Glossary
Abbreviations
| Abbr | Long |
|---|---|
| ADC | Analogue Digital Converter |
| AMT | AudioMagnetoTelluric , e.g. above 1 kHz |
| CSAMT | Controlled Source AudioMagnetoTelluric |
| CSEM | Controlled Source ElectroMagnetics |
| JSON | Java Script Object Notation |
| LMT | Long Period MagnetoTelluric, e.g. below 100 or 1000 s period |
| LSB | Least Significant Bit |
| MT | MagnetoTelluric |
| QC | Quality Control |
| RMT | RadioMagnetoTelluric, e.g. above 50 kHz |
| RR | Remote Reference |
| XML | Extensible Markup Language |
Terms
| Term | Explanation |
|---|---|
| Anti Aliasing Filter | an anti aliasing filter is a low pass filter cutting off frequencies above the Nyquist frequency. The Nyquist frequency is half the sampling rate. The anti aliasing filter is used to avoid aliasing of the signal; that is simply spoken: in case the ADC samples with 1 kHz, but a 4 kHz signal is at the ADC input, the ADC generates an erroneous pseudo signal at lower frequencies like 128 Hz “which isn’t there” |
| dead band | also AMT dead band; frequency range between 1 - 4 kHz where MT data is poor ( low excitation of magnetic fields) |
| MT dead band | between 1 s - 10 s; similar as above but can be resolved by longer recording |
| Radio Filter | a radio filter is basically a low pass filter suppressing high frequency (radio frequency) noise. HF noise can irritate the pre-amplifier (bandwidth); this filter is independent of the anti aliasing filter |
Note: the definitions of RMT, AMT, MT (genral) and LMT are flexible.
- RMT uses simple antennas, 16 / 24 bit high speed ADCs, artificial sources and is used for shallow depths (near surface).
- AMT uses induction coils, 24 bit ADCs, natural sources and is used for intermediate depths to 100 - 300 m.
- MT uses induction coils, 32 bit ADCs, natural sources and is used for depths to 3000 - 5000 m and more
commercial exploration: down to 10 or 100 s \(\Rightarrow\) and get 1 - 3 km depth. - LMT uses induction coils down to 1000 - 4000 s, fluxgate magnetometers from 100 s to ∞ LMT does not care about the first km of depth.
usage: a) volcanic roots (also geothermal roots), b) earthquakes and c) scientific exploration.