Microearthquake and resistivity imaging of the Longonot geothermal prospect,

S. Onacha 1, P. Malin 2, E. Shalev 1, S. Simiyu 3 and William Cumming 4.

Total No of pages (Excluding Cover Page) = 6

1Duke University Earth and Ocean Science, Box 90227, Durham, NC, USA Ph. (919) 681-8889 FAX (919) 684 5833 2 Kenya Electricity Generating Company (KenGen), P.O. Box 785 , Kenya 3Consulting Geophysicist, Cumming Geophysics, 4728 Shade Tree Lane, Santa Rosa, CA 95405

Microearthquake and resistivity imaging of the Longonot geothermal prospect, Kenya

Stephen Onacha 1, Eylon Shalev 1, Peter Malin 1 Silas Simiyu 2 and William Cumming 3

1 Department of Earth and Ocean Sciences, Nicholas School of the Environment and Earth Sciences, Duke University, Box 90235, 103 Old Chemistry Building, Durham, N.C 27708-0227 2 Kenya Electricity Generating Company, Olkaria Geothermal Project, Box 785 Naivasha, Kenya 3 Consulting Geophysicist, Cumming Geophysics , 4728 Shade Tree Lane, Santa Rosa, CA 95405

Summary - This paper presents the results of microearthquake (MEQ) and resistivity studies at the Longonot geothermal prospect in Kenya. The data used was acquired by new MEQ instruments specifically designed to work under the field conditions in Kenya. The results show a good correlation between resistivity, location of microearthquakes, volcanic activity, fluid flow and structures. Analysis of the resistivity data shows high anisotropy close to the boundaries of low and high resistivity anomalies which are also associated with the location of microearthquakes. Based on the data analysis and interpretation, one well has been located in an interpreted fracture zone. The fracture zone is bound by areas of higher resistivity which are attributed to lower fracture porosity and probably lower temperatures. Such areas of deep high resistivity should not be targets for drilling exploration wells. An important finding of these studies that has international implications in the way MEQ data is acquired for geothermal exploration is that MEQ data loggers must be deployed appropriately close to the fracture zone. When the seismometers are located far away from the fracture zone, the MEQ signals are often attenuated before they arrive at the recording stations. This can lead to the erroneous conclusion that geothermal prospects are aseismic. 1.0 Introduction The Longonot geothermal prospect lies the reservoir properties of the geothermal systems. within the Kenya Rift Valley (KRV) which extends The significant finding of most of the studies is that from Tanzania in the south to Ethiopia in the north. the structure of the KRV varies both along and The KRV is part of the East African Rift Valley perpendicular to the rift. Regional seismic (EARV) system that extends from Mozambique in tomographic studies show that the Longonot volcanic the south to the Red Sea in the north (Figure 1). system is associated with low P-wave velocities in The Longonot geothermal prospect is the crust. The areas of low P-wave velocities have associated with the Longonot Volcano which has a been interpreted as zones of partial melt that may well developed 2 km wide summit crater (with a peak form important heat sources for the geothermal altitude of over 2770 masl) and an interpreted caldera systems. Most studies indicate significant spatial covering about 40 km 2. The caldera is postulated to variations in magmatism and tectonism along the Rift have been formed by a regional withdrawal of Valley (Karson and Curtis, 1989). These variations magma along the KRV (Scott and Skilling, 1999). could significantly affect the structures and the fluid The first volcanic eruptions are postulated to have flow mechanisms of the geothermal systems that are occurred at about 0.4Ma with eruption of trachyte found along the KRV. The complex geological, lava and volcanic ash deposits (Scott and Skilling, structural and volcanic setting provides a big 1999). Pumice eruptions also occurred at about challenge to understanding the hydrothermal systems 9150±110 ypb followed by trachytic volcanic in the Kenya Rift Valley. eruptions at about 5650 ybp. An explosive eruption 2.0 Previous studies of volcanic ash and pumice occurred at about 3280±120 ybp (Scott and Skilling, 1999). The Previous studies relevant to geothermal eruption of pumice is important because it signifies exploration at Longonot have been carried out by the the interaction of fluids with magmatic sources that British Geological Survey, KenGen and Duke could be the heat source for the geothermal system. University. The main events of the Longonot volcano The geology of the Longonot geothermal system is include building of an early shield, pyroclastic and dominated by trachytic lava flows and pyroclastics. lava cone, formation of a summit crater and crater Existing geophysical models indicate that floor lava eruptions. the upwelling of the asthenosphere provides the These rocks have high silica content ( ≥ driving mechanisms for the lithosphere uplift and 60%). From geological mapping and analysis of extension. Most of the previous geological, incompatible trace elements (ICE), Clarke et al. geophysical and geochemical studies have been (1990) noted that: focused on understanding the regional deeper crustal structure and therefore are not relevant to evaluating 40 °

