Frequency domain
GsAMT — Grounded-Source Audio-Magnetotellurics
A controlled transmitter fixes the AMT dead band
Frequency band
1 Hz - 10 kHz
Depth range
50 - 2,000 m
Resolution
Moderate
Source
Grounded electric dipole transmitter
Typical cost
Moderate to high
Survey speed
Moderate - transmitter setup then roving receivers
Best terrain
Areas with weak natural signal or high cultural noise
Domain
Frequency domain
Overview
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.
How the physics works
Beyond roughly three skin depths from the transmitter the field behaves like a plane wave and standard MT processing applies. Closer in, the near-field term dominates and apparent resistivity rises as 1/f, producing the characteristic 45-degree upturn on log-log plots.
Where it is used
- Epithermal and porphyry targeting under cover
- Geothermal exploration in noisy terrain
- Structural mapping and fault detection
- Groundwater exploration in hard rock