Geothermal handbook: PlanninG and Financing power Generation t e c h n I c a L r e p o r t 2 / 2

Seismic exploration,
well known and used in the oil and gas industry, is a geophysical
method that uses “waves” from the surface to map subsurface structures like faults
and cracks, which are important because they often are conduits for hot steam and
fluids. Most drillings for geothermal resources would be targeted to hit at least one
subsurface fault; by using directional drilling methods, it is possible to hit more than one
F I g u r E 2 . 2
Resistivity Cross Section through a Geothermal Field in Iceland
Source | ICEIDA 2010.
l
l
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l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
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250
200
200
150
100
50
NJ-1
1
NG-7
NG-1
0
Resisvity
Alteraon
Unaltered Rocks
Smecte-Zeolite Zone
Mixed-layer clay Zone
Chlorite Zone
Chlorite-Epidote Zone
>25 m
10 - 25 m
2 - 10
 
m
,
Low-resisvity cap
High-resisvity core
Elevaon (m a.s.l.
)
NW
SE
Distance (m)
400 -
200 -
- 200 -
- 400 -
- 600 -
0 -
0
500
1000 
1500
2000

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C h a p t e r 2
At this stage, the pre-feasibility study initiated in early Phase 1 is finalized. Interpretation of old 
information and results from new surveys are used to develop a preliminary reservoir model, which 
estimates reservoir properties, such as permeability, flow parameters, temperatures, thickness, and 
areal extent.
After completing the surface exploration, the first step in evaluating the resource is to carry out 
a volumetric resource assessment, which can be improved with information gained from the test 
drillings during Phase 3. For this purpose, probabilistic simulations (e.g., Monte Carlo) are often 
applied to perform the volumetric assessment. Long term testing of productive exploration wells will 
define the expected productivity of future wells, as well as yield information on the pressure response 
(drawdown) of the reservoir to fluid production. The pressure response can be used for lumped 
parameter modeling of the reservoir to predict the future response of the reservoir during utilization. 
This is necessary to plan for the next steps in developing the geothermal resource; making the first 
estimate on potential; and deciding where to focus the work within the exploitation license area. At 
the conclusion of this stage, a detailed geo-scientific report is developed covering the explored 
area, including a conceptual model of the geology of the geothermal field. The report should present 
recommendations as well as preliminary development strategies for the area.
29 
Costs for Phase 2 (i.e., for conducting MT’s, TEM’s, seismic or drilling gradient holes) depend on the 
size and accessibility of the geothermal site and the availability of necessary tools and equipment. 
While minimum exploration costs for a geothermal site would in many cases be US$1 to 2 million, each 
gradient well could add US$ 0.5 to 1 million to that figure. Since all geothermal fields and projects are 
different, it is difficult to generalize the required investment costs for Phases 1 and 2.