GsAMT
Grounded-Source Audio-Magnetotellurics
GsAMT (and the closely related CSAMT) replaces the unreliable natural field with a grounded dipole several kilometres away. Signal quality improves dramatically, but the geometry must stay in the far field or the apparent resistivities are biased.
Crew
Transmitter operator plus 2 roving receiver pairs
Mobilisation
2-3 days: transmitter siting, grounding, receiver sync
Daily coverage
4-8 stations per receiver crew per day
Equipment and specifications
| Transmitter | Grounded dipole or large loop, 20-40 kW, 1 Hz - 10 kHz controlled waveform |
|---|---|
| Grounding | Multiple electrode arrays watered and salted to keep contact under 10 ohm |
| Receivers | Same E/H recorders as AMT with transmitter-synchronised timing |
| Sync | GNSS clocks on transmitter and every receiver |
| Offset control | Receivers kept in the far-field zone, typically 3-10 km from the transmitter |
Setup sequence
- 01
Site the transmitter
Central to the block, on conductive ground, with legal access for the full campaign.
- 02
Ground it properly
Water and salt the electrode arrays; measure and log contact resistance each morning.
- 03
Prove the far field
Check a test station against natural-source AMT to confirm plane-wave conditions.
- 04
Roll the receivers
Occupy stations in leapfrog order, recording each transmit frequency sweep.
- 05
Process
Compute impedance per frequency against the transmit log; splice with AMT for the low band.
Safety
Energised grounded dipole
Signed and fenced electrode arrays, radio clearance before energising, lock-out on the transmitter.
Public access to electrodes
Continuous watch or physical barriers wherever the public can reach.
Long cable runs
Elevated or buried crossings at tracks; daily insulation checks.
Generator noise and exhaust
Hearing protection, ventilation, CO monitoring.
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 control | Perfect, constant transmit amplitude. | Ground contact and generator load make the transmit moment drift; log it continuously. |
| Near-field | Plane-wave assumption always holds. | Stations too close to the transmitter break the assumption and bias resistivity. |
| Logistics | None. | Transmitter siting and permits usually decide where the survey can happen. |
| Repeatability | Identical parameters return an identical curve every time. | Coupling, moisture and temperature move the response between repeats. |
Field QC checklist
- · Transmit current logged for every frequency and station
- · Far-field check station matching natural-source AMT within 10%
- · Electrode contact resistance under 10 ohm at the transmitter