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Figure 1: Location of the Longonot hydrothermal system within the Kenya Rift Valley (KRV) which is part of the East Africa Rift Valley (EARV) that stretches from Mozambique in the South to the Red Sea in the north. The KRV is generally a NS trending feature that is bound by NNW and NNE faults. The Longonot Volcanic system is composed of pit crater and a caldera covered by pyroclastic rocks.

(a) Recent volcanic activity tapping a large studies of Longonot with a view drilling exploration fractionating magma chamber. wells to a depth of 2000m. (b) A Tectono-Volcanic Axis (TVA) together Between July 10 and September 30, 2000, with the caldera walls form major structures Duke University and KenGen, conducted a in this area. However, other structures are microearthquake monitoring experiment around probably covered by pyroclastics (600m). Longonot volcano (see Figure 1). The study was (c) The Longonot crater seems to be at the sponsored by the United States Department of Energy intersection of NE and NW trending (DOE) geothermal resources program, KenGen, and regional faults. These structures may the Incorporated Research Institutes for Seismology strongly influence the regional (IRIS) program. The goal was to characterize the hydrogeology while the faults within the geothermal resource by studying the associated KRV may control the local hydrogeology. microearthquakes (McCausland et al., 2000; Shalev High permeability shear zones will act as et al., 2000). The choice of MEQ sites was motivated channels of fluid flow. The fault patterns by the results of electrical resistivity surveys may therefore significantly determine the (Onacha, 1998), which identified a potentially active overall productivity of the wells. geothermal system in the southern part of Mt. (d) Geothermal fluid indicators include Longonot. The monitoring experiment located only fumaroles and altered grounds. The 23 microearthquakes during the three months fumaroles occur within the summit crater, recording. southern part of caldera rim and to the 3.0 Data Acquisition southeast Based on the above findings, in 2000, Kenya Schlumberger DC resistivity, Transient Electricity Generating Company (KenGen) carried Electromagnetic (TEM), Magnetotelluric (MT) and extensive geophysical, geochemical and geological microearthquake (MEQ) data at Longonot were acquired in different stages between 1998 and 2004. DC resistivity measurements were acquired for trending profiles across the Longonot TVA (see current electrode spacing of 6000m. The TEM Figure 2) using WinGlink program (Rodi and measurements were acquired by the central loop Mackie, 2001) to determine which features in the configuration with a square loop of 300m. In the inversion models were important.

MT data acquisition, magnetic coils and potential 204000 206000 208000 210000 212000 214000 216000 218000 220000 222000 224000 226000 228000

Scale 1:125000 electrodes were oriented in orthogonal NS and EW 0 2500 5000 7500 MT Sounding 9904000 TDEM Sounding 9904000 m C a directions. The electric field was measured using lead l d e MEQ Location ra chloride porous pots. Some sites straddled known 9902000 9902000

ter Cra fractures in order to study the effect of open fractures 9900000 9900000

L36 on the MT data. The ground conductivity was 9898000 9898000 L32 L25 improved by placing the electrodes in a salt water and L27 9896000 L39 9896000 L26 L30 L37 L23 L34 L35 clay mixture. The Phoenix MTU-5A equipment was L24 used for data acquisition in the frequency range of 9894000 L22 9894000 L20 9892000 T 9892000

