Estimation of the depth of investigation in the magnetotelluric method from the phase

Abstract

Proper computation for depth of investigation (DOI) of the magnetotelluric (MT) method is vital because the standard approximate depth transforms provide the depth corresponds to the interpreted conductivities instead of the actual DOI of MT. Previous works on DOI estimation of the MT method are based on 1D inversion. Although the previous works are significant, the methods of DOI calculation are lengthy and affected by non-uniqueness of inverse modeling. Therefore, to overcome these problems, we developed a simple and direct approach of DOI computation for the MT method using phase, which applies to a layered earth. For this purpose, we calculate the instantaneous phases of electric and magnetic fields, which correspond to the minimum evaluation frequency using the solution of the 1D electromagnetic (EM) diffusion equation and instrument sensitivity. From the calculated instantaneous phases, we derive an expression that connects the phase-lag between the electric and magnetic fields at the minimum evaluation frequency to its corresponding DOI. To examine the effectiveness of our method, we applied the new approach to 1D synthetic and field data and compared the calculated DOIs with the DOIs obtained from a previous method. We determine that our new approach is faster in computation and overcomes the effect of non-uniqueness of inverse modeling, unlike the previous methods.

Lithospheric Conductors Shed Light on the Non‐Uniform Destruction of North China Craton

“…Data quality for most stations is good, although a few MT sites near densely populated area suffer from high cultural noise, especially in TNCO and ENCC, which were excluded in the 3-D inversion. We applied the latest formula (Borah & Patro, 2019) to estimate the depth-of-investigation (DOI) for the used data before 3-D inversion, and the results showed that the DOIs of BBMT and LMT data are almost greater than 60 and 150 km, respectively (Figure S1 in Supporting Information S1).…”