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Simple methods for assessinggroundwater resources in low

permeability areas of Africa

Groundwater systems and water quality

Commissioned Report CR/01/168N


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The full range of Survey publications is available from the BGSSales Desks at Nottingham and Edinburgh; see contact detailsbelow or shop online at www.thebgs.co.uk

The London Information Office maintains a reference collectionof BGS publications including maps for consultation.

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publications; this catalogue is available from any of the BGS SalesDesks.

The British Geological Survey carries out the geological survey ofGreat Britain and Northern Ireland (the latter as an agency

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projects. It also undertakes programmes of British technical aid ingeology in developing countries as arranged by the Department

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The British Geological Survey is a component body of the NaturalEnvironment Research Council.

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BRITISH GEOLOGICAL SURVEY


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Helping to meet International Development Targets is thenumber one challenge for the water sector. To increaseaccess to sustainable supplies of safe water for the poor,groundwater resources need to be exploited successfully,sustainably and economically on a much larger scale thanthey are now.

Exploiting groundwater on a larger scale requiresknowledge to be transferred more widely and effectively.Unfortunately, techniques for finding and exploitinggroundwater are often hidden in scientific journals or

project reports, and not widely disseminated. This manualaims to be a first step in providing useful information for

project engineers working on rural water supply projects insub-Saharan Africa. The focus of the manual is on lowpermeability aquifers, where groundwater is difficult tofind. The techniques have been tested by the BritishGeological Survey for their effectiveness and by WaterAidstaff for usability. However the manual is not static andsuggestions and comments would be most welcome forfuture editions.

The manual is an output from a U.K. Department forInternational Development project (R7353 Groundwaterfrom low permeability rocks in sub Saharan Africa).

Alan MacDonald

April 2002

Preface


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Many individuals both in BGS and elsewhere contributed tothe development of this manual. Much of the material has

been tested and improved by WaterAid staff in Nigeria,Ghana or Tanzania. Colleagues at BGS also providedadvice and helpful comments. Thanks to DFID for

providing funding for the project

British Geological SurveyJohn Chilton: reviewing the manualJude Cobbing editingRoger Peart advice on geophysical aspects

Dave Beamish advice on geophysical modelling

WaterAidNick Burn (London)Kitka Goyol (Nigeria)Gordon Mumbo (Ghana)Herbert Kashililah (Tanzania)

Thanks also to Petros Eikon for providing the geophysicalmodelling software and Prof John Barker (UniversityCollege London) for his helpful comments on the bailertest.

Acknowledgements


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Preface i

Acknowledgements ii

Contents iii

1 Introduction 1

2 Groundwater resources in sub-Saharan Africa 22.1 Why groundwater? 22.2 Groundwater resources in SSA 2

3 Reconnaissance 73.1 Experience 73.2 observations - geology and previous success 73.3 Maps and reports 73.4 Siting boreholes and villages on maps 73.5 further techniques 8

4 Techniques for siting wells and boreholes 94.1 Geological Triangulation maps, observationand geophysics 94.2 Community discussions and villageobservation 104.3 When are geophysical techniques required? 104.4 Ground conductivity using FEM 124.5 Electrical resistivity 204.6 Further techniques 22

5 Gathering information during drilling 255.1 What are boreholes and wells? 255.2 Drilling methods and equipment 255.3 Drilling and sampling procedures 27

5.4 Data collection during drilling 296 Assessing the yield of a borehole 35

6.1 Why carry out a pumping test in a borehole? 356.2 Measuring yield during an airlift 356.3 The bailer test 366.4 Constant rate tests 38

7 Simple methods for assessing groundwater quality41

7.1 Groundwater quality 417.2 Simple measures of water quality 437.3 Detailed analysis 43

8 Using the information to build a regional picture45

8.1 Why is understanding the regionalhydrogeology useful? 458.2 Keeping hold of information 458.3 A simple database 458.4 The future? 46

Appendix 1 Fact sheets on techniques and methods47

FIGURES

Figure 1.1 Understanding the groundwater resources canmean the difference between successful andunsuccessful boreholes. 1

Figure 2.1 Groundwater provinces in sub-Saharan Africa. 2

Figure 2.2 Land area and rural population of thedifferent hydrogeological zones of SSA. 3

Figure 2.3 Variation of permeability and porosity withdepth in basement aquifers (based on Chilton &Foster, 1995). 3

Figure 2.4 Cross section of groundwater flow inhighland volcanic areas. 4

Figure 2.5 Groundwater occurrence in consolidatedsedimentary rocks. 4

Figure 2.6 Groundwater occurrence in unconsolidatedsedimentary rocks. 5

Figure 3.1 A GPS being used by WaterAid project staff inNigeria. 8

Figure 4.1 Siting wells and boreholes in areas where it iseasy to find groundwater, where groundwateroccurrence is generally understood, and in complexareas. 9

Figure 4.2 The geological triangulation method. 9

Figure 4.3 Places to visit on a reconnaissance visit to acommunity. 10

Figure 4.4 How ground conductivity methods work. 12

Figure 4.5 The different possible orientations of FEMequipment. 12

Figure 4.6 Choosing survey lines in a community. 13

Figure 4.7 Example of a notebook for a groundconductivity survey using EM34 equipment. 13

Figure 4.8 The EM34 equipment, comprising two coils, aconnecting cable, transmitter and receiver. 14

Contents


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Figure 4.9 The EM34 being used with the coils horizontal.14

Figure 4.11 Resistivity and conductivity of common rocktypes. 15

Figure 4.12 Ground conductivity measured using theEM34 instrument over a pocket of weathered

basement rocks. 16

Figure 4.13 Apparent conductivity measured overincreasing thicknesses of weathered basement usingdifferent coil spacings and orientations. 17

Figure 4.14 Response of the EM34 instrument over a singlevertical conductor. 17

Figure 4.15 Schematic diagram showing the kind ofresponse that is given by ground conductivity (usingthe EM34 instrument) over fracture zones in basement.18

Figure 4.16 Using ground conductivity to map out theboundary between a sandstone and a mudstone in avillage. 19

Figure 4.17 The three main electrode configurations forcarrying out VES. 20

Figure 4.18 Equipment for carrying out resistivity surveys.21

Figure 4.19 Electrodes have to be hammered in andsometimes watered to ensure good electrical contactwith the ground. 21

Figure 4.20 The instrument stays at the midpoint andcables run out to either side. 21

Figure 4.21 Common resistivity curves and theirinterpretation for basement areas 23

Figure 4.22 Simple resistivity curves and a roughinterpretation for sedimentary areas. 24

Figure 5.1 Component parts of a typical borehole. 25

Figure 5.2 A typical hand-dug well. 25

Figure 5.3 Community members standing too close to adrilling rig for safety. 27

Figure 5.4 Equipment required for collecting andanalysing data during drilling. 28

Figure 5.5 Example of field notebook for assessingdrilling conditions. 29

Figure 5.6 Rock chip samples stored in clearly markedplastic bags. 30

Figure 5.7 Rock chip samples stored separately, inorder of collection. 30

Figure 5.8 A cored sample showing an open fracture inlimestone, associated with calcite veining. 31

Figure 5.9 Example of rock chip samples displayed in asectioned pipe. 31

Figure 5.10 An example of the information that can becollected from a routine water supply borehole usingsome of the simple techniques described in thischapter. 32

Figure 5.11 Typical borehole completion diagram,showing lithological and construction details. 33

Figure 6.1 Measuring yield during airlift. 35

Figure 6.2 Community members helping with a bailer testin Edumoga, Nigeria, 36

Figure 6.3 Equipment required for a bailer test. 36

Figure 6.4 Carrying out a pumping test using Whale pumpspowered from a car battery. 38

Figure 6.5 Example of a notebook recording water-levelsduring a constant rate pumping test. 39

Figure 6.6 An example of measuring ? s from recoverydata. 39

Figure 6.7 Measuring ? s from a drawdown curve. 40

Figure 6.8 Any deviations from a straight line in a Jacobplot can give useful information about an aquifer. 40

Figure 7.1 Simple field equipment for estimating water

quality. 43Figure 7.2 Highly corroded rising main from a borehole

with TDS greater than 3000 mg/l. New rising maincorroded within one year. 44

Figure 8.1 Project staff using a groundwater developmentmap produced on a GIS using the techniques describedin this manual. 46

BOXES

Box 3.1 Questions to discuss with someone with previous

experience of the project area. 7Box 3.2 A first field visit. 7

Box 4.1 Summary for surveys to identify deep weatheringin basement. 16

Box 4.2 Summary of surveying for fracture zones inbasement rocks. 17

Box 4.3 Summary of surveying with ground conductivityin sedimentary environments. 19

Box 4.4 Resistivity and Conductivity 22

Box 4.5 Summary of resistivity VES. 22

Box 6.1 Criteria for success used in the bailer test 37

TABLES

Table 2.1 Summary of groundwater potential ofgroundwater domains in sub-Saharan Africa withindicative costs of development. 6

