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Article 3. Electrophysical properties of the medium.

Igor Komarov

Application of GPR for the separation of media is possible due to their differences in the electrical properties. The main electrical properties are the specific electrical resistance ρ and dielectric constant ε.

The electrical resistivity determines the attenuation of electromagnetic field in the medium and, consequently, the depth of research. The lower the damping the greater the depth penetrated field. That is we get a response in the survey environment with greater depth. Virtually all materials except pure metal can be attributed to dielectrics with finite conductivity. In this regard, there is a notion of the relative permittivity.

The contrast of ε in the layers determines the reflectivity of the boundaries. This feature together with the linear dimensions of the surface of the local objects and wave lengths determine the ability of objects to the formation of the diffracted waves.

There is a dispersion of the dielectric constant in a broad range of frequencies, that is frequency dependence of the applied electromagnetic field. This is a very complex relationship, given by the different ways of polarization components that make up the aggregate rock. For practical use of this dependence is determined experimentally. Unfortunately, at present, these definitions are very few and mostly for minerals or mono-mineral media (for example, ice).

The velocity of propagation of electromagnetic waves in a dielectric depends on the dielectric permittivity and magnetic permeability. However for most environments permeability value is close to 1. Therefore the speed of electromagnetic waves in the medium is equal to:
v = c / √ε
where c - velocity of propagation of electromagnetic waves in vacuum (speed of light).
In the GPR is generally measured velocity in cm / ns (centimeters per nanosecond). Thus, the formula for velocity calculation is:
v = 30 / √ε (cm/ns)
Knowledge of velocity is needed to convert the time delay of the reflected pulse in the depth of the reflecting boundary.

Numerous laboratory and field experiments show that in range of meter waves propagation of electromagnetic waves slightly dependent on the frequency and type of soil, but are very dependent on their moisture. The general law is a reduction the resistivity with increasing frequency, moisture, clay rocks and mineralization in the rock.

The wavelength is determined by the ratio of the velocity of the electromagnetic wave to the frequency of the antenna radiation:
λ = V / f (m)

The tables below show the main electrical characteristics of some materials and rocks (with 100 MHz antenna).

Materialε
Air 1
Freshwater 80
Ice 3-4
Seawater 81-88
Dry Sand 3-5
Saturated Sand 20-30
Volcanic Ash/Pumice 4-7
Limestone 4-8
Shale 5-15
Granite 5-15
Coal 4-5
Dry Silt 3-30
Saturated Silt 10-40
Clay 5-40
Permafrost 4-5
Average Surface Soil 12
Dry, Sandy Coastal Land 10
Forested land 12
Rich Agricultural Land 15
Concrete 6
Asphalt 3-5
Source: Conyers and Goodman, 1997.

Material Attenuation (dB/m)
Air0
Fresh water0.18
Sea water330
Dry sandy soil0.14
Wet sandy soil2.3
Dry loamy soil0.11
Wet loamy soil7.9
Dry clay0.28
Wet clay20
Wet basalt5.6
Wet granite0.62
Wet shale45
Wet sandstone24
Wet limestone14

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