Method
Introduction
Magnetotellurics (MT) is a passive electromagnetic method that uses naturally occurring electric and magnetic fields to image subsurface resistivity structure from near surface to crustal depths. It is one of the deepest sounding electromagnetic methods used in exploration and regional geologic investigations.
Signal energy is supplied by global lightning discharges and solar activity. Grounded dipoles and magnetic antennas record the electric and magnetic field response across a broad frequency range, and those data are processed into apparent resistivity and impedance information that can be inverted for depth-dependent structure.
Audio-magnetotellurics (AMT) covers the higher-frequency portion of the MT spectrum and is better suited for shallow to intermediate depth imaging. MT and AMT are often used together to improve coverage from the near surface into deeper targets.
Applications
MT is especially valuable in low-noise environments and in areas where conductive cover masks deeper structure. It is commonly integrated with geology, drilling, gravity, and other EM methods.
- Deep geologic structures and basin architecture
- Mineral, groundwater, geothermal exploration
- Structural mapping beneath conductive overburden or alluvial cover
- Regional framework studies that benefit from depth continuity across broad scales
Survey Design
| Parameter | Description |
|---|---|
| Depth | Depth depends on frequency content and subsurface resistivity structure, ranging from shallow AMT targets to deep MT investigations. |
| Station Spacing | Station spacing is commonly 10-200 m for detailed work, with wider spacing used for regional programs. |
| Scale | Surveys may be acquired as profiles, localized grids, or larger regional arrays. |
| Production | Stations are commonly occupied for hours to overnight, with daily production dependent on layout, access, and recording time. |
Instrumentation
- Receivers: ZEN Receiver and GDP-3224 Multi-Function Geophysical Receiver for AMT programs
- Magnetic Antennas: ANT/4 and related field-proven magnetic sensors
- Electric Field Sensors: Grounded dipoles sized to survey objectives and target depth
Deliverables
- AVGs, EDIs, etc
- 2D and 3D inversion results
Case Studies and Resources
- Contact Zonge for MT and AMT project examples, regional case studies, and deliverable samples.