Magnetic shielding

Broadband seismometers are to some degree sensitive to magnetic fields because all thermally compensated spring materials are slightly magnetic. This may become noticeable when the seismometers are operated in industrial areas or in the vicinity of dc-powered railway lines. Magnetic interferences must be suspected when the long-period noise follows a regular time table. Shields can be manufactured from permalloy ($\mu$-metal) but they are expensive and of limited efficiency. An active compensation is often preferable. It may consist of a three-component fluxgate magnetometer that senses the field near the seismometer, an electronic driver circuit in which the signal is integrated with a short time constant (a few milliseconds), and a three-component set of Helmholtz coils which compensate changes of the magnetic field. The permanent geomagnetic field should not be compensated; the resulting offsets of the fluxgate outputs can be electrically compensated before the integration. Information on a practical installation can be obtained from [stuadmin@geophys.uni-stuttgart.de 1998].