Transient Electromagnetic Method (TEM)

Key Takeaways

• The depth of investigation of a TEM survey increases with time after the primary pulse is turned off, because the eddy currents induced in shallow formations decay faster than those in deeper formations: immediately after the primary field collapses, the secondary field response is dominated by the near-surface resistivity structure; as time increases, the eddy currents diffuse downward through the earth and the measured signal reflects progressively deeper formation resistivity; this diffusion behavior allows a single transmitter-receiver configuration to sound from shallow (a few meters depth, measured at early times of approximately 1-10 microseconds after the pulse) to deep (hundreds or thousands of meters, measured at late times of 1-100 milliseconds), providing a continuous resistivity-depth profile beneath the survey station; the maximum depth of investigation is determined by the signal-to-noise ratio of the late-time measurements, the transmitter loop area and current (larger loops with higher current achieve greater depth), and the ambient electromagnetic noise environment (industrial electrical noise and geomagnetic variations limit late-time sensitivity in urban or high-latitude environments).
• Central-loop TEM configuration, where the receiver is located at the center of the transmitter loop, is the most common survey geometry for shallow to intermediate depth investigations (10-500 meters), because the receiver couples strongly to the secondary field of the downward-diffusing eddy current system and the geometry minimizes the coupling to the primary field (which would otherwise overwhelm the weak secondary signal during and immediately after the transmitter pulse); the central-loop geometry is efficient for resistivity mapping of near-surface hydrogeology, contamination plumes, and shallow geothermal systems; large fixed-loop configurations (transmitter loop of 1-10 km sides with receivers outside the loop) are used for deep petroleum exploration targets (1-5 km depth) and for mineral exploration in shield terranes where deep basement conductors (graphite, sulfide ore bodies) are the exploration target; the transmitter current in large-loop TEM systems is typically 20-50 amperes in loops of 100-1000 meter sides, generating primary magnetic moments of 100,000-25,000,000 ampere-square meters that provide adequate signal penetration to exploration depths of several kilometers.
• TEM is more sensitive to conductive anomalies (low-resistivity targets) than to resistive anomalies, because the method measures the strength of the induced secondary field, which is proportional to the conductivity of the formations carrying the eddy currents: a saline aquifer, clay layer, sulfide ore body, or graphite zone all sustain eddy currents strongly and produce a large secondary field that is easy to detect; a hydrocarbon reservoir or resistive basement, by contrast, suppresses eddy current flow and produces a weak secondary field that appears as an absence of response relative to the background signal — detection of resistive anomalies requires accurate modeling of the background and careful noise correction, which makes direct TEM detection of oil and gas reservoirs challenging in most geological settings; the exception is the borehole TEM (also called through-casing resistivity or borehole transient electromagnetic) method, where the transmitter and receiver are deployed in the borehole, increasing the coupling efficiency to resistive reservoir anomalies and enabling monitoring of water-oil contact movement during production or water flooding.
• Airborne TEM (ATEM) systems carry the transmitter loop and receiver coils on a helicopter or fixed-wing aircraft, enabling rapid resistivity mapping of large survey areas (1,000-10,000 km2 per season) at a fraction of the cost of ground-based TEM or seismic surveys: helicopter-borne systems (such as the VTEM, SkyTEM, and HELITEM systems) tow a 10-30 meter diameter transmitter frame beneath the aircraft at 30-60 meters altitude and measure the secondary field at multiple receiver positions within or below the frame, achieving depth penetration of 300-600 meters in resistive terrain; fixed-wing systems (AeroTEM, GEOTEM) operate at higher altitude and speed, trading depth resolution for areal coverage efficiency; airborne TEM has become the primary reconnaissance tool for mineral exploration in Northern Canada, Australia, and Scandinavia, providing resistivity maps that guide follow-up ground TEM and drilling programs; in the petroleum context, ATEM surveys are used in the Canadian North and Arctic regions to map permafrost thickness, identify near-surface gas hydrate deposits, and characterize shallow geohazards before drilling.
• TEM monitoring of enhanced oil recovery (EOR) and carbon capture and storage (CCS) operations uses repeated surveys at the same location over months to years to detect resistivity changes caused by the movement of injected fluids: injected CO2 is more resistive than the formation brine it displaces, so CO2 spreading in a saline aquifer storage formation produces a resistivity increase detectable by time-lapse TEM surveys; injected steam during SAGD or cyclic steam stimulation heats and mobilizes bitumen, reducing the pore fluid viscosity and increasing formation conductivity in the steam chamber, which is detectable by borehole TEM arrays placed in observation wells around the steam injector; the Sleipner CO2 storage project in the North Sea used repeated surface TEM surveys (among other monitoring methods) to track CO2 plume migration over two decades, demonstrating that TEM monitoring can reliably detect CO2 saturation changes of a few percent in sandstone reservoirs at depths of 800-1,000 meters.