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 |
Chopper
A chopper is a special amplifier design which reduces the 1/f noise.
metronix also used this term for chopper on == LF mode and chopper off == HF mode.
The more precise term would be “switch LF/HF mode”. Today we say switch free” coil, because a “dual-path” amplifier is used.
ADU-07e/ADU-08e/ADU-10e: chopper on @512 Hz and lower, chopper off @1024 Hz and higher.
ADU-11e: chopper on @1024 Hz, chopper off @2048 Hz and higher.
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.
Joblist “Rajasthan”
This ADU-08e joblist records various frequencies over the day with MFS-06e coils.
| sampling_rate | duration | start | stop | DigitalFilter | comment |
|---|---|---|---|---|---|
| 65536 | 1 min | 10:00:00 | 10:01:00 | ||
| 32768 | 6 min | 10:03:00 | 10:09:00 | 4 x offline = 8192 Hz | |
| 4096 | 46 min | 10:11:00 | 10:57:00 | 4 x offline = 1024 Hz | |
| 512 | 04:49 | 10:59:00 | 15:58:00 | with 8x digfil –> 64Hz | 4x , 4x … offline |
| repeat | |||||
| 65536 | 1 min | 16:00:00 | 16:01:00 | ||
| 32768 | 6 min | 16:03:00 | 16:09:00 | 4 x offline = 8192 Hz | |
| 4096 | 46 min | 16:11:00 | 16:57:00 | 4 x offline = 1024 Hz | |
| 512 | 07:49 | 16:59:00 | 00:58:00 | with 8x digfil –> 64Hz | 4x , 4x … offline |
| repeat | |||||
| 65536 | 1 min | 01:00:00 | 01:01:00 | ||
| 32768 | 6 min | 01:03:00 | 01:09:00 | 4 x offline = 8192 Hz | |
| 4096 | 46 min | 01:11:00 | 01:57:00 | 4 x offline = 1024 Hz | |
| 512 | endless | 01:59:00 | … | with 8x digfil –> 64Hz | 4x , 4x … offline |
This joblist is an example to get around 10 kHz and 1000+ s
The catch is that the list 3 times repeats a recording schedule good for 10 kHz and 100 - 1000 s. The user experience is here, that the MT activity strongly varies over the day time, and that using MS-06e coils he has to choose between HF & LF mode.
The list ensures that the top layers are well resolved to at least 3 - 5 km depth, and the last job can get the desired deeper layers, useful for modelling.
The joblist is programmed that way, that whenever the USB drive is plugged into the ADU, older jobs will be terminated and cleared, and the new jobs will be started. This system “jumps in”, tries to even partly to run a job. This guarantees that remote reference can be used for all times.
Thanks to NGRI India for this idea.
Download the Rajasthan_JobList and unzip it to get the original file, copy on or USB drive.