T V 400-0.0025 HZ. The telluric lines were typically 50 V A A 1 9890000 1 9890000 - 70m. The three MT loggers were deployed E 2 N E N everyday in different locations and data was 9888000 9888000 automatically acquired for 17 hours. 9886000 Longonot JGI 9886000 MEQ data was acquired by 15 new 204000 206000 208000 210000 212000 214000 216000 218000 220000 222000 224000 226000 228000 seismographs designed to record low frequency data. The new seismographs consist of a data logger Figure 2: Location of MEQ, TEM and module, geophone module, and an anti-theft “lock- MT stations in the Longonot area. The location of out lock-down” ground auger assembly. The tri-axis the Tectonovolcanic Axis (TVA) which is an on which the geophones are mounted also contains an important structural feature is shown. Two NE- opening for a J-hook which anchors onto a ground SE trending profiles used in the interpretation of auger. This mechanism couples the geophones MT data are also shown acoustically onto the ground and also provides security against vandalism. The electronics and Locating earthquakes was done by first battery are located inside the logger module, and the picking P and S phases with the associated errors. GPS antenna is located on top of the logger. The new The locations of the MEQ stations required to locate logger module houses three 1 Hz geophones. The earthquakes were determined from the internal GPS logger is designed to work as an integrated unit with measurements and cross-checked with locations from the geophone module and powered by an 11.1V, the hand held GPS. The microearthquakes are located 59.4A-Hr Lithium-Ion battery. at a reference datum which is the average of the Data from 100 MT, 88 TEM, 87 DC stations. resistivity soundings and 15 MEQ sites has been used Station corrections were incorporated to to image the Longonot geothermal prospect. The MT account for the elevation differences with respect to and MEQ data acquisition sites during the 2004 field the average elevation. Station corrections are campaign in 2004 and TEM acquired in 1998 are computed by locating many earthquakes with zero shown below (Figure2). corrections and then assigning the average time errors as station corrections. The location of 4.0 Data Analysis microearthquakes was carried by the Hypoinverse- Occam and layered inversions were 2000 code (Klein, 2000) with a parameter input performed by the program TEMTD (Arnasson, interface in Matlab. The initial velocity model was 2006). 1-D models from TEM data were used to obtained from the velocity structure previously used generate equivalent 1-D MT models at the same site. to locate events in the area of Longonot (Simiyu and The synthetic 1-D MT models were then used to Keller, 2000). correct for static shifts (Pellerin and Hohmann, 1990) S-wave splitting of microearthquakes was in both TE and TM modes of the MT data. Resistivity carried out and the results compared to the maps were prepared from the results of 1-D Occam’s polarization directions of MT data at collocated sites models to show the spatial distribution of resistivity (Onacha, 2006). S-wave splitting, which is at fixed elevations. 1-D interpretation was carried out sometimes referred to seismic birefringence, is both for layered and Occam’s inversion using the probably the most diagnostic and measurable entire MT data set after rotation to the principal axes. property of seismic anisotropy (Crampin 1985). These maps together with the 1-D sections have been Anisotropy is usually interpreted as a characteristic of used as the basis for the 2-D modeling. 2-D forward fluid-saturated, stress-aligned cracks. (Crampin, and inverse modeling was carried out on two NE-SW 2005; Elkibbi and Rial, 2005). S-wave splitting was done by searching for the angle that maximizes the amplitude ratio of the horizontal components of the S-waves. After rotating the S-waves to the direction 9910000

parallel and perpendicular to this angle, the time 9905000 delay was obtained by cross correlation of the OLKARIA LONGONOT amplitudes. 9900000 300 200 5.0 Results 150 9895000 100 The regional DC resistivity map at 1000masl 70 50 9890000 derived from 1-D models (Figure 3) shows that the AKIRA 40 Olkaria and Longonot geothermal areas are defined 30 9885000 25 by a low resistivity anomaly while the Suswa field 20 15 shows higher resistivity may be due to a lower degree 9880000 10 of hydrothermal alteration and lower permeability. 7 The map at 1800masl (Figure 4) derived from Occam 9875000 5 SUSWA 3 1-D invariant MT models shows a NE-SW trending 1 low resistivity anomaly at the intersection of the NW- 9870000

