Question 10 of 10: The Magnetotelluric Method — Principles, Equipment and Interpretation

Question 10: The Magnetotelluric Method — Principles, Equipment and Interpretation (16.66% of paper)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Basic principles. The magnetotelluric (MT) method uses naturally occurring, time-varying electromagnetic fields as its energy source, rather than a controlled transmitter — low-frequency fields (below roughly 1 Hz) originate mainly from solar-wind/ionospheric (magnetospheric) current systems, while higher frequencies are driven by global lightning activity trapped in the earth–ionosphere waveguide. These natural, essentially horizontal, time-varying magnetic fields induce telluric (electric) currents in the conductive earth by Faraday induction; the resulting orthogonal electric ($E$) and magnetic ($H$) field components measured together at surface are related by the earth's impedance, from which apparent resistivity is derived (Cagniard's relation): $\rho_a=\dfrac{1}{5f}\left|\dfrac{E}{H}\right|^2$, with $f$ the frequency in Hz, $E$ in mV/km and $H$ in nT. A key property exploited by the method is that the electromagnetic skin depth increases as frequency decreases, $\delta\approx503\sqrt{\rho/f}$ metres, so low frequencies (long periods) probe progressively deeper into the earth — making MT the standard tool for imaging resistivity structure from a few hundred metres down to tens or hundreds of kilometres, well beyond the reach of any active-source electrical method.

Equipment. Two orthogonal, non-polarizing electrode pairs laid out on the ground (typically 50–100 m dipoles) measure the two horizontal components of the telluric electric field; two or three orthogonal induction coil magnetometers (or fluxgate/ring-core sensors for the lowest frequencies) measure the corresponding horizontal (and often vertical) magnetic field components; all channels are logged simultaneously and continuously by a broadband data logger with GPS timing, typically for many hours to days per site to capture the full range of periods needed (from fractions of a second up to thousands of seconds). A second, simultaneously recording "remote reference" station some distance away is commonly used specifically to identify and remove locally generated noise (cultural EM noise, powerlines) that is not part of the true natural source field, since it should be incoherent between the two widely separated stations while the genuine MT signal is coherent.

Interpretation. Time-series $E$ and $H$ records are Fourier-transformed and combined (via robust cross- and auto-power spectral processing) into an apparent-resistivity-versus-period and phase-versus-period sounding curve at each site, forming the full impedance tensor (which also carries directional/dimensionality information — a scalar impedance for a 1-D layered earth, or a full tensor revealing 2-D/3-D structure and geoelectric strike). These sounding curves are then inverted — as a simple 1-D layered model for a first pass, or with 2-D/3-D inversion codes for structurally complex settings — to recover a resistivity-versus-depth (or resistivity cross-section/volume) model of the subsurface, which is cross-validated against independent geological, well-log or other geophysical constraints before being interpreted in terms of crustal structure, faulting, fluid/melt content, or targets such as deep mineralization or geothermal reservoirs.