Controlled Source Audio Magnetotelluric Method (CSAMT)
Time：2023-12-18 Read：207 order
Introduction： By using artificial field sources to excite underground rocks, the potential difference generated during the flow of current is received, and the primary field potential formed by different power supply frequencies is received. Due to the different propagation depths of fields at different frequencies in the formation, the reflected depth also forms a mathematical relationship with frequency. Rocks with different electrical conductivity generate different potentials and magnetic fields during the flow of current. The CSAMT method is an electromagnetic exploration method that uses the differences in electrical conductivity of different rocks to observe changes in the primary field potential and magnetic field intensity.
CSAMT uses controllable artificial field sources. Measure the electromagnetic field component transmitted from an electric dipole source to the ground, with a distance of 1-2km between the two electrode power sources. The measurement is conducted in the range of 5-10k

advantage

Due to its large detection depth (usually up to 2 kilometers) and its dual properties of profile and depth measurement, this method has many advantages:

Firstly, using a controllable artificial field source, the measurement parameter is the ratio of electric field to magnetic field - Kanya resistivity. This enhances anti-interference ability and reduces the influence of terrain.

Secondly, by changing the frequency instead of changing the geometric dimensions, electrical depth measurements at different depths can be carried out, improving work efficiency. Electromagnetic depth measurements at 7 points can be completed simultaneously in one launch.

Thirdly, the detection depth range is large, usually up to 1-2 km.

Fourthly, it has high lateral resolution and can sensitively detect faults.

Fifth, the high resistance shielding effect is small and can penetrate the high resistance layer. Similar to MT and AMT methods, CSAMT method is also affected by static and near-field effects. Multiple static correction methods can be used to eliminate the influence of static effects.

The definition of CAGNIARD resistivity has universal significance and important utility value. This is because within the range of skin roughness of the frequency used, the average multi-layer electrical medium resistivity is equivalent to the uniform electrical medium resistivity represented by the average value. As long as the agreement is met, the calculation and inversion of CAGNIARD resistivity are relatively simple.

QA:

What type of electrode is used for AMT measurement, and how is the distance between MNs set?

The electrode adopts solid or liquid unpolarized electrodes, and the electrode distance is generally selected between 50-300 meters based on the size of the natural electric field signal and the survey depth. The electrode distance is small and convenient for construction; Large polar distance and complex construction; Strong signal, pole distance can be appropriately reduced; Signal weak pole distance can be increased; Survey shallow areas, it is recommended to use small pole distances (50 meters);

Can a large polar distance (>100 meters) frequency range be used for deep exploration?

In theory, it is dc-32 kHz, with 6 frequency bands.

What is the amplitude range of a natural electric field?

A few millivolts per kilometer.

How to interpret natural electric fields?

Calculate the apparent impedance and apparent impedance phase using the ratio of electric field to magnetic field.


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