The main stages of any filed survey are equipment calibration, data acquisition and primary data processing. The observations can be carried out on 2D- (profile) or 3D-geometry grid. The grid step is selected depending on the required resolution of imaging. Initial EM data processing is performed in the field using the original software consistent with the equipment. We carry out the observations using a number of techniques and methods.
Magnetotelluric (MT) observations are carried out using Phoenix MTU equipment. The frequency range of the MT recording depends upon the particular technique: audio-MT (from 10 kHz to 1Hz) or broadband MT (from 400 Hz to 0.0001 Hz). The array is installed using GPS receiver and magnetic compass. All 5 components of the MT field are typically recorded. We use self-made non-polarizable electrodes for electrical field measurements. Remote reference technique could be used to suppress the EM noise, if any.
Transient sounding (TEM) measurements are performed with TEM-FAST or CYCLE-5 systems. Typically we use single loop or loop-in-loop array for vertical magnetic field excitation and measurement. This technique allows to obtain the decay curve in a time range from 10 mcs to 1 ms.
Depending on particular CSEM technique a wide range of observation schemes, arrays and equipment can be used including Phoenix MTU-5/5A for CSMT.
DC resistivity measurements are carried out using profiling, median gradient, vertical electrical sounding and electrotomography techniques. A number of arrays, including Shlumberger, Wenner, dipole, median gradient and multi-electrode arrays are utilized for observations. A variety of transmitters and receivers could be used for low-frequency electric field excitation and measurement including ASTRA-100, MARY-24, IMVP-8, SysCal Pro, etc.
Induced polarization (IP) observations could be carried out using classical IP technique or Differential Normalized Method (DNM) technique. The classical scheme (both in frequency and time domain) presumes the usage of median gradient or Shlumberger array. The DNM observations (time-domain) are performed with a special array including two transmitter lines AB and two perpendicular receiver lines. The ASTRA-100 and VP-1000 are used as transmitters, MARY-24, IMVP-8 as receivers.