Difference between revisions of "OR/19/035 Technical information"

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Latest revision as of 09:59, 21 January 2020

Fellgett, M W, Kingdon, A, Fernie, A, Baker, G, Mehta, M, and Henderson, A F. 2019. Oilfield in a box: The Hutton Field dataset. British Geological Survey Internal Report, OR/19/034.

Definitions[edit]

The Atlas country hydrogeology maps provide a summary of the hydrogeology and geology for 48 countries in Africa. A total of 38 country maps have been released in an initial phase. An additional 10 will be released later.

Each map has two themes:

  • geology, with geological categories that reflect significant hydrogeological units (MacDonald et al. 2010[1]); and
  • a hydrogeology classification that combines aquifer type and productivity.

For three countries in Africa, no Atlas country maps have been developed: Comoros, Namibia and South Africa. This is because:

  • for Comoros: the base geology map at 1:5 million scale is not at a high enough resolution to provide useful information for the country.
  • For Namibia and South Africa: other digital maps of geology and hydrogeology, developed outside the Africa Groundwater Atlas project, provide better, larger scale information for both these countries, so that developing new Atlas maps with less detail would not be helpful.

Map scale[edit]

The maps provide a national-scale overview for each country, at a nominal scale of 1:5 million. This means that 1 cm on the maps is equivalent to 50 km on the ground. This makes them suitable for viewing at the following maximum sizes:

A2 — for the largest countries in Africa, such as Algeria and the Democratic Republic of Congo
A3 — for countries of a size similar to Sudan and Mali
A4 — for countries of a size similar to Kenya and Zambia
A5 — for countries of a size similar to Ghana and Tunisia
A6 — for the smallest countries in Africa, such as Burundi and Lesotho

The maps are not suitable for providing detailed information on geology and hydrogeology at a sub-national (e.g. catchment) scale.

Data and attribute field descriptions[edit]

Each map shapefile contains at least two attribute fields (Table 1), describing:

  • geology, (both bedrock and superficial/unconsolidated geology), with geological categories that reflect significant hydrogeological units (MacDonald et al. 2010[1]); and
  • hydrogeology, as a combined classification of aquifer type and aquifer productivity.
  • Unconsolidated sediments: Three of the country maps (Botswana, Mauritania and Sudan) also include an additional, third attribute field that highlights where very extensive unconsolidated sediments (largely dune sands) overlie bedrock (Table 1). For these maps only, the geology attribute field highlights only the underlying bedrock geology. Some other countries in Africa also have extensive covers of unconsolidated sediments, but in Version 1.0 this has not been shown separately, primarily because of: the complexity of displaying three dimensional geology on a two dimensional map; the availability of geological information and understanding; and the need to adapt each country map to reflect national approaches to geological categorisation.

The combination of aquifer type and aquifer productivity to classify the hydrogeology of each country provides a practical description that will be understood by hydrogeologists. Definitions of aquifer type and aquifer productivity are summarised in Tables 2 and 3, with a summary of the map attribute codes describing the combined hydrogeology categories.

Aquifer type is defined in terms of the hydrogeological environment, or the geological characteristics that largely control the nature of groundwater flow and storage in the aquifer. Four main categories have been distinguished, with subdivisions of some of the categories (Table 2). The key feature of the aquifer type classification is the dominant way that groundwater flows through, and is stored in, aquifers: e.g. in weathered zones; as intergranular (or matrix) flows through pores; or through fractures; or through karstic (solution-enhanced) features.

Aquifer productivity has been estimated by using borehole yield data as a proxy, which are the most widely available data for aquifers in Africa. Available quantitative borehole yield data from across Africa were synthesised, and six main categories of aquifer productivity were distinguished, based on approximate ranges of yields (in litres/second or L/s) (Table 3). These ranges are assumed to relate to the average yields that would be obtained from a single, effectively sited and developed borehole, to an appropriate depth (which will vary depending on the aquifer type), in the relevant aquifer. In reality in most cases, the aquifer productivity categories used in the Atlas maps are a largely qualitative assessment, because of the scale at which these maps have been developed; the heterogeneity of most of the aquifers; and the limited availability of aquifer properties data. The category codes applied, therefore, often span more than one of the ranges shown in Table 3.

For each country, the hydrogeology codes are described in text form in the associated legend, saved as a layer file.

Table 1    Attribute fields in the Africa Groundwater Atlas country hydrogeology maps.
Attribute field heading code Explanation of attribute values
GLG Geology: the geological descriptions reflect significant hydrogeological units at a national scale for each country, and where possible reflect geological nomenclature used (at this scale) in each country. The geological descriptions can include some or all of: geological age; lithological description; and whether sediment is consolidated or unconsolidated.
HGComb Hydrogeology Combined: a combination of Aquifer Type (Table 2) and Aquifer Productivity (Table 3). Formed by: Aquifer Type Code‐Aquifer Productivity Code.

