Transient Electromagnetic (Time-domain EM) and NanoTEM
A TEM survey involves generating an electromagnetic field by transmitting a time-varying current into a transmitter loop. A magnetic antenna measures the response of the secondary magnetic field induced by eddy currents to obtain information on the resistivity (ohm·m) of the subsurface. Conductors are ideal targets because they will produce measurable b-field responses with respect to time after transmitter turn off.
We observe the induced voltages from the secondary field at the receiver, where each decay waveform is measured in discrete time intervals, or time gates, that vary from microseconds (NanoTEM) to milliseconds. A slow rate of decay after the transmitter turn-off indicates a more conductive subsurface, where a quick decay indicates resistive material.
The TEM survey may be customized based on the target. The configuration of the transmitting loop is determined based on the orientation and depth of the target to optimize the coupling potential of the EM field with the target body. Measurements are made along a profile within or outside of the loop. Each station is occupied for a matter of minutes, depending on the transmitted frequency and stacking procedures, where frequencies often range from 1–32 Hz.
The NanoTEM system is designed to image very shallow responses (<5 m–10’s of m) by utilizing a very fast turn off time. The standard TEM system may image depths of 10s–1000s of meters depending on the size of the current loop and power from the transmitter.
Applications
TEM is a good survey to utilize in the presence of conductive metallic minerals and when the orientation and depth of the target is well understood. The depth of investigation is larger than most electromagnetic methods.
- VMS and magmatic Ni-Cu-PGE deposits
- Mafic Layered Complexes
- Skarn Deposits
- CRDs and other types of deposits where sulfides can be massive
- Salt Water Intrusion mapping
- Groundwater and hydrogeologic structure mapping
Survey Design
The efficacy of a TEM system is determined by understanding the orientation, depth, and conductive nature of the target. The goal is to design a transmitter loop that will provide maximum coupling with the conductor.
- Depth: The depth is dependent on loop size and power output from transmitter.
- Station Spacing: 5–50 m, based on desired lateral resolution
- Configurations: Fixed In-loop, Moving Loop, Borehole
- Production: Multiple stations are acquired per day, depending on terrain and acquisition parameters (frequency, number of stacks, number of cycles per stack upon observing ambient noise).
Instrumentation
- Receiver: GDP-3224 Multi-Function Geophysical Receiver
- Magnetic Antenna: TEM/4, NanoTEM
- Transmitter: GGT-10/30, ZT-30, NT-20
Deliverables
- Measured data are plotted as profiles of estimated resistivity vs depth
- Results are presented in cross-section form
Survey Life Cycle
Case studies and Resources