TDEM

The Crone Pulse EM system is a TDEM system, and is used exclusively by Crone and its affiliated companies for contract surveys. The Crone Pulse EM system has been developed for use in the Mineral Exploration and Mining Industries with durability, accuracy, and longevity in mind; leading to the discoveries of numerous world class mineral deposits globally.

The TDEM method is a geophysical technique in which electric and magnetic fields are induced by transient pulses of electrical current and the subsequent decay response is measured. TDEM methods are generally able to determine subsurface electrical properties, but are also sensitive to subsurface magnetic properties in applications like UXO detection and characterization. TDEM surveys are very common surface EM technique for mineral exploration, groundwater exploration, and for environmental mapping, used throughout the world in both onshore and offshore applications.

EM diagram

Two fundamental electromagnetic principles are required to derive the physics of TDEM surveys: Faraday’s law of induction and Lenz’s Law. A loop of wire is usually energized by a direct current. At some time the current is cut off as quickly and accurately as possible. Faraday’s law dictates that a nearly identical current is induced in the subsurface to preserve the magnetic field produced by the original current (eddy currents). Due to ohmic losses, the induced surface current dissipates – this causes a change in the magnetic field, which induces subsequent eddy currents. The net result is a downward and outward diffusion of currents in the subsurface which appear as an expanding smoke ring when the current density is contoured. These currents produce a magnetic field by Faraday’s law. At the surface, the change in magnetic field (flux) with time is measured. The way the currents diffuse in the subsurface is related to the conductivity distribution in the ground. This is a basic view of the physical principles involved. When conductive bodies are present, the diffusion of the transients change. In addition, transients are induced in the conductive bodies. This is only the most basic overview.

secondary field