Augmenting the magnetotelluric response function: an integrated geophysical approach

Abstract

We show, in a typical groundwater study, a well know problem in electromagnetic exploration that near-surface (<40 m deep) electrically conductive layers can mask deeper resistive features. This is also true for the magnetotelluric (MT) method since the MT method is naturally sensitive to conductive bodies, especially in the near-surface, and insensitive to resistive features. To solve this problem, we use the electrical resistivity tomography (ERT) method to constrain the near-surface electrical resistivity structure missing in the MT method. The combination of ERT data to a depth of 40 m with audio magnetotelluric (AMT) data starting at a depth of ~30 m will improve lateral and vertical resolution of the top 30 m and reduce the masking effect, of near-surface conductors, on deeper resistors. Using Maxwell’s equations and Ohm’s Law we derive the well-known current density equations for both the MT and ERT methods. We then equate both the MT and ERT current density equations, through the common layer conductivity value in Ohm’s Law. The resulting newly derived equation is simplified so that a pseudo-MT impedance tensor can be calculated from ERT data. The resulting pseudo-MT impedance tensor is used to calculate both the phase and resistivity values for each ERT data point used in the conversion. The pseudo-MT data is then combined with MT data at the same location to form, what we named, the augmented ERT-MT response function. The augmented ERT-MT response function is then inverted, resulting in an improvement of up to ~ 90 % in recovered electrical resistivity structures. This improvement is observed in multiple resistivity inverse models when compared to the true electrical resistivity structure for both the synthetic layered one-dimensional (1-D) case and a two-dimensional (2-D) half-space. Both 1-D and 2-D synthetic models, using the augmented ERT-MT response function, mapped resistive features below conductive layers that are absent in resistivity inversions using MT data only. The augmented ERT-MT response function is applied to AMT data collected over an electrically resistive dyke beneath a conductive overburden. The augmented ERT-MT response function again assisted in mapping the resistive (~1000 Ω∙m - ~10000 Ω∙m) dyke that was absent in the AMT only resistivity inverse model. The augmented ERT-MT response function is used on a second site to map an aquifer system contained in the Malmani Group dolomites. As expected, greater detail is seen in the near-surface (< 100 m deep) in the augmented ERT-MT response function resistivity inverse model while retaining the deeper features from the standard AMT resistivity inverse model. The fault zone, ~120 m along the profile at the second research site, is seen in the resistivity inverse model using the augmented ERT-MT response function, which is missing in the AMT resistivity inverse model. Therefore, for both conductive and resistive features, the augmented ERT-MT response function improved the AMT resistivity inverse models by recovering features that are generally masked by near-surface conductive layering for both the synthetic and case study experiments.