Geophysics

Electrical conductivity

Magnetotellurics is an electromagnetic geophysical method used to image the geological subsurface. It works by measuring natural variations of electrical and magnetic fields.

Geological materials have a high resistivity, as they are poor electrical conductors. However, the presence of geothermal fluids increases the conductivity of subsurface materials (low resistivity).

This difference can be used to map the subsurface geology, detect faults and cap rock, and discover geothermal resources.

Seismology

Geothermal fields can be characterised by increased levels of seismicity (i.e. micro-earthquakes).

Active seismicity (studying the propagation of man-made vibrational waves through the earth) can be used to investigate fault structures, delineate geothermal fields, and model complex geological systems.

Gravity
magnetics

Gravity studies use changes in density to characterise subsurface properties, find faults, and create 3D models.

It can also be used to monitor field depletion and detect cold water invasion. Pore density is affected by fluid flow, and therefore changes the localised gravity in a subsurface material.

Elevation, latitude and weather conditions can also affect gravity measurements, so must be taken into account when collecting data.

Magnetics

An aeromagnetic survey is carried out using a magnetometer aboard or towed behind an aircraft. As different rock types differ in their content of magnetic minerals, a magnetic map shows the geological structure of bodies of rock, as well as the presence of faults and folds.

Magnetism can also identify the Curie point temperature - where crustal rocks lose their magnetisation. Areas with subsurface geothermal activity have a high temperature gradient and heat flow, so the Curie point depth for a geothermal resource will be shallow compared to surrounding non-geothermal areas.