SE trending caldera boundary and the TVA. This 9865000 map probably shows the near surface alteration 185000 190000 195000 200000 205000 210000 215000 220000 225000 patterns which seem to correlate with the area of Figure 3: Regional DC resistivity at 1000masl recent volcanic activity. The MT resistivity at 204000 206000 208000 210000 212000 214000 216000 218000 220000 222000 224000 ohm.m 3000mbsl (Figure 5) probably shows the heat source MT Sounding 3000 9904000 25 TDEM Sounding 9904000

0 2000 C 3 for the geothermal system to the south of the a ld MEQ Location e 1500

r 1 7 10a 5 0 9902000 9902000 1000 Longonot crater. The deep low resistivity coincides

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2 er 5 5 t 2 a with the southern extension of the TVA that is r 0 C 3 600 9900000 0 9900000 1 15 10 400 aligned by recent volcanic centres. The TVA 2 200 9898000 0 7 9898000

1 150 therefore correlates with the surface alteration and the 5

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100 7 5 0 1 5 9896000 0 9896000 10 2 postulated fracture zone that may be buried by recent 70

15 5 0 5 0 20 2 4 1 2 50 7 0 0 3 2 5 volcanic activity. The maps show considerable spatial 5

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1 variations in resistivity. 0 3 1 150 5 20 0 0 25 9892000 9892000 40 T 0 V 5 7 T 20 A common feature in the interpretation of 0 A 100 V 150 A 200 1 15

0 9890000 400 7 9890000 resistivity from Mt. Longonot is the existence of a 00 6 800 10 1000 high resistivity surface layer of recent volcanic rocks 7 9888000 9888000 5 or pyroclastics with variable resistivity depending on Scale 1:125000 3 0 2500 5000 7500 Longonot JGI 9886000 Resitivity at 1800masl 9886000 the age of the rocks and proximity to the fracture m zone. The second layer with extensive low resistivity 204000 206000 208000 210000 212000 214000 216000 218000 220000 222000 224000 is associated with an interpreted smectites-zeolite Figure 4: Resistivity at 1800masl from TEM and clay layer. The base of the clay cap probably MT data correlates with a geothermal reservoir at a 6.0 Discussions temperature between 200 and 240˚C. This is likely to be the case because a shallow well drilled to the SW The main objective was to explore a new part of the Longonot caldera struck steam that has area for exploration drilling in the KRV. In this geochemical indicators of a hydrothermal reservoir at study, the MT data set provided suitable information 240˚C. The 2-D resistivity model (Figure 6) shows a for targeting geothermal wells below the clay cap deep low resistivity zone, which is interpreted as the within a NW-SE trending fracture zone that coincides heat source for the geothermal system at a depth of with a Tectonovolcanic axis associated with recent more than 5km. volcanic eruptions. Analysis of S-wave splitting shows that The low seismic activity may be attributed station L37 recorded 49 S-wave splitting events while to the deployment of the MEQ stations far away from station L34 recorded only 12 events. The field the fracture zone imaged by the interpretation of deployment was only for about three weeks. The S- resistivity . This reasoning is supported by the wave splitting again shows dominant polarization of observation that two of the stations located close to the fast S-wave in the NE-SW direction and in the the interpreted fracture zone recorded the highest NW-SE direction (Figure 7). The S-wave splitting number of microearthquakes with significant S-wave directions are similar to those recorded by the MT splitting. From similar studies at Krafla in Iceland data strike directions (Figure 8). (Onacha, 2006), it has been established that the network should be located close to the source of 206000 208000 210000 212000 214000 216000 218000 220000 222000 224000 earthquakes. 9902000 9902000 MT Sounding a r e d l a C

W Caldera Wall 9900000 9900000 Well Site ohm.m 204000 206000 208000 210000 212000 214000 216000 218000 220000 222000 224000

9904000 9904000 3000 r L36 Crate MEQ Location 2000 9898000 9898000 L32 DUKE2000 1500 L25 C L27 9902000 a 9902000 ld e 1000 ra JGI2004

800 L39 er 9896000 9896000 rat L26 L30 9900000 C 9900000 600 L37 400 L23 L34 L35 200 L24 9898000 9898000 150 9894000 E Caldera L22 9894000 LN1 100 70 9896000 9896000 50 L20 40 9892000 9892000 9894000 9894000 30 25 20 9892000 9892000 9890000 9890000

