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

Gravity and Self Potential Results In the OAS Report

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.

Figure 2: SP Map (mV) from “Network Method”.

Figure 3: Normalized 2D SP Current Source Locations.

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

Figure 5: SP Inversion Current Sources in 3D Using Homogeneous Resistivity Assumption (10x vertical exaggeration)

Figure 6: Vertical W-E Section of 3D Current Sources. Bath Springs at ~1500m.

Bath

Figure 7: Vertical S-N Section of 3D Current Sources. Farms Soufriere ~2500m.

Farm Estate~Deep Harbor

Figure 8: Bouguer Anomaly Map (mGal). (2.6 g/cc background)

mga

l

Figure 9: Polynomial Residual Gravity Map (mGal).

Figure 10: Bouguer Gravity Map with Faults and Cones from Huttrer (1998).

FaultCone

Figure 11: Polynomial Gravity Map with Faults and Cones from Huttrer (1998).

FaultCone

Figure 12: Possible Extent of Gravity Body From Bouguer Map.

Gravity Body

Figure 13: Possible Extent of Gravity Body From Polynomial Gravity Map.

Gravity Body

Low

V. High

V. High

V. High

LowHigh

Figure 14: Tertiary Map of Bouguer Gravity.

Low

Low

V. High

V. High High

Figure 15: Tertiary Map of Polynomial Gravity.

Figure 16: Bouguer 3D Surface.

N

N

NN

0

1

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3

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5

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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

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

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.

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

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

Self Potentials: Further Analysis

Farm EstateBath Springs

3D SP Source Inversion Using Homogeneous Resistivity Assumption (5x vertical exaggeration)

Farm EstateBath Springs

3D SP Source Inversion Using 2D Resistances Overa Homogeneous Halfspace (5x vertical exaggeration)

-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)

“3D” SP Source Inversion Using Interpolated SP Data and Interpolated Resistances in a Single Layer Model

Gravity: Further Analysis