Method
Introduction
A transient electromagnetic (TEM) survey generates a controlled electromagnetic field by transmitting current through a loop and measuring the decaying secondary magnetic field after current turnoff. That decay is controlled by subsurface resistivity, making TEM particularly effective for conductive targets.
Decay responses are sampled in discrete time gates that range from microseconds to milliseconds depending on system design. Slow decay typically indicates more conductive material, while rapid decay is more typical of resistive ground.
TEM survey geometry is designed around target orientation, expected depth, and desired coupling. Standard TEM systems are used for shallow to deep investigations, while NanoTEM systems are optimized for very shallow imaging with very fast transmitter turnoff.
Secondary EM responses from conductors
Applications
TEM is widely used where conductive bodies are the primary target and where survey geometry can be tuned to maximize coupling.
- VMS and magmatic Ni-Cu-PGE deposits
- Mafic layered complexes and skarn deposits
- Carbonate replacement and other massive sulfide-bearing systems
- Salt-water intrusion mapping
- Groundwater and hydrogeologic structure mapping
Survey Design
| Parameter | Description |
|---|---|
| Depth | Depth depends on loop size, transmitter power, target conductivity, and the noise environment. |
| Station Spacing | Station spacing commonly ranges from 5-50 m depending on the desired lateral resolution. |
| Configurations | Common geometries include fixed in-loop, moving-loop, and borehole configurations. |
| Production | Multiple stations can typically be acquired per day, with production controlled by terrain, loop logistics, frequency, and stacking requirements. |
Instrumentation
- Receiver: GDP-3224 Multi-Function Geophysical Receiver
- Magnetic Antenna: TEM/4, NanoTEM
- Transmitters: ZT-100 (with off-shelf generator), GGT-10, GGT-30, ZT-30, and NT-20
Deliverables
- .TEM file format of measured data
- Profiles of each component
- 1D inversions
Case Studies and Resources
- Contact Zonge for TEM and NanoTEM project examples, target-specific applications, and supporting references.