Geophysical Investigation Of Ground Water (siwes Report)

                                                                                      CHAPTER 3

GROUND WATER & GROUND WATER


EXPLORATION

3.2.1


INTRODUCTION   

  Water is life. There


is no gain saying that without water there would be no life of any kind on


earth. Apart from air, it is the most important. It sustains human life and


constitutes about 70% of its weight .It tremendous use and advantage is worth


of note in domestic, industrial, agriculture, navigational and recreation


sectors as well as energy in form of hydropower generation.

  Although water is


about 70% (by volume) of potable water supply is through underground water


exploitation (wells, boreholes, tube wells wash bore). This is so because the


unit  cost of providing and sustaining


water through well is less. In addition to this, the underground water is pure,


natural and     free from contamination. It does not


require treatments (addition of additive such as chlorine, potassium, alum and


others) which have been proved to have their cumulative effect on the body


chemistry.

  Groundwater exploration is gaining more and


more importance in Nigeria owning to the ever increasing demand for water supplies,


especially in areas with inadequate surface water supplies. Already, ten


percent of the world’s population is affected by chronic water scarcity and


this is likely to rise to one-third by about 2025 (WHO, 1996). The water


scarcity experienced by the people, led to the search for surface water supply.


Surface water, which mostly occurs as rivers are subjected to pollution. It is


sad to say that most of the rivers in Nigeria are highly polluted, the


pollutants being inadvertently introduced by man via industrial and petroleum


exploration activities. Despite the reported favorable ground water situations


the world over, the Nigeria situation appears to be restricted by the fact that


more than half of the country is underlain by sedimentary formations. These


rocks comprise mainly sand stones, shales, clays and hard crystalline


impervious rocks which are either igneous and limestone (Offodile, 1983).

  The first alternative opened to man is ground


water, which may be defined as “water in the zone of saturation and from which


wells, springs and underground run off are supplied”. This water is trapped by


geological formations (Palacky et al., 1981). Many dug wells that were sunk in


the study area without an initial proper investigation failed and so were abandoned.


There are several reasons for the failure of boreholes and these include


inadequate or lack of pre drilling investigation, lack of expertise on the part


of personnel handling the drilling and sometimes lack of proper development of


a successfully dug hole.

Hence, a systematic and


scientific approach to the problem is therefore essential for the study area in


order to overcome these problems.        

 


Groundwater is water located beneath the ground surface in soil pore


spaces and in the fractures of lithologic formations. A unit of rock or an


unconsolidated deposit is called an aquifer when it can yield a usable quantity


of water. The depth at which soil pore spaces or fractures and voids in rock


become completely saturated with water is called water table. Groundwater is


often withdrawn for agricultural, municipal and industrial use by constructing


and operating extraction wells. Groundwater is also widely used as a source,


for drinking supply and irrigation in food production (Zekster and Everett,


2004). Naturally, 53% of all population relies on groundwater as a source of


drinking water. In rural areas the figure is higher. Basically, Lagos which is the


study area located in the south-west region of Nigeria with dense population


living along the coast have a problem with inhabitants gaining access to


groundwater at the same depth.

  GROUND WATER EXPLORATION

Electrical resistivity method of


geophysical techniques happens to be the most preferred method in groundwater


potential.

 


Vertical Electrical Sounding (VES) is a geoelectrical common method to


measure vertical alterations of electrical resistivity. The method has been


recognized to be more suitable for hydro geological survey of sedimentary basin


(Kelly and Stanislav, 1993).

 


The electrical resistivity technique involves the measurement of the


apparent resistivity of soils and rock as a function of depth or position. The


most common electrical technique needed in hydro geologic and environmental


investigations is vertical electrical soundings (resistivity sounding). During


resistivity surveys, current is injected into the Earth through a pair of


current electrodes, and the potential difference is measured between a pair of


potential electrodes. The current and potential electrodes are generally


arranged in a linear array. Common arrays include dipole-dipole array,


pole-pole array, Schlumberger array and the Wenner array. The bulk average


resistivity of all soils and rock influencing the current. It is calculated by


dividing the measured potential difference by the input current and multiplying


by a geometric factor specific to the array being used and electrode spacing.


In a resistivity sounding, the distance between the current electrodes and the


potential electrodes is systematically increased, thereby yielding information


on subsurface resistivity from successively greater depth. The variation of


resistivity with the depth is modeled using forward and inverse modeling


computer software.