In the development of a ground-water supply the first problem is to locate an aquifer capable of yielding the needed supply. A desirable first step is to learn all that is known about the ground-water system in the region and particularly in the specific locality where development is desired. In most areas available information and experience will not be sufficient to guarantee the outcome of the proposed development, and the next step will be field exploration by all methods that can aid in delineating the areal distribu- tion, depth, thickness and potential yield of aquifers. The methods of exploration have been discussed in detail in numerous textbooks (Ramensky, 1947; Todd, 1959; Kli- mentov, 1961; Davies and de Wiest, 1966; Heath and Trainer, 1968; Bindeman, 1969), and have been summarized in several short articles (Thomas and Peterson, 1967; Ineson and Gray, 1969). Pertinent high-lights of some of these methods are discussed in the following sections of this chapter.
3.1.1
Methods
3.1.1.1 Inventory
All operations for ground-water exploration require topographic maps of adequate accuracy and at a suitable scale. If such maps do not exist, all operations have to com- mence with the preparation of such maps. In unexplored areas airphotogrammetric maps, or maps based on radar imagery, are important.
In order to define a preliminary programme for the exploratory work, as for example field reconnaissance, geophysics and test drilling, it is necessary, prior to all operations for ground-water exploration, to examine and bring together existing material, either published or unpublished, such as geological, hydrogeological, hydrological and geo- physical reports and papers, geological and hydrogeological maps, logs of drillings of all kinds, chemical analyses of water, hydrological data from existing observation and other wells including information on location, height above sea level, depth and construction of the wells. Additionally information about the outflow of springs and surface run-off, analysed data from pumping tests, hydrological and geological data from underground and open-pit mining and from waterworks, meteorological data, etc., should be considered.
Such material may be found in the archives of the geological surveys, water authorities, waterworks, water-management offices, mining industry, oil and gas industry, firms concerned with geophysical research, drillers, and with laymen interested in natural sciences.
3.1 page I
Ground-wuter studies
Experience and a critical attitude are commonly necessary for determining whether and to what degree the material gathered is affected by personal opinion, e.g. in the case of logs compiled by drillers, or to what degree personal interpretations are included in reports and papers.
There is no rigid recipe for sifting and processing the above-mentioned data. It may be advisable to enter the data on punched cards, so that they are available for processing by computer. Interpretations emerge which have been illustrated in tables, maps, dia- grams, cross-sections and in photographs. This material should be clear and easy to handle so that it assists in field-work.
Finally, it is part of an inventory to be acquainted with such matters as vegetation, morphology, utilization of land, and the drainage pattern on the basis of existing maps or other material.
3.1.1.2 Field reconnaissance
From the results of the inventory, the nature and extent of the needed investigations in the field become evident.
The existing topographic maps are examined in the field with respect to their accuracy;
if inaccuracies prove excessive, the preparation of reliable maps is initiated.
The next step is the examination and compilation of the information obtained during the inventory of the geological conditions, namely by examining the reliability of existing geological maps, or if those do not exist, by independently surveying outcrops and carrying out routine geological surveys (Fent, 1949; Leggette, 1950). In many cases it may be necessary to compile geological maps. The scale of such maps depends, on the one hand, on the scale of existing topographic maps, and, on the other hand, on the size covered by the area of investigation and on the objective of the ground-water exploration.
Important clues to the principal features of the hydrological conditions are obtained by water-level and temperature measurements in existing wells and by water sampling.
With the help of simple methods carried out either in the field or in the office it is possible to determine the p H value, the total and carbonate hardness, and the concentration of C1- and SO4-- ions of ground waters.
In the case of an adequate density of observation points and sufficiently exact altitude indications on the topographic map, a ground-water contour m a p can be constructed providing important information of the general ground-water conditions, using only those data from water-level measurements which apply to the same aquifer. It is advisable, therefore, to measure the depths of the wells and to make inquiries as to the kinds of rocks and sediments which have been penetrated. Popular expressions are examined critically and, if possible, interpreted geologically.
