Page 80 - Geothermal Energy Systems Exploration, Development, and Utilization

56 2 Exploration Methods
simultaneous measurements of the strength of the magnetic field variations at
the surface of the earth and the strength of the electric field component at right
angles in the earth. Because the direction of polarization of the incident magnetic
field is variable and not known beforehand, it is common practice to measure at
least two components of the electric field and three components of the magnetic
field variation to obtain a fairly complete representation. Another assumption in
MT is that the displacement currents can be neglected since conduction currents
dominate the electromagnetic behavior. The dominant diffusive process makes
it possible to obtain responses of volumetric averages of the measured earth’s
resistivity. Measurements are usually represented as MT-apparent resistivity and
phase as a function of frequency.
The investigation depth is a function of the electrical resistivity ρ of the earth and
angular frequency, ω, of the EM field. Since earth is a conductor, the electromagnetic
wave is governed by a diffusion process in the earth. This implies that the field
strengths attenuate (decrease exponentially) with depth. A reasonable measure of
the penetration scale length is the skin depth δ, which corresponds to the depth
at which the amplitude of the incident electromagnetic field has attenuated by a
factor of 1/e.
A useful approximation for a uniform half-space of resistivity ρ is given as:

√ √ δ ρ
δ ≈ 500 ρ/f D = 2 = 356 f (meters) (2.11)
where D is the so-called investigation depth and f the frequency (f = ω/2π).
The skin depth relation shows that investigation depth depends not only on
frequency but also on the resistivity of the subsoil.
Depending on the measurement frequency range and thus investigation depth,
MT methods are named differently. MT measures in the frequency range 1 to
10 −6 Hz, where studies focus on imaging crustal and mantle geological targets.
Natural electromagnetic source energy is usually adequate to ensure the full
frequency spectrum. In the mesoscale frequency range, from 1 to 105 Hz, the
method is referred to as audiomagnetotelluric (AMT). A controlled electromagnetic
source (CSAMT, see below) is commonly used at higher frequencies to prevent
low signal-to-noise ratios where cultural noise and a weak natural signal may be
present. At the very shallow scale radiomagnetotellurics (RMTs) measurements
in the frequency range of 15 to 250 kHz using a radiotransmitter allow detailed
characterization within the first tens of meters of depth.
At each MT station, five measurements (channels) are recorded. These are the
magnetic field in two horizontal directions and in the vertical direction, and the
electric field in two horizontal directions, the horizontal measurements being
perpendicular (e.g., north and east). A typical MT station for data acquisition
consists of two pairs of electrodes set up as orthogonal dipoles with lengths
between 50 and 100 m, and three magnetometers (typically flux gates or induction
coils) also set up in orthogonal directions (two horizontal, the same as the electric
dipoles, and vertical) as sketched in Figure 2.6. The two dipoles measure the
electric field fluctuations in the horizontal directions from the potential difference