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Instrumentation

Long-Period Magnetotelluric Instrument(LEMI-417M)

Each MagIE site is currently using a LEMI-417M to record geomagnetic field and geoelectric field (where possible) strength. These instruments are designed for long-period magnetotelluric soundings (<1 Hz). Each instrument is comprised of a fluxgate magnetometer, a set of electrodes (although not always present), a recording unit and a GPS antenna for timestamping. The LEMI-417M samples once every second and writes binary files to a flashcard. In the case of Birr, it was not possible to record electric field strength due to the proximity to Birr town

The 3-axis fluxgate magnetometer is on the left, recording device is in the middle, with electrodes in the right background and data logger in the right foreground.

The fluxgate magnetometer consists of two separate coil windings wrapped around ring core of a magnetically permeable alloy. An alternating current is applied to one winding, known as the drive winding. This constant changing of magnetic field strength of the AC current in the drive winding induces a current in the other winding, the sense winding. In the absence of an external magnetic field, the net currents will equal and cancel out. With the addition of an external field, the half core generating a magnetic field in the direction of the external field will saturate sooner than the other half, and the magnetic fields from the hemispheres will no longer cancel out. This will lead to a net change of flux in the sense winding, and a voltage will be induced proportional to the strength of the external field. The magnetic field strength can be calculated from this Voltage. To get a 3D representation of the magnetic field strength in the LEMI-417, three separate fluxgate magnetometers are used to get the Bx ,By and Bz components of magnetic field strength.

Key Features:

  • High resolution of 0.01nT
  • Has a measurement range of ±68’000nT
  • Low temperature drift of <0.2 nT per ◦C.
  • Very Low power consumption of <1.2W – allows device to be operated by mains power supply or batteries for more isolated locations