The advantages of complementing MT profiles in 3-D environments with geomagnetic transfer function and interstation horizontal magnetic transfer function data: results from a synthetic case study

Abstract

: The advantages of complementing MT profiles in 3-D environments with geomagnetic transfer function and interstation horizontal magnetic transfer function data: results from a synthetic case study. -Geophysical Journal International, 207, 3, pp. 1818-1836 S U M M A R YAs a consequence of measuring time variations of the electric and the magnetic field, which are related to current flow and charge distribution, magnetotelluric (MT) data in 2-D and 3-D environments are not only sensitive to the geoelectrical structures below the measuring points but also to any lateral anomalies surrounding the acquisition site. This behaviour complicates the characterization of the electrical resistivity distribution of the subsurface, particularly in complex areas. In this manuscript we assess the main advantages of complementing the standard MT impedance tensor (Z) data with interstation horizontal magnetic tensor (H) and geomagnetic transfer function (T) data in constraining the subsurface in a 3-D environment beneath a MT profile. Our analysis was performed using synthetic responses with added normally distributed and scattered random noise. The sensitivity of each type of data to different resistivity anomalies was evaluated, showing that the degree to which each site and each period is affected by the same anomaly depends on the type of data. A dimensionality analysis, using Z, H and T data, identified the presence of the 3-D anomalies close to the profile, suggesting a 3-D approach for recovering the electrical resistivity values of the subsurface. Finally, the capacity for recovering the geoelectrical structures of the subsurface was evaluated by performing joint inversion using different data combinations, quantifying the differences between the true synthetic model and the models from inversion process. Four main improvements were observed when performing joint inversion of Z, H and T data: (1) superior precision and accuracy at characterizing the electrical resistivity values of the anomalies below and outside the profile; (2) the potential to recover high electrical resistivity anomalies that are poorly recovered using Z data alone; (3) improvement in the characterization of the bottom and lateral boundaries of the anomalies with low electrical resistivity; and (4) superior imaging of the horizontal continuity of structures with low electrical resistivity. These advantages offer new opportunities for the MT method by making the results from a MT profile in a 3-D environment more convincing, supporting the possibility of high-resolution studies in 3-D areas without expending a large amount of economical and computational resources, and also offering better resolution of targets with high electrical resistivity.

Magnetotelluric characterization of the Alhama de Murcia Fault (Eastern Betics, Spain) and study of magnetotelluric interstation impedance inversion

“…In the analysis of the responses and inversions using Z, ZI and qZ, we followed a methodology similar to the work of Campanyà et al (2016), which focused on the interstation horizontal magnetic tensor (H) (the relationship between the horizontal magnetic fields at two locations), but we adapted it to the calculation of interstation impedances (ZI). Departing from different synthetic models, we calculated the corresponding impedances (Z) and interstation impedances (ZI), using one or several configurations (relative locations between local and neighboring sites); and then performed three types of inversion: inverting Z, inverting ZI, and inverting qZ.…”

Subsurface Characterization of the Pennsylvanian Clare Basin, Western Ireland, by Means of Joint Interpretation of Electromagnetic Geophysical Data and Well‐Log Data

“…The fact that the initial model was based on the results from the Z inversion facilitated the convergence of Z , T , and H during the joint inversion. Due to the complexity of the geology at depth, the reference sites for the H data were selected after the inversion of the Z data, choosing regions with no major anomalies below the reference site, as recommended by Campanya et al ().…”

“…Figure b2 shows the distribution of the electrical resistivity values within each cluster as histograms. The bin width for the histograms, 0.15 at log10 scale, was selected based on the resolution achieved when constraining the subsurface electrical resistivity values using electromagnetic geophysical methods from a synthetic study similar to the model derived from the present study (Campanyà et al, ; Table ). These histograms underpin the interpretation of the electrical resistivity model of Figure .…”

Crustal structure of a young collision zone: the Arabia–Eurasia collision in northeastern Turkey investigated by magnetotelluric data

“…The error of the phase tensor data was also taken into consideration, as suggested by Booker (2014). In order to estimate skew angle errors, ten thousand skew values were computed by utilizing different values of all components of impedance tensor (Z xx , Z xy , Z yx and Z yy ) which are randomly distributed within the total range of the assumed error, following the approach of Campanyà et al (2016). For periods smaller than 0.1 s, most of the ellipses show weak polarization and the small skew values (mostly < [− 2° 2°]), indicating that 1D or 2D structures are dominant.…”