Table 3.1 Information available from differentorganisations. 8

Table 4.1 Summary of common geophysical techniquesused in groundwater investigations (from MacDonaldet al. 2000). 11


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Table 4.2 Electrical conductivity of different mudstonesmeasured in Nigeria. 19

Table 4.3 Advantages and disadvantages of usingresistivity VES. 20

Table 5.1 Summary of drilling methods and theirapplicability to different environments. 26

Table 6.1 Advantages and disadvantages of measuringyield during airlift. 36

Table 6.2 Table for assessing the success of a boreholefrom a bailer test. If the maximum drawdown andtime for half and three quarters recovery are all lessthan quoted here (for the correct borehole diameterand pumping rate) then the borehole is likely to besuccessful. 37

Table 7.1 World Health Organisation guidelines fordrinking water quality: inorganic chemicals of healthsignificance in drinking water. 41

Table 7.2 World Health Organisation guidelines fordrinking water quality: inorganic substances and

parameters in drinking water that may give rise tocomplaints from customers. 42


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In many areas throughout Africa, a staggering proportion ofwells and boreholes fail. Failure can occur for a number ofreasons inadequate maintenance and communityinvolvement, poor engineering or a lack of water. Often itcan be difficult to work out the exact reason after the event.However, in many geological environments the impacts of

poorly sited and designed boreholes and wells are a majorconcern to funding agencies, implementing institutions andlocal communities. In such areas, good supplies ofgroundwater cannot be found everywhere, and boreholesand wells must be sited and designed carefully to make useof the available groundwater. To appropriately site anddesign water sources, the groundwater resources of an areaneed first to be investigated to understand how water occursin the ground.

In this manual we present some techniques that allow aquick assessment of groundwater resources withoutrequiring much expertise or expense. Some of thetechniques are old and established while others are new.However, all techniques have been tested by BGS (andothers) in assessing groundwater resources in Africa. Thismanual does not claim to be a detailed textbook forhydrogeologists there are enough already (see reading listat the end of the chapter). Rather it is meant as a practicalaid for those involved in the practice of rural water supply,

particularly in Africa. Little training or equipment isrequired for the tests and they can all be carried out in a

short space of time.The manual is divided into six sections. The first gives

an overview of the groundwater resources of sub-SaharanAfrica (SSA) and discusses the scope and detail ofinvestigations required in different geologicalenvironments. The remaining chapters describe simpletechniques for assessing groundwater resources, from basicreconnaissance to assessing the yield of a borehole. In theappendix are summary sheets of the most commontechniques which can be photocopied and used in the field.

FURTHERREADING ON HYDROGEOLOGY

Price M. 1996. Introducing Groundwater. Stanley Thornes,278 pp

Freeze, R A, and Cherry, J A, 1979. Groundwater. PrenticeHall, Englewood Cliffs

Fetter, C W, 1994. Applied Hydrogeology. Macmillan,New York.

1 Introduction

Figure 1.1 Understanding the groundwater resources can mean the difference between successful and

unsuccessful boreholes.


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2.1 WHY GROUNDWATER?

Groundwater is well suited to rural water supply in sub-Saharan Africa (SSA). Since groundwater responds slowlyto changes in rainfall, the impacts of droughts are often

buffered. In areas with a long dry season, groundwater isstill available when sources such as rivers and streams haverun dry. The resource is relatively cheap to develop, sincelarge surface reservoirs are not required and water sourcescan usually be constructed close to areas of demand. Thesecharacteristics make groundwater well suited to the more

demand-responsive and participatory approaches that arebeing introduced into most rural water and sanitationprogrammes.

Groundwater has excellent natural microbiological

quality and generally adequate chemical quality for mostuses. Nine major chemical constituents (Na, Ca, Mg, KHCO3, Cl, SO4, NO3 and Si) make up 99% of the solutecontent of natural groundwaters. The proportion of theseconstituents reflects the geology and history of thegroundwater. Minor and trace constituents make up theremaining 1% of the total, and their presence (or absence)can occasionally give rise to health problems or make themunacceptable for human or animal use.

2.2 GROUNDWATER RESOURCES IN SSA

Figure 2.1 shows a hydrogeological map of SSA. Broadly,SSA can be divided into four hydrogeological provinces:Precambrian basement rocks; volcanic rocks;unconsolidated sediments; and consolidated sedimentary

2 Groundwater resources in sub-Saharan Africa

PreCambrian basement rocks

Volcanic rocks

Consolidated (postPreCambrian) rocks

Unconsolidated sediments(mainly Quaternary)

Lakes

Robinson Projection

Hydrogeological Domains

Figure 2.1 Groundwater

provinces in sub-Saharan

Africa (based on a numberof sources see MacDonald

and Davies 2000).


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rocks. Groundwater occurs in different ways in these four

provinces, and different methods are required to find andsustainably develop groundwater in each. Basement rocksform the largest hydrogeological province, occupying 40%of the 23.6 million square kilometres of SSA (Figure 2.2);volcanic rocks are the smallest hydrogeological provincewith only 6% of the land area. The relative importance ofthe hydrogeological provinces is best indicated by the rural

population living in each one. Rural communities are mostdependent on local resources for water supply, sincetransporting water over significant distances is prohibitivelyexpensive and difficult to manage. Figure 2.2 shows thenumber of rural people living in each of the four mainhydrogeological provinces in SSA.

A brief description of each hydrogeological zone isgiven below. This is a summary of information given in acompanion report describing the hydrogeology of sub-

Saharan Africa (MacDonald & Davies 2000).

1. Crystalline basement rocks occupy 40% of the landarea of SSA; 220 million people live in rural areasunderlain by crystalline basement rocks. Theycomprise hard crystalline rocks such as granite andmetamorphic rocks. The occurrence of groundwaterdepends on the existence of a thick weathered zone(the uppermost 10 - 30 m) or deeper fracture zones(Figure 2.3). Much of the basement has beenweathered or fractured. Groundwater in the shallowweathered zone can be exploited with boreholes, dugwells and collector wells; groundwater in the deeperfracture zones can only be exploited using boreholes.

Borehole and well yields are generally low, but usuallysufficient for rural demand. Good sites for wells andboreholes can be found using standard geophysical

Land area of hydrogeological

zones in SSA

0

10

20

30

40

50

basement volcanic consolidated

sedimentary

unconsolidated

sediments

hydrogeological zone

landarea(%)

Rural population living on different

hydrogeological zones in SSA

0

50

100

150

200

250

basement volcanic consolidated

sedimentary

unconsolidated

sediments

hydrogeological zone

ruralpopulation(millions)

Figure 2.2 Land area and rural population of the different hydrogeological zones of SSA.

Figure 2.3 Variation of permeability and porosity with depth in basement aquifers (based on Chilton & Foster, 1995).


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techniques. Groundwater is generally of good quality(occasional elevated sulphate, iron or manganese), butis vulnerable to contamination.

2. Volcanic rocks occupy 6% of the land area of SSA,and sustain a rural population of 45 million, many ofwhom live in the drought stricken areas of the Horn ofAfrica. Hard black basalts and ash deposits make upmany of the volcanic rocks. Groundwater occurs inzones of fracturing between individual lava flows andwithin volcanic rocks which have themselves beenhighly fractured or are porous (Figure 2.4). Yields can

be highly variable, but are on average sufficient for

rural domestic supply and small scale irrigation.Groundwater in mountainous areas can be exploitedthough springs, wells and boreholes. Where the rocksare hard and the fracture zones deep, only boreholesare possible. Geophysical methods are not routinelyused to site boreholes and wells, but may be valuablein certain circumstances. Groundwater quality cansometimes be poor due to high fluoride concentrations.

3. Consolidated sedimentary rocks occupy 32% of theland area of SSA and sustain a rural population of 110million. Consolidated sedimentary rocks comprisesandstone, limestone and mudstone and often form

Figure 2.4 Cross section of groundwater flow in highland volcanic areas.

Figure 2.5 Groundwater occurrence in consolidated sedimentary rocks.


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thick extensive sequences. Sandstone often containslarge amounts of groundwater, particularly wherefractured or friable; limestone can also containsignificant groundwater. Mudstones, which maycomprise up to 65% of all sedimentary rocks are pooraquifers, but groundwater can still sometimes be foundin harder more fractured mudstone (Figure 2.5).Where aquifers and groundwater levels are shallow,

wells can be used. However where the aquifers aredeep, boreholes must be used and need to be carefullyconstructed and gravel packed to avoid ingression ofsand. Geophysical methods can easily distinguishsandstone from mudstone and between hard and softmudstone. Where sandstone or limestone aquifers areextensive and/or shallow, careful siting is often notrequired for domestic water supplies. Groundwaterquality is generally good, but can be saline at depth, orhave localised elevated sulphate, iron or manganese.

