Multi-frequency calibration of grounded electric lines in MT data acquisition systems

The sensitivity of a well-grounded electric line has a constant real value, whereas that of a poorly grounded electric line is a complex, frequency-dependent function. To account for this, one can apply a multi-frequency in-situ electric line calibration method, which may be regarded as a natural extension of the classic receiving unit (“box”) and magnetic sensor (“coil”) calibration procedures. Without such correction, the inversion of seemingly high-quality magnetotelluric data may yield models with false conductive and resistive structures. The proposed electric line calibration method requires accurate estimation of the complex impedances of each grounded dipole over a wide frequency band. This procedure can be performed either using the built-in impedance analyzer of modern magnetotelluric instrumentation (such as NORD receiver by Nord-West Ltd., used in this study) or a simple handheld LCR meter. Field experiments demonstrate the effectiveness of the proposed approach in correcting magnetotelluric data acquired with poorly grounded electrodes of various types (graphite or brass, with or without additional resistive load). Furthermore, the impedance behavior of real poorly grounded electric lines is found to follow the generalized Cole–Cole model rather than the simpler models previously used.