SouthernRocks association with the Chilean company Advanced Geophysical Technologies S.A. (AGT) has provided over 6 years of seamless development of the gDAS24 24-bit distributed array system and our long term commitment to AGT provides SouthernRock with exclusive and dedicated availability of the gDAS24 technology.
The system has been designed with a focus on simplicity and flexibility. Each gDAS-24 receiver records time series data using 24-bit ADC's at operator selectable sample rates between DC and 16kHz on each of its 4-channels according to a pre-programmed schedule stored in memory and controlled by synchronous GPS timing and slaved crystal oscillators. Oversampling provides an increase in resolution of up to 29 bits at the low end of the sample rate. Field data is recorded and periodically backed-up to a solid state memory drive and data is later downloaded to a PC for processing. Any number of 4-channel gDAS24 might be located along any survey line, distributed amongst a grid, or remote site, installed with respective electric field dipoles and / or magnetic antennas depending on the survey design.
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 2^20 samples, using sample frequencies between 32Hz and 16kHz 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.