4. Unconsolidated sediments cover 22% of the landmassof SSA; at least 60 million rural people live on these

sediments, but many of those living on less productiverocks types are close to small unconsolidatedsedimentary aquifers (UNSAs) associated with rivervalleys. UNSAs comprise a range of material fromcoarse gravel to silt and clay. Groundwater is foundwithin gravel and sand layers (Figure 2.6). Yieldsfrom thick deposits of sand and gravel can be high,sufficient for domestic supply and agriculturalirrigation. Where aquifers and groundwater levels areshallow, wells can be used and boreholes installedusing hand drilling. However where the aquifers aredeep, boreholes must be used and need to be carefullyconstructed and gravel packed to avoid ingress of sand.

Geophysical methods can easily distinguish sand andgravel layers and can be used to indicate the thicknessof UNSAs. In large UNSAs, little siting is required.

Groundwater quality problems can occur in UNSAsdue to natural geochemistry and contamination, such ashigh iron, arsenic and elevated nitrate.

There are many exceptions to the general models presentedhere, and areas exist in each of the hydrogeologicalenvironments where groundwater is not easily found. Moreresearch and experience is required to help refine the

models and shed light on the groundwater potential ofdifferent environments. Two of the most widespread

problematic areas are poorly weathered basement rocks andsedimentary mudstones. Research into the potential forgroundwater in these rocks types is limited, and water

projects in these areas are rarely successful.The basic models for how groundwater occurs in the

various hydrogeological environments presented abovehave been developed from research and experience both inAfrica and other similar hydrogeological areas worldwide.Table 1 gives a summary of the current knowledge of thegroundwater resources of each hydrogeologicalenvironment. Indicative costs of developing a groundwater

source are given to help reflect the implications for ruralwater supply of the varying hydrogeological conditions andthe current knowledge base of different aquifers. Thetechnical capacity required to develop groundwater alsochanges with the hydrogeology: in some environments littleexpertise is required, while in others considerable researchand money is required to develop groundwater.Throughout the remaining chapters we discuss whichtechniques are important for the various hydrogeological

provinces.

FURTHERREADING

MACDONALD, A M AND DAVIES, J. 2000. A brief review ofgroundwater for rural water supply in sub-Saharan Africa. BritishGeological Survey Technical Report WC/00/33.

Figure 2.6 Groundwater occurrence in unconsolidated sedimentary rocks.


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FOSTER, S S D, CHILTON, P J, MOENCH, M, CARDY, F ANDSCHIFFLER, M. 2000. Groundwater in rural development, WorldBank Technical Paper No 463, The World Bank, Washington D C.

CHILTON, P J AND FOSTER, S S D. 1995. Hydrogeologicalcharacterisation and water-supply potential of basement aquifersin tropical Africa. Hydrogeology Journal 3 (1), 36-49.

EDMUNDS, W M AND SMEDLEY, P L. 1996. Groundwatergeochemistry and health: an overview. In: Appleton, J D, Fuge, R& McCall, G J H. (eds), Environmental geochemistry and health,Geological Society Special Publications, 113, 91-105.

Table 2.1 Summary of groundwater potential of groundwater domains in sub-Saharan Africa with indicative

costs of development.

Costs* and technical difficulty**of developing groundwatersources

GroundwaterDomains

GroundwaterSub-Domains

GroundwaterPotential

AverageGroundwaterYields

GroundwaterTargets

RuralDomesticSupply

Small ScaleIrrigation

Highlyweatheredand/or fracturedbasement

Moderate 0.1- 1 l/s Fractures at thebase of the deepweathered zone

-

# - ##

-

## - ###

BasementRocks Poorly

weatheredand/or sparselyfracturedbasement

Low 0.1 l/s Widely spacedfractures andpockets of deepweathering

###

Generally not

possible

Mountainousareas

Moderate 0.5 5 l/s Horizontal fracturezones betweenbasalt layers.More fracturedbasalts

-

# - ###

-

# - ###

Volcanic RocksPlains orplateaux

Moderate 0.5 5 l/s Horizontal fracturezones betweenbasalt layers.More fracturedbasalts

-

# - ###

-

# - ###

Sandstones Moderate -High

1 20 l/s Porous orfracturedsandstone

-

# - ##

-

# - ##

Mudstones Low 0 0.5 l/s Hard fracturedmudstones;igneous intrusionsor thinlimestone/sand-stone layers

-

## - ###

Generally notpossible

Consolidatedsedimentaryrocks

Limestones Moderate 1-10 l/s Karstic and

fracturedlimestones

-

## - ###

-

# - ##

Large basins Moderate -High

1 20 l/s Sand and gravellayers -

# - ##

-

# - ##

Unconsolidatedsediments Small

disperseddeposits, suchas riversidealluvium

Moderate 1 20 l/s

-

# - ##

-

# - ##

*The approximate costs of siting and constructing one source, including the hidden cost of dry sources: = < 1000; = 1000 to 10 000 and = > 10 000.

** The technical difficulty of finding and exploiting the groundwater is roughly classified as: # = requires little hydrogeological skill; ## = can applystandard hydrogeological techniques; ### = needs new techniques or innovative hydrogeological interpretation.


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Every good football manager and army general knows theimportance of reconnaissance. Only with accurateintelligence on key parameters is it possible to plan astrategy for success. The same is true for water projects.Before a water project can be planned, key socio-economic,institutional and physical information must be gathered inthis manual we are only concerned with the physical issues.There is no point in planning a groundwater project if thereis no groundwater available, or in buying sophisticatedexploration equipment where groundwater is ubiquitous. Inthis chapter we describe some simple ways to carry outsimple, effective reconnaissance.

3.1 EXPERIENCE

It is always helpful to find someone who has worked in thearea before. Not only can they give their own opinion ofthe area, but they can help point in the direction of other

projects in the area or maps and reports that might havebeen written. Box 3.1 gives a list of information that wouldbe useful to discuss with someone who has experience inthe area. However, all advise and information given should

be treated cautiously and always checked in the field. In ourexperience, people can often give misleading information intheir enthusiasm to be helpful.

3.2 OBSERVATIONS - GEOLOGY AND

PREVIOUS SUCCESS

The first visit to a project area is very important. It is at thistime that lasting impressions are made. The project is alsoat its most fluid in the early stages so design alterations aremuch easier. If possible a visit should be made when thewater problems are at their worst during the height of thedry season. Several days should be spent visiting different

parts of the area, trying to get a balanced overview of the

water problems in the area. This will be easier if maps canbe gathered before such a visit. The aim must be to covermuch ground and make a rapid assessment, rather thanmaking a detailed assessment in only one or two areas.Box 3.2 lists information that should be gathered.

3.3 MAPS AND REPORTS

Useful information often exists hidden away on peoplesshelves or locked in government filing cabinets. For mostareas it should be possible to gather basic information, suchas topographic and geological maps. Often otherinformation exists, such as aeromagnetic maps, aerial

photographs and even hydrogeological maps. If otherprojects have been carried out in the area there will beproject reports, and possibly databases of boreholes.Universities are also good sources of information. GeologyDepartments of nearby Universities will often know mostabout the current geology, and may have undertaken studies

in the area. International consultants can often be usefulsources of information and may have access to informationthat is now not available in country and also have access toacademic literature. BGS for example has an extensivelibrary of maps and reports from throughout the world andas a DFID resource centre offers a free enquiry service for

people in developing countries. Table 3.1 lists differentorganisations that are useful to contact, and the informationthat may be available in each.

3.4 SITING BOREHOLES AND VILLAGES ON

MAPS

Knowing where most of the villages are is an important partof the planning phase of a project. It is then possible to plotthem on maps and therefore estimate what geology

3 Reconnaissance

Box 3.1 Questions to discuss with someone with

previous experience of the project area.

What was their involvement in the area?

How easy is it to find groundwater what was their successrate?

What do they know about the geology?

Do they have records/reports of borehole drilling, or the project ingeneral?

Ask them to draw a map of the area, showing geology andeasy/difficult areas to find water.

Any poor water quality in the area?

What techniques would they consider for finding groundwater?

What other projects do they know of and people knowledgeableabout the area?

Box 3.2 A first field visit.

Take a GPS (see section 3.4), magnifying glass, hammer, waterEC meter and any maps. If you have space also take a water-level dipper, pH meter and compass-clinometer.

Drive along main roads cutting across the area and stop wherethere is rock at the surface (often in river valleys). Examine anddescribe the rocks, and take a sample. Take a GPS reading ofany stop you made so it can be marked on the map.

At representative villages discuss carefully the water supplyproblems. Walk to the dry season water source and try to workout why there is water there, measure the water EC and pH. Noteany successful or dry wells and boreholes and measure depth,depth to water level, EC and pH. Discuss any previousunsuccessful drilling and work out the geology for the village fromsamples at the bottom of wells, or nearby river valleys.

Note areas (discussing with local NGOs or government officials)where there are successful wells and boreholes, and areas whereboreholes and wells have been unsuccessful.


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underlies each village. At the reconnaissance stage thisgives a good idea of the proportion of villages underlain byeach geological unit. In the same manner each improvedwater source (such as borehole or improved well) can belocated on maps, and each abandoned borehole. This willhelp assess which areas are easy to find groundwater andwhich are difficult.

