1. Exploring the relationship between Frequency and Depth
Action: Set Layer 1 Resistivity ($\rho_1$) to a middle value like 100 Ω·m. Slowly move the Survey Frequency slider from the far right (high frequency) to the far left (low frequency).
Question to help understand: Watch the green dashed "Skin Depth" line on the cross-section. How does the penetration depth change as you lower the frequency? Based on this, if you needed to map a deep geological feature, would you tune your instrument to a high or low frequency?
2. The effect of Ground Conductivity on Penetration
Action: Set the Survey Frequency to exactly 1.0 kHz. Now, slowly move the Layer 1 Resistivity ($\rho_1$) slider from 1000 Ω·m (resistive) all the way down to 1 Ω·m (highly conductive).
Question to help understand: How does the skin depth react as the ground becomes more conductive? Imagine you are doing an environmental survey over a highly conductive saltwater spill; will your EM signal penetrate deeper or shallower compared to clean, dry soil?
3. Reading the FDEM Apparent Resistivity Chart
Action: Set $\rho_1$ = 10 Ω·m, $h_1$ = 30 m, and $\rho_2$ = 1000 Ω·m. Move the frequency slider back and forth and watch the white dot track along the green chart.
Question to help understand: At very high frequencies, the apparent resistivity curve flattens out near 10 Ω·m. At very low frequencies, it points toward 1000 Ω·m. Explain why the instrument reads mostly Layer 1 at high frequencies and mostly Layer 2 at low frequencies.
4. Diffusion Speed and Resistivity
Action: Set $\rho_1$ to 10 Ω·m (conductive) and click the Play "Smoke Ring" button. Watch the speed of the expanding orange ring. Next, change $\rho_1$ to 1000 Ω·m (resistive) and click play again.
Question to help understand: In which type of ground does the "smoke ring" (the diffusing EM field) travel faster? Why do highly conductive materials act to slow down the downward diffusion of the magnetic field? (Hint: Think about eddy currents opposing the change).
5. Detecting Layer Boundaries in the Time Domain
Action: Set $\rho_1$ = 10 Ω·m, $h_1$ = 50 m, and $\rho_2$ = 1000 Ω·m. Click Play "Smoke Ring". Keep your eyes on both the cross-section and the white dot moving along the orange "Voltage Decay vs Time" chart.
Question to help understand: Notice the exact moment the expanding smoke ring crosses the dashed boundary line at 50m. What happens to the shape and slope of the decay curve on the chart at that exact time?
6. The Influence of Layer Thickness
Action: Keep $\rho_1$ = 10 and $\rho_2$ = 1000. Play the animation once with $h_1$ = 30 m, and then play it again with $h_1$ = 150 m.
Question to help understand: How does changing the thickness of the top layer change the timing of the bend in the decay curve? How would a geophysicist use this specific time-delay in the field to calculate the depth to bedrock?