magnetotelluric prospecting

magnetotelluric prospecting Naturally occurring variations in the Earth's magnetic field induce eddy currents in the Earth that are detectable as electric (or telluric) field variations on the surface. The magnetotelluric (MT) method is an electromagnetic (EM) technique for determining the resistivity distribution of the subsurface from measurements of natural time-varying magnetic and electric fields at the surface of the Earth. The ratio of the horizontal electric field to the orthogonal horizontal magnetic field (termed the EM impedance), measured at a number of frequencies, gives Earth resistivity as a function of frequency or period, resulting in a form of depth sounding. This is a wide-band depth-sounding technique; the frequencies utilized range from about 10⁴ to 10⁻⁶ hertz (Hz). The method is suitable for shallow as well as deep geological investigations. The depth of investigation in MT is a function of subsurface resistivity and frequency (or the inverse, period) of the EM signals. The depth of sounding can be roughly related to frequency by the use of the skin depth, which is defined asδ ≊ 0.5 (resistivity/frequency)^0.5 km,

where resistivity is expressed in ohmmetre and frequency in hertz. MT can probe great depths (up to 600 km) by utilizing sufficiently long periodicities.

Natural time-variations of the Earth's magnetic field of frequencies greater than 1 Hz are of atmospheric origin and are caused by global thunderstorm activities. Some thunderstorm energy is converted into EM fields which are propagated with slight attenuation in the Earth–ionosphere interspace as a guided wave. At a large distance from the source this is a plane wave of variable frequency. The MT signals of frequencies lower than 1 Hz are of magnetospheric origin (that is, they result from the interaction of solar wind and the Earth's magnetic field).

In the scalar MT method, the electric field in one horizontal direction and the magnetic field in an orthogonal horizontal direction are measured simultaneously at a surface location. The tensor MT method is more popular. This requires simultaneous observation of two orthogonal horizontal (usually north–south and east–west) components of the magnetic field (Hx, Hy) and the electric field (Ex, Ey) and the vertical magnetic field component (Hz) at a surface location (Fig. 1). Magnetic sensors are squid magnetometers or induction coils consisting of several hundred turns of fine copper wire wound over a ferrite core or mu-metal of high magnetic permeability. Electric sensors are non-polarizing electrodes: commonly copper rods inserted in supersaturated copper sulphate solution or lead rods in supersaturated lead chloride solution in porous pots.

Data processing entails converting the field records into EM impedance (Z) data and determining apparent resistivity and phase information from Z for the various observational frequencies. The EM wave impedance is obtained as Z = E/H. This is a complex quantity (it has amplitude and phase). Since two mutually perpendicular horizontal components of E and H are measured in the field, Z can be defined for any given direction in tensor MT. For example, the electric field in the north–south direction and the magnetic field in the east–west direction can be used to define the impedance Zxy = Ex/Hy, while the electric field in the east–west direction and the orthogonal magnetic field component can be used to define the impedance Zyx = Ey/Hx. Zxy and Zyx are equal if the subsurface is homogeneous and isotropic. If the ground is heterogeneous, both impedances differ in magnitude and phase, as in the example presented in Fig. 2. In general, the magnetotelluric impedance is a tensor whose values depend on the direction of measurement. The components of the impedance tensor can be rotated mathematically to attain maximum values in the direction of linear conductive subsurface structures (and minimum values in the perpendicular direction). In MT parlance, the direction in which the impedance tensor elements are maximized during rotation is termed the geoelectric strike or azimuth of the impedance tensor. It usually coincides with the trend of a major geological feature.

The magnitude of Z is used to define an interpretative quantity called the apparent resistivity, ρ_a, which for a given EM wave frequency or period (T) is obtained as, ρ_a = 0.2T|Z|².

Apparent resistivity is in ohm.m (symbolically written as ωm) when the wave period T is in seconds (T is the reciprocal of frequency in hertz), E is in mV/km, and H is in nanotesla or gamma.

The phase of Z (symbolically denoted by θ) is obtained as the ratio of the imaginary to real parts of the complex impedance, i. e. θ = imag Z/real Z and is in degrees.

The relationship between the vertical magnetic field Hz and the horizontal magnetic components can also be used to estimate the geoelectric strike (and to define directions of current concentrations or induction arrows in MT parlance). The differences in the strike directions determined by impedance tensor rotation and vertical–horizontal magnetic field relationships serve as indicators of the three-dimensionality of subsurface conductivity structures.

Mathematical modelling techniques exist for interpreting MT data (commonly apparent resistivities and phases). When the subsurface consists of horizontally stratified rock sequences, the resistivity varies significantly only with depth and the resistivity structure is said to have a one-dimensional (1-D) physical property distribution. The parameters sought in 1-D interpretation are layer resistivities and thicknesses, or alternatively conductivity–thickness products. When the geological sequences are not horizontally stratified, the resistivity structure is described as being three-dimensional (3-D) and may vary in all directions. There are, however, many situations where tectonic processes have imparted strong structural lineations in subsurface materials (e.g. major faults, angular unconformities, and discordant intrusive bodies) so that there is a well-defined strike. The gross geoelectric structure is then taken to be approximately 2-D (i.e. it changes only across strike and at depth). 1-D, 2-D, and 3-D data interpretations are carried out by interactive forward or automatic inverse numerical modeling on computers.

Geological and environmental applications

Mineral, hydrocarbon, and geothermal resources and contaminant dispersal processes in the subsurface are all genetically related by water and heat. The physical property most affected by changes in subsurface fluid content and temperature is electrical resistivity. Hence, MT can be used in routine exploration for natural resources and in environmental investigations.

The geological basis for employing MT in hydrocarbon exploration is the fact that prospective basins tend to have successions of source or cap rocks (dominantly shaly formations) and reservoir rocks (typically sandy formations and fractured crystalline rocks such as limestones and dolomites), which have contrasting electrical properties: shales and other fine clastic rocks are highly conductive (generally <30 Ωm), whereas sandstones and carbonate rocks are highly resistive (usually >50 Ωm). In a typical hydrocarbon trap, oil and gas are insulators and rest on highly conductive brines; such a trap may sometimes provide good targets for the MT method especially in occurrences at shallow depth. The MT method also has good potential for imaging deep sedimentary basins, since a broad depth range (approximately 10 m to 600 km) can be achieved by selecting appropriate wave frequencies or periodicities. A frequency bandwidth of about 10⁻⁴ to 10³ Hz (i.e., 10⁻³–10⁴ seconds) is used in typical basin-evaluation studies. Basin exploration entails the determination of structural and stratigraphical variations down-dip. This can be achieved by performing depth soundings over a network of stations in the prospective region. The interpretative model will define the thickness of the sedimentary cover, the approximate distribution of the resistive and conductive members of the sedimentary sequence, and hence the possible location of reservoir and source or cap rocks.

The MT method also finds application in studies of large-scale geological features such as crust and mantle structure, the structure of mountain belts, and the structure of major rift valleys. It can also be used for imaging ancient and modern volcanoes.

Maxwell A. Meju

Bibliography

Cagniard, L. (1953) Basic theory of the magnetotelluric method of geophysical prospecting. Geophysics, 18, 605–35.
Vozoff, K. (1972) The magnetotelluric method in the exploration of sedimentary basins. Geophysics, 37, 98–141.