37 Appendix-6 Results of Geophysical Survey 1. Introduction Geology Of
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Appendix-6 Results of Geophysical Survey 1. Introduction Geology of the survey areas in Masaka, Mukono and Kayunga Districts is mostly composed of weathered zone of Uganda-Toro formations covering the basic rocks of gneiss-granite composite rocks. The targeted ground water formation is considered to be in the fractured zone developed in the boundary of this basic rocks and weathered formations. Ground water development in this project aims to provide tubewells with handpump units; however, the hydro-geological examination conducted in these areas has expressed concerns about the following two points at the stage of utilization: 1) TDS (Total dissolved solids) of ground water might be too high for use. 2) Water table could be too deep for pumping up by a handpump. Taking these two points into consideration, the following studies were carried out geological and geo-structural analysis by using topographic maps and geological maps, lineament analysis by using satellites imageries as well as geophysical investigations by using magnetic and electrical methods. 2. Satellites Imageries Analysis 2.1 General Satellite imageries data were obtained in order to find out faults and fractured system in the survey area by analyzing lineaments as follows: 1) Satellite imagery used: LANDSAT No.7 ETM+ False color image 2) Path number and date of images taken are as follows; Details of Images District Path Row Date Masaka 172 060 Dec.31,1999 Mukono 171 059 Nov.27,2001 Kayunga 171 060 Nov.27,2001 3) Combination of band Blue : Green : Red= Band 1 : Band 4 : Band 5 4) Scenery mosaic Digital mosaic process was made for sceneries (171-059 and 171-060) taken near the boundary of Mukono and Kayunga Districts. 2.2 Results of Analysis Geological maps referred were Uganda Geological Map compiled by the Department of Geological Survey and Mines of Uganda(1966,1:1,500,000). Basic color tone used in scenery band combination is shown in the following table, and lineament analysis map in Table A6.1 A - 37 and A6.2. Basic Color Tone Used in Scenery Band Combination Color Tone Object Green Green land Blue Water zone Flesh – Reddish purple Barren-dry grass land Bluish purple Swamp (1) Masaka District 1) Lineaments Lineaments with a length of 2-4kms were observed. They dominantly extend in NE-SW to WNW-ESE trending. Comparatively high density lineaments were detected in the central to the southern part of Masaka District. Especially a high density lineament was observed in the central to western part of Pre-Cambrian sediments (K-A) distributed area. 2) Faults Faults structures are seen around the boundaries of Pre-Cambrian sediments group (K-A) and Pre-Cambrian gneiss group (G-C) in the central to the western part of the Mukono District, and of Pre-Cambrian metamorphic group (B-T) and Pre-Cambrian gneiss group (G-C) in the northern part of Mukono District. The extension of the former faults is NE-SW direction and the latter is WNW-ESE. (2) Mukono District 1) Lineaments Lineaments with a length of 2-4kms were observed. In the southern part of Mukono District within Pre-Cambrian metamorphic group (B-T) in parallel to Lake Victoria, E-W trending lineaments are dominant, while in the central to the northern part of Mukono District, NE-SW to NW-SE trending lineaments are dominant. Either lineament is harmonious with or diagonal to the trending of Pre-Cambrian metamorphic group (B-T) or faults system. 2) Faults Faults and fracture patterns are mainly observed in the boundary of Pre-Cambrian metamorphic group (B-T) and Pre-Cambrian gneiss group (B-T), and within the distributed area of Pre-Cambrian gneiss group in the central-northern Mukono District. Dominant direction of these faults is NW-SE, which is diagonal to the topographic trending developed in Pre-Cambrian metamorphic group (B-T). (3) Kayunga District 1) Lineaments Lineaments with a length of 2-4kms were observed. NE-SW to NW-SE trending lineaments are dominant. Comparatively high density lineaments are located in the Pre-Cambrian gneiss group (GC) in the southern part of Kayunga District. Dominant trending of lineament here is in a direction of NE-SW. 2) Faults No topographic pattern of fault is recognized in the imagery of Kayunga District. A - 38 3. Geophysical Exploration 3.1 General Magnetic and resistivity surveys were adopted as geophysical exploration.Survey stations were selected from the hydro-geological point of view at a representative point in a village. One site per village was selected with 150 villages in total, where both magnetic and resistivity surveys were conducted. The detailed information for each survey is shown below, and the list of villages where geophysical exploration was conducted is shown in Tables A6.1, and Fig.A6.1 and Fig.A6.2. (1) Magnetic profiling survey