With the advent of GPS (global positioning system) it isnow very easy to locate wells and villages on maps. GPS

are small inexpensive pieces of equipment about the size ofa mobile phone or calculator (Figure 3.1). When switchedon they track the position of satellites and from thisinformation can accurately locate where they are on theground. They can give a read out in decimal degrees or inmany local grid systems. For greatest accuracy they should

be set up to the same grid system as the maps on which theinformation will be plotted.

3.5 FURTHER TECHNIQUES

3.5.1 Satellite interpretation

It is sometimes useful to use information from satellites tohelp create a base map for the area. This is a specialisedtechnique and will require the input of a good consultant oruniversity. A satellite image contains information from thelight spectrum and is interpreted to help give an indicationof changing conditions on the ground. Under goodconditions changes in geology can sometimes be observed.Fracture zones, rivers and roads are interpretable withexperience. Information from satellite images can be

presented on maps at about 1:50 000 scale. Although

useful for a reconnaissance of an area, satellite imagescannot normally be used by themselves for siting wells orboreholes.

3.5.2 Geographical information systems (GIS)

GIS are excellent tools for water supply projects. Theyallow map information to be combined, analysed and

presented in many different ways. This means that tailormade maps can be easily created for different projectstakeholders. However, to set up a GIS demands specificexpertise and considerable effort to get all the data in theappropriate format. Universities and consultants howevershould be able to carry out this work.

To create a GIS for an area available data are put intodigital form. That means digitising topographic andgeological maps and making sure they are in the same mapregistration. Once this is done other information, such asvillage locations can be added and plotted on top. Once

some investigations have been done preliminarygroundwater potential maps could be drawn up and printedfor use in the field.

Figure 3.1 A GPS being used by WaterAid project staff

in Nigeria.

Table 3.1 Information available from different

organisations.

Institution Data & Information

Mapping Institute Topographic maps, aerial photographs

Geological Survey Geological maps, aerial photographs,hydrogeological maps, aeromagneticmaps

Rural WaterDepartment

Databases of boreholes, reports ofprevious projects or statewide surveys

Universities Geological maps, local research in thearea.

Local NGOs, localgovernment

Databases of boreholes, consultantsreports, records of those who haveworked in the area.

International Geological

Organisations (such asBGS)

Geological maps, Consultants reports,

academic literature.


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Of all techniques in this manual, these are most dependenton geology. A technique that may give 90% success in one

geological environment may be worse than useless in

another. No technique or piece of equipment is consistently

useful in all environments. Different hydrogeological

environments also demand different levels of siting. Where

groundwater is easily found (Figure 4.1) little siting isrequired for wells and boreholes and hydrogeological

considerations are of little priority. In other areas

groundwater is not ubiquitous, but siting methods are well

established and standard techniques can be used (e.g.

weathered basement rocks). However, there are many

hydrogeological environments which are complex and no

standard techniques are available for siting wells andboreholes. In these areas, geophysical and other techniques

must be tested to provide new rules of thumb that are

appropriate for that environment.In this chapter we describe several useful techniques for

siting wells and boreholes, and rules of thumb for

interpreting data in most hydrogeological environments.

4.1 GEOLOGICAL TRIANGULATION MAPS,

OBSERVATION AND GEOPHYSICS

For an accurate assessment of the potential for groundwaterat a village it is important not to rely on just one technique

or approach. Maps can often be wrong, community

discussions can be misleading and geophysical surveys

cannot be interpreted properly unless the geologicalenvironment is known first. An approach that has beenused successfully in many groundwater projects is to use a

combination of maps, observation and geophysics. We

have called it geological triangulation.

1. Maps. Villages should be located accurately on

available geological and topographic maps. The co-ordinates of each village are determined using a global

positioning system (GPS). Once located, the map

provides an indication of the basic geology at the

village site.

2. Observation. The local geology must be examinedwith care and discussed with the local community. The

nature of the rocks should be noted. Local wet and dry

season sources of water need to be visited, as should

any locations that the community considers as possible

groundwater sources. Rock samples need to be

collected from local rock exposures; and rock spoil

from shallow wells examined. More information on

carrying out village observation is given in Section 4.2.

This geological information should be used to up-date

scanty map information.

4 Techniques for siting wells and boreholes

Figure 4.1 Siting wells and boreholes in areas where it

is easy to find groundwater, where groundwater

occurrence is generally understood, and in complex

areas.

Hydrogeology generally understood

Hydrogeology complex

Easy to find groundwater

sandslimestonesandstone

Weatheredcrystalline basement

Mudstone, hard sandstone, unweathered

1.Map

s

G

PS

and

geolo

gy

map

2.Observatio

n

wells

stream

s,rocks

asking

communities

3. Geophysics EM34

resistivity

magnetics

aerial photographs

Figure 4.2 The geological triangulation method.


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3. Geophysics. Geophysical surveys can be undertaken atsites based upon geological observations made withinthe village. The type of geophysical survey depends onthe rock types present. The survey results shouldsupport the observation data, confirming the type ofrock present. The survey results can then be collatedwith observed data to identify targets for boreholes orwells (see Chapter 4.3).

4.2 COMMUNITY DISCUSSIONS AND

VILLAGE OBSERVATION

Tapping into the experience and knowledge of localcommunities plays an integral part in understanding thegeology of a village. Community members have thegreatest experience of the surrounding environment and thehistory of water development within their village.Discussions and observations at a village are usually to helpanswer the following questions: what is the rock type at thevillage? Has there been any exploration there before?

Where are there current wet and dry water sources? Usefulpeople to meet are any well diggers, women who fetchwater and children.

What is the rock type of the village?

Prepare by looking at the geological map for the areaand what sorts of rock are likely to be there.

In discussion find out where the rocks in the area areexposed (children often know the best)

Visit any wells that have been dug and identify the soilrock profile with depth. Examine a sample of thedeepest rock. Find from well diggers how hard therock is.

Visit any borehole sites (failed or working) and lookfor any rock chippings from drilling.

Visit places where rocks are exposed make sure theyare the true bedrock and not just rocks that have beencarried into the area. Good places to look are rivervalleys and small hills (see Figure 4.3).

Observe boulders in the village used for seats, grindingstones etc. and find out where they have come from.

Hints: concrete looks very much like sandstone; do not befooled by rocks that have been brought into the area orwashed down in rivers. Take samples where possible andget a second opinion.

Where are there current wet and dry season sources?

Prepare by looking at any old files, reports or databasesfrom previous projects.

Discuss with community members all sources of waterwithin the village.

Visit well sites, measure water levels and salt contentof the water. Find out how much water is taken atdifferent times of the year particularly at the peak of

the dry season, and in drought. Visit boreholes, discuss depth with community (e.g.

number of rods used in its drilling, or lengths of screenand casing in its construction) and yield at differenttimes of the year.

Visit pond or river sources. Measure salt content ofwater. Discuss how many people use the source, atwhat time of year, how much and for what purpose.

This information should give some indication of thelikelihood of groundwater in the area. If some wells and

boreholes have considerable groundwater throughout theyear, or pond sources have a high salt content and aresustainable throughout the year, then groundwater is likelyto exist in the area.

What are the sources of pollution around the village?

Information should also be gathered on potential sources ofpollution in and around the village. However, care and tactmust always be used when discussing sources of pollution.Most water projects should have a sanitation component,with qualified staff and individual techniques to discusssanitation. Things to look out for are: Type and location of on-site sanitation Burial grounds

Cattle pens Market areas

One of the greatest barriers to tapping into this knowledgeis language. Not only different spoken languages, butradically different ways of describing things. Those withexperience of geology or water engineering describe rocksand water in a certain manner which is alien to most other

people. It is always important to find an example of what isbeing described. It may take more time, but helps reduceuncertainty.

4.3 WHEN ARE GEOPHYSICAL TECHNIQUESREQUIRED?

Geophysical techniques are required if maps andobservation alone do not give sufficient information to helpsite a successful well or borehole. They measure physical

properties of rocks (hence the name geophysics). They donot directly detect the presence of water, and cannot beused as a failsafe method for siting wells and boreholes.All they do is help interpret what rocks are present in thearea, and in some instances help locate where they may bemore fractured.

There are many different geophysical techniques

available and countless pieces of equipment. Many requiresophisticated equipment or complex analysis and aretherefore not appropriate for use in rural water supply

programmes. A comprehensive list of different techniquesis given in Table 4.1. The two most useful in the context of

Successfulwells

Dry borehole

River(dry season

source)

Stream

Rocks

Village

Rocks

Figure 4.3 Places to visit on a reconnaissance visit to a

community.


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11

rural water supply in sub-Saharan Africa are electricalresistivity and ground conductivity. Magnetic techniquesare also sometimes useful. The working and analysis ofthese three methods using the most common equipment aredescribed below.

Table 4.1 Summary of common geophysical techniques used in groundwater investigations (from MacDonald et

al. 2000).

