Electric Prospection

VISUALIZING THE SUBSOIL WITH LAPLACE AND POISSON EQUATIONS

VERTICAL ELECTRIC SOUNDING (VES)

A VES allows to get 1D information of the soil through the application of pulses of current as stimulus and the simultaneous register of the difference of potential generated as a way of response. Using Laplace equation is possible to deduce the stratigraphic profile. Starting from a set of data "stimulus/response" gotten in the outdoor work. Notes:

The penetration depth depends on the dipolar opening. It is said that russians have used this method in Siberia with dipoles of 80 Km
While the dipolar opening is growing a greater electrical power will be needed in order to get an accurate lecture of V, due to soil volume that participates in the electrical conduction is greater
This method allows to characteristize the subsoil, to detect underwater, to qualify the strate of rocky base, to know the subtopography, etc.

DIPOLE DIPOLE ARRAY (DDA)

The DDA gives 2D information of the soil which is very used in mining to detect zones with high concentration of metallic minerals. The phase of the soil generates an "apparent" map of the subsoil, which must be converted into a "real" map using Laplace equation.

Notes:

Sometimes depending on the kind of exploration, the information given in Res apparent map can be enough. Converting an apparent map into a real map has obviously an increasing in the cost
It is said that "good" geophysicists can mentally deduce the real map just looking at the apparent map

IP-Res Study

Concept: The application of electric current to the field and the reception in the form of a potential difference. Therefore, the field configuration is deduced through Maxwell's equation.

Methods

To map the resistivity in 1, 2 and 3D
To measure the resistivity in function of the frequency (Induced polarization in the frequency domain)
To study the electric potential decay (induced polarization in the time domain)
To study the induced EMF, etc.

Applications

Identification of strata with mining potential
Indispensable support for the field net
Water detection
Strata mapping

MORE:
* Interpretation of Electric Prospecting, April/20/2004 (Spanish)

ELECTRIC METHODS OF PROSPECTING

Some geoelectric methods measure the electric fields generated spontaneously in the Earth crust. Others measure the electric fields artificially generated due to the application of an electric current (continuous or alternating) of low frequency (about 10 Hz).

i) The Resistivity Method used in mining, geology and hydrology, allows to study the anomalies of electric resistivity, what can be correlated with the presence or absence of mineralized volumes.
MORE

ii) The Method of Induced Polarization quantifies the capacity properties of the Earth crust (presence of "mini-condensators") as it could be the case of sulphurs diseminated. | See Aplication

iii) The Autopotential Method is related with the natural currents present in the Earth crust due to spontaneous chemical reactions and to the presence of underground water (NOTE: During 2003, Daniel Kwok, Larry Kostiuk et al discovered that the water flows create an electric current)

Drilling - Sounding (1D Resistivity)

Dipole Dipole (2D Resistivity in the ZY plane)

Gradient (2D Resistivity in the XY plane)

Companies that do Geoelectric Prospecting
Geodatos

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RESISTIVITY VALUES FOR COMMON ROCKS (Ohm*m)
* Mineral with metalic lustre: 0.1 m - 10
* Igneous rocks: 10 - 100 M
* Metamorphic rocks: 10 - 1000 M
* Sedimentary rocks: 1 - 100 K
* Unconsolidated sediments: 0.1 - 100 K

MYSTERY
What is the relationship between Fourier and the IP effect?