Controlled Source Electromagnetics (EM) is a method that uses electromagnetic fields transmitted from artificial sources to study the subsurface geology and it is used to map conductive targets, such as ore bodies, saltwater intrusions, clay lenses and pipelines.  The main advantage of Controlled Source EM is that it gives the interpreter the ability to model the location, depth and geometry of a target with a precision greater than any other geophysical method.

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In this method, an inducing magnetic field is generated by transmitting time varying electric currents through a loop of wire.  This inducing field propagates through the earth until it encounters a conductive body.  To repel this invasion, electric currents are created on the surface of the conductor to create a secondary field that is equal and opposite to the inducing field within the conductor.  Over time, these inductive currents start to lose energy to heat, and as they lose this energy, the secondary field decays.  The higher the conductivity of the target, the less energy gets lost to heat, and the lower rate of decay of this secondary field.

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If enough spatially distributed measurements of this secondary field can be collected, the size and geometry of the induced current paths that sit on the conductive target can be modelled. If the full decay of this secondary field is observed, the conductivity of the target can also be modelled.

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In Airborne EM the survey platform for the primary inducing magnetic field is typically either a helicopter or a fixed wing aircraft.  Currently Helicopter EM is dominating base metal exploration as Helicopter EM systems travel slower, and lower to the ground, yielding better signal to noise ratios and a better depth of penetration.  

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EM Processing, Anomaly Picking and Inversion

EM processing involves separating the primary from the secondary fields, and enhancing the signals while identifying and removing sources of noise.  At Platform Geoscience, we scour through the data on a profile-by-profile basis, to identify and characterize all anomalous signals that could represent geologic targets. Any anomaly that is identified as being interesting by the client is then modelled by hand to derive a potential target geometry.

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Inversion is a technique using forward modelling, that estimates the distribution of physical properties in the subsurface to understand the observed data.  A model of the subsurface target area is built using known geological information, and the geophysical response is calculated. The model’s response is compared with the observed data to test and refine the model’s accuracy to an acceptable standard.

 

Platform Geoscience has over 75 years of combined experience interpreting Airborne EM data.  With its wealth of knowledge, the team at Platform knows what to be looking for when interpreting data.   The Platform team also uses a unique approach, developed over its years of firsthand experience modelling EM data from hundreds of thousands of line kilometers collected on and above the surface, when developing inversion models.