Some examples are:

B‐L    Basement Aquifer – Low Productivity
CSF‐M/H    Consolidated Sedimentary Aquifer dominated by Fracture Flow – Moderate to High Productivity
CSIF‐M(V)    Consolidated Sedimentary Aquifer with mixed Intergranular and Fracture Flow – generally Moderate Productivity but noticeably variable
CSIFK‐H/VH    Consolidated Sedimentary Aquifer with mixed Intergranular and Fracture Flow and Karstic Flow – High to Very High Productivity
U‐L/H    Unconsolidated Sedimentary Aquifer – Low to High Productivity
U‐H(*)    Unconsolidated Sedimentary Aquifer – High Productivity
(* denotes further information, which is described in the associated legend/layer file)
UNCON Extensive unconsolidated sediments overlying bedrock (currently only for Botswana, Mauritania and Sudan). Mainly aeolian/dune sands.
Table 2    Definitions and descriptions of aquifer type categories.

Aquifer Type

Aquifer type is defined in terms of the hydrogeological environment, or the geological characteristics that largely control the nature of groundwater flow and storage in the aquifer. Four main categories have been distinguished, with subdivisions of some categories.
Category Subcategory Definition Attribute Field Code
Unconsolidated Sedimentary Unconsolidated sedimentary aquifers with dominantly intergranular flow U
Consolidated Sedimentary Consolidated Sedimentary Fracture Aquifers with dominantly fracture flow CSF
Consolidated Sedimentary Intergranular Aquifers with dominantly intergranular flow CSI
Consolidated Sedimentary Intergranular/Fracture Aquifers with significant intergranular and fracture flow CSIF
Consolidated Sedimentary Karst (sometimes also Consolidated Sedimentary Fracture Karst or Consolidated Sedimentary Intergranular Fracture Karst) Aquifers with significant karstic features (sometimes also with significant intergranular and/or fracture flow) CSK (sometimes also CSFK or CSIFK)
Igneous Igneous Intrusive Intrusive igneous aquifers, often granitic I
Volcanic Volcanic aquifers V
Basement Crystalline basement aquifers with typical weathered/fractured aquifer properties B
Table 3    Definitions and descriptions of aquifer productivity categories.

Aquifer Productivity

Aquifer productivity is defined based on a synthesis of borehole yield data, which is the most widespread available data for aquifers in Africa. Six main categories have been distinguished, based on approximate ranges of yields (in litres/second or L/s). These ranges are assumed to relate to the average yields that would be obtained from a single, effectively sited and developed borehole in the relevant aquifer.
Category Approximate range in yield (l/s) Attribute Field Code
Very High >20 VH
High 5–20 H
Moderate 2–5 M
Low to Moderate 0.5–2 LM
Low 0.1–0.5 L
Very Low <0.1 VL

How the atlas country hydrogeology maps were created[edit]

The Atlas country hydrogeology maps were developed by modifying and attributing a digital Africa-wide geology map at 1:5 million scale, using available hydrogeological and geological information.

The 1:5 million scale geology map was originally published by UNESCO (Furon and Lombard 1964[2]), and was digitised by, and is made available through, the USGS (Persits et al., 2002[3]). It can be accessed and downloaded in shapefile format via the USGS (details in References). The digitised geology map includes some linework defining coasts that was created using Esri© data, used with permission of Esri©.

This 1:5 million scale Africa-wide geology map (Furon and Lombard 1964[2], Persits et al, 2002[3]) was used by the British Geological Survey to develop three Africa-wide quantitative groundwater maps, released in 2012 (Table 4). These are available to download in digital GIS-enabled format (as xyz tab-delimited text files) from the British Geological Survey (www.bgs.ac.uk/research/groundwater/international/africanGroundwater/maps.html) and can also be viewed on an online portal hosted by IGRAC (www.un-igrac.org/special-project/africa-groundwater-portal). These maps are underpinned by systematic data and literature reviews and case studies. More detail on their development is available on their download webpage (www.bgs.ac.uk/research/groundwater/international/africanGroundwater/maps.html) and in a number of documents, particularly MacDonald et al. (2010[1], 2012[4]), and also Bonsor and MacDonald (2010a[5], 2010b[6], 2011[7]).

