EXPERIENCE AND PROSPECTS OF MAGNETOTELLURIC SOUNDING APPLICATIONS IN SEDIMENTARY...

N.A. Palshin, E.D. Aleksanova, D.V. Yakovlev et al.

GEOPHYSICAL RESEARCH, 2017, V. 18, № 2, P.27-54.DOI: 10.21455/gr2017.2-2

EXPERIENCE AND PROSPECTS OF MAGNETOTELLURIC SOUNDING APPLICATIONS IN SEDIMENTARY BASINS

©2017 N.A. Palshin1, E.D. Aleksanova2, A.G. Yakovlev3, D.V. Yakovlev2, R. Breves Vianna4

1 Shirshov Institute of Oceanology, RAS, Moscow, Russia

2“Nord-West”, Ltd., Moscow, Russia 3 Lomonosov Moscow State University, Moscow, Russia

4LASA Prospecções S.A., Rio de Janeiro, Brazil

Studies of sedimentary basins using the magnetotelluric (MT) sounding method have been carried out for more than 50 years. Over the past fifteen years the number of MT surveys increased manifold, which prompted the development of MT data acquisition technology, inversion and interpretation methods and the efficiency of the method achieved a new level. MT sounding method have become in high demand in the market of geophysical services. The application of MT sounding is especially effective in the regions with basalt traps, salt tectonics and folded belts, i.e. in the areas where seismic methods are confronted with certain difficulties.

The paper deals with special features of sedimentary rocks electrical conductivity and its dependency on petrophysical and hydrophysical parameters: clay content, porosity, fluid salinity and temperature. Specific requirements to MT studies technology in sedimentary basins are formulated: wider period range in use and significantly smaller distances between acquisition sites to compare with deep academic studies. MT analysis and interpretation technology includes: dimensionality analysis, static shift correction, multistage data inversion and geological interpretation of resistivity structure. The use of a priori geological and geophysical information is essential.

The results of a geological interpretation of MT sounding data obtained in the Paraná Basin and in the Eastern Siberia (the biggest inland areas of basalt traps) are given as examples. MT data interpretation enables to study structure of sedimentary basins in details and to identify main geological formations. Also the results of MT investigations in the Taimyr, where several new objects with good prospect for oil and gas were revealed and resistivity anomalies probably related to gas hydrates were singled out, are presented. The basic directions of the further development of the MT method to enhance the efficiency of studying sedimentary basins were formulated. Keywords: magnetotelluric sounding, sedimentary basin, basalt traps, salt dome tectonics, fold belts, reservoir and sealing properties, joint and constraint inversion.

Introduction

Magnetotelluric (MT) sounding method began its development in 1960s, mainly in application to sedimentary basin imaging (cf. [Berdichevsky, 1960, 1980; Bezruk et al., 1964; Keller, 1968; Vozoff, 1972; Dmitriev et al., 2014]).Herewith, a substantial part of the widely known academic research and publications in this field have had a primary focus on the deep structure of the Earth’s crust and upper mantle which became possible due to the method’s large depth of investigation and relative simplicity of long-period data acquisition [Bedrosian,2007; Korja, 2007]. Nevertheless, use of MT for sedimentary basins’ imaging has become widespread, with huge number of sites acquired under the framework of commercial exploration projects. In recent years Russian geophysical companies alone have carried out magnetotelluric acquisition at as many as dozens of thousands of stations. Similarly, intense MT exploration is under way in China, Canada, Kazakhstan, Uzbekistan, where service companies have a large number of modern magnetotelluric instruments. However, results of those studies mostly remain undisclosed and little known to the academic community.



Of course, seismic methods still play the leading role in sedimentary basins’ exploration due to high resolution, but in a number of quite common geological settings magnetotellurics turns out to be more efficient.

First of all, this takes place if the structure contains “rigid” seismic reflection boundaries in the near-surface part of the section and/or an inversion of seismic velocities is observed. Such settings are typical for sedimentary basins with flood basalts coverage, which are often oil and gas bearing [Patro et al., 2005; Berdichevsky et al., 2015; Patro, 2017]. The largest basaltic fields are found on the ocean floor; platform magmatism is less than 10% of the total volume of Phanerozoic igneous rocks and is mainly represented by the trap series of the tholeiitic flood basalts. The intrusive trap formation consists of sills and dikes; the former may reach hundreds of meters in thickness. The presence of numerous sills and dikes in the sedimentary formations, on the one hand, ensures the existence of oil traps of various types. On the other hand, eruption-related heating can affect the process of generation and migration of hydrocarbons [Gajula, 2008].The largest continental sedimentary basins of this type are those of Eastern Siberia, the Paraná Basin and the northwestern part of Hindustan [Saunders et al., 1992]. The formation of basaltic traps in these regions is associated with mantle plumes that existed during the disintegration of Pangaea supercontinent at the boundary between the Paleozoic and Mesozoic (Siberian trap), as well as during the split of the Gondwana supercontinent in the Mesozoic (traps of the Paraná Basin and northwestern Hindustan) [Zonnenshain, Kuzmin, 1983; Khain, Lomize, 2005; Patro, 2017].

Another type of sedimentary basins, where MT is efficient, is associated with salt-dome tectonics (see, for example, [Aleksanova et al., 2009]). Salt diapirs are found in many sedimentary basins, having salt deposits overlain by formations with sufficient thickness, as well as in fold belt areas. Classical salt diapirs (domes) are formed due to gravitational instability in regions that have not undergone significant tectonic stress; however some salt domes are found in tectonically active regions. The first type includes areas of salt-dome tectonics in the Gulf of Mexico, the north of continental Europe and the North Sea, the Caspian depression, while the Middle East salt diapirs, found in Iraq, Iran and the Arabian Peninsula belong to the second one [Hudec, Jackson, 2007].