Geophysical

technique

What it measures Output Approximate

maximumdepth ofpenetration

Comments

Frequencydomain EM(FEM)

Apparent terrainelectrical conductivity(calculated from theratio of secondary toprimary EM fields)

Single traverse linesor 2D contouredsurfaces of bulkground conductivity

50 m Quick and easy method for determiningchanges in thickness of weathered zones oralluvium. Interpretation is non-unique andrequires careful geological control. Can alsobe used in basement rocks to help identifyfracture zones.

Transient EM(TEM)

Apparent electricalresistance of ground(calculated from thetransient decay of

induced secondaryEM fields)

Output generallyinterpreted to give 1Dresistivity profile

100 m Better at locating targets through conductiveoverburden than FEM, also better depth ofpenetration. Expensive and difficult tooperate.

Groundpenetratingradar (GPR)

Reflections fromboundaries betweenbodies of differentdielectric constant

2D section showingtime for EM waves toreach reflectors

10 m Accurate method for determining thickness ofsand and gravel. The technique will notpenetrate clay, however, and has a depth ofpenetration of about 10 m in saturated sand orgravel.

Resistivity Apparent electricalresistivity of ground

1-D verticalgeoelectric section;more complexequipment gives 2-Dor even 3-Dgeoelectric sections

50 m Can locate changes in the weathered zoneand differences in geology. Also useful foridentifying thickness of sand or gravel withinsuperficial deposits. Often used to calibrateEM surveys. Slow survey method andrequires careful interpretation.

Seismicrefraction

P-wave velocitythrough the ground

2-D vertical section ofP-wave velocity

100 m Can locate fracture zones in basement rockand also thickness of drift deposits. Notparticularly suited to measuring variations incomposition of drift. Fairly slow and difficult tointerpret.

Magnetic Intensity (andsometimes direction)of earths magneticfield

Variations in theearths magnetic fieldeither along atraverse or on acontoured grid

30 m Can locate magnetic bodies such as dykes orsills. Susceptible to noise from any metallicobjects or power cables.

VLF

(very lowfrequency)

Secondary magneticfields induced in the

ground by militarycommunicationstransmitters

Single traverse lines,or 2D contoured

surfaces.

40 m Can locate vertical fracture zones and dykeswithin basement rocks or major aquifers


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4.4 GROUND CONDUCTIVITY USING FEM

4.4.1 How ground conductivity instruments work

FEM methods measure the bulk electrical conductivity

of the ground by passing an alternating electromagneticfield over and through the ground and measuring thesecondary electromagnetic produced. Figure 4.4 illustratesthe basic principles. The time varying electromagnetic fieldgenerated by the transmitter coil induces small currents inthe earth. These currents generate a secondaryelectromagnetic field, which is sensed (along with the

primary field) by the receiver coil. The ground conductivity(or apparent conductivity) is then calculated by assuming alinear relation between the ratio of secondary and primaryfields). Over a sub-horizontally layered earth, the responsewill represent a weighted mean (related to depth) of therocks within the range of investigation.

The coils can be orientated either vertically orhorizontally, as shown in Figure 4.5. Different orientationchanges the direction of the inducing field and what theinstrument is sensitive to. For vertical coils the readinggives a good estimate of the electrical conductivity (inmS/m sometimes quoted as mmhos/m). The maximumcontribution is from the ground surface, and the responsereduces with depth; the average depth of penetration isabout 0.5 - 0.7 x the coil spacing. Horizontal coils do notgive a good estimate of electrical conductivity beyondabout 30 mS/m. In fact the highest reading the horizontalcoils can give is 65 mS/m. However, when the instrument

is used with horizontal coils it is sensitive to verticalconductors, such as vertical fractures - often good targetsfor groundwater. When passing over a vertical anomaly anegative response is given. Sometimes this can actuallygive readings of less than zero, which at first can be ratherconfusing!

When the coils are horizontal, the readings are verysensitive to misalignment of the coils. With vertical coilsthe instrument is not sensitive to misalignment of the coils,

Induced eddycurrents

Secondary fieldsgenerated byeddy currents

Primary EM field

TransmitterCoil

Receiver Coil

Figure 4.4 How ground conductivity methods work.

Horizontal Coils

Transmitter Receiver

Vertical Coils

Transmitter Receiver

Figure 4.5 The different possible orientations of FEM

equipment.


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13

but is rather more sensitive differences in intercoil spacings.

4.4.2 Carrying out a survey

General principles for carrying out a ground conductivitysurvey are outlined below. A more detailed accountspecifically for the EM34 instrument is given in theAppendix.1. Walk round the village where the survey is to be

carried out and try to locate good survey lines. Bear inmind where the community may want the borehole orwell sited and where field geological information is(e.g. rock exposures or existing wells etc.). Twosurveys roughly at right angles extending outside thevillage often provide the best information (Figure 4.6)

2. Find an area to set up the equipment free fromsignificant influences such as power lines or metalroofs. Choose the appropriate coil separation (andfrequency) for the survey. Generally a 20-m coilseparation is most useful for reconnaissance. Carry outthe daily checks on the equipment, such as battery andnulling. (The sheets in the Appendix explain how totake a reading with the EM34 instrument). Once theequipment is set up satisfactorily the survey can start.

3. Start at a noticeable feature (e.g. a large mango tree) atone end of a survey line. Always have the coils in thesame order, for example with the receiver alwaystrailing the transmitter this avoids confusion overdistances. All distance, comments and reading should

be recorded from the receiver.4. Roughly measure distances. The coil separation

indicated by the instrument is usually a good enoughmeasure. Make sure that noticeable features arerecorded accurately at least every 40 m, so that anyreadings from the instrument can be pin pointed on theground.

5. Keep a detailed notebook. Figure 4.7 shows anexample of a page from a notebook. At the start ofeach survey general information should be given suchas the date, location, surveyors, equipment and coilseparation. The start of the survey should be carefullydescribed. To record the readings the SDVHC systemis very useful: this stands for Station, Distance,

Vertical coil reading, Horizontal coil readings andComments. Make sure that any metal objects (such astin roofs) or problems (such as misalignment ofhorizontal coils) are carefully recorded.

Figures 4.8 to 4.10 show the EM34 instrument being usedin the field.

4.4.3 Interpreting FEM survey results (using the

EM34 instrument)

Interpreting ground conductivity results depends on thegeology and hydrogeological targets. In crystalline

basement rocks (and volcanic rocks) the main targets areeither the weathered zone, or deeper fractures. Insedimentary rocks (both consolidated and unconsolidated)the targets are generally sandstones and sometime fractures.Interpreting data relies on understanding the electrical

properties of rocks and minerals. Several factors affect theelectrical conductivity of the ground: The electrical conductivity of the rock minerals

generally very low The volume of water in the rock (porosity and

saturation) The salt content of groundwater The amount of clay in the ground clay is highly

conductive.

It can be very difficult to distinguish a useful freshgroundwater resource from the presence of salty water orclayey rocks by the use of ground conductivity methodsalone. This is why geological triangulation is so important

the geophysical data must be interpreted in the light of thegeology that is thought to underlie the area. Figure 4.11shows the electrical conductivity of common rock types.

River

Stream

Village

Geophysicssurveys

Figure 4.6 Choosing survey lines in a community.

Date: 12 Feb 2001Village: Egori Ukpute 34.321N 12.012ESurveyors: Bitrus Goyol

Alan MacDonald

Em34 - 20 m coil separationreceiver trailing

S D V H C

1 0 m 31.2 33,4

2 20 m 32.1 35.33 40 m 34.3 32.24 60 m 34.1 34.65 80 m 32.2 32.26 100 m 32.2

31.17 120 m 29.2 31.18 140 m 26.1 24.29 160 m 23.2 22.110 180 m 23.8 26.111 200 m 19.2 20.212 210 m 19.3 18.2

Bearing 210 degs

large mango tree

ant hill left

H- coils uneven

dry borehole Right

Bearing 260 degs

path left @145 m

dry tree Right

32.3

small kitchen left

Starting at the large mangotree, roughly 1km to theNorth of the village at theedge of the river.

Figure 4.7 Example of a notebook for a ground

conductivity survey using EM34 equipment.


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14

Figure 4.8 The EM34 equipment, comprising

two coils, a connecting cable, transmitter and

receiver.

Figure 4.9 The EM34 being used with the coils

horizontal.

Figure 4.10 The EM34 with coils

vertical. Note that the operator

with the receiver is also taking the

readings.


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To help interpret ground conductivity data manydifferent case examples were examined of targets in

basement and sedimentary rocks. These case studies werethen generalised and modelled1 to give typical responses forthe main targets in basement and sedimentary rocks.

CRYSTALLINE BASEMENT ROCKS

Crystalline basement rocks and volcanic rocks generally donot conduct electricity (see Figure 4.11) because they havelittle primary porosity. In these rocks we are generallytrying to identify thick weathering or deep fracture zones.Ground conductivity has proved very useful in theseenvironments and there are many examples of theirsuccessful use (see further reading at the end of this

chapter). Before using any of these interpretativetechniques, geological triangulation must have establishedthat the community is likely to be underlain by basement orvolcanic rocks.

