Questions: Magnetotelluric Methods and Electromagnetic Induction

Question 1

An MT survey aims to image the mantle at depths of 100+ km. Compared to a shallow crustal survey, this deep survey should emphasize which signals?

• A. Higher-frequency signals, because they carry more energy and travel faster through rock
• B. Lower-frequency signals, because electromagnetic skin depth increases with decreasing frequency, allowing deeper penetration
• C. The same frequency range, because depth penetration in MT is controlled by electrode spacing rather than frequency
• D. Higher-frequency signals, because the mantle is more conductive than the crust and attenuates low frequencies

Answer: (B)

Skin depth — the depth at which EM signal amplitude decays to 1/e of its surface value — is proportional to the square root of (resistivity / frequency). Lower frequencies penetrate more deeply because they vary more slowly and are less attenuated by the conductive Earth. A survey targeting the mantle at 100 km depth requires periods of hundreds to thousands of seconds, while a shallow crustal survey uses periods of seconds to tens of seconds. This frequency-depth relationship is the fundamental operating principle of MT.

Question 2

At a site above a volcano, apparent resistivity drops sharply at frequencies corresponding to ~10 km depth. What is the most likely geological interpretation?

• A. A rigid cold lithospheric block is preventing fluid migration at that depth
• B. A high-conductivity anomaly — possibly a magma chamber or a zone of aqueous fluids — exists at approximately that depth
• C. The signal at those frequencies is contaminated by electromagnetic noise from electrical infrastructure
• D. A resistive intrusion at depth is displacing the surrounding conductive country rock

Answer: (B)

MT measures apparent resistivity as a function of depth-proxy (frequency). A drop in apparent resistivity indicates a conductive layer at the corresponding depth. Beneath a volcano, high conductivity at ~10 km depth is the classic signature of partial melt or fluid-saturated rock — either a magma chamber or a zone of hydrous fluids released by dehydration of the descending slab. Dry rock is highly resistive; the presence of melt or saline fluids lowers resistivity by orders of magnitude, making MT particularly sensitive to magmatic and volcanic systems.

Question 3

Magnetotelluric surveys require an artificial electromagnetic transmitter to generate the fields used to probe subsurface conductivity.

• T. True
• F. False

Answer: False

MT is a passive method — it uses natural electromagnetic fields generated by two external sources: solar wind interactions with the magnetosphere (producing long-period, low-frequency signals) and global lightning activity (producing higher-frequency signals in the 'audio' MT range). This is a major practical advantage: no heavy transmitter equipment is needed, and the natural sources provide signals at all useful frequencies simultaneously. Active-source EM methods (like controlled-source MT) do use transmitters for specific applications, but standard MT does not.

Question 4

In magnetotelluric surveying, low-frequency measurements probe deeper than high-frequency measurements because electromagnetic skin depth increases with decreasing frequency.

• T. True
• F. False

Answer: True

The skin depth formula δ ≈ 503√(ρ/f) meters (where ρ is resistivity in Ω·m and f is frequency in Hz) shows that δ increases as frequency decreases. A period-1000-second signal (0.001 Hz) penetrates roughly 30 times deeper than a period-1-second signal in the same rock. This is why MT sounding curves — apparent resistivity vs. period — function as depth profiles: short periods sample shallow structure, long periods sample deep structure.

Question 5

Explain how the frequency-dependent penetration depth of electromagnetic waves allows a magnetotelluric survey to image conductivity structure at multiple depths using only surface measurements.

Model answer: Electromagnetic skin depth increases with decreasing frequency: low-frequency (long-period) signals penetrate deeply before attenuating, while high-frequency (short-period) signals are attenuated within shallow depths. By simultaneously recording electric and magnetic fields across a broad range of frequencies at the surface, MT captures the integrated response from different depth intervals — each frequency band carrying information about conductivity at the corresponding skin depth. Computing apparent resistivity and phase from the impedance tensor at each frequency produces a sounding curve that is effectively a depth profile: short-period data constrains shallow conductivity, long-period data constrains deep conductivity.

This is directly analogous to how seismic surveys use different frequency bands to image at different depths, but the physical mechanism is electromagnetic induction rather than wave propagation. The practical result is that a single MT station, deployed for hours to days while recording natural field variations, simultaneously gathers information spanning depths from meters to hundreds of kilometers.