magnetotelluric and transient electromagnetic ... - Orkustofnun
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Wameyo 424Report 215. TEM AND MT RESISTIVITY SURVEY IN MENENGAI5.1 Geological setting6The Kenya Rift Valley is aprominent feature that is ofgreat geographic andgeological interest. It is a partof the East Africa Rift system,an intra-continental divergentzone where rift tectonics areaccompanied by activevolcanism.SeveralQuaternary volcanoes arefound within the Kenyasegment of the rift floor.Some of these volcanic centresare dotted with hydrothermalactivities and are pictured tohost geothermal systemsdriven by magmatic heatsources (Mungania, 2004).Menengai geothermal prospectis located within an areacharacterized by complextectonic activity associatedwith a rift triple junction. Thisis at the zone where theNyanza rift joins the mainKenya rift (Figure 13).Menengai is located about 20km south of the equator. Themajor structural systems in thearea are the Menengai caldera,the Molo tectonic axis and theLongitudes543210-1-2-3-4-5UGANDAL. VictoriaTANZANIACentral IslandEburuOlkariaSuswaSilaliPakaKorosiL. BaringoMenengaiLongonotL. MagadiShomboleKENYAMt. KenyaVolcanic centreRift faults-632 33 34 35 36 37 38 39 40 41 42LatitudesSolai graben (Figure 14). Menengai caldera is elliptical with the major and minor axis measuring about12 and 8 km, respectively. The ring structure has been disturbed by the Solai graben faults on thenortheast end and one fracture at the south-southwest end of the caldera wall extending southwards(Figure 14). The Molo TVA / Ol rongai fracture system intersects the Menengai caldera on the northnorthwestpart. The caldera floor covers an area of about 88 km 2 , and is partially covered by youngrugged lava flows that are post caldera in age. Most of the lavas are from fissure eruptions that flowedout of fracture openings. An extensive area around the caldera is to a large extent covered bypyroclastics erupted from centres that are associated with Menengai volcano.The Molo TVA is quite a prominent structural feature represented on the surface by a zone with aconcentration of faults and fractures along which volcanic eruptions have taken place (Figure 14). Theother structure that is of significance in this area is the Solai tectonic axis. It comprise numerousfaults/fracture systems trending N-S. It cuts through the caldera from the northern end and extendssouthwards beneath the Menengai volcanic pile. This fracture system is important as a hydrologicalcontrol and possibly contributes to permeability enhancement of the brittle lava formations underlyingthe Menengai eruptives.NyanzaNorth IslandBarrierNamarunuEmuruangogolakBogoriaSouth IslandNairobiOlorgesailleETHIOPIAMt. KilimanjaroFIGURE 13: Map of Kenya showing location of Menengaigeothermal prospect and other Quaternary volcanic systemsalong the Kenyan rift axisSOMALIA
Report 21 425 Wameyo100000009995000Solai9990000Olbanita swampKampi ya motoNorthings (m)99850009980000KabarakBahati997500099700009965000LEGENDMajor fault(rift scarp)Rift floor faultsInferred faultsLava eruptioncentre160000 165000 170000 175000 180000 185000Eastings (m)Nakuru townFIGURE 14: Map of Menengai geothermal prospect showingthe caldera outline, volcanic centres and other main structures5.2 TEM surveyA total of 62 TEM soundings, covering an area of about 50 km 2 , were carried out in the Menengaiprospect area using a central loop TEM array. About 38 soundings were done within the caldera floorand its surroundings, while the remaining soundings were scattered in other parts of the prospect field(Figure 15).The equipment that was used for this survey (purchased from Zonge Engineering Co. except for thepower generator) comprised the following units:• Transmitter - GGT-3 (maximum loop current of 12 amps)• Transmitter controller - XMT-16• Data logger - GDP-16 (three channel unit)• Receiver coil - TEM/3 (receiver moment of 10000 m 2 )• Voltage regulator - VR-1• Power source - 5HP Honda generator (120 Vac at 400Hz)In all the soundings, a 300 m x 300 m transmitter wire loop was used. A half-duty square wavecurrent was transmitted at frequencies 16, 8, 4 and 2 Hz. Logarithmically spaced sampling gates wereused with 16 Hz having 25 gates starting at 36.14 μs to 12.18 ms; 4 Hz had 31 sampling gates startingat 36.14 μs to 48.42 ms and 2 Hz had 34 gates starting at 84.18 μs to 96.85 ms. At each repetition rate,several repeated transients were stacked and stored in a memory cache inside the data logger and werelater transferred to a personal computer (PC) for processing.