Identifying deep weathering

Figure 4.12 illustrates a common scenario in basementrocks. The main target for a well or borehole is where theweathered zone is thickest. The response of the EM34instrument with 10, 20 and 40 m coil separations over thisweathered pocket is also shown. Unweathered basement

1 Modelling was carried out using EMIGMA - a 3D EMcoupling package, and EMIXP a simple 1D EM package

rocks have low electrical conductivity (< 1mS/m) sincethey contain little water or clay. Weathered basement oftenhas conductivity of about 10 mS/m since it comprises

gravels with some clays. The soil zone has been modelledas 1 m thick with low conductivity the most commonscenario found in SSA. For completeness the models werererun with a conductive soil (20 mS/m) which did notsignificantly affect the results.

Deeper weathering is clearly indicated by highconductivity measured by all coil separations andorientations. Therefore, the best target for a well or

borehole in the weathered zone would be at distance 150 mwhere the weathering is thickest and the measurementshighest. Some analysis methods suggest that the best targetis where the horizontal coil readings become greater thanthe vertical coil readings. The modelling clearly shows that

this does not need to be the case. In fact, readings with thehorizontal coils are often uncertain because they aresensitive to errors in alignment of the coils therefore morefaith should be put in the vertical coil measurements.

Rarely is there opportunity to carry out surveys at allcoil spacings and orientations. Which configuration givesthe best information for changes in weathering of interest tothe hydrogeologist 10-50 m? Figure 4.13 showsconductivity measurements from different coil orientationsover different thicknesses of weathering. Although deeperweathering is indicated by higher readings in allconfigurations, the various coil separations and orientationshave different sensitivities to changes at depth. The 10-mcoil separation is not good at differentiating differences inthe thickness of weathering below about 10 m. The 40-mcoil separation is good at differentiating deep changes inweathering, but not shallow changes. Therefore, the 20-m

granite

w eathered granite

dolerite

gabbro

basalt

schists

slate

marble

consoldiated mudstone

conglomerate

sandstone

limestone

alluvium/sand

clay

1 10 100 1000 104

105

106

107

108

109

1101001000 10-5

10-6

0.1 0.01 0.001 0.0001

Electrical Resistivity (Ohm-m)

Electrical Conductivity (mS/m)

Figure 4.11 Resistivity and conductivity of common rock types.


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coil separation with vertical and horizontal coils is probablythe best configuration for initial surveys, repeated with

40-m if time allows.

Considerably higher readings in a basement area areoften associated with surface clays, such as those found in

dambos. These are generally in valleys and will give rise tohigh readings with 10 and 20-m coil separations. These aregenerally not great sites for wells and boreholes because ofthe swelling clay near to the surface. Comments in thegeophysics notebook should help interpret high readings asdambos, since they are easily identified in the field. In adambo ground conductivity may be greater than 30 mS/m,which is higher than would ever be measured in clay-freeweathered basement.

Identifying fracture zones

The other main target in basement areas is fracture zones.These are deep fractures (generally greater than 20 m)associated with faults and tectonic movement. Fracturezones tend to be more conductive to electricity than the hostcrystalline rock since they contain water and also the

Box 4.1 Summary for surveys to identify deep

weathering in basement.

Carry out a survey with 20-m coil separations using both thevertical and horizontal coil configurations.

If time permits, repeat with 40-m coil separations over areas withhigh readings.

Deeper weathering and therefore good targets for wells andboreholes are indicated by high readings in vertical andhorizontal coils (around 10 mS/m).

Higher readings (20 mS/m and above) in valleys are probablydue to clay development in the weathered zone in dambo likestructures these are not often good targets for wells andboreholes.

Horizontal coil readings are sensitive to misalignment of coils,therefore more faith should be placed in vertical coil readings.

0

1

2

3

4

5

6

7

8

9

10

11

0 50 100 150 200 250Measuredapparentconductivity(mS/m)

distance (m)

0

5

10

15

20

25

30 0 50 100 150 200 250

depth(m

)

0 mS/mWeathered basement(conductivity = 10 mS/m)

Unweathered basement(conductivity = 1 mS/m)

Soil(conductivity= 1 mS/m)

distance (m)

10 m separation; Vertical coils

10 m separation; Horizontal coils





Figure 4.12 Ground conductivity measured using the EM34 instrument over a pocket of weathered basement

rocks.


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SEDIMENTARY ROCKS

Sedimentary rocks conduct electricity. Electricity canmove through water in pore spaces in sands and sandstonesand also along the surface of clay minerals. This ambiguitymakes interpreting FEM data in sedimentary areas difficult.Good ground control is required to help interpret the data sothat clay can be distinguished from saturated sandstone.The geological triangulation method described above formsa useful strategy for gathering ground control information.

The different scenarios described below can be used foreither consolidated or unconsolidated sedimentary areas.

Locating sands within clay

Sands and sandstones are generally much better targets forgroundwater supply than clays and mudstones. Electricalmethods can easily distinguish clays and mudstones fromsands and sandstones: pure clays and mudstones tend tohave much higher electrical conductivity. Therefore usinga rapid survey method such as EM34 can help identifyareas underlain by sandstone or sands. Figure 4.16 showshow a boundary between sandstone and mudstone can bemapped within a village. Pure sands and sandstones withlittle clay content should give electrical conductivity belowabout 20 mS/m. Only the vertical coil reading should be

compared, since horizontal coils are insensitive to changesin electrical conductivity at high conductivities.

It can be more difficult to distinguish sandstones fromclays and mudstones when the sandstone layers are thin.The change can be noticeable if the sand is shallow, butmay easily be missed in overall variations in groundconductivity. Undertaking surveys at different coil-spacings can help interpretation of the data. For example ifthe 10 m coil spacings indicated low conductivity, but the40 m coil spacings high conductivity, this would suggest a

shallow low conductivity (possibly sand) layer.

Identifying different types of mudstone

In chapter 2 we reported the results of some new researchwhich indicated that groundwater may be found in certaintypes of mudstone. Where mudstones have been slightlymetamorphosed, fractures (and therefore groundwater) aremore likely. Soft mudstone contains very little usablegroundwater. Soft mudstone can easily be distinguishedfrom hard mudstone by measuring the electricalconductivity of the rocks. Soft mudstones have highconductivity, hard mudstones have low conductivity. Thereason for these changes is the amount of smectite clay

present. Table 4.2 gives typical electrical conductivitiesmeasured in various mudstones in Nigeria.

0

1

23

4

5

6

0 50 100 150 200 250Apparentcond

uctivity(mS/m)

distance (m)



distance (m)

0

5

10

15

20

25

30

0 50 100 150 200 250

depth(m

)

Weathered basement(conductivity = 10 mS/m)

Unweathered basement(conductivity = 1 mS/m)

Soil(conductivity= 1 mS/m)

Figure 4.15 Schematic diagram showing the kind of response that is given by ground conductivity (using the EM34

instrument) over fracture zones in basement.


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Table 4.2 Electrical conductivity of different mudstones

measured in Nigeria.

Type of mudstone Conductivityrange (mS/m)

Mean Conductivity(mS/m)

Soft smectite mudstone(little groundwater)

80 - 270 140

Moderately hard mudstone

(groundwater in largefracture zones)

40 - 110 50

Hard mudstone(groundwater generallyavailable)

4 - 23 10

Identifying fractures

Large fracture zones, similar to those targeted in basementareas, can also be useful targets in sedimentary areas,

particularly if the sandstones do not have significantprimary porosity and permeability. Since sandstonesgenerally have low conductivity they can be located in asimilar way to that described for basement areas.

Fractures zones in mudstones behave in a differentgeophysical way to basement or sandstone. Mudstones arehighly conductive, therefore there is not much contrast

between the conductive faults and the host rock. In fact, insoft mudstone, fracture zones may actually be less

conductive to electricity than the host rock. In hardmudstones, fracture zones may not be distinguished at allusing FEM. In moderately hard mudstones, fractures can

be identified as negative anomalies, or a generally noisyprofile using horizontal coils.

Box 4.3 Summary of surveying with ground

conductivity in sedimentary environments.

Carry out a survey with 20-m coil separations using both thevertical and horizontal coil configurations.

Geological triangulation is vital to help interpret any data.

Use the vertical coil readings for comparisons of electricalconductivity, and horizontal coil measurements to look forfractures.

Sandstone (sand) can be distinguished from mudstone (clay) bylow conductivity measurements (< 20 mS/m).

In a mudstone area, hard mudstone can be distinguished fromsoft mudstone by lower conductivity. If conductivity is very high(> 60 mS/m) then the mudstone is likely to be soft and of no usefor groundwater.

Fractures in both sandstone and harder mudstone can beidentified by noisy horizontal readings, and in particular negativeanomalies.

If time permits, repeat with 40-m coil separations over areas with

noisy readings.

