements geological data & information as well to reveal current dynamic processes. • We start with: data acquisition taken along a line or traverse at an interval(s) then forming a profile. • Followed by data reduction from raw data

• Then we deduce a larger image based on its anomaly and create a model.

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Vulkanologi & Geologi Geothermal

Geophysics can help geothermal exploration • Geothermal system requires specific geological condition that allow deep circulation of ground water to extract heat from the heat source • Geophysics are useful to explore such geological condition, and can directly detect and delineate the sub-surface geothermal water

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Vulkanologi & Geologi Geothermal

Geophysical investigations targets • To assess the dimension of the reservoir (i.e. extent, thickness) • Provide information on • The structure of the reservoir (depth, upflow zones, lateral outflow) • Production zones • Natural heat balance • Geological settings of the geothermal system

EXTENT

DEPTH

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Vulkanologi & Geologi Geothermal

Geophysical parameters of geothermal systems Hot geothermal fluids • temperature • mineralization • fluid phase & gas content • fluid movement Temperature Electrical resistivity Seismic ground noise Self electrical potential (SP)

ACTIVE GEOTHERMAL SYSTEMS

Interaction of thermal fluids & host rocks • hydrothermal alteration • mineral deposition

Electrical resistivity Magnetisation Seismic velocity Density

INACTIVE GEOTHERMAL SYSTEMS

Geological features • • • •

Faults & fractures Caldera collapse Lithology & stratigraphy (deep) basement variation • Shallow/deep intrusions Electrical resistivity Magnetisation Seismic velocity Density

WITHIN GEOTHERMAL SYSTEMS

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Vulkanologi & Geologi Geothermal

Geophysical methods applied in geothermal exploration Heat flow surveys

Remote sensing

Gravity

Magnetics

DC Resistivity

Magnetotelluric (MT)

Seismic (active/passive)

Borehole geophysics

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Vulkanologi & Geologi Geothermal

Heat flow surveys • Main target is to understand heat balance of the system (in its natural state) • Techniques: • Measurement of heat discharge (i.e. convective, conductive, evaporative) from active surface manifestations (e.g. hot springs, geysers, fumaroles, steaming ground) • Estimation of heat discharge by concealed flow

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Vulkanologi & Geologi Geothermal

Remote sensing • Main target is to map the distribution of surface/shallow temperatures and geological setting and structures (local/regional) • Techniques: • Infra red imagery (satellite data, aerial surveys, shallow/surface temperatures) • Satellite and aerial photos (local/regional geology) • Spectral imaging, radar altimetry, LIDAR (Light Detection and Ranging), etc

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Density

Vulkanologi & Geologi Geothermal

• Density measurement can yield: • lithologies • faults and open cracks because the rocks are saturated with water

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Vulkanologi & Geologi Geothermal

Gravity survey • Main target is to map sub-surface density variation to obtain information on lithology and geological structures • Techniques: • Measurements of very small variations in the earth gravitational field across the study area

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Vulkanologi & Geologi Geothermal

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Vulkanologi & Geologi Geothermal

Gravity acquisition

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Vulkanologi & Geologi Geothermal

Magnetics • Main target is to detect and delineate hydrothermal alteration and obtain lithology and structures. • Techniques: • Mapping of local disturbance of geomagnetic field • Map Concealed shallow alteration • Airborne survey (it is rapid, cover details of large area and cost effective)

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Vulkanologi & Geologi Geothermal

DC (direct current) resistivity • Main target is to map low resistivity ground at shallow level • Proxy for clay content • Proxy for fluid

• Techniques: • Direct current (I) is injected into the ground and the resulting potential difference (DV) is measured obtaining resistivity of the ground • Depth of penetration is limited by the practical distance between electrodes

Resistivity = 1 / Conductivity

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Vulkanologi & Geologi Geothermal

Factors controlling resistivity • Temperature

• Resistivity of a sample rock/fluid decreases at higher temperature

• Reservoir fluid salinity

• In ionic solution, conductance is related to ionic mobility and controlled by viscosity

• Porosity & liquid saturation • Archie’s Law

• Rock matrix conductivity

• In geothermal we have clay as alteration product

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Vulkanologi & Geologi Geothermal

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Vulkanologi & Geologi Geothermal

DC Resistivity limitation • Limited depth penetration • We need long arrays to get good depth • AB/2 = 1000 m gives depth about 300m

• Requires large power sources for greater depth • Not reliable in mountainous terrain

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Vulkanologi & Geologi Geothermal

Magneto-telluric (MT) • Main target is to detect deep electrical structures • Techniques: • Recording natural electromagnetic waves (MT waves) at ground surface over a wide range of frequency. • Primary MT waves interact with the ground, producing secondary waves which depend on electrical resistivity of the ground • Depth of penetration is related to the frequency of the recorded MT waves

Vulkanologi & Geologi Geothermal

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Layered 1D models 3D Inversion model

Layered 2D models

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Vulkanologi & Geologi Geothermal

MT limitation • Relict alteration: thick and very extensive conductor, but slightly less conductive than upflow zone • Hydrologically controlled conductor: flat lying conductive body but not real convecting geothermal system • Structurally controlled reservoir (e.g. Sumatra): shows different patter, less permeability, upflow focus along small number of faults

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Vulkanologi & Geologi Geothermal

Seismology • depends upon seismic waves, which travel in the Earth. • The amplitude of seismic waves changes for two main reasons. • One is that the wave front usually spreads out as it travels away from the source and, because the energy in it has to be shared over a greater area, the amplitude decreases. • The second is when some of the wave energy is absorbed. This occurs if the rock is not fully elastic.

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Vulkanologi & Geologi Geothermal

Seismic: active & passive • Main target is to map velocity structure using sub-surface seismic and locate 3D of seismic noise & micro-EQ • Techniques: • Active seismic (shot points) • Refraction seismic – sub-surface seismic • Reflection seismic – velocity structures

• Passive seismic (natural seismic signals) • Seismic ground noise (tremors) – deep thermal fluid activities • Micro-EQ survey – identification of fractures and ground movement direction • Seismic tomography – sub-surface velocity structures

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Vulkanologi & Geologi Geothermal

Micro-EQ survey • Clutters of micro-EQ data in geothermal survey will point out the location of geothermal reservoir. • Boiling process occurring in the reservoir causes the rock properties to change and

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Vulkanologi & Geologi Geothermal

Ground Penetrating Radar

GPR Data for surface alteration of silica sinter

• Similar to seismology but rather to implement electromagnetic energy.

Lynne et.al., 2017

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Vulkanologi & Geologi Geothermal

Key benefits of geophysical investigations • Direct information on deep sub-surface structures • Cost – much less than the cost of drilling an exploration well • Detailed cover of large area • Improvements of 3D conceptual model • Contributions to reservoir modelling