Table 4    Africa-wide quantitative groundwater maps released by BGS in 2012
(For more information see www.bgs.ac.uk/research/groundwater/international/africanGroundwater/mapsDownload.html)
Map Description
Groundwater (aquifer) productivity The groundwater productivity map indicates what borehole yields can reasonably be expected in different hydrogeological units. The ranges indicate the approximate interquartile range of the yield of boreholes that have been sited and drilled using appropriate techniques.
Groundwater storage Groundwater storage was estimated by combining estimates of saturated aquifer thickness and effective porosity of aquifers across Africa. For each aquifer flow/storage type an effective porosity range was assigned based on a series of studies across Africa and surrogates in other parts of the world.
Depth to groundwater Depth to groundwater was modelled using an empirical rules‐based approach, where depth to groundwater was assigned according to rainfall and aquifer type, as well as proximity to rivers.

The Atlas country hydrogeology maps are a further development of the Africa-wide quantitative groundwater maps released in 2012, using the hydrogeological understanding captured in those maps, and where possible incorporating additional geological and hydrogeological information from individual countries. They reflect current understanding of national geology and hydrogeology, given the scale of the geological base map linework and the availability of geological and hydrogeological information and, where possible, they reflect different national approaches to geological and hydrogeological categorisation.

Where possible, the maps have been validated in collaboration with hydrogeologists from the relevant countries who are co-authors on the relevant country pages of the Africa Groundwater Atlas (co-authors are referenced on the relevant Atlas country page and on this page http://earthwise.bgs.ac.uk/index.php/List_of_Authors).

Geology[edit]

Where relevant and possible, the geological classifications and descriptions in the Atlas country maps have been developed further from the modified geological base map (Persits et al. 2002[3]) used to create the Africa-wide quantitative maps (MacDonald et al. 2010[1]). Modifications were made to reflect additional geological understanding and nomenclature at a country level. The main modifications made are changes to geological formation names. Less often, changes were made to classifications of geological age or lithology.

Hydrogeology[edit]

Where relevant and possible, the aquifer type and aquifer productivity classifications used in the Africa-wide maps have been updated for the Atlas country maps, based on available hydrogeological information and understanding for each country. The main modifications are updates to the hydrogeological classifications based on new and/or more detailed available information at a country scale. Wherever possible, the maps have been validated by co-authors of the relevant country pages. The separate aquifer type and aquifer productivity classifications were combined to produce a new hydrogeological classification, as described in the previous section (Technical Information: Data and attribute field descriptions).

Clipping to country boundaries[edit]

The maps were clipped to each country using a digital map of country/national boundaries provided by and used with permission of GADM (GADM Version 3.6, country level (Level 0) boundaries).

Map legends[edit]

Each shapefile is provided with layer files with separate map legends for geology and hydrogeology. These legends use consistent colours to represent similar geological units (on the basis of geological age and rock type) and similar hydrogeology (on the basis of aquifer type). Within the legend colours used for the hydrogeology maps, stronger (brighter) colours represent higher productivity aquifers and weaker (paler) colours represent lower productivity aquifers. The colours used for any individual geological unit or aquifer in one country are not necessarily identical to those for a similar aquifer in another country, but it is intended that similar enough colours have been used to allow more easy comparison between similar hydrogeological environments in different countries. A summary of the general legend colour schemes is given in Table 5.

Table 5    Summary of colour schemes for geology and hydrogeology legends.

Geology

General geological age/rock type Indicative colour
Quaternary unconsolidated Blue
Tertiary or Tertiary‐Quaternary undivided sedimentary (often dominantly unconsolidated) Yellow
Cretaceous sedimentary (also sometimes Upper Mesozoic‐Tertiary) Light Green
Early Mesozoic sedimentary Mid Green
Palaeozoic sedimentary Dark Green
Karstic sedimentary or limestone (where differentiated) Turquoise Blue
Volcanic (all ages) Bright Red
Igneous intrusive or undivided Mid Red
Precambrian basement undifferentiated or craton Mid Brown
Precambrian Mobile/Orogenic Belt Dark Brown
Precambrian metasedimentary Pale Pink
Precambrian metavolcanics Purple

Hydrogeology

Within these legend colours, stronger (brighter) colours represent higher productivity aquifers and weaker (paler) colours represent lower productivity aquifers.
Aquifer type Indicative colour
Unconsolidated sedimentary Blue
Consolidated Sedimentary Fracture Orange
Consolidated Sedimentary Intergranular Green
Consolidated Sedimentary Intergranular/Fracture Purple
Consolidated Sedimentary Karst Turquoise blue
Igneous (Intrusive or Volcanic) Red
Basement Brown

Coverage[edit]

Africa Groundwater Atlas country hydrogeology maps, Version 1.0, are currently available for 38 countries, as shown in Figure 1 and Table 6.