The yields of springs should be determined approximately, preferably measuring the temperature of the water and taking samples for simple chemical analyses.
During dry periods it would be useful to carry out approximate measurements of the surface run-off in suitable places.
If observation wells or stream-gauging stations are maintained in the area to be investi- gated, they should be examined to determine whether they are operating effectively.
For example, there should be adequate hydraulic connexion between an observation well and the aquifer. This is examined by pouring a slug of water into the well and meas- uring the rapidity with which the head build-up dissipates with time.
3.1.1.3 Photogeology
Aerial photographs at scales from 1:1,000 to 1:100,000 commonly provide the only means of correcting unreliable topographical maps and turning them into usable material, but experienced staff and, in many cases, considerable expenditure of time are necessary for these operations.
Defining the ground-wuter régime
The application of stereoscopic aerial photographs has gained steadily in importance.
Patterns, grey-tones or gradations in colour, relief and micro-relief make it possible to observe differences in geology, soils, soil moisture, vegetation and land use, and to draw boundaries between the various units observed. They also provide information on such features as the nature of the rock units, soils and vegetation types. Dips and strikes of the beds, axes of anticlines and synclines, faults, etc., are clearly discernible (Ray, 1960).
Thus photogeological maps can be prepared which increase greatly the efficiency of the subsequent geological field surveys.
An experienced photogeologist can differentiate between various rock and soil types, for instance between coarse-grained and fine-grained clastic material, indicating at the same time whether they are permeable or impermeable, and mapping simultaneously their area and distribution. By combining this geological information with morphological evidence, areas of probable ground-water recharge and ground-water discharge can be delineated.
Intensity of fracturing and fracture patterns are commonly significant since they may have great influence upon the porosity and permeability of the rocks (Lattman and Parizeh, 1964; Seker, 1966). In some circumstances aerial photographs can provide an indication of the relative depth to ground water by indicating the soil moisture content.
Spring levels can be identified as well as ground-water discharge zones in marshy areas.
The presence of an identifiable association of phreatophytes or plants which have a high transpiration capacity and derive their water directly from the water table, indicates a water level close to the land surface (Victorov et al., 1964). Halophyte associations or plants which have a high tolerance for soluble salts in the soil and ground water, often combined with white efflorescences, indicate the occurrence of brackish or saline ground water. Xerophyte plants capable of subsisting on a low content of soil moisture indicate a considerable depth of the water table (Howe, 1958; Schumm, 1968).
Besides conventional black-and-white photography, colour films as well as special films and filters are used sometimes. The latter serve to record only the image of a portion of the visible light spectrum or to fix also radiation on the infra-red side of the spectrum, producing for instance infra-red photography and infra-red colour (or 'false- colour') photography. Certain contrasts are enhanced on these special types of photo- graphy, in particular in soil moisture and in plant associations (Robinove, 1969).
Several other parts of the electromagnetic spectrum can be used for surveying the earth's surface from the air for geological and hydro-geological purposes. Because there is no emulsion sensitive to wave-lengths of more than 1 micron (near infra-red) other sensors must be used to observe and record the radiation. Moreover, we are restricted to the application of those wave-lengths for which atmospheric absorption is at a minimum.
O f practical importance for hydrogeological studies are three line-scan systems pro- ducing images of the earth surface on the basis of ultra-violet and blue light (less than 0.5 micron), medium to far infra-red radiation (3.5 to 5 micron and 8 to 13 micron bands) and radar (various bands in the 3 to 300 mm range) respectively.
The potential for hydrogeological studies of these systems is being prospected at present. Imagery with ultra-violet and blue lights can be useful for water-pollution studies.
