Instrumentation, em

Abstract

Recordings of natural or man-made variations in the magnetic field may be made with fluxgate sensors or induction coils, depending on the frequency of interest. In terms of signal to noise, the crossover between the two sensors is about 0.01 Hz, with induction coils used for measurements higher than 1 mHz and fluxgates for measurements lower than 1 Hz. Exact frequencies depend on the particular equipment and application. Electric field sensors are typically made using pairs of nonpolarizing electrodes to make contact with ground or water, separated and connected to amplifiers by 10–100 m of wire. At high frequencies simple metal rods may suffice as electrodes. Modern electronics and timing derived from the global positioning system have made data collection using autonomous loggers standard for both land and marine MT and CSEM data acquisition.