Almost all salt-dome tectonics areas are oil and gas bearing basins, often large. The oil and gas deposits may be associated either with tops or slopes of the salt domes, as well as with the subsalt formations. In the latter case, the salt plays a role of seal or cap rock. Salt diapirs have a complex shape, often with steeply dipping boundaries and high seismic velocity contrasts. Inside the diapir, no reflectors are usually observed. All of those things make the interpretation of seismic time sections tricky and complicated (especially if the subsalt formation is the main target).

For MT, the presence of salt domes is not an obstacle, and its large depth of investigation makes it possible to infer reliable information about the subsalt sedimentary structure.

Magnetotellurics has certain advantages when applied to map the structure in folded regions, where the geological boundaries are characterized by large dip angles, which also makes interpretation of seismic data fairly challenging [Berdichevsky et al., 2015]. Taimyr and Zagros fold belts (northeast coast of the Persian Gulf), northwest part of Colombia, the



Chaco Basin in Bolivia and Paraguay are among the examples of promising oil-and-gas-bearing folded regions.

Taking the abovementioned into account, in the territory of Russia, the main regions where one can expect MT to be efficient, are: Eastern Siberia (Fig 1, I), Caspian Region (II), Taimyr-Yenisei-Khatanga Region (III) and Timan-Pechora Province (IV).

Fig. 1. Regions of effective application of the MT method in Russia (contoured by a thick dashed line): I – Eastern Siberia, II – Caspian Region, III – Taimyr-Yenisei-Khatanga Region, and IV – Timan-Pechora Province

In most cases, MT surveys conducted in sedimentary basins are aimed at the solution of

the tasks as following: regional-scale studies along the reference geophysical lines, prospecting and exploration of hydrocarbons; prospecting and exploration of deep aquifers; prospecting for geothermal energy sources.

The first two of those tasks are traditionally of the primary focus, with largest amounts of magnetotelluric studies being done for solving them.

Electrical conductivity of sedimentary rocks

The main result of the MT studies is a subsurface resistivity image – a two- or three-dimensional model describing the distribution of electrical resistivity inside the structure (medium). The ultimate and most important stage of the research is the geological interpretation of the resistivity model obtained. When electromagnetic (EM) methods are applied for hydrocarbon exploration, interpretation is based on an understanding of the mechanisms controlling electrical resistivity (or conductivity) of sedimentary rocks. Note that with the significant progress achieved recently in understanding the nature of the electrical conductivity of the rocks of the Earth’s upper mantle (see, for example, [Yoshino, 2010]), the features of the sedimentary rocks’ electrical conductivity, studied in detail by borehole methods, often remain poorly known to the specialists involved in conventional surface EM prospecting.

Conductivity can be represented as a product of charge carrier concentration, charge value and carrier mobility:



here n is the charge carrier concentration; ρ is charge value;μ is charge carrier mobility; N is number of charge carrier types.

Depending on the charge carrier type, three main conductivity mechanisms can be distinguished: electronic, ionic, and semiconductor. We will focus on ionic conductivity in detail, since this particular mechanism is dominant in sedimentary rocks. To a first approximation, we can assume that the sedimentary rock consists of a non-conducting matrix and a conducting fluid (saline solution), whose properties define the conductivity of the rock. Charge carriers in solutions (fluids) are positive and negative ions, formed from the molecules of salts, acids, and alkalis when being dissolved by water. In this simple model, the electrical conductivity of a sedimentary rock depends on fluid’s conductivity and its volume, with latter being defined by porosity coefficient. Thus, the resistivity of the sedimentary rock mainly depends on only two quantities - the electrical conductivity of the fluid and the rock porosity. These obvious facts are reflected in a well-known Archie's empirical law [Archie, 1942]:

, where ρs is the in-situ resistivity of the rock; ρf is fluid resistivity, F is effective porosity.

However, laboratory measurements of electrical conductivity of samples of different sedimentary rocks have shown that in many cases this simplest relation does not hold. It turns out that at least two other factors, the fluid saturation and the matrix configuration (pore space geometry), also have a significant influence. Thus, the Archie’s ratio can be rewritten in a more complete form: , where Sf is the fluid saturation; m is the cementation parameter; n is the saturation parameter; a - tortuosity factro, usually taken as 1.

The Archie’s law or a similar empirical relation proposed by V.N. Dakhnov [1941] can be used to study the petrophysical and hydrophysical properties of most of terrigenous sedimentary rocks.

It should be noted that the electrical conductivity is affected only by the so-called effective porosity - the ratio of the volume of connected pores to the total volume of the rock. Thus, the model includes a pore connectivity parameter, whose estimation is fairly challenging. Obviously, if all the pores are disconnected, the electrical conductivity of the rock is no longer dependent on the porosity. The effect of matrix structure is also very important.

It turned out that for clay rocks containing a significant number of particles with an effective diameter of less than 10 micrometers, the above mentioned relations are barely applicable, since the electrical conductivity of such rocks weakly depends on the salinity (conductivity) of the fluid (Fig. 2). This occurs due to the fact that the pores in clay rocks have subcapillary dimensions, which results in arising of the so-called "surface" conductivity mechanism.

Electrical double layers formed at the contact between the solution and the matrix have dimensions comparable to those of the pore (capillary), and the solution in the capillary has higher electrical conductivity than the fluid in the volume. At the same time, the concentration of ions at the interfaces increases significantly, parallel conducting channels are formed, resulting in the elevation in rock conductivity. This effect is especially pronounced if the fluid



initially has low ion concentration and, consequently, its conductivity is relatively low [Ryzhov, Sudoplatov, 1990].

Thus, the electrical conductivity of sedimentary rocks depends on the clay content, pore space geometry, porosity, fluid saturation, and, finally, the electrical conductivity of the pore fluid. The first four factors characterize the petrophysical and hydrophysical properties of the rock. The latter one is the most important, since it’s the main factor controlling the electrical conductivity of the majority of non-clay sedimentary rocks.

Fig. 2. Dependence of the electrical resistivity of sandy-clayey sediments on the concentration of NaCl in the pore fluid (log-log scale) 1 - pure sand; 2-7 - sand with different clay content (2 - 10%, 3 - 15%, 4 - 20%, 5 - 30%, 6 - 40%, 7 - 60%); 8 - pure clay. Modified from [Ryzhov, Sudoplatov, 1990].