Airborne infra-red scanning at scales 1 :5,000 to 1 :25,000 recording differences of apparent surface temperatures of 0.5" C with a total range of 10" C is applicable under a variety of conditions during the day or night and may give information on geology, soils, soil moisture and ground-water circulation. S o m e relationships between the ground-water and the temperature régimes are given in Sections 3.2.2.3 and 5.5. Radar imagery produces images at scales of 1: 100,000 and smaller and is useful for rapid regional surveys with the additional advantages that it can be done under all weather conditions and that the imagery programme contains five independent categories of choices in scale, direction of flightlines with respect to area to be surveyed, altitude of flying, wave-
Groimrl-wafer studies
length and polarization. The image shows ‘pseudo-relief’, which facilitates its use for geological analysis (Robinove, 1968
;
Williams and Ory, 1967).3.1.1.4 Geophysics
Geophysical methods have been used in ground-water exploration, especially in areas associated with non-indurated sediments, and they serve, in the first instance, for planning efficient and economical test-drilling programmes. Although it is probable that geo- physical investigations will never replace drilling, they can provide information about the principal features of the underground structure in such a way that the number of drillings may be held to a minimum and the depths of the exploratory boreholes may be estimated beforehand (Dobrin, 1952; Dakhnov, 1962; Ogilvi, 1962; Matveev, 1963).
During the last few years it has become increasingly apparcnt that the application of geophysical methods is generally most successful if, before the operations are started, a team of one hydrogeologist and one geophysicist co-operate closely (Carpenter and Bessarab, 1964). The hydrogeologist should be sufficiently acquainted with the physical principles of the geophysical methods in use to realize the possibilities and limitations of the methods with respect to the particular problem with which he is concerned. Under the direction of the hydrogeologist working with him, the geophysicist obtains as accurate an indication as possible of the presumed geological structure of the area to be investi- gated and thus is able to arrange an appropriate observational programme and to evaluate his measurements in accordance with known facts (McGuinniss and Kempton, 1961 ; Ogilvi, 1962).
T w o geophysical methods are generally applied : the electrical-resistivity method and the seismic-refraction method.
The electrical-resistivity method has been most widely used in ground-water investiga- tions, partly because the equipment is inexpensive, portable and easy to operate. It is based on the different electrical resistivities of the various rocks and sediments, resistivities which depend on the material, density, porosity, pore size and shape, and the physical properties of the included water, such as water content, quality and temperature. The method is especially adapted for locating subsurface salt-water boundaries, because the decrease in resistance of the salt water becomes apparent on a resistivity-depth curve.
For measuring the resistivities of the different rocks beneath the surface and thus for distinguishing the sequence of strata, the Wenner or Schlumberger electrode configuration is used in most cases: an electric current is fed into the ground through two currznt electrodes inserted at the land surface. An electrical field thereby develops, the charac- teristics of which are determined by the sequence and physical proportions of the strata.
As the electrode spacing is increased the resistance between them is relatable to rocks at increasing depths. Centred between the two current electrodes is another pair of electrodes separated by a distance equal to half the current electrode spacing. The drop in electrical potential is measured at the two inner electrodes. From the current intensity and the potential difference an apparent resistivity is obtained, values of which are plotted on double-logarithmic paper against half the electrode spacing. This resistivity-depth curve is interpreted for depths to interfaces between strata and for resistivities of the layers either by means of theoietically calculated standard curves which have already been compiled (as, for example, Mooney and Wetzel, 1956; Pylaev, 1968) or by means of theoretical curves which can be produced by a computer and adapted to the problems concerned.
The electrical-resistivity method does not provide satisfactory quantitative results if the various beds are thin: either cumulative effects are obtained or anomalous resistivities are measured, the interpretation of which is extremely difficult or impossible. Similarly, results may not be satisfactory if the differences between the resistivity values of the
3.1 poge 4
Defining the ground-water régime
several layers are insufficiently great. The method is most effective within a hundred metres below the land surface, and increasingly ineffective at greater depths.
Seismic methods involve creating a small shock at the earth’s surface and measuring the time for the resulting sound or shock wave to travel known distances. Seismic waves may be reflected or refracted at any interface where a velocity change occurs; the seismic reflections may provide information on geologic boundaries thousands of metres below the land surface, wh.ereas seismic refraction methods cover only a few hundred metres in depth and are more commonly used in ground-water exploration. Seismic-wave velocities are governed by the elastic properties of the rocks through which they pass.
