EM Anomalies
EM Anomalies
A 3D image of the intrusive rocks at Thor (green) and the South and North Tusks shown in yellow (<70 ohm*m). The black dots are EM apparent resistivity anomalies that indicate conductive rock formations.
What Do EM Anomalies Look Like?
Across EM methods, anomalies show up as localized departures from background conductivity. These patterns are visible in airborne EM maps, conductivity-depth images, and 3D inversion models. For example, EM surveys are widely used to detect base metal sulfides by identifying conductivity anomalies generated around sulfide bodies.
Helicopter airborne magnetic and magnetotelluric survey being undertaken at Thor by Expert Geophysics. The rugged topography at Thor makes this the best method for achieving total EM coverage.
This system uses a passive electromagnetic source as the transmitter, and loops do no need to laid out on surface. The low frequencies that are collected make it possible to map the physical characteristics of rocks down to great depths - commonly 1 to 2km below the surface.
These survey are commonly followed up by ground geophysical surveys that can add greater detail to the airborne anomalies.
This system uses a passive electromagnetic source as the transmitter, and loops do no need to laid out on surface. The low frequencies that are collected make it possible to map the physical characteristics of rocks down to great depths - commonly 1 to 2km below the surface.
These survey are commonly followed up by ground geophysical surveys that can add greater detail to the airborne anomalies.
A profile (Z-Component) of an EM-37 electromagnetic survey showing time channels Z5 through Z-21. Both of these anomalies are EM responses due to massive sulfide mineralization.
What are EM Anomalies and What Do They Mean at Thor?
Measuring Electromagnetic 'Anomalies'
Electromagnetic anomalies are zones in the subsurface that respond differently to an applied electromagnetic field because they contain materials with unusual electrical conductivity or magnetic properties. In mineral exploration, these anomalies are often the first clues that something geologically interesting—possibly an ore body—is present.
There are numerous types of electromagnetic surveys, and four different types have been used at Thor. The first survey utilized a transmitter and receiver that was towed under a helicopter in the 1980's. The second type are Very Low Frequency ("VLF") surveys that are conducted using fixed VLF submarine transmitters. The Third type was done in 2007 and involved a series of large transmitter loops laid out on surface called an EM-37 Survey.
The fourth was an airborne Magnetotelluric survey that was completed in 2022. This uses global lightning activity, especially equatorial thunderstorms to produce low‑frequency EM waves that penetrate deep into the Earth. It also uses solar wind interactions with Earth’s magnetosphere, which generate ultra‑low‑frequency variations. These natural fields act as a passive, continuous transmitter. The MT instruments simply measure how the Earth responds to these fields. This is why MT can image very deep structures—the low frequencies from global lightning and magnetospheric activity penetrate tens of kilometers.
There are numerous types of electromagnetic surveys, and four different types have been used at Thor. The first survey utilized a transmitter and receiver that was towed under a helicopter in the 1980's. The second type are Very Low Frequency ("VLF") surveys that are conducted using fixed VLF submarine transmitters. The Third type was done in 2007 and involved a series of large transmitter loops laid out on surface called an EM-37 Survey.
The fourth was an airborne Magnetotelluric survey that was completed in 2022. This uses global lightning activity, especially equatorial thunderstorms to produce low‑frequency EM waves that penetrate deep into the Earth. It also uses solar wind interactions with Earth’s magnetosphere, which generate ultra‑low‑frequency variations. These natural fields act as a passive, continuous transmitter. The MT instruments simply measure how the Earth responds to these fields. This is why MT can image very deep structures—the low frequencies from global lightning and magnetospheric activity penetrate tens of kilometers.
Although the representation of EM anomalies differs based on the method being used, they can be generalized as follows:
Primary field enters the ground. A transmitter sends a time‑varying magnetic field into the Earth. This is the primary field. Conductors generate induced currents