S3.5 Results of Geophysical Survey Geophysical surveys were carried out during the first fiscal year field survey and the second fiscal year field survey for different survey objectives and by changing survey methods. During the first fiscal year field survey, with objectives of identifying the thicknesses and depths to basements of alluvium layers distributed along valleys having well developed groundwater, vertical electrical sounding (VES) was carried out. On the other hand, during the second fiscal year field survey, to select candidate sites for new borehole drillings in areas where piped water supply is difficult, electromagnetic prospecting and vertical sounding were conducted. Periods of conducting these surveys were from January to February 2009 for the first survey and from July to August 2009 for the second survey. S3.5.1 First Fiscal Year Field Survey (1) Survey Method In the study area of the Eastern Province, Rwamagana and Bugesera Districts, where water supply is covered by existing piped water schemes and in the future, extensions of these schemes and other piped schemes are expected, were excluded as target areas for the survey. At the remaining the 4 districts of Nyagatare, Gatsibo, Kayonza and Kirehe, geoelectrical surveys were carried out at 84 locations distributed over 18 secteurs along alluvium distributed valleys where groundwater pumping has high possibilities. For electrode configuration, survey depths were relatively shallow at 100m and the Wenner method was used in which interpretations of survey results can be made by the direct reading method. Specifications for the geoelectrical surveys are as follows. 1) Survey equipment: ABEM SAS 300 Terrameter 2) Electrode configuration: Wenner 4 electrode method, survey depths of 100m S3-32 Figure S3-23 Electrode Configuration (2) Survey Results From geological maps and results of hydrogeological field reconnaissance, the following 4 geologies are presumed to be distributed in the survey area: 1) surface soils, 2) sedimentary layer (talus sediments along valleys, alluvial sediments), 3) weathered layer, and 4) basement rocks (granites, quartzites, schists, phyllites). From results of geoelectrical surveys, resistivity characteristics can be summarized as follows. 1) Depending on the dryness of the surface, surface soils usually show resistivities of less than 100Ωm. 2) Although the difference between alluvium layer and weathered layer does clearly appear, resistivities are between 50 and 200Ωm, and the lower resistivities are alluviums and higher values are weathered layers. 3) Results of geoelectrical surveys in quartzites and granites show trends of abrupt rising of resistivities in proportion to depths. In schist and phyllite layers, due to influence of clayey weathered material on the surface, low resistivities are seen in shallow depths and rise in lower depths, but resistivities become stable after a certain depth. Average depth to basement rocks is 45 m, depth to weathered layer is 24.7m, average weathered layer thickness is 21.9m, average thickness of alluvium layer is 22.8m and average surface soil cover is 1.8m. Weathered layers above basement rocks are anticipated to be the aquifers. The resistivity ranges shown in Table S3-14 were used as standards for interpretation in determining the structure of each layer. S3-33 Table S3-14 Resistivity Ranges used during Interpretation Rock Resistivity (Ω-m) Rock Resistivity (Ω-m) clays 5 - 100 quartzites 60 - 10000 dry sands 100 - 5000 limestones 300 - 50000 wet sands 40 - 100 phyllites / schists 10 - 500 gravels 90 - 5000 granites / gneisses 1000 - 10000 sandstones 35 - 4000 weathered granites 50 - 300 Source: Electric resistivities (Parasnis 1997, and others) The locations of the geoelectrical surveys are plotted in Figure S3-24. The results of the surveys are shown as resistivity analysis cross-sections in Figure S3-25 and 26, and ρ-a curves are indicated in the Databook. Furthermore, geoelectrical survey locations and existing borehole (handpump) locations are shown on the geological map (Figure S3-27), and assumed geological profiles in the north-west to south-east directions are shown in Figure S3-28. From interpretations of the cross sections and resistivity values, distribution depths of the alluvium and weathered zones are between 10m and 90m with an average depth of 45m. S3-34 Figure S3-24 Location Map of Geoelectrical Survey (1st Year) S3-35 Figure S3-25 Resistivity Analysis Cross Section, 1st Year (1/2) S3-36 Figure S3-26 Resistivity Analysis Cross Section, 1st Year (2/2) S3-37 Figure S3-27 Location Map of Geophysical Surveys and Existing Boreholes S3-38 Figure S3-28 Predicted Geological Profiles S3-39 S3.5.2 Second Fiscal Year Field Survey (1) Survey Method For selection of the survey area, areas which will not be