Question 6 of 10: Electrical vs. Electromagnetic Methods

Question 6: Electrical vs. Electromagnetic Methods (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.

Shared property. Both families are primarily sensitive to bulk electrical resistivity $\rho_e$ (or its reciprocal, conductivity $\sigma$), controlled mainly by pore-fluid content, salinity and clay content.

Physics. DC resistivity uses GALVANIC (direct, ground-contact) current injection: a known current $I$ is driven into the ground through one pair of electrodes and the resulting potential difference $V$ is measured at a second pair, giving an apparent resistivity $\rho_a=k\,V/I$ where $k$ is a geometric factor set by the electrode array. Electromagnetic methods instead use INDUCTIVE coupling and require no ground contact: a transmitter loop or coil carries a time-varying (or stepped, for TDEM) current that generates a primary magnetic field, which induces eddy currents in any conductive ground below; those eddy currents in turn generate a secondary field that a receiver coil measures, its amplitude and phase (or decay rate, for TDEM) depending on the ground's conductivity.

Instruments and arrays. Resistivity instruments are simple current-source/voltmeter systems deployed on four (or more, for a multi-electrode ERT system) ground-planted electrodes in a standard array — Wenner, Schlumberger, dipole-dipole — each trading depth of investigation, lateral resolution and signal strength differently as electrode spacing is expanded. EM instruments range from simple fixed-geometry ground conductivity meters (e.g. EM31/EM34-class, fixed transmitter-receiver coil separation) to airborne frequency- or time-domain systems (fixed-wing or helicopter-towed "bird") with a transmitter and one or more receiver coils at a fixed offset.

Procedures/advantages and disadvantages. Because EM needs no ground contact, it is far faster to survey (walking or flying continuously) and works in settings where electrical methods fail outright — paved ground, very dry/resistive sand, or frozen/permafrost terrain where electrode coupling is poor. However, EM's depth of investigation is fundamentally limited by skin depth $\delta\approx503\sqrt{\rho/f}$, which SHRINKS as the ground becomes more conductive, so EM struggles to see BENEATH a thick conductive surface layer (the signal is absorbed before it reaches depth). Electrical resistivity, by contrast, gives better depth control (via deliberate electrode-spacing expansion) and works well through a conductive overburden because the current is injected directly rather than having to induce through it, but is slow (each electrode must be planted and moved) and fails where ground contact cannot be made.

Case history. A time-domain EM survey was used to rapidly map the lateral extent of a saline-intruded aquifer along a coastal groundwater well-field, exploiting EM's speed and non-contact operation over difficult (partly paved, partly very dry dune sand) terrain that would have made a comparable resistivity survey impractically slow; the resulting conductivity map directly guided placement of new supply wells away from the intruded zone.