Electromagnetic Survey With Magnets and Compass

This simple demonstration models how geophysicists use magnetic surveys to locate ore bodies beneath the Earth's surface. By burying small magnets under sand to represent magnetic mineral deposits, and moving a compass over the surface, students can detect hidden “ore bodies” from the deflection of the compass needle.

1. Fill a tray about 3-4 cm deep with dry sand.
2. Bury several small magnets beneath the surface at different depths and positions. Keep their locations hidden from observers.
3. Smooth the sand surface so no magnet positions are visible.
4. “Fly” the compass slowly over the sand surface, following straight survey lines across the tray (like an aerial survey).
5. Observe and record any deflection of the compass needle as it passes over certain areas.
6. Mark these positions on a simple grid map to represent zones of magnetic anomaly.
7. Once the survey is complete, uncover the magnets and compare the detected positions to the actual buried locations.

• None available.

• Use magnets of different strengths to simulate ore bodies of varying sizes.
• Bury magnets at different depths and compare the magnitude of compass deflection.
• Create a contour map of magnetic field strength using multiple compass readings.

• Handle strong magnets carefully to avoid pinching fingers or damaging electronics.
• Keep magnets away from computers, credit cards, and magnetic storage devices.
• Do not use loose iron filings — they can stick to magnets.

• How does the compass needle behave near a magnet compared to the background field?
• Why do magnetic surveys help locate buried ore bodies?
• How would the strength and depth of a real ore body affect the magnetic anomaly measured?
• What other geophysical survey methods (e.g., electrical, seismic) can detect underground structures?
• How is this model similar to and different from real airborne magnetic surveys?