18-Geol-A7 Applied Geophysics · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2018 — 18-Geol-A7 Applied Geophysics. Three-hour, closed-book exam; approved Casio or Sharp calculator permitted. The paper offers a choice of six of the following ten questions, each worth 16.66% of the total mark, and every question requires an essay-format answer — this is a genuinely all-essay sitting with no numeric data, formula sheet, or figure supplied in the source. All ten questions are answered below so the set stands as a complete study resource for choose-N-of-M exams.
Reference texts: Telford, Geldart & Sheriff, Applied Geophysics (2nd ed.) — the primary reference for every method touched in this paper (electrical/EM methods, seismic refraction/reflection, radiometrics, magnetics, gravity, well logging); Kearey, Brooks & Hill, An Introduction to Geophysical Exploration (3rd ed.) — survey design, array geometry, data acquisition and processing; Blakely, Potential Theory in Gravity and Magnetic Applications — magnetic-mineral behaviour and gravity reduction (Q5, Q7); Selley & Sonnenberg, Elements of Petroleum Geology — well-logging tool context (Q8).
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.
Data collection. The magnetotelluric (MT) method uses the Earth's own natural, time-varying electromagnetic field as its energy source — high-frequency signal from global lightning activity (the Schumann resonance band) and lower-frequency signal from solar-wind/magnetospheric interaction with the ionosphere — rather than an artificial transmitter. At each station, two orthogonal horizontal electric-field components ($E_x$, $E_y$) are measured with pairs of non-polarizing electrodes buried in shallow pits, and the corresponding orthogonal horizontal magnetic-field components ($H_x$, $H_y$), plus usually the vertical component $H_z$, are measured with induction coils or fluxgate magnetometers. Because deeper penetration requires longer-period (lower-frequency) signal, and natural-field energy at very long periods is comparatively weak, a station targeting deep structure must record continuously for many hours to several days to build up an adequate signal-to-noise ratio at the longest periods needed.
Processing and interpretation. Time-series $E$ and $H$ recordings are converted to the frequency domain (Fourier/spectral analysis), and for each period the impedance tensor $Z$ (relating $E$ to $H$: $E=ZH$) is computed by cross-spectral analysis; using a simultaneously-recorded remote reference station (uncorrelated local noise but correlated distant natural-field signal) substantially improves the robustness of this estimate against cultural/instrumental noise. From $Z$, apparent resistivity $\rho_a(T)=\frac{T}{5}|Z|^2$ and phase $\phi(T)$ are computed and plotted against period, as above; these curves (or, for multiple stations, a full impedance-tensor dataset) are then inverted — 1-D layered inversion for a simple stratified setting, or 2-D/3-D inversion for structurally complex settings — to recover a resistivity-vs-depth (or resistivity volume) model. Static-shift correction (removing near-surface, small-scale galvanic distortion of the apparent-resistivity curve) is a standard, necessary processing step before inversion.
Case history. MT is the workhorse deep-imaging tool for geothermal exploration: a geothermal reservoir's hot, saline, and often clay-altered fluids and cap rock are markedly more conductive than the surrounding unaltered crystalline basement, producing a strong, deep resistivity low that MT resolves far better than any surface DC-resistivity method (whose penetration is limited by achievable electrode spacing) — MT survey grids are routinely used to map geothermal reservoir extent and cap-rock geometry ahead of exploration drilling. The same deep-penetration advantage is used in porphyry-copper exploration under thick, resistive volcanic cover, and in basin-scale hydrocarbon exploration to map deep sedimentary structure beneath resistive volcanic or salt sequences that attenuate seismic energy.
