MT/AMT
Magnetotellurics / Audio-Magnetotellurics
MT records orthogonal natural electric and magnetic fields and forms the impedance tensor. Because no transmitter is needed, the only limit on depth is how long you record. AMT is the same technique in the audio band for shallower targets.
Crew
2 geophysicists per 2-4 station rotation, plus a remote-reference watch
Mobilisation
2-3 days: instrument sync, remote-reference site, permits
Daily coverage
2-4 stations per crew per day (long-period sites take 1-3 days each)
Equipment and specifications
| Receiver | 5-channel 24-bit recorder, 10 kHz - 0.001 Hz, GNSS-locked clock |
|---|---|
| Magnetometers | Induction coils for AMT band; fluxgate for long-period MT |
| Electrodes | Non-polarising Pb-PbCl2 or Cu-CuSO4 pots, 50-100 m dipoles |
| Remote reference | Second full station 5-50 km away recording synchronously |
| Power | Solar panel plus deep-cycle battery for multi-day sites |
Setup sequence
- 01
Site selection
Flat, quiet ground away from fences, powerlines, pumps and traffic.
- 02
Lay the dipoles
Orthogonal N-S and E-W dipoles, electrodes buried in wet bentonite, contact resistance under 2 kohm.
- 03
Bury the coils
Level, buried and thermally insulated, oriented to true north; log the azimuth.
- 04
Sync and start
Confirm GNSS lock on both local and remote stations before recording.
- 05
Record
Overnight minimum for AMT; several days for long-period MT to reach the dead band.
- 06
Process
Robust remote-reference processing to apparent resistivity and phase curves.
Safety
Remote lone working
Check-in schedule, satellite communicator, vehicle recovery kit.
Wildlife and livestock
Fenced instrument pits, site briefing, first-aid kit with snake/sting protocol.
Trenching for electrodes
Hand tools only, service clearance, backfill and reinstate.
Lightning
Stop work and disconnect coils when storms are within 10 km.
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. |
| Source field | Ideal plane wave at every frequency. | Natural source strength varies with solar and thunderstorm activity; the 1-5 kHz dead band can be unusable. |
| Recording time | Instant. | Hours to days per site — the schedule driver for the whole survey. |
| Dimensionality | Clean 1D layered response. | Static shifts and 3D distortion must be diagnosed before inversion. |
| Repeatability | Identical parameters return an identical curve every time. | Coupling, moisture and temperature move the response between repeats. |
Field QC checklist
- · Electrode contact resistance under 2 kohm at both ends of every dipole
- · Coherency above 0.8 across the target band after remote reference
- · Smooth apparent-resistivity and phase curves with consistent 45-degree phase at long period