A geothermal system with chloride hotsprings and fumaroles is always prioritized during the exploration stage. These thermal manifestations bring the deep reervoir fluids to the surface, and they are useful to study the reservoir. Bicarbonates warm springs are not so useful since they bring no information about the deep fluids. On the other hand a solfatara field with a massive sulphur sublimation is not favorable since it exhibits the presence of undeground acidic fluids. Once a high enthalpy benign system is shown by the fumaroles and the chloride boiling springs, a detailed structural geology mapping with remote sensing assist is performed. A magnetotellurics (MT) survey is also deployed to image the geometry and the location of the reservoir based on their resistivities. Two third of the MT stations should be located over the prospect, while the remaining one third is deployed outside the prospect to image the background. A gravity survey is performed to image the density structures and to interpret the general structure of the geology. Both MT and gravity datasets are then modeled through 3-D join inversions. When the size of the prospect is considered commercial, exploration wells are then proposed.
At the early stage, the budget for geophysical works is often tight, thus the number of the MT stations becomes limited. In this case, it is better to align the stations into two or three lines. Sections with reasonable numbers of MT sites showing clear subsurface images are more useful when compared to many MT lines with less MT sites on each section. The figure below shows an MT survey consisting just tens of sites. Later, when interesting figures are observed, and the management is willing to put more money, more MT stations can be collected that leads to 3-D inversions.
An example of an MT survey with limited budget. MT sites shown by dots.
Magnetotellurics are electromagnetic waves that originate high in the atmospehere, travel down through the air, and penetrate deep into the earths' interior. The waves are utilized for the studies of the earth's structure. And among them is for geothermal prospecting. Geothermal systems have a distinctive clay cap made of mostly smectites over a hot reservoir. This electrically conductive smectites keep most of the heat inside the reservoir. Geothermal explorations seek for this conductive layer together with the underlying reservoir through magnetotelluric studies. The method measures electric and magnetic fields at the earth surface in time series. The records are then converted into frequency based records, and with some signal processing works an apparent resistivity curve as a function of frequency is produced. This curve is then modeled to resolve the underground resistivity structure. The following table shows a simple layered-earth structure together with the corresponding MT curves. The content of the table could be modified and the Run button simulates the outputs. Have fun! Imam Raharjo.
The amount of funds in the geothermal development stage is huge when compared to the explorations phase, therefore the development should be based on a successful exploration. A fortunate exploration work demonstrates the size of the geothermal resource, the nature of the permeability, the thermodynmics of the reservoir, and the fluid content. This findings will secure the development. On the other hand, saying NO for development is also a successful exploration. This statement will prevent a massive loss. A doubtful YES for development shows a poor exploration work. It carries many un-answered questions in the exploration phase into both technical and a financial problems in the development stages.