Least counts:
For the 16-bit instrument, ±4.5V corresponds to 2^16, so the least count is 137.33 microvolts per count.
For the 24-bit instrument, ±4.5V corresponds to ±5242880 counts, so LC is 0.8583 microvolts per count.
Various sensor, filter, and amplifier response functions must be taken into account when processing data:
The heart of the high-frequency seafloor MT system is a high-gain, AC-coupled amplifier designed originally for controlled-source applications. It is normally run at a nominal gain of 1,000,000 in the pass band on an e-field antenna length of 10 m.
High-gain seafloor E-field amplifier: Picture or Table
We put commercial magnetic sensors in pressure cases to extend their depth capabilities to 7000m. However, induction in the metal case at high frequencies has an effect on the response of the coil. EMI distribute individual calibration files for the BF4 coils, which have a nominal gain in the pass band of 0.3 V/nT.
Effect of seafloor pressure case on BF-4 sensor: Picture or Table
Anti-alias filters usually appear on all 16-bit channels, and so for MT work where ratios of fields are used it is not normally necessary to correct for these filters. However, in seismic work and whenever time series data need to be compared with data from other instruments, the filter response must be taken into account within about a decade of the nyquist frequency. Also, we sometimes use these filters as front-end, high-cut stages to remove troublesome 60 Hz powerline noise. In these cases, an extra filter stage appears in the magnetic field measurements only and needs to be taken into account. NO ANTI-ALIAS FILTERS ARE USED ON THE 24-BIT INSTRUMENT.
Frequency devices anti-alias filters: Picture or Table
For the 16-bit logging systems, we apply x30 or x100 gain to the BF-4 magnetic sensors using an AC-coupled post-amplifier. NO MAGNETOMETER POST-AMPLIFIERS ARE USED ON THE 24-BIT INSTRUMENTS.
Magnetometer post-amplifier: Picture or Table