The electrical conductivity of pore fluids is primarily determined by the concentration

and chemical composition of the solution compounds (salts, acids and / or alkalis) and temperature. The fluid composition, as well as the concentration of the substances forming the solution, is very diverse and depends on the geological structural features, the geological history of the region, and also on geomorphological, meteorological and hydrological factors. In the areas with current or recent tectonic activity, carbon and nitrogen waters are often found. Deep sedimentary formations in fold belt areas (foredeep basins) are commonly associated with highly mineralized fluids, often enriched with hydrogen sulphide. Calcium chloride and sodium chloride solutions are widespread in the deep layers of sedimentary basins. In general, the composition of fluids mainly includes chlorides, bicarbonates and carbonates of metals of sodium, calcium, potassium, and magnesium. The sodium chloride content can reach up to 90% of the total saline content of solution, therefore in most cases we can treat the pore fluid as a sodium chloride solution.

With increase in solution concentration, its electric conductivity initially increases as well, then reaches a certain maximum value and begins to decrease. This dependence is very clearly pronounced for strong electrolytes and is much less clear for the weak ones. The presence of a maximum on the conductivity-versus-concentration graphs is explained by the fact that in the dilute solutions of strong electrolytes the ion velocity weakly depends on the concentration and initially increases approximately in proportion to the number of ions. However, further growth of concentration leads to increase in ion interaction, which reduces the velocity of ion motion. For weak electrolytes, the presence of a maximum on the curve is due to the fact that the degree of dissociation decreases with increasing concentration, and



once a certain concentration is reached, the number of ions in the solution begins to increase more slowly than the concentration. In practice, a decrease in electrical conductivity with increasing concentration is observed only in brines, and is extremely rare in pore fluids.

With increasing temperature (t), the resistivity of the fluid (ρt) decreases due to an increase in the ion velocity caused by the decrease in both fluid viscosity and ion solvation (thus we have an increase in conductivity).The fluid resistivity can be expressed as

follows: , hereρ18°C is the resistivity of the fluid at 18°C; a is a dimensionless constant equal to 0.025 for NaCl solution.

Obviously, this dependence of fluid resistivity and, consequently, the bulk rock resistivity on temperature(along with mapping the conductivity anomalies caused by hydrothermally altered rocks), enables the application of EM prospecting methods for geothermal exploration. The resistivity of sedimentary rocks depends on many factors and, as a rule, an unambiguous interpretation of the observed anomalies in terms of geological structure is hardly possible without additional petrophysical and hydrophysical information. In many cases, when data or at least some estimates on lithology, petrophysical and hydrophysical properties of sediments (clay content, pore space geometry, fluid saturation, typical salinity of brines) are available, it becomes possible to estimate the effective porosity as reservoir quality index (or clay content as cap rock quality).

Due to the ability of estimating the reservoir properties of sedimentary rock and/or cap rock properties, based on electrical conductivity, magnetotellurics finds its application in the prospecting and exploration of hydrocarbons and aquifers.

Magnetotelluric data inversion and joint geological interpretation

The most important part of any geophysical study is the reconstruction of the physical models of the structure from the observed data, i.e. inverse problem solution or data inversion. The ultimate phase of the research is the interpretation of the obtained physical models in terms of geological structure and the construction of geological and/or lithological models. At both the inversion and interpretation stages, the use of available geological and geophysical data makes it possible to increase the efficiency of the survey and the reliability of the results obtained.

As already noted, the seismics is the most efficient and high-resolution geophysical method applied to map the sedimentary basins’ structure. Seismic data are more sensitive to geological features (the boundaries of layers with different elastic properties), while magnetotelluric responses depend on petrophysical and hydrophysical properties of the rock formations. Therefore, it is obvious that the joint use of the results of seismics and magnetotellurics, as complementary methods, makes it possible to significantly increase the reliability and accuracy of the geophysical studies’ results. One of the most consistent and efficient approaches of combining those two methods is the joint data inversion. In this context, two main techniques may be distinguished - true joint inversion and constrained inversion.

In the first case, the measured data of two (or more) methods are inverted using a single algorithm in one optimization procedure. As a result, the final model contains, mainly, the elements that provide a sufficient contribution into all input data [Haber, Gazit, 2013]. Such



an approach is effective only if the model parameters, resolved by different methods, are connected with each other through some relation. A typical example is a joint inversion of seismic and gravity data, based on the relation between seismic velocity and the rock density. Unfortunately, a similar relation between seismic velocity and electrical resistivity is either weak or absent. It should be noted that application of joint inversion to magnetotelluric and seismic data can potentially lead to the situation, in which significant part of the information contained in both seismic and magnetotelluric data doesn’t show up in the models.

Nevertheless, there are certain geological settings, for example, the evaporite formations, for which the relationship between seismic velocity and electric resistivity does exist and realistic models can be derived from joint inversion (see, for example, [Medina et al., 2012]).

In other situations, for example, in salt-dome tectonic settings, a joint inversion of gravity and EM data can be very efficient due to the fact that the salt formations are characterized by high electrical resistivity and low density compared to those of the host sedimentary rocks. In this particular case, joint inversion is justified and efficient, since the salt-associated anomalies of the electrical conductivity and excess density exhibit good spatial correlation (see, for example, [Jegen et al., 2009]).

The basic idea of the constrained inversion is to introduce some model features which had been derived from data of other geophysical method(s) (for example, through fixation of certain parameters) into inverse problem solution. Most often this approach utilizes seismic data as a priori information for the inversion of data from other methods. In particular, seismic reflectors are often identified as resistivity boundaries, and once their geometry is specified, it is not allowed to change throughout the EM inversion process. In this case, the inversion is limited to determining the electrical conductivity distribution within layers, whose geometry is fixed.

