Broadband Magnetotelluric Instruments for Near-surface and Lithospheric Studies of Electrical Conductivity: A Fennoscandian Pool of Magnetotelluric Instruments

A set of three new tensor audiomagnetotelluric – magnetotelluric (AMT-MT) instruments have been assembled in Oulu. Their design is based on the MTU2000 magnetotelluric equipment that was developed earlier in the Uppsala University. The new instruments are broadband systems covering the period range from 0.001 s to d.c. Induction coil magnetometers are used to measure magnetic field in the period range of 0.001–1000 s and fluxgate magnetometers for periods longer than 10 s. The electric field is measured with two 50–200 m long orthogonal electric dipoles using non-polarisable Pb/PbCl2 electrodes. The MT systems are equipped with high dynamic range (24 bit ADC) data recorders having a maximum sampling frequency of 3000 Hz and possibility for dual band burst recording. Monitoring of the system health with SMS messages helps to maintain long period observations allowing continuous measurements and reducing the maintenance efforts and costs. The three new instruments extend the Fennoscandian pool of homogeneous MT equipments into 14 systems and allow synchronous array measurements. This makes it possible to use multisite data processing techniques and therefore considerably improve the data quality, especially in electromagnetically noisy regions. The instrument pool can be used in a broad range of applications to study the crust and upper mantle.