Question 5 of 10: Time-Domain vs. Frequency-Domain Geophysical Measurements

Question 5: Time-Domain vs. Frequency-Domain Geophysical Measurements (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.

Time-domain (TD) measurement. A transient (usually bipolar square-wave or trapezoidal) current is driven through a transmitter, then abruptly switched off; the receiver records the resulting decaying response over a sequence of discrete time gates after the transmitter current has stopped. For TDEM, the decaying secondary voltage induced by eddy currents in the ground is sampled at logarithmically-spaced gates; for time-domain IP (TDIP), the decaying secondary (overvoltage) potential after current shut-off is integrated to give chargeability, $M=\frac{1}{V_p}\int_{t_1}^{t_2}V_s(t)\,dt$, in ms.

Frequency-domain (FD) measurement. A continuous sinusoidal (or multi-frequency) current is driven and maintained, and the receiver measures the amplitude and phase (or in-phase/quadrature components) of the resulting field while the transmitter is still on. For FDEM, in-phase and quadrature secondary-field components are measured relative to the (much larger) primary field at one or more discrete frequencies; for frequency-domain IP (FDIP), apparent resistivity (or phase) is measured at two or more frequencies and compared — either as a percent frequency effect, $\mathrm{PFE}=100\,(\rho_{low}-\rho_{high})/\rho_{high}$, or as phase lag between current and voltage (spectral IP measures this across a wide frequency band and fits it to a Cole–Cole model).

Advantages/disadvantages — EM. TDEM's off-time measurement means it never has to separate a small secondary field from a much larger, co-located primary field (a persistent problem for FDEM), so TDEM is generally better at detecting good (highly conductive) and deep conductors, whose late-time decay persists after weak-conductor responses have died away, and one sounding's multiple time gates give a natural range of depth resolution analogous to a resistivity VES; disadvantages are a need for a powerful transmitter (a large current step to get a measurable decay) and slower per-station acquisition. FDEM instruments (e.g. the Geonics EM31/EM34 slingram systems) are simpler, lighter and can be operated continuously while walking, making them fast for shallow reconnaissance mapping (landfills, groundwater conductivity, utility location); their disadvantage is a shallower, more coupling-limited depth range and the need for multiple discrete frequencies (rather than one continuous time-gated decay) to approximate a depth sounding.

Advantages/disadvantages — IP. TDIP requires only a simple switched-DC-like waveform and one integrated chargeability number per reading, making it the more common, lower-cost field method for routine ore/porphyry exploration; it is, however, less diagnostic of the physical/chemical cause of chargeability than the frequency-domain approach. FDIP (especially spectral IP across many frequencies) resolves the full Cole–Cole decay-time-constant spectrum, which can help distinguish metallic-sulphide (electrode) polarization from clay/membrane polarization through their different time-constant/frequency behaviour — a genuine advantage for discriminating economic sulphide mineralization from disseminated clay alteration — but requires more sophisticated, phase-sensitive receiver electronics and longer acquisition per station.