Question 5 of 9: Time-Domain vs. Frequency-Domain Geophysical Instruments

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

Both classes of instrument are electromagnetic induction methods that energize the ground with a transmitter loop or wire and measure the induced (secondary) response in a receiver, but they differ fundamentally in HOW the transmitter is driven and WHEN the response is measured, and the trade-offs that follow are among the most commonly tested topics in applied EM.

Frequency-domain (FDEM) instruments — e.g. the ground conductivity meter (EM31/EM34), Slingram-type systems, and airborne fixed-wing FDEM systems — drive the transmitter loop with a continuous sinusoidal current at one or several DISCRETE FREQUENCIES and measure the secondary field CONTINUOUSLY, alongside the (much larger) primary field, resolved into in-phase and quadrature components. Advantages: because the receiver reads continuously, the system can be walked or flown at a steady pace, giving fast, dense, real-time profile coverage (excellent for reconnaissance mapping of lateral conductivity variation); the instrumentation is comparatively simple and inexpensive; and a fixed transmitter-receiver coil geometry gives quantitative apparent conductivity directly from the response, useful for engineering/environmental mapping (e.g. delineating a landfill leachate plume). Disadvantages: because the secondary field must be measured in the PRESENCE of the primary, the transmitter-receiver separation and orientation must be held extremely precisely (or a bucking coil used) to null the primary, which limits the achievable transmitter moment (and hence depth of investigation) for a portable system; depth information from a single frequency is limited, so multiple frequencies (each penetrating to a different nominal depth) are needed to approximate a depth profile, and the resulting depth resolution is still coarser than a true time-domain sounding.

Time-domain (TDEM) instruments — e.g. Geonics PROTEM/TerraTEM ground systems and airborne systems such as VTEM/AeroTEM — instead switch a DC or ramped current in the transmitter loop OFF abruptly (or on a step) and measure the decaying secondary (eddy-current) voltage during the transmitter's OFF time, when NO primary field is present at the receiver. Advantages: because there is no primary field to null, the transmitter and receiver can even be the same coincident loop, simplifying geometry and allowing a large transmitter moment (hence good depth penetration and signal-to-noise) for a given instrument size; the induced eddy currents diffuse progressively DEEPER into the ground with increasing time after switch-off, so a single transient decay curve inherently contains DEPTH information (early time gates respond to shallow structure, late gates to deep structure), which is exploited directly in 1-D layered inversion of the decay curve; and TDEM is comparatively insensitive to a conductive near-surface layer masking a deeper conductor, because the shallow response has largely decayed away by the time gates that see the deep target are sampled. Disadvantages: the instrumentation (accurate fast-switching transmitter, wide-dynamic-range receiver, precise timing) is more complex and costly; because measurement happens during the OFF time, the system cannot be walked/flown and read continuously in quite the same seamless way as a CW FDEM system, and very early-time gates are compromised by the finite turn-off ramp time, limiting the shallowest resolvable depth.

A practical example of the trade-off: reconnaissance mapping of a shallow, laterally variable aquifer or contaminant plume is well suited to a fast FDEM ground conductivity meter, whereas confidently detecting and estimating the depth to a well-conductive but deep massive sulphide body (where a shallow conductive overburden could otherwise mask the target in FDEM data) is far better served by a TDEM system, which is why most modern airborne base-metal exploration surveys are flown with TDEM (e.g. VTEM) rather than FDEM systems.