Controlled Source Radiomagnetotellurics

In this section:
Why we use controlled source?
 

In the RMT method success of the survey is highly depends on the existence of the radio transmitters and they placement. The signals of the most powerful VLF transmitters are available for receiving around the world. Nevertheless, they works in very narrow frequency range 10-30 kHz. It is good only for profiling, not for sounding. The most low-frequency radio transmitters have frequencies about 11-12 kHz - it limits the depth of investigation. High-frequency transmitters have relatively low power and they signals are receivable in relatively narrow circle (about first hundreds of kilometers). The map of radio transmitters obviously indicates that the most appropriate regions for RMT survey are Europe and Central America. In the Siberia, Africa and South America it is very little chance of successful RMT survey. Moreover, even in the survey area exists many radio signals we can use only signals with appropriate bearings relative to receiving antennas. Some radio transmitter works during the schedule and can be switched off.

Problems mentioned above can be solved using the controlled source of EM field - generator of the alternating current. Here is the list of main advantages of using the controlled source:

  • More stable signal and increased quality of data.

  • The survey lines can be orientated arbitrary.

  • More regular and controlled mesh of frequencies.

  • Increased depth of investigation by using lower frequencies (lower then 11-12 kHz).

An illustration of these advantages is shown in Figure 1. The left plot shows the sounding corves updated using the signals of broadcast radio transmitters. The right plot shows the sounding curves obtained at the same point using a controlled source (grounded wire).

Figure 1. Camparisou of souding curves obtained using signalf of radio transmitters (left) and controlled source (right).

Types of the sources of the EM field
 

Now the following types of sources of electromagnetic field are usually used in the CSRMT method:

  • horizontal magnetic dipole, HMD (vertical loop),

  • horizontal electric dipole, HED (grounded wire). 

The first type of the source is used by geophysicists from Sweden [Mehta et al., 2017]. The second type of the source is used by geophysicists from Russia [Saraev et al., 2017].

Advantages of the horizontal magnetic dipole:

  • The source antennas very compact and portable. First tens of meters is the maximum linear size of loops.

  • No groundings. You can to work on the arbitrary surface. Also all of parameters of the loop are predictable and tuning the contour into the resonance is easy to automatization process.

  • Because of using the tuning into the resonance, it is possible to use compact and low consumption lab transmitters.

Disadvantages of the horizontal magnetic dipole:

  • Little broadcasting (up to one kilometer).

  • Because of increasing the inductive resistance following the increasing of the frequency, it is difficult to use high frequencies (more than first tens of kHz). Inductance of the loop is proportional to the area of the loop and squared number of turns. Nevertheless, it is limitation for remote regions only. In urban regions the high frequencies are usually covered by rediotransmitters.

  • Little size of the antenna provides several restrictions of the length of the line of measurements.

Advantages of the horizontal electric dipole:

  • Significant broadcasting - up to 3-4 km.

  • High frequencies are not a problem (see Figure 1).

  • Usually, the length of the bipole is about 500-1000 m. This allows measurements over long lines without changing the position of the source.

Disadvantages of the horizontal magnetic dipole:

  • Сumbersome equipment. In this case, the EM transmitter is powered by a 220 V network (or from a gasoline generator). For the source antenna, wires with a cross section of 3-4 mm are used. Usually a kilometer of such wire has weighs about 50-70 kg.

  • It is necessary to install cluster grounding. But this is a problem only in the case of linear objects of investigation. At the area researches it is possible to spend some hours on installation of a transmitter and to work with it though the whole week.

Structure of the EM field of the controlled source
 

In this section, we briefly discuss the main characteristics and features of the EM field of a controlled source using the example of a horizontal electric dipole. The features discussed here are also valid for sources of other types. Let's consider the features of EM field for the example of a normal field for a simple model of a homogeneous half-space with a resistivity of 100 Ω*m and relative permittivity 4. We will consider the case where the source and the receiver are located on the ground-air boundary.

Preliminary it is necessary to defined the terminology. The electromagnetic field has very different and complicated interaction with the geological environment. That is why the specifics and meaningfulness in the terms we use are very important in order to avoid confusion and misunderstanding.

Here we need an expression for the squared wave number of the EM field:

 

All symbols are known: w - angular frequency, m - absolute magnetic permeability, e - absolute dielectric permittivity, s - conductivity, i - imaginary unit. Here we recall the wavelength to wave number relation:

So, let's specify some important terms we will use here and below.

  • Near-field zone - the spatial-frequency area where EM field is equal to DC field.

  • Far-field zone - the spatial-frequency area where the EM field equals to plane vertically incident wave.

  • Transition zone - the spatial-frequency domain located between the near-field and far-field zones, where the EM field depends on many factors: the type of source, frequency, relative source-receiver position, electromagnetic parameters of the medium, etc.

  • Quasistationary zone - spatial-frequency area where the EM field propagates "instantaneously" witch means that the wavelength in the air is much bigger than the source-receiver distance.

  • Wave zone - spatial-frequency area where the EM wavelength is comparable or less than the source-receiver distance and it is necessary to take into account the displacement currents in the air and the "propagation effect".

  • Low-frequency field - EM field, whose frequency is so low that the density of displacement currents in the earth is much less than the density of conduction currents in the earth and the dielectric permittivity of rocks can be neglected.

  • High-frequency field - EM field whose frequency is so high that the density of the displacement currents in the earth is comparable to, or greater than the density of the conduction currents in the earth and it is necessary to take into account both conductivity of the rocks and the dielectric permittivity.

Here it is nesessary to paying attention to the following moments.