Method: Horizontal magnetic profiling by using a Proton magnetometer Profile length: 200m plus with an interval of 10 meters spacing Number of villages: 150 sites Equipment: G-856 made by Geometrics, USA Analysis: Horizontal magnetic intensity profiling (2) Horizontal electrical profiling Survey At an anomalous point detected by magnetic survey, horizontal resistivity profiling was carried out to clarify the underground resistivity structure. Method: Shlumberger horizontal profiling Profile length: 200m with an interval of 20 meters spacing Electrode spacing: AB/2=40 m. or 60 m., MN/2=5 m. Number of villages: 150 sites Equipment: SAS300C made by ABEM, Sweden Analysis: Horizontal apparent resistivity profiling (3) Vertical electrical sounding At an anomalous site detected by magnetic survey and profiling horizontal resistivity surveys, vertical electrical sounding was carried out to know underground resistivity structure. Method: Shlumberger array Electrode spacing: AB/2=1-120 m., MN/2=0.5-10 m. Number of villages: 150 sites Equipment: SAS300C made by ABEM, Sweden Analysis: 1-D inversion analysis 3.2 Exploration Results After data processing and analysis, results related to aquifer, fractures, and water quality (salinity) were extracted. The results of observation are shown in Tables A6.2, and Fig.A6.3 and Fig.A6.4. For convenience of interpretation, resistivity values were categorized as shown below. 10 100 500 in Ohm-m Extremely low Low Intermediate High The vertical electrical sounding curves identified in the survey were generally classified in three A - 39 types of patterns: 1000 0 20 Group Ⅰ: Conductive basement is detected at the 40 100100 depths overlain by a (m) DeptDepth h (m) 60 layer with a APPAA(ohm-m) RESIS RENT APPAREAPPA TIVITY (ohm-m) NT RESIS RE PPARERENT TIVITY SISTIVITY (ohm-m) NT RE SISTIVITY (ohm-m) resistivity of 50-100 80 ohm-m, which may 10 100100 11 1010 100 10001000 1010 100 1000 AB/2 (m) correspond to a AB/2 (m) RESISTIVITY (ohm-m) promising aquifer or UGANDA JICA BISHI METAL Kasambya fractured rich zone. Data Set: MA-2 Date: Mar,2003 MASAKA District Equipment: SAS Sounding: 1 Azimuth: 0 4 0 1010 Group Ⅱ: Conductive layer covers the surface 20 and the resistivity 40 decreases downwards 1000 Depth (m) (m) with no conductive 60 APPARENT RESISTIVITY (ohm-m) APPARENTA RESISTIVITY PPARE NT RE SISTIVITY (ohm-m) basement at depths. 80 100100 100100 If the lowest layer 11 1010 100 10001000 1010 100 1000 AB/2 (m) shows about 100 RESISTIVITY (ohm-m) UGANDA JICA ohm-m, it may BISHI METAL Kitokolo DaDa ta Set : MA-52 Date: Mar,2003 MASAKA District indicate a good Equipment: SAS Sounding: 1 Azimuth: 0 4 0 aquifer or fractured 1010 zone, but if it is over 20 200 ohm-m, it is assumed that a 40 1000 promising aquifer Depth (m) (m) 60 cannot be expected. (ohm-m) APPARENTA RESISTIVITY PPARE NT RE SISTIVITY (ohm-m) 80 Group Ⅲ: Conductive basement 100100 100100 4 11 1010 100 10001000 100100 10001000 10 AB/2 (m) is detected at a RESISTIVITY (ohm-m) shallow depth but UGANDA JICA BISHI METAL Lukalu becoming resistive at Data Set: MU-5 Date: Mar,2003 MUKONO District Equipment: SAS Sounding: 1 Azimuth: 0 depths due to a continuous increase in resistivity. No promising aquifer or fracture zone could be expected. Resistivity structures in Masaka, Mukono and Kayunga Districts can be characterized as follows: (1) Masaka District A total of 63 villages were measured by geophysical methods. Most of the area is covered by Pre-Cambrian gneiss-granite composite rock. Partly in the north, Uganda-Tolo formation A - 40 consisting of metamorphic rocks such as gneiss, hornblende, phyllite are seen, while in the southwestern edge of the area, Karague-Ankorean formation consisting of sediments such as sand and mudstone is distributed. Resistivity pattern shows generally middle-low-high from the surface, and low resistivity corresponds to weathered zone. Depth to the basement rock ranges from 15 to 74 meters with an average of 45.8 meters, which is generally deep. Regarding water quality, no extremely low resistivity was detected, which suggests that salinity is not high and therefore, water quality is good enough as far as salinity is concerned. (2) Mukono District A total of 46 villages were measured by geophysical methods. Most of the area is covered by Pre-Cambrian gneiss-granite composite rock. Partly in the south, Uganda-Tolo formation consisting of metamorphic rocks such as gneiss, hornblende, phyllite are seen. Resistivity pattern shows generally high - low - high from the surface, and the low resistivity corresponds to weathered zone. Depth to the basement rock ranges from 11 to 78 meters with an average of 38.8 meters. As for water quality (salinity), extremely low resisitivity of 4 ohm-m was detected at Mu-14, which is probably caused by high salinity water. (3) Kayunga District A total of 41 villages were measured by geophysical methods.