- Page 3 and 4: Report 21 411 Wameyowhere ρ = Bulk
- Page 5 and 6: Report 21 413 Wameyoσ = 1 σ w +
- Page 7 and 8: Report 21 415 Wameyowhere μωf= 4
- Page 9 and 10: Report 21 417 WameyoV∞ 2eωt− i
- Page 11: Report 21 419 Wameyo23μ ⎡0 2μ
- Page 14 and 15: Wameyo 422Report 2122−6T Ex10μ T
- Page 18 and 19: Wameyo 426Report 21100000009995000N
- Page 20 and 21: Wameyo 428Report 21FIGURE 17: Field
- Page 22 and 23: Wameyo 430Report 21Elevation (m)W20
- Page 24 and 25: Wameyo 432Report 21Elevation (m)200
- Page 26 and 27: Wameyo 434Report 21SWMenengai Crate
- Page 28 and 29: Wameyo 436Report 21Menengai Crater2
- Page 30 and 31: Wameyo 438Report 21REFERENCESArchie
Wameyo 424Report 215. TEM AND MT RESISTIVITY SURVEY IN MENENGAI5.1 Geological setting6The Kenya Rift Valley is aprominent feature that is ofgreat geographic <strong>and</strong>geological interest. It is a partof the East Africa Rift system,an intra-continental divergentzone where rift tectonics areaccompanied by activevolcanism.SeveralQuaternary volcanoes arefound within the Kenyasegment of the rift floor.Some of these volcanic centresare dotted with hydrothermalactivities <strong>and</strong> are pictured tohost geothermal systemsdriven by magmatic heatsources (Mungania, 2004).Menengai geothermal prospectis located within an areacharacterized by complextectonic activity associatedwith a rift triple junction. Thisis at the zone where theNyanza rift joins the mainKenya rift (Figure 13).Menengai is located about 20km south of the equator. Themajor structural systems in thearea are the Menengai caldera,the Molo tectonic axis <strong>and</strong> theLongitudes543210-1-2-3-4-5UGANDAL. VictoriaTANZANIACentral Isl<strong>and</strong>EburuOlkariaSuswaSilaliPakaKorosiL. BaringoMenengaiLongonotL. MagadiShomboleKENYAMt. KenyaVolcanic centreRift faults-632 33 34 35 36 37 38 39 40 41 42LatitudesSolai graben (Figure 14). Menengai caldera is elliptical with the major <strong>and</strong> minor axis measuring about12 <strong>and</strong> 8 km, respectively. The ring structure has been disturbed by the Solai graben faults on thenortheast end <strong>and</strong> one fracture at the south-southwest end of the caldera wall extending southwards(Figure 14). The Molo TVA / Ol rongai fracture system intersects the Menengai caldera on the northnorthwestpart. The caldera floor covers an area of about 88 km 2 , <strong>and</strong> is partially covered by youngrugged lava flows that are post caldera in age. Most of the lavas are from fissure eruptions that flowedout of fracture openings. An extensive area around the caldera is to a large extent covered bypyroclastics erupted from centres that are associated with Menengai volcano.The Molo TVA is quite a prominent structural feature represented on the surface by a zone with aconcentration of faults <strong>and</strong> fractures along which volcanic eruptions have taken place (Figure 14). Theother structure that is of significance in this area is the Solai tectonic axis. It comprise numerousfaults/fracture systems trending N-S. It cuts through the caldera from the northern end <strong>and</strong> extendssouthwards beneath the Menengai volcanic pile. This fracture system is important as a hydrologicalcontrol <strong>and</strong> possibly contributes to permeability enhancement of the brittle lava formations underlyingthe Menengai eruptives.NyanzaNorth Isl<strong>and</strong>BarrierNamarunuEmuruangogolakBogoriaSouth Isl<strong>and</strong>NairobiOlorgesailleETHIOPIAMt. KilimanjaroFIGURE 13: Map of Kenya showing location of Menengaigeothermal prospect <strong>and</strong> other Quaternary volcanic systemsalong the Kenyan rift axisSOMALIA
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