Method (ohm·m)
Introduction
The magnetotelluric (MT) method is a passive imaging technique that utilizes naturally occurring electromagnetic fields to examine subsurface structures to depths of up to 10’s of kilometers. This is the deepest sounding electromagnetic exploration method.
The signal source comes from global-scale lightning discharges and solar wind, and the resolution is governed by subsurface resistivity contrasts as a function of frequency. Using grounded dipoles and sensitive magnetic field antennas, the electrical and magnetic fields are measured over a suite of frequencies (0.001-1,000Hz) and produce apparent resistivity measurements (ohm*m) that represent the geologic structures at depth. Each station is occupied for multiple hours, often overnight.
The Audio-Magnetotelluric (AMT) method encompasses the higher frequency band of the MT spectrum (>1Hz) and is better suited for shallower imaging (from a few meters to a few kilometers in depth). AMT is often used in tandem with MT.
Applications
MT excels in regions with little to no cultural interference and is capable of seeing through conductive overburdens and alluvial basins, and is a great supplementary tool to other geophysical surveys :
- Deep geologic structures (500+m deep), with near-surface detail up to approx. 500m (from a zonge flyer… not sure about this just trying to supply both depth and lateral resolution capabilities)/
- Mineral, groundwater, geothermal exploration
- Mining and water resource management
Survey Design
| Parameter | Description |
|---|---|
| Depth | 100’s-1000’s meter. Function of resistivity structure and frequency (low f = deep) |
| Station Spacing | Often 10-200m. The received signal strength is proportional to the station spacing |
| Scale | Localized grid or profiles of electrodes, regional scales |
| Cost | Mid cost. |
| Production | Stations are typically acquired overnight, where approximately 4-8 sites are deployed per day. |
MT Station
Instrumentation
- Receiver:
- ZEN Receiver, (
- (AMT) GDP-3224 Multi-Function Geophysical Receiver
- 24-bit multi-channel
- Magnetic antenna: ANT/4, ….
Deliverables
Processed MT data in the form of sounding plots and EDI files.
Two-dimensional inversions as sections of inversion resistivity versus depth
3D model and horizontal depth or elevations slice of Resistivity data
Survey Life Cycle
- Site selection based on noise/environmental conditions
- Deployment of dipoles and magnetic antennas
- Long-duration recording of natural EM fields
- Quality control of time series data
- Impedance calculation and inversion
- Interpretation and integration with geology
MT Survey Design
Case Studies and Resources
- [Zonge MT/AMT Projects]
- [Geothermal and Mineral Exploration Examples]
- [Links to Scott’s Info and Tucson Projects]