Where a seismic wave passes through rocks of contrasting properties, its velocity is changed at the boundary, which can thus be identified. Increased porosity tends to decrease wave velocity, but increased water content increases it. The refraction method can be used to determine the depth of unconfined ground water in loose sediments (Levshin, 1961). Because seismic methods require special equipment and trained techni- cians, they have been used only to a limited extent in ground-water exploration, although they are widely used in explorations for oil and natural gas. In ground-water exploration the seismic-refraction method lias proved especially successful in exploring the total thickness of non-indurated sediments above bed-rock in buried valleys or in wide basins on consolidated rocks. It is advisable to combine the electrical-resistivity method with the seismic-refraction method, since commonly the electric resistivity of the basement rocks does not differ significantly from that of the overlying non-indurated sediments.
3.1.1.5 Drilling and logging1
Large production wells are generally constructed by drilling, of which the methods most commonly in use are the cable tool or percussion, hydraulic rotary and reverse rotary.
Each method is particularly suited for drilling in some materials and not in others, and each is also adaptable to a wide variety of conditions. There are many variations in detailed techniques used, so that well drilling has become an art which may be skilfully adapted to local conditions. Construction methods differ regionally and among individual drillers. Many drillers have equipment in sufficient variety to utilize the techniques best adapted to the materials encountered in the well. The techniques of well drilling (Bennison, 1947, p. 123-232; Anderson, 1951 ; Moss, 1958; Stow, 1963 ; ICulichikhin, Vozdvizhensky, 1966) have progressed considerably during the past fifty years. Experience, sometimes bitter, has demonstrated again and again that successful, economical development of significant ground-water supplies by wells requires the services of experienced, well- equipped drillers.
It is important that complete drilling logs are obtained for production wells even though test drilling may have been done in advance. These add to local geophysical information as well as providing a basis for design and development decisions for the particular well under construction.
Rotary drilling. A rock-cutting tool at the end of a drill pipe of smaller dimension is rotated while seated on the bottom of the borehole. The drill cuttings may be brought to the land surface by flushing with a circulating fluid, although they may be drawn up with the tool.
Hydraulic rotayy drilling. When applying the hydraulic rotary method, either water or a fluid m u d (clay or bentonite suspension), or compressed air is pumped down through the drill pipe; the drilling fluid leaves the tool directly above the bottom of the borehole, and when rising in the borehole carries the drill cuttings with it.
The method of bringing the cuttings up to the land surface by means of compressed air may be applied either above or below the ground-water level; in the first case drill
1
1
1
1. See also Chapter 9 for a further discussion of logging.
3.1 page 5 Rev. I (1975)
Ground-water. stuclies
dust is obtained; in the second a mixture of air/water/drill cuttings. With the pneumatic- drill system it is, however, impossible to support the borehole walls. The method is thus suitable only for rigid rocks yielding small-sizecl cuttings, and for small borehole diameters.
With the help of a drilling fluid, on the other hand, it is possible to construct boreholes in almost all types of rocks, The head difference between the drilling fluid and the ground- water reservoir is kept sufficiently high so that the walls of the borehole remain stable;
thus no casing is necessary. However, the diameter of the drill hole must be chosen so that the rate of rise of drilling fluid remains higher than the settling rate of the drill cuttings. If rocks with clay or other fine-grained components are penetrated, the drilling fluid incorporates the finest particles which are also circulated by the hydraulic pump.
Because the water is circulated continuously, in most cases through a settling pit, a fluid m u d is the result. W h e n heavy drilling m u d is used, the drill-hole diameter may be larger, because the settling of the cuttings is lower. Because of these advantages, drilling mud, with the addition of finely ground fire-clay, chemical additives and weighting material is used commonly from the beginning of drilling. Special muds are produced for particular purposes.
The behaviour of the ground water in the borehole, as, for example, in water-level fluctuations during the drilling, cannot be observed when drilling m u d is used. It is
The behaviour of the ground water in the borehole, as, for example, in water-level fluctuations during the drilling, cannot be observed when drilling m u d is used. It is