by duygu kiyan, DIAS

Uncovering Hidden Heat beneath County Antrim

The GEMINI Magnetotelluric Survey Launches

23 June 2025

This week marks the launch of magnetotelluric (MT) fieldwork for the GEMINI project! While the 3D Consulting-geo GmbH field crew begins deployment in County Antrim, our DIAS team is setting up a base station near the low-noise environment of Gougane Barra. Over the next two months, we will be deploying MT stations at 120 sites across County Antrim, each one helping us see deep beneath the surface.

What is  Magnetotellurics?  Magnetotellurics—often shortened to MT— is a geophysical method used to explore the Earth's interior without the need for drilling. It is a passive geophysical technique, which means we do not send any energy into the Earth ourselves. Instead, we measure natural changes in the Earth’s electric and magnetic fields. These tiny, naturally occurring signals reveal a key physical property: how well the rocks beneath our feet conduct electricity.

Did you know?

The term ‘magnetotelluric’ comes from the combination of ‘magneto-’ referring to magnetic fields and ‘telluric’ meaning Earth currents.

Why does that matter? Every type of rock has a different resistivity, or resistance to electrical flow (the inverse of electrical conductivity). Some things, like salty water or metal-rich minerals, conduct electricity easily: they have low resistivity. Others, like dry granite or quartz, are much more resistant. But resistivity does not just depend on the type of rock. It is also affected by temperature and fluids in the rock’s pore spaces. That makes MT especially useful in places where these factors matter, such as geothermal areas. In other words, MT helps us locate hot water reservoirs and fractured, fluid-filled zones deep within the Earth’s crust.

How do we measure it? In the field, we set up two main types of sensors:

1. Magnetic sensors: These are wire coils, around 144cm long—called induction coils—that measure changes in the Earth’s magnetic field. We install them in three directions: north-south, east-west, and vertical.

2. Electric sensors: These are pairs of electrodes buried in the ground, spaced about 100 meters apart. They measure voltage differences caused by telluric currents—natural electric currents that flow through the Earth when the magnetic field fluctuates.

By recording how the electric and magnetic fields vary together, we can estimate resistivity beneath each station. Once all the data are collected, the next step is to create a detailed model of the subsurface.

What’s Next? The GEMINI MT survey is just getting started. Over the coming weeks and months, we will be uncovering new insights into the geothermal potential and geological structure of County Antrim.
Stay tuned for more updates, behind-the-scenes field photos, and a first look at what we are discovering beneath the surface!

Images: A magnetic coil being buried in County Antrim during last summer’s pilot study; Plan view diagram of the MT sensor layout (Schmoldt J-P. 2011, PhD Thesis)

DIAS is a GEMINI partner.