Method (ohm·m)
Introduction
CSAMT surveys are utilized to measure the calculated resistivity structure of the Earth. A transmitter dipole is deployed at a large distance away from the receiver grid, transmitting a controlled electrical signal over a range of frequencies. The orthogonal components of the electric and magnetic fields are measured at the receiver with grounded dipoles and a magnetic antenna. The resistivity values relate to the geological response of the ground, providing information on resistivity contrasts of more or less resistive rock packages or structures.
Data are generally collected in the far-field, where depth of investigation is frequency-dependent, but recent 3D CSAMT inversions now incorporate near-field data to improve resolution and depth coverage.
Zonge is producing 3D CSAMT inversions which includes the near-field data and expands the depth of the model.
CSAMT Propagation Zones – Near-field, Transition, Far-field
Applications
CSAMT surveys are a common tool for identifying resistivity contrasts and imaging geologic structures.
They are effective in both reconnaissance and detailed targeting stages of exploration, and complement other geophysical techniques.
This method can be applied to:
Depth to bedrock and geological structures or lithologies
Silicified structures in host rocks or fluid pathways
Fault and shear zones with silicification, altered or brecciated material
Water-table trends, aquitard imaging
Contaminant locations for mineral, hydrology and petroleum investigations
Survey Design
| Parameter | Description |
|---|---|
| Depth | The depth is dependent on transmitted frequency and resistivity of the subsurface, often in the 100’s of meters range. Vertical resolution is often estimated to be 5–20% of the profile length. |
| Dipole Length | Often 10–200 m. The received signal strength is proportional to the station spacing. |
| Scale | Localized grid or profiles of receiver arrays, 4–6 dipoles acquired per measurement. |
| Production | Profiles are completed on a scale of days, with approximately 5–15 receiver arrays measured per operator per day. |
CSAMT 4-Channel Array
Instrumentation
- Receiver: GDP-3224 Multi-Function Geophysical Receiver (24-bit, multi-channel)
- Magnetic Antenna: ANT/6
- Transmitters: GGT-30 (30 KVA), GGT-10 (10 KVA)
- Generators: ZMG-30 or ZMG-9
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Survey Life Cycle
- Planning and layout of transmitter and receiver grids
- Deployment of dipoles and magnetic antennas
- Data acquisition across frequency bands
- Quality control and initial processing
- 1D/2D/3D inversion modeling
- Interpretation and integration with geology
Deliverables
- Pseudosections of observed Cagniard resistivity and impedance phase
- 2D inversion sections (resistivity vs. depth)
- Plan maps of inversion results at various elevations
- 3D resistivity models with horizontal depth slices
Insert sample 2D and 3D outputs or images here
Include figure or graphic for 3D model example
Case Studies and Resources
- [Link to Scott’s Info]
- [Tucson Project Summary]
- [Relevant Case Studies]
- [Zonge Application Notes or Technical Papers]