High conductivity(mudstone)

Low conductivity(sandstone)

Geophysical survey

Exploratory borehole

100 m North

B

Apparentconductivity(mS/m)

Distance (m)

A B

20 m horizontal coil

20 m vertical coil

Line ofgraph above

Figure 4.16 Using ground conductivity to map out the boundary between a sandstone and a

mudstone in a village.


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4.5 ELECTRICAL RESISTIVITY

4.5.1 How the resistivity method works

The resistivity technique is the longest establishedgeophysical method used to site wells and boreholes inAfrica. It has been used successfully for more than 50

years.There are two main survey modes: profiling and depthsounding. Resistivity profiling is a relatively slow processand has largely been superseded by EM conductivitytraversing for detecting lateral variations. Methodscombining both profiling and depth sounding are nowavailable and can give good insight into the geology.However, such surveys are complex and require specialistequipment and interpretation; for this reason they are notgenerally appropriate for small rural water supply projects.The most common resistivity survey method used in Africais vertical electrical depth sounding (VES for short). Thisgives a one-dimensional profile of the resistivity beneath

the midpoint of the survey. Electromagnetic methods (suchas FEM described above) can give similar informationwhen carried out at different coil spacings. However, sinceresistivity is so widely used it is discussed here.

Ground resistivity is measured by passing an electricalcurrent through the ground and measuring the potentialdifference between two points. Ohms law is then used tocalculate the resistance. The resistance is then multiplied

by a geometric factor (normally called a K factor) tocalculate resistivity. Resistivity is in fact the inverse ofconductivity (see Box 4.4). Figure 4.11 shows theresistivity and associated conductivity values for commonrock types. To carry out a depth sounding (VES),

electrodes are expanded about a single point. When theelectrode spacing is very wide, the electric currents passdeeper into the ground and are therefore measuring theresistance deeper into the ground. A depth sounding

provides information only about one point (the midpoint ofthe survey). The technique assumes that there are no largelateral variations in the rock type. Table 4.3 shows theadvantages and disadvantages of the VES resistivitymethod.

Table 4.3 Advantages and disadvantages of using

resistivity VES.

Advantages Disadvantages

Can identify layers of differentresistivity (in other wordschanges with depth).

Can penetrate deep into theground.

Not affected by tin roofs etc.

Highly susceptible to badelectrode connections.

Difficult to interpret

Laborious and slow.

Cant locate vertical fracturezones

Only takes a reading at onepoint

4.5.2 Carrying out a resistivity survey

There are different electrode configurations available for

carrying out a VES. The three most common areSchlumberger, Wenner and Offset Wenner (seeFigure 4.17). In all cases the electrodes are moved to setdistances on either side of a midpoint. The Appendix hasdatasheets showing what the distances are, and thecorresponding K values for the distances. Some tips forcarrying out a survey are given below and illustrated inFigures 4.18 to 4.20).

1. Choose an area for the inner electrodes that looksfairly homogeneous.

2. Always make sure the battery is fully charged andcarry a spare.

3. Make sure the electrodes are hammered well intothe ground and water them. At large electrodespacings a salt solution can be used.

Midpoint

Schlumberger

Outer electrodesare widely spacedInner electrodes

spacing is small

Wenner

Four electrodesare evenly spaced

Current Electrode

Potential DifferenceElectrode

Two Wenner arraymeasurements are

taken and then averaged

Offset Wenner

First measurementuses four electrodesto the left.

Second

Measurementuses fourelectrodes tothe right

Figure 4.17 The three main electrode configurations for

carrying out VES.


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4. For small electrode spacings a small currentshould be used (< 5 mA). Check that theresistivity is roughly the same at several currentsettings. If the current is too low, readings willvary.

5. At larger electrode spacings, higher currents arerequired since they have farther to travel.

6. Plotting the data as you go along allows you to see

if any readings are anomalous. A resistivity curveshould always look quite smooth.

7. As with all geophysical methods a notebookshould be kept. If there are any doubts aboutreadings, make sure this is clearly marked.

Figure 4.18 Equipment for carrying out resistivity

surveys.

Figure 4.19 Electrodes have to be hammered in

and sometimes watered to ensure good electrical

contact with the ground.

Figure 4.20 The instrument stays at the midpoint

and cables run out to either side.


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4.5.3 Interpreting resistivity data

Resistivity data are interpreted by plotting the apparentresistivity against electrode spacing on a log-log scale.This should produce a smooth curve. Detailed quantitativeinterpretation can be carried out by computer using avariety of computer packages (such as Resixplus) orusing type curves. These are described in many text books

and are not considered in detail here. However, a roughinterpretation can be given by just looking at the shape ofthe apparent resistivity curve. For Offset Wenner andWenner the curve can be interpreted directly, for aSchlumberger array, however, the curve will be in severalsegments. These can be combined into one smooth curve,

by using the crossover points (readings taken with the sameouter electrode spacing but different inner electrodespacing). Starting with the right-hand segment, move it upor down to match the cross over points of the segment tothe left. Repeat the process (always moving the right-handsegment to meet the left-hand one) until you have onesmooth curve.

Figure 4.21 shows some common curves for basementareas and a rough interpretation beside them. For basementareas, the main target is the low resistivity weathered zone.Therefore if the curve shows low resistivity at largeelectrode spacings, then the weathered zone is likely to bedeep.

Figure 4.22 shows some common curves forsedimentary areas. In an area with sandstone andmudstone, the main target will usually be the sandstones.Sandstones (or sands) at depth will be indicated by higher

resistivity measurements at the wider electrode spacings;mudstones or clays will be indicated by low resistivity atlarge electrode spacings. If trying to differentiate hardmudstone from soft mudstone, then lower resistivityreadings (sometimes less than 10 ohm-m) will generally bysoft mudstones, which will not usually contain water.

4.6 FURTHER TECHNIQUES

4.6.1 Magnetic methods

Magnetic methods in geophysical exploration involvemeasuring the intensity of the earths magnetic field.Variations in the magnetic field are complex and oftenhighly localised, due to differences in the magnetic

properties of rocks near to the surface. This makes thetechnique useful and sensitive for identifying certain typesof rocks, but can also make the data quite difficult toanalyse.

The earths main magnetic field originates from electricalcurrents in the liquid outer core. This magnetic field can beapproximated by a dipole (bar magnet) located at theearths centre and inclined at 110 to the spin axis. Magneticmaterials can cause localised anomalies in the earthsmagnetic field. There are many different forms ofmagnetisation but generally the most significant forgeophysical surveys are ferrimagnetic and ferromagnetic.Ferromagnetic materials are strongly magnetic. Althoughthey do not occur naturally, such materials are in commonusage (e.g. steel in bridges, vehicles etc.) and can give largeanomalies on surveys. Some rock minerals are also

magnetic (known as ferrimagnetic), e.g. magnetite,maghaemite and pyrrhotite. When these are contained inrocks in sufficient quantities, they can cause measurableanomalies in the earths magnetic field. In generalsedimentary rocks have the lowest magnetic susceptibility;and basic igneous rocks the highest.

Magnetic methods are only useful if igneous rocks needto be identified. They can be useful for locating dykes (insome environments to avoid drilling in them and others,such as mudstones, to target them for drilling). Other usescan be to identify the extent of lava flows in volcanic rocks.

Magnetic surveys are relatively simple to carry out.Modern proton precessor magnetometers are easy to use

and 5-10 km of survey can be carried out in one day. Thedata are plotted on graph paper. Any significant anomaliesare either due to the presence of metal or igneous rocks. Acareful record must be kept in a field notebook of any metalin a 50 m radius, such as metal roofs, cars, bridges etc.This allows the data to be analysed confidently.

4.6.2 Aerial Photographs

Aerial photographs can be very useful for identifying localgeological conditions around a village or community. Theyare interpreted with the use of a stereoscope which gives a

three dimensional picture of the ground at a scale ofanything from 1:5000 to 1:24 000. They are often excellentfor identifying fracture zones, which appear as subtle linearfeatures. Changes in geology can also be often inferred

Box 4.4 Resistivity and Conductivity

Resistivity is the inverse of conductivity thereforehigh resistivity means low conductivity. Resistivity inohm-m is related to conductivity (in mS/m) by theformulas:

Resistivity = 1000 / (conductivity)

Conductivity = 1000 / (resistivity)

Box 4.5 Summary of resistivity VES.

Resistivity involves passing electrical currents into the groundthrough electrodes and measuring the resistance.

Electrodes are expanded about a midpoint to allow the currentto penetrate deeper into the ground.

There are different electrode arrays the most common areSchlumberger, Wenner and Offset Wenner.

Resistivity is highly susceptible to errors in arid areas mainlydue to poor electrode contacts with the ground.

Resistivity data can be interpreted quantitatively on computeror using type curves.

Rough qualitative interpretation can be given by looking at theshape of the apparent resistivity curve produced from the data.

Much of the same information can be given by repeatingelectromagnetic surveys (using EM34 equipment) withdifferent coil spacings.


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from the image. However, aerial photographs are oftenvery difficult to get hold of in Africa. Even if they are

available, they are often viewed as a security risk and notsold.