Figure 1    Countries for which Africa Groundwater Atlas country hydrogeology maps (Version 1.0) are available.
Table 6    Countries for which Africa Groundwater Atlas country hydrogeology maps (Version 1.0) are available, with legend language.
Country Legend Language1
Algeria E, F
Angola E, P
Benin E, F
Botswana E
Burkina Faso E, F
Cameroon E, F
Central African Republic E, F
Chad E, F
Cote d’Ivoire E, F
Democratic Republic of the Congo E, F
Djibouti E, F
Ethiopia E
Gabon E, F
Gambia E
Ghana E
Kenya E
Lesotho E
Madagascar E, F
Malawi E
Mali E, F
Mauritania E, F
Morocco E, F
Mozambique E
Niger E, F
Nigeria E
Republic of the Congo E, F
Senegal E, F
Sierra Leone E
Somalia E
South Sudan E
Sudan E
Tanzania E
Togo E, F
Tunisia E, F
Uganda E
Western Sahara E, F
Zambia E
Zimbabwe E

1 E = English/anglais; F = French/français; P = Portuguese/Português

Limitations[edit]

The Atlas country hydrogeology maps provide a national (country) scale overview of the hydrogeology and geology, with geological categories that reflect significant hydrogeological units. They are suitable for viewing at sizes from A2 down to A6, depending on the size of the country (see the section Map scale, above). They are not suitable for providing detailed information on geology and hydrogeology at a sub-national (e.g. catchment) scale.

Like all maps, the country hydrogeology maps are a two-dimensional representation of the complex three-dimensional hydrogeological reality. In most cases, the maps show the uppermost aquifer only. If there are other aquifers at depth, these are not shown. In some cases, where a major aquifer is overlain by relatively low permeability unconsolidated sediments (that don't form an important aquifer), the overlying unconsolidated sediments are not shown on the map.

The maps reflect current understanding of national geology and hydrogeology, given the scale of the geological base map linework and availability of geological and hydrogeological information.

Because the maps reflect hydrogeological and geological terminology used in individual countries, the names and descriptions of hydrogeological or geological units that cross country (national) borders may not agree on both sides of the border (resulting in some country border ‘edges’ that have different colours/attribution).

The maps do not necessarily show the same information as other hydrogeology or geology maps available for any country.

Future development[edit]

The Atlas country hydrogeology maps (Version 1.0) may be updated in future. Any future updates will be made via the BGS website and supporting information provided.

Feedback[edit]

Feedback from users is always welcome. Please provide any feedback, and report any errors or problems, to AfricaGwAtlas@bgs.ac.uk

References[edit]

  1. 1.0 1.1 1.2 1.3 MacDonald, A M, Ó Dochartaigh, B É, Bonsor, H C, Davies, J, and Key, R. 2010. Developing quantitative aquifer maps for Africa. British Geological Survey Internal Report IR/10/103. http://nora.nerc.ac.uk/id/eprint/501779/
  2. 2.0 2.1 Furon, R, and Lombard, J. 1964. Explanatory note: Geological Map of Africa (1/5,000,000) / Note explicative: carte geologique de l'Afrique (1/5 000 000). UNESCO and the Association for African Geological Surveys (ASGA). UNESCO, Paris, 1964.
  3. 3.0 3.1 3.2 Persits, F, Ahlbrandt, T, Tuttle, M, Charpentier, R, Brownfield, M, and Takahashi, K. 2002. Map showing geology, oil and gas fields and geologic provinces of Africa, Version 2.0. USGS Open File report 97–470 A.
  4. MacDonald, A M, Bonsor, H C, Ó Dochartaigh, B É and Taylor, R G. 2012. Quantitative maps of groundwater resources in Africa. Environmental Research Letters, 7 (2), 024009. https://doi.org/10.1088/1748-9326/7/2/024009
  5. Bonsor, H C, and MacDonald, A M. 2010a. Groundwater and climate change in Africa: review of aquifer properties data. British Geological Survey Internal Report IR/10/076. http://nora.nerc.ac.uk/id/eprint/501776/
  6. Bonsor, H C, and MacDonald, A M. 2010b. Groundwater and climate change in Africa: review of recharge studies. British Geological Survey Internal Report IR/10/075. http://nora.nerc.ac.uk/id/eprint/501776/
  7. Bonsor, H C, and MacDonald, A M. 2011. An initial estimate of depth to groundwater across Africa. British Geological Survey Open Report OR/11/067. http://nora.nerc.ac.uk/id/eprint/17907/