What is the Best method for Siting

Techniques for Siting

Many techniques are available for siting drilled water wells, and the most commonly used ones are summarized in this section. There is no ideal single technique that fits best to every condition. Instead, the use of different techniques should always be tailored to the local conditions and rock types, and in particular to the three situations of increasing hydrogeological complexity set out in section 3.4. Some of the techniques may look simple, but considerable skill and experience is required to understand and interpret the results correctly. Therefore, it is strongly recommended that the techniques always be used by a trained hydrogeologist or technician and carried out under the supervision of one. The most often applied techniques are summarized below.

Remote sensing:
The use of aerial photography, side-looking airborne radar (SLAR) and satellite imagery has a powerful role in identifying geological boundaries and hydrologically significant features (such as deep fractures) which may not be visible on the ground. Such remote techniques always require an independent check at the ground surface by field reconnaissance, by geophysical survey or by drilling (known as ground truth) to have confidence in the findings obtained remotely.

Remote sensing can be very useful, but its use should always be determined by realistic expectations of what it might or might not indicate.

The most likely applications are firstly in the planning and reconnaissance stage and secondly for narrowing down target areas or locating specific features for geophysical survey, for locating and delineating communities requiring water supplies and identifying existing supply sources.

Hydrogeological field surveys:
If indications of the potential for using groundwater have been obtained from maps, documents, satellite images, hydrogeological field reconnaissance provides the
opportunity to check this. Thus, the mapped geological formations should be confirmed from rock exposures (for example in river beds and road cuttings). Local topography and geomorphology can influence groundwater occurrence, storage and flow and enhance groundwater recharge and help to produce favourable sites. These should be observed and noted. Vegetation cover can reflect geological conditions and indicate the presence of shallow groundwater.

The field reconnaissance should locate and examine existing dug and drilled wells to verify the information about yields and water levels already collected from secondary sources. Their operating status and condition should be noted, together with any visual evidence of water quality constraints such as iron staining or fluoride impacts, and any likely sources of pollution. These observations should be supplemented by information from local communities who are likely to be very
knowledgeable about their local environment and their water sources. Older members of the community, for example, may be able to indicate scoop holes that have dried up, or they may recall vegetation patterns prior to deforestation. Such information should include the normal seasonal variations and more severe drought impacts on yields and groundwater levels in both traditional and improved sources as well as information about water quality constraints. All of the information collected in the field reconnaissance should be carefully recorded (using a logbook, drawings, photographs, GIS, other field equipment). To collect such information effectively, field surveys need the participation of trained community workers who can converse in the local language. If the survey finds sound evidence of groundwater potential, then sites can be selected without the need for additional investigations using geophysics. This is likely to the case for most of the areas with simple hydrogeological conditions of unconsolidated sedimentary materials and shallow groundwater. The survey should also enable a choice to be made on the hydrogeological suitability of areas or locations for dug wells or boreholes where both are envisaged within a program.

Establishing a siting approach based on a combination of existing information, remote sensing and field survey can be highly cost effective for these conditions, but will only be successful if it is led by an experienced hydrogeologist. Moreover, qualified personnel with hydrogeological knowledge are needed to decide when such an approach cannot be applied with confidence and additional investment in geophysical surveys is needed, and then to plan, implement and interpret the surveys.

Geophysical surveys:
Geophysical surveys are by far the most commonly used techniques in well siting. These techniques measure the physical properties of rocks, such as their resistivity and conductivity, magnetic fields and sonic properties. Most cannot directly detect the presence of water. Instead, the contrasts in sub-surface (rock and water) properties are interpreted in relation to geological features that are expected to facilitate groundwater storage and movement.

In favorable circumstances, these techniques can detect vertical fractures in hard rock, layering in horizontal formations, and contrasts between dry and wet rock and between fresh and saline water. As with remote sensing, ground truth is needed, and use of geophysics should always be determined by realistic expectations of what can be achieved. Although geophysical surveys can assist in locating productive sites, they are often included automatically in a tender in the hope that they will produce something useful. This approach rarely rewards the effort put in, and there are many cases of geophysics having added nothing to the reliability of well siting. While there are many geophysical techniques, those most commonly used for well siting are the electrical resistivity and electromagnetic (EM) methods, with seismic refraction
and magnetic techniques also having some applications.

The resistivity method has been used for many years and can be employed in two distinct ways. The first is a Vertical Electrical Sounding (VES) in which depth variations in subsurface resistivity at fixed point can be interpreted in terms of a sequence of geological layers. The electrodes are expanded in an array about this central point. The second is a constant separation traverse in which the electrode array is moved across the ground to provide qualitative information about lateral changes in subsurface rock types and structures. Resistivity profiling has largely been replaced by the electromagnetic (EM) methods, which provide better information about lateral changes in resistivity much more quickly and cheaply.

Electrical resistivity and electromagnetic methods are the two most widely used geophysical survey methods. The equipment is relatively inexpensive, robust and not difficult to operate in the field. A widespread consequence of this is the routine use of such equipment by field technicians who do not have geological training. However, in order to obtain the best results, the interpretation of data from these (or any other) geophysical methods requires experience and triangulation with local hydrogeological knowledge. All the information obtained is entered into a computer that analyses the measurements with special software. The range resistivity’s are very large. The values given in figure 2.5 are only informative: the particular conditions of the site may change the resistivity values. For example, dry coarse sand or gravel may have a resistivity like that of igneous rock, whereas weathered rock may be more conductive than the soil that overlaying it.

Since the resistivity of the soil or rock is controlled primarily by the pore water conditions, there are wide ranges in resistivity for any particular soil or rock, such that resistivity values cannot be directly interpreted in terms of soil type lithology. Commonly, however, zones of distinct resistivity can be associated with specific soil or rock units on the basis of local outcrops or borehole information. It is the enormous variations in rock and mineral electrical resistivity that makes resistivity techniques attractive.

In fact, siting which is carried out by inexperienced operators and analysts can actually reduce the likelihood of finding water. If the interpretation is poor, it may be better to drill at random.