The ability to detect a signal is dependent on the magnitude of the source signal, the geometry of the system, and material parameters, as well as the measurement instrument resolution. The analytical models discussed here are useful in estimating exploration limitations, as well as in the determination of the required sensitivity of the receiver for an expected target and in interpretation of the measured data.
This discussion considered only a narrow range of conductivities and simple model geometry. In practice, conductivities may occur between 10 μS/m and 100 S/m. The wave equations have a dependency on both dielectric permittivity and magnetic permeability; neither of which were considered in detail.
Dielectric permittivity is a measure of electric polarization: charge displacement from a neutral resting position under the influence of electric fields. A material dielectric constant is given as a multiplier of the free space dielectric permittivity. Although often assumed to have a value of 1 (which is approximately true for the atmosphere), a value of 5-7 is more typical for common earth materials, with a range of 2 – 10 not uncommon. The calculations discussed herein assumed a relative value of 3-4. This parameter usually has little significance in low frequency situations where conductivity contributes a significantly greater proportion of charge movement.
Magnetic permeability is a measure of magnetic polarization: the ability to form magnetic domains in the presence of a magnetic field. The majority of earth materials are diamagnetic or paramagnetic with permeability constants approximately equal to one. However, there are naturally occurring ferromagnetic materials with large constants; magnetite being perhaps the most prevalent. These materials can enhance applied magnetic fields, having relative values in ranges of 100s – 10,000s. While not common, the presence of these materials can produce significant transient responses to an electromagnetic exploration. Iron, nickel, and cobalt are the most common ferromagnetic materials; exploration in regions where these materials may be present need to take the effect of μ ≠ μo into account – the expressions were developed with the assumption that μ = μo.
The analyses presented herein are simplified approximations, but the predicted responses to simplified models place the required instrument sensitivity limits in the acceptable range of generalized exploration parameters. Instruments having the ability to detect magnetic field strengths in the range of 0.01 – 1.0 nT are capable of use in a typical geophysical environment for magnetic or electromagnetic exploration.
And that’s a wrap.
