Regional-scale MT studies
Along with the seismics, magnetotellurics (MT) is one of the leading methods for continental and regional-scale geophysical exploration. The main goal of the continental-scale MT exploration projects is to image the Earth crust and upper mantle below a whole continent. Such projects usually last for decades and are characterized by the typical station spacing about 10-50 km and maximum period for transfer functions’ estimation about 10 000 s (e.g., USArray, AusLAMP). On the other hand, the regional-scale MT exploration projects generally aim to image the Earth crust and its sedimentary cover within a given continental region. Such projects last for years and are characterized by typical station spacing of 1-5 km and maximum period for transfer functions’ estimation about 3 000 s (e.g., examples below). In many cases the regional-scale MT measurements are accompanied by the time-domain EM to achieve better resolution of the upper part of the section and also for the static shift correction of the acquired MT data.
All-Russian network of reference geophysical profiles
Regional-scale magnetotelluric studies in Russia are being conducted as a part of governmental projects initiated by the Russian Ministry of Natural Resources. Nord-West Ltd. is taking active part in these surveys and since 2010 has already acquired more than 50 000 MT sites. An example of resistivity image and geological interpretation of a small part of the 3-SB profile (Siberian platform, in the interfluve between Podkamennaya Tunguska and Nizhnyaya Tunguska rivers) is given below.
The main goal of this survey was to construct a resistivity model of the sedimentary cover by the means of joint interpretation of MT, seismic (CMP), well logging and potential fields’ data. The model was then analyzed to identify the electrical conductivity anomalies associated with good reservoir rock properties in the Lower Paleozoic, Vendian (Neo-proterozoic) and Riphean (Mezo-proterozoic) formations.

Regional-scale MT studies in Russia. Solid black lines represent profiles where acquisition of MT data was completed, while dash-dotted magenta lines show the planned profiles
Three structural levels are identified within the sedimentary cover:
1. Suprasalt formation consisting of layers with varying resistivity. Highly resistive layers and regions are in good correlation with the intrusive bodies (traps), revealed by the borehole data.
2. Salt formation consisting of halogen-carbonate sediments with generally high resistivity (hundreds of Ohm-m), but often with conductive inclusions probably associated with the zones of rock alteration caused by magmatic intrusions.
3. Subsalt formation represented by the Vendian and Riphean (Mezo-proterozoic) sediments. A correlation between the increase in thickness and the decrease in resistivity is observed for terrigenous formation containing high-porosity sandstones (Vanavara formation). In some parts of the area Vendian sediments have low resistivity, which can be explained by the increased fracturing of carbonate rocks.

Resistivity image with geological interpretation for a section of the 3-SB regional profile in the Eastern Siberia: 1 – seismic reflectors by CDP data; 2 – tectonic fault zones revealed by seismic; 3 - the top of the halogen-carbonate formation (Litvintsev formation) determined from CDP and borehole data; 4 - high-resistivity basement according to MT data; 5 - wells; 6 - intrusive rocks according to borehole data; 7 – hydrocarbon fields; 8 – MT stations

Regional MT exploration in Parana Basin, Brazil
This MT & TDEM survey has been carried out in 2014 by Nord-West Ltd. in cooperation with LASA Prospecções S.A. under a contract with the Brazil’s National Agency of Petroleum, Natural Gas and Biofuels. The main goal was to assess the prospects of oil and gas potential in the region and delineate licensed areas. The terms of reference included mapping diabase intrusions, estimating the thickness and depth to bottom surface of the traps, imaging the lateral inhomogeneity of the traps, identifying fault zones, studying the sedimentary formations beneath the traps, discriminating the layers according to their resistivity, estimating the lateral inhomogeneity of the entire resistivity structure, mapping the surface of the crystalline basement, identifying the low resistivity regions in the Earth's crust.

Regional-scale MT exploration in Brazil. Legend: 1, 2 – location of the survey lines;

3 - sedimentary basin boundary ; 4 - Bauru sandstones; 5 – Serra Geral basalts;

6 – Mesozoic-Paleozoic sediments; 7 - crystalline basement; 8 - Ponta Grossa arch.

Resistivity image obtained for the Line 3 (see the map above) is shown to the right. The data clearly reveal inhomogeneous high-resistive formation interpreted as Serra Geral traps overlaid by the Bauru sands; vertical and horizontal heterogeneities of the basaltic formation are imaged. The thickening of the high-resistivity layer with two supplying vertical channels, traced in the central part of the line, corresponds to the well-known Ponto Grossa Arch, which is distinctly identified based on the magnetic prospecting data and, according to the geologists, represents the fan-shaped dyke complexes. The depth to the crystalline basement in the survey area was found varying from 3 km in the southeast to 5.5 km in its central part. The obtained results are in good agreement with the available borehole lithological and logging data. More details on this and other MT surveys of Nord-West Ltd. in thick sedimentary basins could be found in [Palshin et al., 2017].

1 - main boundaries; 2 - other boundaries; 3 - regions with a significant content of intrusive rocks; 4 - regions with predominance of magmatic rocks in the Serra Geral formation; 5 - zones of altered rocks; 6 – MT stations; 7 - borehole positions and charts: 8 - Serra Geral formation; 9 - Ponta Grossa formation; 10 - Precambrian rocks; 11 - intrusive rocks