Depth of investigation of Magnetotelluric method from phase

Ujjal K Borah and Prasanta K Patro

In the present study, we have established a new method to calculate the Depth of Investigation (DOI) of magnetotelluric (MT) method from phase which overcomes the drawbacks of the previous method. For the purpose, we have related the phase data for the lowest evaluation frequency to the corresponding depth mathematically, taking the basic concept of electromagnetic (EM) induction in a stratified earth and instrument sensitivity into account. We tested the effectiveness and reliability of our method by applying the new method to synthetic and field data and comparing the calculated DOIs with the DOIs obtained from a previous method. This study shows that the new method is faster in computation and overcomes the effect of non-uniqueness of inverse modelling, unlike the previous methods and proves to be better choice for DOI calculation.

Fig. Depth of Investigation (DOI) calculation using Spies (1989) approach and its comparison with the new approach. (a) One dimensional model obtained after Marquardt inversion (Marquardt, 1963). (b) Apparent maximum depth with Z = Zmax. The point of intersection of both curves gives the maximum DOI (Spies, 1989). The green circle represents the DOI obtained from the new approach. This provides a comparison with the DOI from the approach of Spies (1989).



For further details:

Borah, U.K. and P.K. Patro, (2019). Estimation of the depth of investigation in the magnetotelluric method from the phase. Geophysics, 84(6), E377-E385.