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Geophysical Exploration for Geothermal Pre-Feasibility
Studies in NevisGravity and Self Potential Geophysics
ByFrank 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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Dee
p H
arbo
r
Ocean R
oad
Government Road
Craddocks
Farms
BathStonyGrove
Brow
n Hill
ChurchGround
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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Figure 4: Normalized SP Current Sources. Faults and Cones from Huttrer (1998) and Temperatures from GeothermEx (2004).
56
5349
54
54
47
4036
38
41
41
FaultsConesSoufrieres
50 Temp (C)
46
100
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Figure 5: SP Inversion Current Sources in 3D Using Homogeneous Resistivity Assumption (10x vertical exaggeration)
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Figure 6: Vertical W-E Section of 3D Current Sources. Bath Springs at ~1500m.
Bath
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Figure 7: Vertical S-N Section of 3D Current Sources. Farms Soufriere ~2500m.
Farm Estate~Deep Harbor
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Figure 8: Bouguer Anomaly Map (mGal). (2.6 g/cc background)
mga
l
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Figure 9: Polynomial Residual Gravity Map (mGal).
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Figure 10: Bouguer Gravity Map with Faults and Cones from Huttrer (1998).
FaultCone
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Figure 11: Polynomial Gravity Map with Faults and Cones from Huttrer (1998).
FaultCone
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Figure 12: Possible Extent of Gravity Body From Bouguer Map.
Gravity Body
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Figure 13: Possible Extent of Gravity Body From Polynomial Gravity Map.
Gravity Body
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Low
V. High
V. High
V. High
LowHigh
Figure 14: Tertiary Map of Bouguer Gravity.
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Low
Low
V. High
V. High High
Figure 15: Tertiary Map of Polynomial Gravity.
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Figure 16: Bouguer 3D Surface.
N
N
NN
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0
1
2
3
4
5
6
7
0 500 1000 1500 2000 2500 3000 3500 4000 4500
Distance (m)
Gra
vity
(m
Ga
l)
Figure 17: Data Line for Gravity Modeling From Bouguer Map.
SW NE
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Figure 18: Data and Model Fits. (1) Shallow Limit and (2) Deep Limit.
0
1
2
3
4
5
6
7
-4000 -3000 -2000 -1000 0 1000 2000 3000 4000
Distance (m)
Gra
vity
(m
Gal
)
3.0g/cc
2.2 g/cc 2.2 g/cc
3.0 g/cc
2.2 g/cc
(2)(1)
0
1
2
3
4
5
6
7
-4000 -3000 -2000 -1000 0 1000 2000 3000 4000
Distance (m)
Gra
vity
(m
Gal
)
Depth = 200 mWidth = 3000 mThickness = 300 m
Depth = 1250 mWidth = 600 mThickness = 2300 m
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0
1
2
3
4
5
6
7
-5000 -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 5000
Distance (m)
mG
al
1 Km
0.4 Km
2.25 Km
3 Km
0.2 Km
– o = 0.6 g/cc
DataModel
Figure 19: Data and Model Fit for Realistic Gravity Body.
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Figure : Possible Extent of Bouguer Gravity Body with SP Current Sources.
10056
54
54
47
40
4953
41
46
36
38
Low Gravity
Low Gravity
High Gravity
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Figure : Possible Extent of Polynomial Bouguer Body with SP Current Sources
10056 54
54
47
40
4953
41
46
36
38
High Gravity
Low Gravity
Medium Gravity
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Self Potentials: Further Analysis
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Farm EstateBath Springs
3D SP Source Inversion Using Homogeneous Resistivity Assumption (5x vertical exaggeration)
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Farm EstateBath Springs
3D SP Source Inversion Using 2D Resistances Overa Homogeneous Halfspace (5x vertical exaggeration)
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-62.63 -62.625 -62.62 -62.615 -62.61 -62.605 -62.6 -62.595 -62.59
17.115
17.12
17.125
17.13
17.135
17.14
17.145
longitude (deg)
lati
tud
e (d
eg)
Interpolated Resistance Data (log10
)
2
2.5
3
3.5
4
Interpolated SP Data (mV)Interpolated Resistance Data (log10 Ohms)
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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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