Monday, 3 July 2017

The Importance Of Geophysics In Groundwater Investigation

Before we delve into this, we need to first of all define some terms.

What is Geophysics?      It is the use of Physics to solve Geology problems.

What then is Geology?  Simply the study of the Earth. This includes the structure and the rocks.

Geophysics is usually employed when it involves probing the subsurface. In physical Geology, you are dealing with the surface geology, i.e. the structures, forms, and the lithofacies that are visible on the surface.

To solve environmental issues, mining, archeological, engineering (Geotechnical) and geological (hydrogeological) problems, geophysics is employed. It is achieved by passing an electric current via two current electrodes and the potential drop between the two current electrodes is measured by a voltmeter. From this, the resistance is deduced. Here, the current (I) is already known and the potential drop (V) is known as well.
Given the formula:          
V=I×R                                                                           ------- (1)
Making R the Subject:    R=V/I                                    ------- (2) {where R is measured in Ohms}

The essence of carrying out this is to determine the intrinsic properties of the geologic materials present in that environment. What the geophysicist is interested in per see is the Apparent Resistivity. This is usually affected by the presence minerals, groundwater, and porosity.

For groundwater investigation, Electrical Method is used. It involves the introduction of current via wires to the electrodes i.e. Vertical Electrical Sounding (VES). There are two main arrays for this: The Schlumberger array and Wenner array. Although each array has its own limitation, it will help you infer how the subsurface looks like.

There are however some factors that determine the accuracy of your results and are:

  1. The accuracy of the equipment (terrameter) used.
  2. The competency of the geophysicist.

Fig 1: ASSEMBLAGE OF THE EQUIPMENT(Terrameter) for Geophysical Investigation.

From the readings gotten and interpretation made, the rock-bearing-water is ascertained and the depth of penetration is known.

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