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TRAINING ON SURFACE EXPLORATION STUDIES FOR GEOTHERMAL RESOURCES AND
DEVELOPMENT OF CONCEPTUAL MODELS
UNDER THE AUSPICES OF INTERIM PROJECT COORDINATION UNIT OF THE AFRICA
GEOTHERMAL CENTER OF EXCELLENCE
OVERVIEW OF GEOPHYSICAL METHODS USED IN GEOTHERMAL EXPLORATION
Antony Wamalwa, Geothermal Development CompanyEmail:- [email protected]
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Source of Heat- Magma?
Porosity and Fracture permeability
Fluid Circulation –Thermal fluids and Recharge
How deep is the hydrothermal reservoir (base of cap)
GEOTHERMAL SYSTEM
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GEOPHYSICAL METHODS
To detect and delineate geothermal resources
Location of exploitable reservoirs and the sitting of drillholesthrough which hot fluids at depth can be detected
Estimate properties of the system (temperature, permeability, heat sources etc.)
Geophysics together with geology, geochemistry , temp measurement will contribute to the conceptual model
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WHY GEOPHYSICSInvestigate the physical chemical properties of the rocks
What we measureDensityMagnetic susceptibilityAcoustic wave velocityElectrical resistivity
What we want TemperaturePermeability and porosityFluid content/Medium (heat transfer)
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GEOPHYSICS DATA
The goal is to develop a conceptual model for geothermal system
Data can be processed though inversion or forward modelling
Geophysical information can be presented as
Anomaly maps2D cross-sections3D blocks
A=B+CQuestion:- If A=10 what is B and C
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SURFACE GEOPHYSICAL EXPLORATION METHODSPotential Field Methods Gravity Methods Magnetic Methods
Resistivity Methods Direct Current Methods (DC) Transient Electromagnetic Methods (TEM) Magnetotelluric Methods (MT) Spontaneous Potential (SP) etc
Seismic Methods Passive Seismic (Earthquake ) Active Seismic (Refraction,Reflection)
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GEOPHYSICS DATA PROCESSING
Data processing can be done singly of together
Data processed to describe the subsurface geology
All geophysical data must be constrained to make geologic sense
For geothermal exploration the data should show
• Where is the fractured reservoir
• Where is the cap rock
• Where is the heat for the system
• How if fluid flow
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POTENTIAL FIELD METHODSGravity method
Magnetic Method
Gravity Method is guided by the Newton's Law of attraction between two bodies
Considers the Earth Magnetic field
Geothermal exploration takes advantage of the alteration caused by the interaction of thermal fluids with the volcanic rocks
Although not widely used in geothermal exploration
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GRAVITY METHODS
Gravity Meter and Global Positioning System Magnetometer
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GRAVITY METHODS
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Gravity Data Analysis
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Gravity Data Analysis
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Gravity Data Analysis
Gravity plot
along the Kenya
rift
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Gravity Data Analysis
Gravity model along the rift valley
High anomalies associated with rift volcanoes
Heavy rocks may imply the heat source for the geothermal system
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Gravity data Analysis
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Gravity data Analysis
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Gravity Data Analysis
Band pass filtered
gravity data
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Gravity Data Processing
3D density Model from gravity data
The density was allowed to vary from 2.4g/cc to 3g/cc
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Gravity Data Analysis
Coso geothermal field
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Gravity Data Analysis
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Magnetic Methods
Magnetic data interpretation can be complicated
Not common in geothermal exploration although it is easy and cheap reconnaissance tool.
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RESISTIVITY METHODS
Investigate for-
• Fluid distribution -liquid/vapor (liquids-low
resistivity, Vapor-high resistivity)
• Hydrothermal alteration and mineral distribution (High and low resistivity)
• Lithology and geological structures/tectonic features (resistivity gradients)
• Regions with magmatic material (low resistivity)
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RESISTIVITY METHODSDirect Current (DC) Methods
Transient Electromagnetic (TEM)
Magnetotelluric (MT) Method
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DC Resistivity
Vertical Electrical Sounding (VES)
DC resistivity is good for very shallow crustal analysis
The penetration depth if only 1/3 of the distance between and A and B
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Time domain Electromagnetic
Receiver coil
Transmitter loop
Transmitter
Receiver
Current is artificially induced in the ground by time varying magnetic field fromcontrolled source into a loop laid on the ground.
The monitored signal is the decaying magnetic field at surface with a receiver atthe center of the loop.
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Magnetotelluric (MT)
x
y
z
Magnetic coils
Electric
dipoles
EH
High frequencies (short periods) probe
shallow depths.
Low frequencies (long periods) probe deep
The resistivity is resolved by the ratio of
orthogonal electric and magnetic fields to
give impedance tensor.
H
Earg;
H
E12
aa
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MT and TEM Equipment deployment
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Resistivity Data Analysis
Quality assurance is key in data collection In MT good site selection Ensure good electrode contact with
the ground Management of Static shift on MT
data
Quality assurance
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Resistivity Interpretation for High Temperature geothermal system
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Resistivity of Menengai caldera
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Resistivity data analysis 1D, resistivity analysis
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Resistivity data analysis 2D, resistivity analysis
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Resistivity data analysis 1D, 2D and 3D resistivity analysis
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SEISMIC METHODSWave propagation
TermsTravel timeArrival timeOrigin time
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SEISMIC METHODSActive Source Seismic Method
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SEISMIC METHODSPassive Source Seismic
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SEISMIC METHODSQuality Check - Instrument Response
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SEISMIC METHODSPicking of seismic phases and magnitude estimation
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SEISMIC METHODSWhy consider Seismic methods in geothermal exploration and development
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Micro-seismic Data Analysis
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Seismic Data Analysis
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Seismic Ray Tracing
Seismic Data Analysis
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Seismic Data Analysis
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Seismic Data Analysis
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Joint Interpretation Geophysical Data
Resistivity and seismic event location- Menengai Kenya
Joint interpretation of micro-seismic data and resistivity
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Joint Interpretation Geophysical Data
Gravity and seismic event location- Menengai -Kenya
Joint interpretation of micro-seismic events and gravity anomaly map
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Joint Interpretation Geophysical Data Joint interpretation of resistivity and micro-seismic data
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Joint Interpretation Geophysical Data Joint interpretation og gravity, microseismic and resistivity data
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Development of Conceptual Model
Information to input in the conceptual Model
Depth to the heat source:- aseismic zone, deeper low resistivity zone and a denser material at depth
The geothermal reservoir zone:-shallow intense seismic zone, fairly high resistivity (30-70ohm.m) and lower density areas
The Clay cap : shallow low resistivity zone
![[GEOTHERMAL RESOURCE] 10-PM...[GEOTHERMAL RESOURCE] 10-PM Geothermal Reservoir Monitoring Using Multi-geophysical Survey Techniques Tsuneo ISHIDO Institute for Geo-Resources and Environment,](https://img.pdfslide.us/doc/110x75/5f23860250a1a91a656a1312/geothermal-resource-10-pm-geothermal-resource-10-pm-geothermal-reservoir.jpg)
