SubSurface Dams - Sustainable Sanitation and Water Management ...

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3-4-3 Geophysical surveyAt the 5 sites selected by the interpretation of satellite images and aero-photographs andpreliminary exploration, electric soundings were carried out in addition to field exploration todetermine the geological structure.At some sites in Tangapore Village and Nare Village, magnetic soundings were also carriedout, whose results only confirmed those of the electric soundings but with less precision.The electric soundings are thus more useful for detecting underground structure at a shallowdepth.(1) Method of electric soundingsThe electric soundings were carried out using the vertical quadripole method (Wenner’smethod). From their results, resistivity profiles were drawn to analyze the undergroundstructure. One of the resistivity profiles thus obtained is shown in Fig. 3.7.For the electric soundings and the analysis of their results, the following points were takeninto account.1) Survey lines for the resistivity profiles, whose length was about 150 to 500 m, wereset across the assumed underground structure. Along the survey lines, the electricsoundings were carried out with an interval of about 50 to 100 m, namely 3 to 10survey points per line.2) At each survey point, a sounding line was drawn parallel to the direction of thesupposed underground structure.3) To obtain a three-dimensional view of the geological structure, 2 to 3 survey lines forthe resistivity profiles were set as far as possible.4) The resistivity of the basement rock (lateritic crust, heavily weathered rock, and freshrock) were determined by lengthening the survey lines to an outcrop of basementrock or to a point where basement rock was definitely present at a very shallowdepth. These resistivities determined largely contributed to the geologicalinterpretation of the resistivity profiles.5) Where there were wells (in particular “dug wells”) allowing observation of thegroundwater level and geological sections, electric soundings were also carried outnear the wells to determine the resistivity of the well site. These resistivitiesincrease the certainty of the geological interpretation of the resistivity profiles.Resistivity determined by electric soundings reflects not only the electrical properties of therock and soil, but also those of the groundwater. Even in the strata composed of identicalmaterials, resistivity may vary remarkably if there is a large difference in their water content.Therefore, resistivity is not sufficient for precisely determining the lithological nature of thestrata. However, resistivities and resistivity profiles obtained from electric soundings carriedout at a large number of points are important clues in estimating the geological structure andthe state of groundwater because zones of almost identical resistivity can be considered tocorrespond to strata with identical lithologies and water content.27
(2) Selection of the subsurface dam sites based on the electric soundingsThe results of the examination of the geological structure of each of the 5 sites on the basis ofthe field exploration and the electric soundings were as follows:a. Saouga (south of Gorom Gorom, Oudalan Province)This site probably had a fossil valley that was an aquifer of shallow groundwater.However, the construction of the subsurface dam would be too large as a demonstrationstudy.b. Nare (south of Tougouri, Namentenga Province)This site probably had a fossil valley that was an aquifer of shallow groundwater.Although constructing a subsurface dam here would be a little too large as ademonstration study, the site was suitable for a subsurface dam.c. Louda (south of Kaya, Sanmatenga Province)A ring-shaped landform had been formed at this site, but no favorable aquifer wasdiscovered.d. Bassneil (north of Korsimoro, Sanmatenga Province)A ring-shaped landform had been formed at this site, but an unconsolidated sedimentlayer that could have been an aquifer was likely to be very thin.e. Tangapore/Kossoden (north of Korsimoro, Sanmatenga Province)This site was of a bottleneck-shaped landform located downstream of a ring-shapedlandform, and the presence of shallow groundwater was suggested by the results of thesurvey of existing wells. The resistivity profiles by electric soundings also indicated thepossible presence of a fossil valley whose size was appropriate for a demonstration study.This site was thus probably suitable for a subsurface dam site.As a result of the survey described above, 2 sites were selected as candidates for thesubsurface dam site: Tangapore Village, Korsimoro District, Sanmatenga Province, and NareVillage, Tougouri District, Namentenga Province.The number of electric soundings carried out at these 2 sites were as follows:- at Tangapore: 58 points on 4 survey lines- at Nare: 95 points on 6 survey lines28
Page 2 and 3: Photo: Construction of the subsurfa
Page 4 and 5: Subsurface DamSmall-Scale Surface D
Page 6 and 7: ContentsIntroduction1. General desc
Page 8 and 9: 1. General description of the Model
Page 10 and 11: Department of the Environmentand Fr
Page 12 and 13: According to the results of an anal
Page 14 and 15: A large amount of waterevaporates f
Page 16 and 17: y adopting an appropriate dam struc
Page 18 and 19: 2-5 "Fossil valley", a suitable sit
Page 20 and 21: temperature of 30 to 34 degrees and
Page 22 and 23: 3-3 Distribution of fossil valleys
Page 24 and 25: 3-4 Surveys carried out in this pro
Page 26 and 27: Satellite image of the area around
Page 28 and 29: 3-4-2 Field explorationAt the 5 sit
Page 32 and 33: Groundwater level assumed from the
Page 34 and 35: 3-5 Detailed field surveys (test dr
Page 36 and 37: Groundwater level isalmost the same
Page 38 and 39: The village is on a peneplain with
Page 40 and 41: In this project at Nare, the "earth
Page 42 and 43: DownstreamFilled back soil3mCoarseg
Page 44 and 45: All the results of the quality test
Page 46 and 47: Photo 4.3: Construction of the subs
Page 48 and 49: 4) Basal stratum of the fossil vall
Page 50 and 51: 6. Observation of the water storage
Page 52 and 53: Table 6.1: Features of the faciliti
Page 54 and 55: PerchedwaterImpermeablelayer“Main
Page 56 and 57: Table 6.3: Annual rainfall in the K
Page 58 and 59: 3) The reserved water level during
Page 60 and 61: Ground surface level: 275.73mGround
Page 62 and 63: 6-4 Analysis of seasonal fluctuatio
Page 64 and 65: SandysiltSiltSand(white)Sand(moist)
Page 66 and 67: (1) Relation between the rainfall w
Page 68 and 69: 4) Subsequently, the cycle in which
Page 70 and 71: 7. Other studiesIn addition to the
Page 72 and 73: these facilities:- 3 mill stations,
Page 74 and 75: Photo 7.3: Water-supply station for
Page 76 and 77: 8. Recommendations for future subsu
Page 78 and 79: advisable to arrange rainfall stati
Page 80:
AcknowledgementsThe Model Project t
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