The latter approach is the most efficient in the joint use of seismic and EM prospecting data. Indeed, the electrical conductivity and seismic wave velocity in sedimentary rocks are, as a rule, of different nature and weakly correlated with each other. On the one hand, in most cases this makes true joint inversion inefficient, but on the other, this fact opens up great opportunities for joint use of seismic and EM prospecting data, as the results of distinct methods complement each other.

The inversion of magnetotelluric data constrained by seismic model is applicable in sedimentary basins due to the presence of subhorizontal layer boundaries, marking a sharp change in sediment lithology. Those boundaries separate contrast values of resistivity and/or seismic wave velocity. Seismic boundaries may or may not coincide with resistivity ones, but even if they don’t they usually have similar shape, which is due to general layering features, typical for sedimentary basins. Identification of such boundaries, if they exist, is crucial for constrained inversion methods, which imply that model geometry (number of layers and their thicknesses) be fixed based on seismic reflectors imaged. With this approach, the inversion procedure is limited to determining the distribution of the resistivity within the specified structure geometry (Fig. 3).



Fig. 3. Stages of magnetotelluric data inversion constrained by structure geometry derived from seismic. Stage I - interpreting seismic data, correlating major reflectors; Stage II - filling the seismic section with rock resistivities derived from MT data; Stage III – construction of a lithological model from the MT resistivity images and the electrical logging chart measured in a reference well.

To avoid the inaccuracy in boundaries’ geometry, a priori geological and geophysical information should be utilized. The most important source of this information is the results of electrical borehole logging. A direct comparison of the resistivity models inverted from MT data with those derived from logging is usually incorrect due to difference in scale.

For a more adequate comparison of the resistivity models obtained from magnetotellurics and electrical logging, further analysis and processing of the logging data can be done using the method proposed in [Pedersen et al., 1988]11. This method accounts for certain features of the logging data as well as for the anisotropy of the electrical conductivity of the rock formations. It utilizes relations, which are essentially similar to the formulas for calculating the equivalent resistance of the circuit containing the resistors connected in parallel (ρL) and in series (ρN). To evaluate circuit total resistance in case of parallel connection of circuit elements, the summation of their conductance (inverse resistance) is applied, while in case of series connection, on should add up the resistances:

whereρk (zi) is the resistivity reading from the logchart at the depth zi; Δzi is the depth

spacing ("elementary layer thickness"). Since logging is most often performed with constant depth spacing, the value of Δzi is constant throughout the chart.

Thus, for ρLwe have the ratio of summated thicknesses (for certain "elementary layers") to their summated conductances. Similarly, (ρNis calculated as a ratio of summated transverse resistances of the "elementary layers" to their total thickness. Given the fact that the logging

1 This method has been successfully applied, in particular, to analyze the logging data from Kola

superdeep borehole [Zhamaletdinov, 2011].



readings are significantly affected by (ρN, these estimates cannot be considered true. However, they yield the range of the possible resistivity anisotropy. The averaging is done either for selected stratigraphic or lithological intervals, or within a moving interval of certain length, comparable with the wavelength. Analysis of the data of electrical (especially induction) logging allows one to distinguish the boundaries of sedimentary formations and identify them in terms of stratigraphy. If acoustic logging data is also available, it becomes possible to analyze the correlation between seismic reflectors and resistivity boundaries.

Unfortunately, in many cases, available geological and geophysical data is insufficient to perform a constrained inversion and an "ordinary" unconstrained inversion is done. In this case, the choice of the initial guess model and the a priori model plays an important role. For sedimentary basin settings, as a rule, a smooth one-dimensional (1-D) resistivity model, derived from either TE-mode or determinant impedance responses, is used as both the initial guess and the a priori model. Inversion is run in several stages, and including the TM-mode responses into inverted dataset allows imaging high-resistivity features, if the structure is essentially 2-D. Multi-stage inversion is a series of successive inversions with different parameters, and the final model from the previous stage is used as initial guess at the next one. Among the inversion parameters there are frequency range, response component type (apparent resistivity and impedance phase for 2 modes, or polarizations, induction vector or tipper, etc.) and response error bars derived from measured data, or error floor, specified by interpreter, and, finally, smoothing parameters. With the appropriate selection of the parameters and the stage sequence, one can achieve an effectively better and reliable solution.

Over the course of the development of magnetotellurics, it has been a gradual transition from profile (two-dimensional) acquisition to array (three-dimensional, 3-D) acquisition, which requires solving the inverse problem in three-dimensional formulation (see, for example, [Siripunvaraporn, 2012, Kelbert et al., 2014]). It should be noted that the 3-D approach has both advantages and disadvantages.

On the one hand, with 3-D inversion, there are no restrictions on the model dimensionality, which allows eliminating the error associated with the use of a two-dimensional or one-dimensional approach when applied to real data acquired in three-dimensional structure. On the other hand, the solution of three-dimensional problems requires not only high quality array observations, but also exceptionally large computing resources. While most two-dimensional inverse problems can be solved on modern PCs in a relatively short time (within several hours per stage), 3-D inversion requires multi CPU systems with large memory, and still the calculation time per one stage can reach up to several days and even weeks.

Obviously, the 3-D inversion can not be considered a universal solution to overcome all the difficulties associated with using 1-D or 2-D approach. For the sedimentary basins’ studies, the combination of the conventional 2-D (or even 1-D) inversion (yielding more detailed models) with a 3-D approach, which makes it possible to estimate the regional 3-D distribution of electrical conductivity, seems to be the most reasonable.



Magnetotelluric survey for sedimentary basins’ mapping: technological features

Field magnetotelluric surveys carried out in sedimentary basins have some specific features, compared to MT applied for deep (crustal and mantle) studies:

the depth of investigation is, as a rule, a few dozens of meters to 10-20 km, i.е. down to the top of the crystalline basement;

requireshigh density of stations and the optimal location of survey lines (at least 5 stations over the given structure, extra stations beyond the boundaries of the exploration area);

the need to ensure high productivity of field work to reduce the cost of the survey; the need to ensure the availability of quality data in the presence of cultural EM noise

and / or low-level magnetotelluric signal. As seen from the mentioned above, there are quite obvious requirements for the

technology of fieldwork. In the majority of sedimentary basins, the total conductance of the sedimentary formation does not exceed 500-1000 S, therefore in order to provide the desired investigation depth it is sufficient to obtain magnetotelluric transfer functions’ estimates within period range of 0.001-0.025 to 1000 s, although in many cases shorter periods (up to 100-200 s) turn out to be sufficient.

