On the detection and characterisation of magmatic brines using magnetotellurics

In the coming decades, it may be possible to exploit magmatic hydrothermal fluids for geothermal energy and the metals they contain. It is essential to find methods for assessing their location, temperature and composition.

The magnetotelluric method allows us to produce electrical conductivity maps and image the areas where these fluids reside.  Electromagnetic waves created by storms and solar winds interact with the earth and produce electrical currents in the rocks, fluids and magma that compose it. These electric currents correspond to the migration of ions under the influence of an electric field. The ion current density is proportional to the electrical conductivity of the medium, which can be very high in magmatic fluids. By recording variations in the electromagnetic field at the surface we can create conductivity maps and thus find potential reservoirs of magmatic fluids.

The conductivity of a heterogeneous medium formed of different components can be deduced from the conductivity of each of these components, their relative proportion and the geometry of the interface. For a porous reservoir filled with connected fluid, the Hashing and Shtrikman formula relates total conductivity to fluid conductivity and porosity.

Thanks to magmatic inclusions and volcanic gases, we have an idea of the elements that can be found in these fluids: Na, K, Ca, Fe, Mg, Al, B, Li, Cu, Zn, Rb, Sr, Mo, Ba, Pb. The objective is then to find the possible compositions that explain the observed conductivity given the pressure and temperature conditions and reservoir geometry.

The conductivity of a complex system can be deduced from the conductivity of simple subsystems. An example of a subsystem is the H2O-NaCl system. It is described by the dissociation reactions of NaCl, HCl, NaOH and H2O. Conductivity depends on the number of charge carriers available and is therefore governed by the equilibrium constants of these reactions. Thanks to conductivity measurements in PT these constants can be determined and the conductivity of the H2O-NaCl system can be predicted for given PTc conditions. To do this, existing theories are used, notably the Debye Huckel Onsager theory.

In an electrolyte solution at equilibrium, the charges are not distributed randomly. They arrange themselves in a way that allows the conductive fluid to be electrically neutral. When the ions are set in motion, this structure slows down their progress, resulting in a frictional force that opposes the electric driving force. Once the ion flow is stationary, the speed of an ion is proportional to the driving force and its mobility, which depends on each ion and the properties of the solvent, such as its viscosity.

We have created a database of PT conductivity measurements for the subsystems of a magmatic brine. Based on this database and existing theoretical models, we are developing a model that predicts the electrical conductivity of brine at a given pressure and temperature. This model can be used to determine the possible compositions of brine based on its conductivity. We plan to use it to characterise brines beneath Mount Pelée, Martinique.