TEM/TDEM
Transient / Time-Domain Electromagnetics
Current in a transmitter loop is abruptly switched off. The collapsing field induces eddy currents that diffuse downward and outward, and the decaying secondary field is measured between transmitter pulses with no primary field present.
Crew
3-4 crew for loop layout plus 1 operator
Mobilisation
1-2 days: loop wire spooling, receiver checks, base station
Daily coverage
3-8 soundings per day depending on loop size
Equipment and specifications
| Transmitter | 20-40 A into a 100-400 m single-turn loop, 1-30 Hz base frequency, fast turn-off |
|---|---|
| Receiver coil | Air-core or ferrite induction coil, calibrated effective area |
| Loop wire | Heavy-gauge insulated cable, spooled on reels with continuity tester |
| Recorder | Multi-gate stacking receiver, microsecond to hundreds of milliseconds |
| Positioning | GNSS on all loop corners and the receiver station |
Setup sequence
- 01
Size the loop
Loop side roughly equal to the target depth; bigger loops for deeper targets.
- 02
Lay and close the loop
Square as terrain allows, corners on GNSS, joints taped and tested for continuity.
- 03
Set the receiver
Centre-loop for depth soundings, offset for lateral profiling; log coil orientation.
- 04
Check turn-off
Record the current waveform — turn-off ramp directly controls the earliest usable gate.
- 05
Stack
Stack until the late gates are smooth; increase stacks rather than gain in noisy ground.
- 06
Invert
Fit a layered model to the decay, using the measured ramp in the forward calculation.
Safety
High-voltage loop turn-off
No handling of loop wire while energised; radio call before each transmit cycle.
Trip hazards from loop wire
Wire flagged at tracks and paths, elevated crossings, brief all site traffic.
Manual handling of reels
Two-person lifts, mechanical reel stands.
Remote-site medical
First-aid kit, satellite comms, evacuation plan filed daily.
Field vs lab
| Aspect | In the simulator | In the field |
|---|---|---|
| Signal-to-noise | Clean synthetic response; noise added only if you ask for it. | Cultural noise, wind and instrument drift set the usable dynamic range. |
| Turn-off ramp | Instant step turn-off. | A finite ramp corrupts the earliest gates and must be measured and modelled. |
| Loop geometry | Perfect square loop. | Terrain distorts the loop; corner coordinates are part of the data. |
| Superparamagnetic / IP effects | Usually ignored. | Can invert the sign of late gates in weathered or clay-rich ground. |
| Repeatability | Identical parameters return an identical curve every time. | Coupling, moisture and temperature move the response between repeats. |
Field QC checklist
- · Loop continuity and current amplitude logged per sounding
- · Repeat sounding within 5% across mid-time gates
- · Late gates above the noise floor after stacking