In this period range, the main sources of magnetotelluric signal are distant lightning activity and geomagnetic pulsations. These two are separated by a "dead" band (nearly 1 to 10 s) with a low level of natural EM variations.

The distant lightning activity occurs in the tropical regions of Africa, South America and Asia. The electromagnetic field generated by lightning propagates throughout the planet along a waveguide formed by the Earth's surface and the ionosphere. This source is subject to some diurnal and seasonal variations, but due to the superposition of several source regions, a sufficiently powerful signal almost always exists in the frequency range of a few Hz to a few kHz (round the clock and in all seasons). Besides, there is a planetary-scale increase in the signal intensity, associated with the changes in atmosphere properties in the morning and evening hours.

Geomagnetic pulsations of several types, characterized by different nature and properties, serve as the main source of natural EM field variations in the period range of 1 to 200-500 s. An important feature of pulsations is their elevated intensity in the morning and evening (local time) hours. It should be noted that, in contrast to polar substorms and magnetic storms, these sources do not have a clear dependence on latitude and solar activity. Dependence on the overall geomagnetic activity is observed only for certain types of pulsations.

Thus, the optimal time for registering the magnetotelluric signal in the period range sufficient to sedimentary cover imaging, is within the interval from the middle of the day to the middle of the next day (local time) with a total acquisition duration being 16-20 hours. It is important that the acquisition should be covering the evening and morning hours (Figure 4). With this schedule, each instrument records one site per day, taking into account the time needed for accessing the site, layout setup and retrieval.



Fig. 4. Optimal duration of electromagnetic field acquisition in case of sedimentary basin’s structure studies. Filled rectangles show the time intervals corresponding to the increased intensity of distant lightning activity and geomagnetic pulsations.

If the survey’s terms of reference includes imaging beyond sedimentary cover (the Earth's crust), and the required depth of investigation is 40-60 km, the acquisition duration may reach 2-3 days. In this case, the maximum period for transfer functions estimation is about 3000-5000 s (rarely up to 10 000 s). It should kept in mind that the long-period variations (with periods of 500 to 3000 s) are induced by polar substorms, whose intensity and frequency depend on the magnetic latitude of the observation site, solar activity, time of year and day. The most intense disturbances occur in the high latitudes, during the equinoxes, and also in the morning and evening time. However, in contrast to distant lightning activity and pulsations, polar substorms may be absent for several days depending on solar activity, which makes it challenging to ensure long-period data acquisition within a limited period of time. From what has been said, it follows that an increase in the depth of investigation to 40-60 km may lead to a significant reduction in productivity and, as a consequence, an increase in the survey cost (per station).

The performance-optimized field crew includes one technician (operating the instrument) and two or three workers, assisting in setting up the layout. If the sites can be accessed by road network, a motor vehicle (AWD truck or compact all-terrain vehicle, "Argo" or similar) is used to move around the area. Depending on the terrain, one crew can make two to four relocations per day. Thus, with the simultaneous operation of two crews, the productivity reaches up to 7-8stations per day. Besides the field crews, an additional specialist (geophysicist) is recommended to be involved in reference station monitoring, who would also be responsible for data quality control and final processing in the field camp.

For electromagnetic field registration, magnetotelluric instruments are used, equipped with induction coils as magnetic field sensors. The modern units, commercially available from Metronix Geophysics, Phoenix Geophysics, and Zonge International match the requirements for acquisition systems, capable of imaging the sedimentary basin structure. These include:

multichannel 24- or 32-bit ADCs with high input impedance front-end for efficient electric field filtering and amplification;

portability and low power consumption, providing at least 1-2 days of continuous recording with a high sampling rate (simultaneous acquisition within several frequency bands) when using a standard car battery;

low-noise induction sensors, capable of measuring the magnetic field over a wide frequency range, 10 kHz to 0.0003 Hz;

operational performance in a temperature range of -40 to +50ºC and in high humidity.



Another important aspect that depends on the terms of reference, is the optimal survey configuration (orientation of the acquisition lines, station spacing along the line and distance between the lines, recording duration). Most often, these parameters are pre-specified by the client, but still some certain rules should be followed during planning:

- survey lines must be perpendicular to the regional structure strike; - at least 5 stations required to map the structural feature, i.e. the size of the target

structure, for example, a salt dome or reef structure, determines the station spacing; - the length of the line should be 5-10 times greater than the maximum target depth; - each line should have not less than 20-30 stations; - it is recommended that additional measurements be carried out beyond the boundaries

of the survey area to cover the resistivity model with data at the line ends (Fig. 5); - survey planning has to take into account the locations of cultural electromagnetic

noise sources (electrified railways, power lines, quarries, pipelines, electric fences, etc.).

Fig. 5. 2-D MT acquisition planning (D is depth of investigation). The dark gray region indicates the subdomain where the resistivity image is reliably recovered with no extra stations beyond the survey line (filled triangles represent the stations of the regular survey grid); light gray regions – the parts of the image requiring extra measurements at sites marked by empty triangles

To increase the survey productivity, in case of weak horizontal inhomogeneity of the structure being mapped, it is permissible to measure the magnetic field components only at every second or every third station, while horizontal components of the electric field be measured at all stations. For the stations lacking magnetic field data, during processing it is taken from the (nearest) neighboring point.

A strict requirement is the use of the remote reference method (that may imply installing of several reference stations) for the purposes of EM noise cancellation. It is important to use up-to-date robust statistical-spectral processing methods for transfer function estimation. In case of 3-D survey, it becomes possible to use some conceptually new approaches to data processing (see, for example, [Smirnov, Egbert, 2012]), which can significantly improve the efficiency and noise-immunity of magnetotelluric studies.

