Electromagnetic Methods
Electromagnetic induction lets us sense subsurface conductivity without ground contact: a transmitter coil drives currents in the earth, and a receiver measures the secondary field those currents produce. Frequency-domain (FDEM) and time-domain (TEM) systems trade depth, resolution, and productivity in different ways.
Learning Objectives
Undergraduate Core: By the end of this module, you will be able to:
- Explain primary-field induction, secondary currents, and receiver response.
- Relate frequency, conductivity, time gate, and skin or diffusion depth qualitatively.
- Compare FDEM and TEM acquisition and select an appropriate system for a target.
- Identify coupling, cultural noise, and limits of apparent-conductivity interpretation.
Graduate Extension
Distinguish qualitative diffusion-depth proxies from Maxwell-equation forward models and evaluate sensitivity, equivalence, and inversion regularization.
Practice this module Teach with active-learning slides
Interactive Lecture
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🖥️ EM System Animation Activity
How transmitter, ground response, and receiver interact in an EM system.
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🖥️ EM Waveforms Animation Activity
Primary and secondary fields, in-phase and quadrature components, and TEM decay.
Topic Apps
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🖥️ Interactive 1D EM Forward Modeling
Model FDEM and TEM responses over layered conductivity structures.
Demo
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⚡ Skin Depth Calculator & Visualizer
See how frequency and ground conductivity set the penetration of EM fields, with the classic skin-depth formula evaluated live.
Classroom Lab
🧰 FDEM plume triage and TEM bedrock comparison — use diffusion-depth teaching proxies to make survey decisions, then identify the assumptions that require a full forward model.
Deep EM Extension
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🌍 Magnetotellurics and Deep EM
Continue from controlled-source FDEM/TEM to natural-source MT, tensor impedance, apparent resistivity, phase, static shift, dimensionality, and deep-crustal interpretation.
Research Code: PyHydroGeophysX
FDEM and TEM forward modeling and inversion
The induction principles here carry directly into quantitative EM modeling. PyHydroGeophysX wraps SimPEG to run frequency-domain (FDEM) and time-domain (TDEM) forward modeling and inversion over layered and 2D conductivity structures.
- TDEM workflow: transient decay and inversion (runs in Colab).
- FDEM workflow (source): frequency-domain response and inversion.