Geophysical Exploration for Geothermal Pre-Feasibility Studies in Nevis

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Geophysical Exploration for Geothermal
Pre-Feasibility Studies in Nevis

• Gravity and Self Potential Geophysics

By Frank Dale Morgan Geo-Caribes Technical Team
Massachusetts Institute of Technology
March 2006
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Gravity and Self Potential Results In the OAS
Report
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Craddocks
Government Road
Church Ground
Bath
Stony Grove
Farms
Ocean Road
Brown Hill
Deep Harbor
Figure 1 Map of Study Area with Data Stations.
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Figure 2 SP Map (mV) from Network Method.
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Figure 3 Normalized 2D SP Current Source
Locations.
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41
38
36
40
41
47
53
46
49
54
56
54
100
Figure 4 Normalized SP Current Sources. Faults
and Cones from Huttrer (1998) and
Temperatures from GeothermEx (2004).
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Figure 5 SP Inversion Current Sources in 3D
Using Homogeneous Resistivity Assumption (10x
vertical exaggeration)
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Bath
Figure 6 Vertical W-E Section of 3D Current
Sources. Bath Springs at 1500m.
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Farm Estate
Deep Harbor
Figure 7 Vertical S-N Section of 3D Current
Sources. Farms Soufriere 2500m.
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mgal
Figure 8 Bouguer Anomaly Map (mGal). (2.6 g/cc
background)
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Figure 9 Polynomial Residual Gravity Map (mGal).
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Fault
Cone
Figure 10 Bouguer Gravity Map with Faults and
Cones from Huttrer (1998).
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Fault
Cone
Figure 11 Polynomial Gravity Map with Faults
and Cones from Huttrer (1998).
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Gravity Body
Figure 12 Possible Extent of Gravity Body From
Bouguer Map.
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Gravity Body
Figure 13 Possible Extent of Gravity Body From
Polynomial Gravity Map.
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V. High
Low
High
V. High
V. High
Low
Figure 14 Tertiary Map of Bouguer Gravity.
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V. High
Low
V. High
High
Low
Figure 15 Tertiary Map of Polynomial Gravity.
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N
N
N
N
Figure 16 Bouguer 3D Surface.
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SW
NE
Figure 17 Data Line for Gravity Modeling From
Bouguer Map.
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Depth 1250 m Width 600 m Thickness 2300 m
3.0 g/cc
2.2 g/cc
3.0 g/cc
2.2 g/cc
2.2 g/cc
Depth 200 m Width 3000 m Thickness 300 m
(2)
(1)
Figure 18 Data and Model Fits. (1) Shallow
Limit and (2) Deep Limit.
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Figure 19 Data and Model Fit for Realistic
Gravity Body.
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38
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Low Gravity
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41
47
53
46
49
High Gravity
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56
54
100
Low Gravity
Figure Possible Extent of Bouguer Gravity Body
with SP Current Sources.
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Medium Gravity
38
36
40
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High Gravity
47
46
53
49
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Low Gravity
56
100
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Figure Possible Extent of Polynomial Bouguer
Body with SP Current Sources
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Self Potentials Further Analysis
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Bath Springs
Farm Estate
3D SP Source Inversion Using Homogeneous
Resistivity Assumption (5x vertical exaggeration)
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Bath Springs
Farm Estate
3D SP Source Inversion Using 2D Resistances
Over a Homogeneous Halfspace (5x vertical
exaggeration)
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Interpolated Resistance Data (log10 Ohms)
Interpolated SP Data (mV)
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3D SP Source Inversion Using Interpolated SP
Data and Interpolated Resistances in a Single
Layer Model
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Gravity Further Analysis
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