Another necessary step is the correction of galvanic distortions caused by the effect of local near-surface resistivity inhomogeneities. In the detailed surveys with station spacing of 200-300 m, statistical methods can be effectively applied for the static shift correction of magnetotelluric apparent resistivity. If the station spacing exceeds 1 km, the correction requires implementing at each MT site a separate EM acquisition with Near-Zone Time-Domain EM sounding method (TDEM or TEM), which has an immunity to galvanic distortions [Strack et al., 2003]. Also, the use of the TDEM data makes it possible to avoid a



systematic bias in determining the level of apparent resistivity curves in the statistical approach, mentioned above.

Regional-scale magnetotelluric studies

The goals of regional-scale MT surveys, as a rule, include mapping the structure of the Earth's crust down to Moho surface, as well as imaging the sedimentary basins crossed by the survey line. Regional-scale magnetotelluric studies in Russia are being conducted as a part of governmental project initiated by the Russian Ministry of Natural Resources. The location of the major regional survey lines in Russia is shown in Fig. 6.

For 2-D regional-scale surveys, the depth of investigation reaches over 40 km, allowing one to map the structure of the sedimentary cover, but also the most of the Earth's crust, and in some cases even the upper mantle. In recent years, similar studies have been completed in other regions of the world - in Brazil, India, China.

Along with seismic, magnetotellurics is one of the leading methods utilized in regional-scale geophysical surveys. For MT acquisition, typical station spacing along the line varies within 1-3 km, while the maximum period for transfer functions’ estimation is about 3000 s.

Fig. 6. Location of the main regional-scale reference survey lines in Russia. Line names are given in accordance with the Program of Multi-Method Geophysical Research at Reference Survey Lines of the Ministry of Natural Resources and Ecology of the Russian Federation. Modified from [Berdichevsky et al., 2015]

In a case history below, we discuss the results of regional magnetotelluric studies performed in the sedimentary basin of Paraná (Southern Brazil) and in Eastern Siberia (Russia). In these regions, the presence of flood basalts (traps) in the upper part of the sedimentary cover is common. In both cases, the surveys have been conducted using a remote reference technique and MTU-5 acquisition system by Phoenix Geophysics Ltd. fitted with MTC-50 induction coils. In Paraná Basin, magnetotellurics have been accompanied by time-domain electromagnetics to achieve better resolution of the upper part of the section and also for static shift correction in MT data.

The studies in the sedimentary Paraná Basin, located in southern Brazil (the states of Mato Grosso do Sul, São Paulo and Paraná) were conducted in 2014 by Nord-West Ltd. (Russia) in cooperation with LASA Prospecções S.A. (Brazil) under a contract with the



Brazil’s National Agency of Petroleum, Natural Gas and Biofuels. The survey’s 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.

MT observations have been performed in the central, deepest part of the Paraná depression along three lines, with station spacing of about 2 km, and recording duration of nearly 40 h. Transfer functions' estimates have been obtained with the maximum period of 2000 s (although shorter periods of about 100 s turned out to be sufficient to solve the problem). TDEM acquisition has been done at each MT station, which allowed imaging the upper section interval (200-400 m down from the surface) and applies an effective static shift correction to the MT apparent resistivity data.

After the static shift correction, the resulting impedance estimates have been analyzed in terms of model dimensionality, and TE and TM components have been identified. Dimensionality analysis, involving phase tensor calculation [Booker, 2014], has shown that a two-dimensional (2-D) approach can be applied for all of the data in short period range (up to 100-200 s).

As a result of multi-stage 2-D inversion, a series of resistivity images have been obtained, with maximum depth limit of 10 km and normalized root mean square error (RMS) not exceeding 2.5.

Fig. 7shows a resistivity model for one of the survey lines along with its geological interpretation, in Fig. 8 a comparison of observed and calculated apparent resistivity and impedance phase data is presented for both TE and TM components.

The main feature of the resistivity models is the presence in the middle part of the section of an intermediate high-resistivity layer located in the depth interval of 2 to 4 km, with a large number of intrusive bodies (sills). Thus, the sedimentary strata lying beneath the basaltic traps of the Serra Geral formation, can be divided into the upper and lower parts with a strong lateral heterogeneity, and subvertical resistive zones associated with secondary altered sedimentary rocks around intrusive bodies (dikes).

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 magnetic prospecting data and, according to geologists, represents the fan-shaped dyke complexes [Piccirillo et al., 1990; Raposo et al., 1991; Frank et al., 2009]. 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.



Fig. 7. Sedimentary basin of Parana, Brazil. Survey area map (a) and the results of geological interpretation of MT data along line 3 (b). Panel a: 1 –location of the lines 1, 2, 3; 2 –location of the line 3; 3 - boundaries of the sedimentary basin; 4 - sandstones of the Bauru formation; 5 – basalts of the Serra Geral formation; 6 –Mesozoic-Paleozoic sedimentary rocks; 7 - crystalline basement rocks; 8 - the position of the Ponta Grossa rise[Frank et al., 2009]. The red rectangle on the smaller-scale map of South America approximately indicates the survey region Panel b: 1 - the boundaries of the main resistivity layers; 2 - extra boundaries within them; 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 - rocks of the Ponta Grossa formation (Paleozoic sedimentary rocks); 10 - rocks of Precambrian crystalline basement; 11 - intrusive rocks.



Fig. 8. Sedimentary Paraná Basin, Brazil.Comparison of the observed (left panel) and calculated (right panel) response data (pseudosections) for line 3.

In 2012-2015 Nord-West Ltd. conducted a series of regional-scale MT surveys in the territory of Siberian platform, in the interfluve between Podkamennaya Tunguska and Nizhnyaya Tunguska rivers. This area lies to the north of the region where the largest Eastern Siberian hydrocarbon fields (for example, the Kovykta gas field) are located and is considered as having a substantial hydrocarbon potential.