FURTHERREADING

TextbooksMILSOM, J. 1996. Field geophysics (2nd Edition). The GeologicalField Guide Series, John Wiley & Sons, Chichester.

1

10

100

1000

10000

0.1 1 10 100 1000

electrode spacing (m)

apparentres

istivity(Ohm-m)

Soil (1000 Ohm-m)

Weathered basement(50 Ohm-m)

Basement(1000 Ohm-m)

10m

1

10

100

1000

10000

0.1 1 10 100 1000


apparentresis

tivity(Ohm-m)

Soil (1000 Ohm-m)


Basement(10 000 Ohm-m)

10m

1

10

100

1000

10000

0.1 1 10 100 1000


apparentresi

stivity(Ohm-m)

Soil (100 Ohm-m)


Basement(1000 Ohm-m)

10m

1

10

100

1000

10000

0.1 1 10 100 1000


apparentresistivity(Ohm-m)

Curve is not smooth. Data beyond8 m electrode spacing are subjectto large errors, probably because ofpoor electrode contacts. Cannot

analyse.

Figure 4.21 Common resistivity curves and their interpretation for basement areas


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REYNOLDS, J M. 1997. An introduction to applied andenvironmental geophysics. John Wiley & Sons, Chichester.

TELFORD, W M, GELDART, L P. AND SHERIFF, R E. 1990. Appliedgeophysics. Cambridge University Press, Cambridge, U.K.,770pp

Papers with good examples

BARKER, R D, WHITE, C C, AND HOUSTON, J F T. 1992. Boreholesiting in an African accelerated drought relief project. In: Wright,E P, Burgess, W G (eds.), The Hydrogeology of CrystallineBasement Aquifers in Africa. Geological Society London SpecialPublication, 66, 183-201.

BEESON, S AND JONES, C R C. 1988. The combined EMT/VES

geophysical method for siting boreholes. Ground Water, 26, 54-63

CARRUTHERS, R M AND SMITH, I F. 1992. The use of groundelectrical methods for siting water supply boreholes in shallowcrystalline basement terrains. In: Wright, E P and Burgess, W G

(eds.), The Hydrogeology of Crystalline Basement Aquifers inAfrica. Geological Society London Special Publications, 66, 203-220.

MACDONALD, A M, DAVIES, J AND PEART, R J. 2001.Geophysical methods for locating groundwater in lowpermeability sedimentary rocks: examples from southeast Nigeria.Journal of African Earth Sciences, 32, 1-17.

1

10

100

1000

10000

0.1 1 10 100 1000

electrode s pacing (m)

apparentresisti

vity(Ohm-m)

Curve is not smooth. Data beyond8 m electrode spacing are subjectto large errors, probably because ofpoor electrode contacts. Cannot

analyse.

1

10

100

1000

10000

0.1 1 10 100 1000



Soil (1000 Ohm-m)

Weathered sandstone(75 Ohm-m)

Sandstone(100 Ohm-m)

10m

1

10

100

1000

10000

0.1 1 10 100 1000



Soil (1000 Ohm-m)

Soft Mudstone(10 Ohm-m)

10m

Figure 4.22 Simple resistivity curves and a rough interpretation for sedimentary areas.


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If the results of reconnaissance and geophysical surveyshave been favourable, drilling a borehole or digging a wellcan get underway. This provides an invaluable opportunityto actually observe what the rocks are like under acommunity. For surprisingly little effort or expense,information can be gathered on the location and nature ofthe water producing horizons.

Where the water table is shallow, and the material soft,communities can dig wells. These wells provide limitedgeological data on the shallow weathered zone. Moredetailed geological and hydrogeological data can beobtained during borehole drilling, and can show thedistribution of weathered, non-weathered and fracturedrocks at greater depths, and groundwater occurrence withinthem. Much useful information can be obtained fromdrilling during ongoing rural water supply projects, withoutthe need for purpose-drilled exploration boreholes. Thischapter describes some simple techniques for collectingdata.

5.1 WHAT ARE BOREHOLES AND WELLS?

A borehole is a narrow-diameter tube drilled into theground at a controlled angle (usually vertical) bymechanical means. Boreholes for rural water supply are

generally drilled to a depth of between 20 and 100 m,although in some situations where the aquifers are veryshallow or very deep they can lie outside this range. The

basic parts of a typical borehole drilled for groundwaterabstraction are shown in Figure 5.1.

A hand-dug well (as the name suggests) is constructedby hand. Typically, they are 1 - 2 m in diameter and from10 to 20 m deep (although they have been constructed toover 100 m!). They tap shallow groundwater and have alarge volume to allow the storage of several cubic metres ofwater. The basic construction of a hand-dug well is shownin Figure 5.2.

Boreholes are best used where the target forgroundwater is thought to be deep and the rocks hard.Hand-dug wells can be used only if the water-bearing rocksare shallow and the rocks soft enough to excavate withsimple hand tools, or with the help of pneumatic hammers.

The construction of hand-dug wells is generally wellunderstood by those involved in community water supply.However, the drilling of boreholes is often thought of asmysterious. To unravel some of this mystery a briefintroduction is given below.

5.2 DRILLING METHODS AND EQUIPMENT

A drilling rig is a crane with a motor and a mast equippedwith the necessary cables and/or slides to allow for the lift,fall and rotation of a drill string for the purpose of drilling a

borehole. The drilling rig must have a mechanism toremove drilled rock cuttings during and/or between periods

of drilling.Various borehole drilling methods have developed

because geological conditions are diverse, ranging fromhard rock, such as granite and gneiss, to completely

5 Gathering information during drilling

Grout

Gravel pack

Casing

Screen

Non-weatheredrock

Soil andweathered rock

Bail plug / sand trap

Figure 5.1 Component parts of a typical borehole.

Caisson(porous concrete)

Water table

Drainage apron Well head

Figure 5.2 A typical hand-dug well.


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unconsolidated sediments, such as sand and gravel, orweathered rock. Particular drilling methods have become

dominant in certain areas because they are most effectivefor the local aquifers. The major water well drillingmethods are summarised in Table 5.1.

The main drilling systems used in sub-Saharan Africaare:

Cable Tool Percussion; Direct Rotary Air/Mud-flush; Rotary Down the hole Hammer Air-flush (DHD).

Appendix 1 gives detailed descriptions of these differentmethods and also the construction of hand-dug wells.

The capacity of a drilling rig depends on its liftingability and the amount of torque that can be applied to thedrilling string. The maximum drill rod and casing/screencarrying capacity of the drilling rig is limited by the lengthof travel along the mast. The width of the drill tableopening limits the range of drilling bit and casing/screen

diameters that can be handled with safety. The choice ofdrilling rig for a particular job should be controlled by:

the geological conditions; the budget of the project; the maximum depth of borehole hole to be drilled,

plus an additional 25%; the types and dimensions (internal and external) of

casing and screen to be used; the depth to water table; whether a gravel pack or formation stabiliser is

required; the accessibility of the borehole drilling sites.

A number of texts describe drilling methods and equipmentin detail, these are listed at the end of the chapter.

Should the borehole be considered suitable forequipping with a hand pump, the productive parts of theborehole may be screened and the rest of the boreholecased. Screen acts as a formation stabiliser in looseweathered materials, as a filtration device to prevent loose

Table 5.1 Summary of drilling methods and their applicability to different environments.

DrillingMethods

Formations towhich method isbest suited

Water tabledepth (m belowground)

Usualmaximumdepth (m)

Usualdiameterrange (mm)

Usual casingmaterial

Remarks

Hand auger Clay, silt, sand andgravel (particle size


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weathered material from entering the borehole where it canblock the pump, and as protection from larger lumps ofrock which may fall from the borehole walls and damagethe pump. In certain aquifer formations, such as hardfractured granite, a formation stabiliser and filter may not

be needed.A 2 - 4 mm washed gravel pack is usually emplaced

around the casing and screen to the full length of the screen.

The gravel in the borehole acts as a formation stabiliser inloose weathered materials, although as noted above, it maynot be necessary to use a formation stabiliser in all aquiferformations. Coarse sand and gravel packs also act as a

borehole filter.Following construction and emplacement of casing,

screen and gravel pack, an air line is inserted to the base ofthe borehole and compressed air used to lift water to thesurface and remove sediment from the screen and finematerial from the gravel pack and surrounding aquiferformation. The borehole is air-lifted for a period of onehour or until the water produced is considered clean. Whilethis operation removes a certain amount of sediment from

the borehole, a full air development procedure involvesmoving the air line up and down the borehole so that alllevels of the screen are equally developed. Additionalgravel is emplaced as required at the top of the boreholefollowing any consolidation during airlifting.

A clay or grout seal is then put at the top of the graveland the space then back filled. A good cement seal most be

placed around the top of the borehole to stop surface waterflowing down the sides of the borehole.

5.3 DRILLING AND SAMPLING PROCEDURES

Drilling and constructing a borehole requires variousprocedures: drilling; installing casing and screen; placing afilter pack (if required); grouting to provide sanitary

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