  1. The far-field and wave zones are not the same. In the far-field zone, the primary field in the earth is negligibly small because of  Joule-Lenz law and only the primary field in the air is significant. Because of the huge contrast of air and ground conductivity, the EM field gose into the earth near to vertical. In this case, the main role is played by the conductivity of the earth. When the field is divided into quasistationary and wave zones, the ratio of the wavelength to the source-receiver distance (the product of the wavenumber magnitude and the distance) is of primary importance. Typically, the wavelength is estimated in non-conductive air.

  2. Quasistationary and low-frequency fields are not the same. A quasistationary field is a field with sufficiently low frequency in order that the finiteness of the wavelength and the propagation velocity of an EM wave can be assumed infinity. But the low-frequency field is a field with sufficiently low frequency in order to neglect the displacement currents in comparison with the conduction currents.

Now we can finish the theoretical discussion and start a visual demonstration of the features of the EM field of the controlled source.

Frequency domain

The CSRMT metod is a kind of frequency soundings, so, we will start to consider the features of the EM field in frequensy domain.

In electromagnetic geophysics it is common to split the EM field into the near-field, transition and far-field zones. Figure 2 shows an illustration of the characteristics of such a splitting. Let us discuss them in some detail. In the near-field zone, the EM field is independent of frequency and the phase of the individual components is zero (relative to the phase of the current in the source). Therefore, the phase difference of the individual components is also zero. This corresponds to the DC field. In the transition zone, the dependence of the field on frequency in the general case is rather complicated and is not described by elementary functions. In the far-field zone, both amplitude and phase of the electric field do not depend on the frequency. The magnetic field decreases with frequency depending on the direction of the component. But the most important property of the far-field zone is the equality of the apparent resistivity to the resistivity of the half-space, and the  impedance phase is 45 degrees. Similarly to a plane vertically incident wave - the basic model of magnetotelluric. Therefore, in the far-field zone of a controlled source of any type, we can approximate the field by a plane vertically incident wave and use well-developed algorithms for processing and inversion of magnetotelluric soundings for controlled-source data. 

 

Figure 2. General structure of the EM field of the horizontal electric dipole.

In practice the splitting of the EM field into near-field, transition and far-field zones is performed using the value of skin-depth of the EM field in the conductive earth d:

In this case the classification is folowing [Zonge and Hughes, 1991]. The estimated position of the boundary of the near-field and transition zone is 0.1d. The estimated position of the boundary between the transition and far-field zone is 4d for the broadside area of the horizontal electric dipole and 5d for the inline area of the horizontal electric dipole. It is important to note that these values ​​are valid for apparent resistivity and are suitable for designing the source placement before the experiment. For the impedance phase, the positions of the boundary between the transition and far-field zones will be further approximately by one skin-depth relatively to apparent resistivity case (see Fig. 2).

The wave zone is characterized by a rather sharp increase of the amplitude of the EM field components and a decrease from the phase relative to the phase of the current in the source. It is important to note that in Figure 2 the source-receiver distance is only 500 m. In this case, the boundary between the wave and quasistationary zones corresponds to a frequency of approximately 50 kHz. This means that when you work with the CSRMT method, you always have a deal with a wave field at high frequencies and a quasi-stationary field at low frequencies. An important property of the wave zone is the equality of the surface impedance to the surface impedance in the quasistationary case. This makes it possible to use unmodified magnetotelluric inversion codes. However, this equation is not satisfied for the ratio of the vertical magnetic field to the horizontal one (tipper).

 

Spatial domain

Let's now consider the dependence of the components of the EM field of a horizontal electric dipole on the distance. Figure 3 shows the dependence of the amplitude of the components Ex and Hy for the inline and broadside areas.

Figure 3. Dependency on distance of Ex and Hy components of the horizontal electric dipole for inline and broabside areas. 1 - near-field and transition zones, 2 - far-sield zone, 3 - quasistationary zone, 4 - wave zone.

In the near-field zone, the electric field Ex (along the moment of the source) decreases as cube of the distance. The magnetic field Hy decreases as squared distance. In the far-field zone, both the electric and magnetic fields decrease as cube of the distance. In this case, in the far-field zone, the amplitude of the horizontal electric and magnetic fields at the broadside area is twice bigger than int the inline case. In the wave zone on the inline area of the horizontal electric dipole, the electric and magnetic fields decrease more slowly, as squared distance. In the broadside area of the source, the electric and magnetic fields decrease even more slowly - as first power of the distance. This leads to a change in the directional diagram of the horizontal electric dipole upon transition from the quasistationary zone to the wave zone. In the quasistationary zone, the maximum field magnitude is in broadside area of the horizontal electric dipole, and in the wave zone, the inline area. As an illustration, Figure 4 shows the directional diagrams of a horizontal electric field around a horizontal electric dipole at different distances.

Figure 4. Directional diagram of the horizontal electric field Ex around the horizontal electric dipole with frequency of current 100 kHz at difference distances. Distance 0.3 km corresponding to quasistationary zone, distance 2 km - wave zone. Dipole is licated in the center of diagrams.

Guidance
 
  1. Survey in the far-field zone can be designed on the basis of model relationships. For a horizontal electric dipole, the boundary of the far-field zone is located at a distance of 4 skin-depths in the broadside area and 5 skin-depths on the inline area.

  2. It is better to measure low frequencies (quasistationary field) in the inline area of a horizontal electric dipole, and high frequencies (wave field) - on the broadside area.

  3. We can use quasistationary MT iversion codes for inversion of CSRMT data measured in the wave zone.

References
  1. Zonge K.L., Hughes L.J. Controlled source audio-frequency magnetotellurics. Electromagnetic methods in applied geophysics. V.2 - Applications. Series: Investigations in geophysics, No 3, 1991, P. 713-809.

© 2020 by Arseny Shlykov