The main goal of the studies was to construct a resistivity model of the sedimentary cover of the northern part of the Baikitskaya anteclise and the southern part of the Kureiskaya syneclise as a result of joint interpretation of magnetotelluric data, seismic CDP data, 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.

For the interpretation purposes, MT data from different surveys have been collected. Those surveys form a network of acquisition lines, with the measurements performed by different contractors during the period from 1999 to 2015. The acquisition duration was about 20 h, and the period range 0.003 to 2000 s. Along with magnetotelluric acquisition, multi-offset TDEM observations have been carried out at line 2 (Figure 9, a) in between the wells Chunskaya-120 and Lebyazhinskaya-2, for which the Impulse-D (SibGeoTech Ltd.) with 500x500m transmitter loop and 40 Amp transmitter current had been utilized. This allowed constructing a 3-D model of the upper part of the section to a depth of 1.5 -2 km along with effective correction for the static shift bias in MT data.



Dimensionality analysis of the model performed using the phase tensor [Booker, 2014] confirmed the applicability of 2-D inversion of the quasi-TE and quasi-TM mode responses. An a priori resistivity model was constructed according to all available geological and geophysical information, enabling identification of those resistivity boundaries, which coincide with known seismic reflectors. 1-D resistivity models, derived from quasi-TE MT data, constrained by seismic models, were utilized as initial guess for the multi-stage 2-DMT inversion.

The results of joint interpretation of MT data at one of the survey lines are presented in Fig. 9, while Fig. 10 demonstrates the agreement between the observed and calculated quasi-TE and quasi-TM data components in terms of apparent resistivity and impedance phase.

Three major resistivity layers are identified within the sedimentary cover: suprasalt, halogen-carbonate (salt) and subsalt. Resistivity images, graphs and maps of the resistivity distributions in specific layers, derived from the resistivity model, revealed the following features of the resistivity structure.

1. A sequence of layers with varying resistivity can be distinguished within the suprasalt formation. Highly resistive layers and regions are in good correlation with the intrusive bodies (traps), revealed by the borehole data. These high-resistivity features, spread all across the survey area, have variable thickness and location depth.

2. Sediments of the halogen-carbonate formation generally have high resistivities (hundreds of Ohm-m), but often include conductive regions that may be associated with the zones of rock alteration caused by magmatic intrusions.

3. The subsalt formation is represented by the Vendian and Riphean (Mezo-proterozoic) sediments. Resistivity variations are prominent in the Vendianrocks. 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. Relatively conductive Riphean formation has resistivity ranging from tens to a few hundreds of Ohm·m and is found within two large troughs of near North-South strike: the Pre-Yenisei and Angara-Kotui.

Resistivity variations are consistent with the existing concepts of facies substitution and differences in sedimentation conditions in different parts of the basins. In the bottom part of the section, a high-contrast boundary is identified, which may be interpreted as top of either the crystalline basement or high-resistivity low porosity Riphean formation.

The results obtained in two different sedimentary basins with massive trap coverage, provide a convincing evidence for the high efficiency of the MT method application in the described geological settings.



Fig. 9. Eastern Siberia. Survey layout map (a) and the final model interpreted form MT data at 3-SB line ("Altai-Severnaya Zemlya") (b) Panel a: 1 - the boundaries of the survey area; 2 – seismic 2-D CDP and 2-D MT lines utilized for joint interpretation; 3 – fragment of the 3-SB line; 4 – seismic 2-D CDP lines; 5 - deep wells used for interpretation; 6 - hydrocarbon fields. Panel b: 1 – seismic reflectors as imaged 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 - well position; 6 - intrusive rocks according to borehole data; 7 – hydrocarbon fields’ locations; 8 – MT stations/



Fig. 10. Comparison of the observed (left panel) and calculated (right panel) response data (pseudosections) for the fragment of 3-SB line. See Fig. 9, a for the line location

Application of the magnetotellurics for assessment of oil and gas potential prospects of the Taymyr Peninsula

Since 2005 Nord-West Ltd. has been participating in multi-method geophysical studies in the territory of Taymyr Peninsula (North of Siberia, Russia), funded by Rosnedra Federal Agency, with the primary goal being to reveal new hydrocarbon-promising areas, planned for thefuture detailed geophysical exploration and subsequent licensing. To date, the total length of the completed survey lines counts nearly 20 thousand linear kilometers.

The main components of the geophysical technology applied for the studies of both the Mountain Taymyr and the Yenisei-Khatanga Trough, are the 2-D CDP seismic survey and the magnetotellurics. Also, the previously collected data of gravity and magnetic prospecting have been utilized for the interpretation.

In the years 2005-2009, joint geophysical survey was conducted in the western part of the Mountain Taymyr (Taymyr fold belt), which is a collision fold belt formed at the end of the Paleozoic. In the Mesozoic stage, it experienced a significant transformation, associated with intraplate deformations. At its current state, the Mountain Taymyr is a system of folded and southward thrust sheets, mainly represented by the Paleozoic formations.

MT acquisition was conducted along the seismic lines using Phoenix Geophysics MTU-5 instruments. The duration of continuous field recording was 15-25 hours, which allowed obtaining MT response data in the period range of 0.025-2000 s. The average station spacing



was nearly 2-3 km, however smaller spacing was used around the line intersections. In total, 609 stations, distributed over 11 lines with overall length of about 1300 km have been collected.

Correction of the static shift effect caused by the small-scale near-surface inhomogeneities, was made by statistical normalizing method, which implies spatial smoothing of the impedance tensor components’ magnitude at the specified period. For each station, the normalizing (correction) coefficient was then calculated, used to shift the apparent resistivity level over the entire frequency range (which in mathematical terms is equal to the multiplication of the apparent resistivity by the correction factor).

Static shift correction was followed by the dimensionality analysis of the obtained impedance estimates and impedance decomposition into TE and TM modes. For all survey lines, the resistivity images (down to 25 km depth) have been constructed as a result of multistage 2-D bimodal inversion of magnetotelluric data.

