Glossary

Abbreviations

Table 4 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

Table 5 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.