Glossary

Aquifer: A permeable geological formation or body that will yield water in economical amounts.

Confined Aquifer: An aquifer overlain by an impermeable layer such that the piezometric head rises above the top of the aquifer.

Cross-hole tomographic: involves the measurement of the travel times of seismic ray paths between two or more boreholes in order to derive an image of seismic velocity in the intervening ground.

appropriate for a few shallow (50 m) targets of interest, or where one is interested in identifying gross lateral velocity variations or changes in interface dip [17]. Though Seismic Refraction yields lower resolution than Seismic Reflection and Seismic Cross-hole tomographic, it is however chosen over Reflection as they are inexpensive and help to determining the depth to the water table (buried refractor) and to the top of bedrock, the gross velocity structure, or for locating significant faults. The buried refractor is usually saturated and has a greater velocity than the

Cross-hole transmission (tomography) data acquisition is possible using several techniques amongst which includes seismic techniques, electrical resistivity, electromagnetic, radar with seismic being the most common. Majority of cross – hole tomographic seismic data have been collected for research however the those collected over extremely high resolution of up to 0.5 m are better suited for site characterization. Figure 14 shows a typical example of seismic cross-hole survey. The multiple sampling of the intra-wellbore area permits very detailed estimation of the velocity structure [19]. As seismic P-wave velocities can be related to lithological and hydrogeological parameters as discussed above, this extremely high resolution method is ideal

In conclusion, Aquifers could be classified into confined or unconfined aquifers on the basis of the presence or absence of the positioning of water table. Its characterization is a function of variations in subsurface petro-physical properties (porosity, hydraulic conductivities, and permeability) measured using geophysical techniques like electrical resistivity, electromagnetic

Having considered, what aquifers are and their characterization based on petro-physical properties of the aquifers. It is also essential to note that these properties help in selecting suitable techniques for aquifer exploration, characterization and its exploitation. However the most widely used and suitable of these techniques is the electrical methods particularly use of the electrical resistivity technique because of its speed, reliability and the fact that it is more

Aquifer: A permeable geological formation or body that will yield water in economical

Confined Aquifer: An aquifer overlain by an impermeable layer such that the piezometric

for detailed stratigraphic and hydraulic characterization of interwell areas [20].

unsaturated equivalent soil unit and the bedrock surface [18].

induction, ground penetrating radar and Seismic techniques.

economical in terms of use for exploration and exploitation.

head rises above the top of the aquifer.

6. Conclusion

28 Aquifers - Matrix and Fluids

7. Recommendations

Glossary

amounts.

Dielectric Constant: A measure of the separation (polarization) of opposite electrical charges within a material that has been subjected to an external electrical field.

Effective Conductivity: The coefficient multiplying the expected value of the head gradient to yield the expected value of the flux.

Electrical Profiling: An electrical survey made at several surface locations using a constant electrode separation distance. Electrical profiles, also called constant spread profiles or resistivity profiles, provide information about lateral changes in apparent resistivity.

Electrical Resistivity: A measure of the ability of electrical current to flow through materials, measured in Ohm-m. Electrical resistivity is an intrinsic property of a material and is the inverse of electrical conductivity.

Electrical Sounding: An electrical survey made at a single surface location by moving electrodes progressively farther apart. Electrical soundings, also called expanding spread profiles, provide information about apparent electrical resistivity as a function of depth.

Normal Moveout Correction (NMO): Adjusting seismic or radar velocity estimates to flatten the parabolic appearance of reflectors due to offset between sources and receivers.

Radargram: A picture of the subsurface profile (graph like) representing a profile length along x-axis and y-axis or A radar image of mineral deposits or a planetary surface.

Reflection: Energy that bounces back from a surface due to a change in physical properties, such as seismic impedance in the case of sound waves resolution.

Transmitter: A transmitter is an electronic device used to produce radio waves in order to transmit or send data with the aid of an antenna during a geophysical investigation.

Receiver: A receiver is a device used to receive signals and decode signals and transform them into information the computer understands during a geophysical investigation.

Seismograms: Is an instrument used for measuring earthquake (seismic) signals which could also be adapted to be used for other geophysical investigations. These are held in a very solid position either on the bedrock or on a concrete base.

Seismic Refraction: Is a geophysical principle governed by Snell's law. Used in the fields of engineering geology, geotechnical engineering and exploration geophysics, seismic refraction traverses are performed using a seismograph(s) or geophone(s) in an array and an energy source.

Seismic Reflection: Is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the earth's surface from reflected seismic waves. It's the most common geophysical methodology used for oil and gas exploration which exhibits the highest degree of technical sophistication in terms of both data acquisition and signal processing capabilities.

Unconfined Aquifer: An aquifer that has no overlying confining impermeable layer.