Joint interpretation of resistivity and seismic images with the involvement of gravity and magnetic prospecting data made it possible to clarify the understanding of the geological structure of the region, in particular, to identify a series of large previously unknown geological objects that aren’t exposed at the earth’s surface, and identify hydrocarbon-prospective are as well.

Fig. 11 shows the geophysical studies results along one of the survey lines. In the middle part of the line, a large Gydan-Taymyr Trough has been mapped, with the thickness of the sedimentary formation reaching 20 km (filled with about 10 km of Paleozoic sediments and the Upper Riphean sequence of comparable thickness).

A joint analysis of the seismic and resistivity models has shown that the largest anomalies of the seismic wave field coincide with the resistivity anomalies; taking the available paleoreconstructions into account, it can be assumed that these anomalies correspond to reef structures and manifestations of salt-dome tectonics (Fig. 12).

It was concluded that the distinct-boundary high-resistivity zones correspond to the regions of loss of correlation between the seismic reflectors and resistivity boundaries. In one case, those anomalies are accompanied by low gravity field, which suggests that they might be associated with low-density anhydrite bodies (salt domes), whileother certain resistive regions fall within the elevated gravity field zones, which is the evidence for their carbonate (reef) origin.

Magnetotellurics plays an important role as a part of the geophysical technology used to study the Mountain Taymyr. MT studies made it possible to predict a generalized lithology pattern of the Phanerozoic formations in the area, to clarify the location of the bottom of the traps and Jurassic-Cretaceous sediments, to identify the non-outcropped intrusive bodies in the upper part of the section, to make important conclusions about the oil and gas potential of the area, and to outline the strategy for further studies. Application of magnetotellurics confirmed the existence of the previously-unknown sufficiently large structures, assessed as prospective in terms of oil and gas potential.



Fig. 11. Seismic image (a), resistivityimage (b) and final model interpreted in terms of geological structure (c). The black lines indicate the major faults.

In 2014-2015 a joint seismic 2-D CDP and MT survey was carried out in the west of the

Yenisei-Khatanga regional trough. Five survey lines have been acquired on the right bank of the Yenisei River, filling the "white spot" on the map of the western part of Taymyr Peninsula in terms of the exploration maturity (Novotaymyrarea). The lines have crossed a number of well-known oil, gas and gas condensate fields. The station-to-station distance was 500 m, which made it possible to increase the detail of the resistivity models obtained.

Fig. 13 demonstrates the resistivity image for one of the lines along with its geological interpretation. In its northern part the line passes over the Baikalovskya oil and gas condensate field, in the central part - the Payakhskaya oil field, in the southern part it crosses the Muksukhinskaya structure. At the time of the described survey, the latter one hasn’t yet undergone hydrocarbon saturation tests.



Fig. 12. An example of the identification of salt domes based on the joint analysis of resistivity and seismic images. The top surface of the salt domes is shown with a black line.

Fig. 13. Resistivity image inverted from the data acquired in Novotaymyr area and its geological interpretation. Red dashed ellipses represent the boundaries of revealed regions of increased resistivity (see text). Modified from [Afanasenkov et al., 2015].

The most important outcome of the studies is the identification of high-resistivity anomalies in the depth interval of 200-700 m over known hydrocarbon fields and the Muksukhinskaya structure. The boundaries of these anomalous regions are indicated in Fig. 13 with red dashed ellipses. The resistivity reaches 35-40 Ohm-m within the anomalies, while for the host rock it is about 10-15 Ohm-m. All the anomalies have been verified and confirmed by electrical logging data (log charts are shown in Figure 13).

Analyzing the nature of the identified resistivity anomalies, it can be assumed that the most likely explanation for their origin is the accumulation of gas hydrates (methane) under the seal of permafrost rocks located in the near-surface layer. Such accumulations may be formed above the hydrocarbon fields in the gas hydrate stability zone at depths of 200-800 m due to the vertical migration of volatile hydrocarbons through the fractured zones [Afansenkov, Volkov, Yakovlev, 2015]. If this mechanism is confirmed, the existence of increased resistivity zones at depths of 400-800 m can be considered as a new prospecting indicator, which would justify the efficiency of magnetotellurics in prospecting and exploration of hydrocarbon deposits in permafrost settings.



Conclusions and future development of the method

1. Magnetotellurics is an effective method for mapping the geological structure of sedimentary basins. MT is widely used in regional-scale studies, oil and gas prospecting and exploration, as well as for the solution of the problems related to hydrogeology and geothermal resources exploration in many countries around the world.

2. The most important advantage of the method is a wide depth range of the imaging and the capability of evaluating the petrophysical and hydrophysical properties of sedimentary rocks.

3. Magnetotelluric studies of sedimentary basins have a number of differences from deep studies of the Earth's crust and upper mantle, which is related to a reduced depth of investigation (from dozens of meters to 10-20 km) and, accordingly, a shorter period range (from 0.0025 to 1000 s), and a denser observation grids with station spacing varying from 50-100 m to 2 km.

4. The application of the MT method is especially efficient in the regions with flood basalts and salt-dome tectonics, as well as in fold belt areas.

Further development of the method requires the solution of the following important

problems: - expanding the frequency range of measurements toward high frequencies in order to

improve correction methods for the near-surface distortions and overall reliability of interpretation;

- reducing the cost of the survey using multi-channel autonomous MT instruments; - further development of the measurement techniques and multisite robust data

processing methods with the aim of increasing the noise immunity of the method; - development of the technology of three-dimensional MT surveys and optimal

combination of the 3-D approach with conventional 1-D and 2-D schemes; - development of the methods of joint and constrained inversion capable of handling

MT and other geophysical data (seismic, well logging, gravity).

Acknowledgments The authors are thankful to the Brazil’s National Agency of Petroleum, Natural Gas and

Biofuels for the kind permission to publish the results of magnetotelluric studies in the Paraná Basin in southeast Brazil, and are also grateful to the reviewers for their helpful remarks and comments.

This work was carried out with the organizational and financial support of Nord-West Ltd., the Russian Foundation for Basic Research (project no. 16-05-00791-a) and the State Research Order no. 149-2014-0031.

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