covered by piped water schemes in the future, areas which have high possibilities for being allocated as imidugudu, or areas for new development of imidugudu along with borehole siting were discussed with districts to determine the survey locations. Also, fault lines in the assumed survey areas were estimated from satellite images and geological maps. Furthermore, survey points were screened from areas most suitable for groundwater development in topographically low areas. Detailed topographies of the screened sites were reconfirmed from topographical maps and satellite images. Then, measurement directions for geomagnetic survey were determined and using the WADI-VLF equipment, changes in horizontal underground structure were measured along 300 to 600m lengths. As a result of measurements, 2 points from positive anomaly sections where the existence of fault structures can be assumed were selected and vertical geoelectrical surveys (VES) were carried out to measure changes in resistivities in the vertical direction. The survey was conducted at measurement depths of 100m using electrode intervals same as the first year survey, the Wenner configuration. The total length of the geomagnetic survey was 19,860m, and VES was carried out at 56 points. Specifications of the survey are shown below. 1) Survey equipment For electromagnetic survey, ABEM WADI-Electromagnetic VLF For geoelectric survey, ABEM SAS 300 Terrameter 2) Electromagnetic survey interval and length Survey interval: 10m Survey length: 300～600m Survey extent: 19,860m 3) Electrode configuration: Wenner 4 electrode method, survey depths of 100m Electrode intervals are the same as the first year survey S3-40 (2) Survey Results From results of the VES survey, in Nyagatare District, the average depth to the basement is 39.94m, depth to the weathered layer is 23.94m, and thickness of the weathered layer is 16m. In Gatsibo District, depth is 55.0m with 35.81m thickness of the weathered layer, and for Kayonza District, depth of 49.83m while thickness of the weathered layer is 28.23m. At Nyagatare District, since the weathered layer thickness is thin and depth to the basement is shallow, the underground valley structure necessary for groundwater retention is undeveloped and hydrogeological conditions are poor. Locations of geophysical surveys are shown in Figure S3-29 while resistivity cross sections from VES survey results are shown in Table S3-30 and 31. In the survey area, locations having positive anomalies and thick weathered layers were selected as candidate points for borehole drilling and these coordinates are shown in S3-32. These points become a criterion for future imidugudu allocation and locations for borehole drilling when a new umudugudu development is decided. Analyses of geophysical surveys will be summarized in the Databook, but an example of the VLF survey results is shown below.. Table S3-15 Example of VLF Survey Results Location: 1-1 Date: 08/06/2009 Location: 1-2 Date: 08/06/2009 Village: Rukundo Profiles: Latitude Longitude Village: Rukundo Profiles: Latitude Longitude Sector: Rwimyaga Start profile: S01 16' 53,9" E30 26'39,9" Sector: Rwimyaga Start profile: S0116'59.0" E3026'24,9" District: Nyagatare End profile: S01 17' 05.8" E30 26'27.5" District: Nyagatare End profile: S0117'38.1" E3025'57.2" WADI Interpreted data WADI Interpreted data 60 60 40 40 20 20 0 0 -20 -20 VES 1-2 -40 VES 1-1 -40 -60 -60 0 10 20 30 40 50 0 10 20 30 40 50 Interpreted VLF data Interpreted VLF data Station Station Station interval Interpreted WADI Value(kHz) Station interval Interpreted WADI Value(kHz) Station interval Interpreted WADI Value(kHz) Station interval Interpreted WADI Value(kHz) 10m Profile:50 Remarks 10m Profile:50 Remarks 10m Profile:51 Remarks 10m Profile:51 Remarks 0 2.9 26 0.0 0 1.5 26 10.3 1 -8.9 27 -5.8 1 8.2 27 0.5 2 -6.2 28 -27.9 2 12.5 28 -15.4 3 2.7 29 -27.9 3 13.9 29 -17.9 4 2.8 30 19.4 4 12.3 30 -14.7 5 -1.9 31 -28.9 5 10.1 31 -8.5 6 -1.1 32 6.9 6 8.8 32 -6.1 7 5.8 33 5.5 7 -11.0 33 -14.4 8 4.2 34 0.1 8 -49.3 34 -2.7 9 -0.4 35 0.0 9 -49.5 35 1.2 10 -4.4 36 -14.5 crosses at station 12 10 -9.8 36 -6.9 11 -4.1 37 -57.4 of profile43 11 -4.1 37 -1.5 12 -2.5 38 -32.4 12 -3.1 38 13 -3.0 39 17.8 13 -8.8 39 14 -3.3 40 -1.8 14 -0.7 40 15 6.5 41 -2.8 15 26.6 41 16 8.0 42 36.2 16 53.8 42 17 0.5 43 12.6 17 41.6 43 18 3.5 44 18 11.0 44 19 14.0 45 19 5.8 45 20 24.8 46 20 -1.2 46 21 26.4 47 21 -4.1 47 22 18.2 48 22 -0.5 48 23 5.0 49 23 -1.2 crosses at station 25 49 24 2.2 50 24 -14.1 of profile50 50 25 0.0 51 25 -5.5 51 S3-41 Figure S3-29 Location Map of Geoelectrical Survey (2nd Year) S3-42 Figure S3-30 Resistivity Analysis Cross Section, 2nd Year (1/2) S3-43 Figure S3-31 Resistivity Analysis Cross Section, 2nd Year (2/2) S3-44 Figure S3-32 Recommended Borehole Locations and Coordinates S3-45 S4.