Method reference
Eight ways to see through the ground
Each method exploits a different part of the electromagnetic spectrum, from switched direct current up to gigahertz radar. The right choice depends on target depth, host conductivity and the resolution you actually need.
CSEM
Controlled-Source Electromagnetics
Marine and land resistivity mapping with a towed transmitter dipole
- Band
- 0.05 - 10 Hz
- Depth
- 500 - 4,000 m
- Resolution
- Coarse (hundreds of metres)
- Cost
- High
GPR
Ground Penetrating Radar
High-frequency radio pulses for centimetre-scale shallow imaging
- Band
- 10 MHz - 2.6 GHz
- Depth
- 0.1 - 30 m
- Resolution
- Very high (centimetres)
- Cost
- Low
AEM
Airborne Electromagnetics
Helicopter and fixed-wing conductivity mapping at survey scale
- Band
- 300 Hz - 200 kHz
- Depth
- 20 - 400 m
- Resolution
- Moderate (tens of metres)
- Cost
- High per day, low per line-km
MT/AMT
Magnetotellurics / Audio-Magnetotellurics
Natural-field soundings from the near surface to the mantle
- Band
- 10 kHz - 0.001 Hz
- Depth
- 50 m - 100 km
- Resolution
- Decreases with depth
- Cost
- Moderate
GsAMT
Grounded-Source Audio-Magnetotellurics
A controlled transmitter fixes the AMT dead band
- Band
- 1 Hz - 10 kHz
- Depth
- 50 - 2,000 m
- Resolution
- Moderate
- Cost
- Moderate to high
TEM/TDEM
Transient / Time-Domain Electromagnetics
Switch the current off and watch the ground decay
- Band
- Equivalent 1 Hz - 100 kHz
- Depth
- 20 - 1,500 m
- Resolution
- Good for conductors
- Cost
- Moderate
RIM/BHR
Radio Imaging / Borehole Radar
Hole-to-hole radio tomography of the rock in between
- Band
- 50 kHz - 30 MHz
- Depth
- 10 - 300 m between holes
- Resolution
- High between holes
- Cost
- Moderate - needs boreholes
IP/ERT
Induced Polarisation / Electrical Resistivity Tomography
Galvanic current injection for resistivity and chargeability sections
- Band
- DC - 10 Hz
- Depth
- 2 - 500 m
- Resolution
- High near surface, degrades with depth
- Cost
- Low to moderate