gDAS Distributed Array System
Typically, the system has been used for highly efficient coverage with an Offset Pole-Dipole 3D-IP array with coincident MT data acquisition, Vector/Tensor IP and MT, or just MT surveying at distributed sites or in EMAP-style surveys. For MT surveying usually each gDAS-24 acquisition system records time series data during the night, taking advantage of the often better night-time conditions for MT data acquisition, over multiple intervals of 219 or 220 samples, using sample frequencies between 16Hz and 32kHz to provide impedance results from and beyond 0.001Hz through to 8kHz. Recorded time series data is post-processed to provide estimates of the transfer function of the subsurface from which the apparent resistivity and impedance phase are derived, with subsequent 1D or 2D inversion modelling providing resistivity imaging from near surrface to depths of several kilometres and often even tens of kilometres. The arrays are then relocated during the day or the same setup is used for acquisition oftime series IP data simultaneously across the whole gDAS24 array while transmitting a Time or Frequency Domain signal through the required pole o dipole sources. Simultaneous acquisition of Remote magnetic field data in conjunction with the MT results provide the inferred local electric field data for telluric cancelation of the time series IP records while transmitter current monitoring provides a cross reference for stacking and averaging of the secondary decays before the calculation of chargeability and apparent resistivity.