T 15

T V V A Scale 1:100000 A 10 1 0 2000 4000 6000 9890000 9890000 7 5 9888000 m 9888000 3 206000 208000 210000 212000 214000 216000 218000 220000 222000 224000 9888000 9888000 204000 206000 208000 210000 212000 214000 216000 218000 220000 222000 224000 Scale 1:125000 Figure 8: Average MT strike direction at each of 0 2500 5000 7500 Longonot JGI m Occam Resistivity at 3000mbsl the sites at the Longonot hydrothermal system for the frequency range of 320-100 Hz. The locations Figure 5: Resistivity at 3000mbsl. The location of the caldera and crater boundaries are shown. microearthquakes and the proposed exploration There are 2 main strike directions in the NW and well (LN1) are shown. NE directions. The NW direction is the dominant strike direction. Southern Edge of Caldera

Fracture Zone Ohm.m

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L 2000 L 2000 500 recorded only 38 events located mainly on the 300 200 1000 1000 150 boundary between the deep low and high resistivity 100 70 0 0 50 close to the interpreted fracture zone above the 30 -1000 -1000 20 interpreted heat source. This suggests that the 15

Depth (m) Depth 10 -2000 -2000 7 earthquakes are probably generated by a combination 5 -3000 -3000 2 of fluid movement and tectonic activity. The clay cap -4000 Heat Source -4000 is probably formed by the interaction of meteoric -5000 -5000 water and gases from the deep hot geothermal -6000 -6000 2000 4000 6000 8000 10000 12000 14000 Distance (m) reservoir. The interaction of the gases and meteoric 2 N NE LONGONOT Horizontal Scale 1 : 75000 LONGONOT JGI water may form acidic condensates that alter rocks Vertical Scale 1 : 75000 NE2 into clays. The recharge and outflow directions in the Figure 6: 2-D resistivity model along a NE-SW Longonot geothermal system are probably controlled profile across the Mt. Longonot hydrothermal by the regional NW-SE and NE-SW faults. system in Kenya. The model shows an interpreted The resistivity is probably consistent with heat fracture and heat source. alteration mineralogy patterns and therefore it can be used to infer reservoir temperatures. The near surface high resistivity can be attributed to altered pyroclastics and lava rocks. The second low resistivity layer may be interpreted as a smectite- zeolite zone (<200°C) up to an elevation of 1500masl. A mixed-layer clay zone (200-230°C) probably occurs up to an elevation of 1000masl. The higher resistivity below 1000masl in the interpreted fracture zone is attributed to a chlorite-epidote zone (230-280°C). The deep higher resistivity on both sides of the interpreted fracture zones is attributed to lower formation temperatures and lower permeability. Figure 7: The main directions of S-wave splitting The interpretation of the 2-D model are in NW-SE and NE-SW. S-wave polarization obtained from MT data after static stripping and length and thickness are proportional to the using the existing TEM data to correct for the static number of events. shift shows a deep low resistivity zone below an interpreted fracture zone. The deep low resistivity is interpreted as a partially molten magma that could be Naivasha, Kenya. : Ministry of Energy of the heat source for the geothermal reservoir at Kenya. Longonot. This is supported by the location of the Crampin, S. (1985). Evaluation of Anisotropy by microearthquakes above the low resistivity indicating Shear-Wave Splitting. Geophysics, 50(1), 142- that the low and high resistivity boundary might be 152. the transition to partially molten rocks. Crampin, S., & Peacock, S. (2005). A review of The interpreted fracture zone above the heat shear-wave splitting in the compliant crack-critical source has been proposed as a target for drilling anisotropic Earth. Wave Motion, 41(1), 59-77. exploration wells. However, it is also noted that the Elkibbi, M., & Rial, J. A. (2005). The Geysers areas with very low resistivity within the interpreted geothermal field: results from shear-wave splitting fracture zone could be the core of the fracture zone analysis in a fractured reservoir. Geophysical and therefore a less attractive drilling target because Journal International, 162(3), 1024-1035. the low resistivity could be as a result of fault gauge Karson, J., and Curtis, P. (1989). Tectonic and and clay which could be impermeable. magmatic processes in the Eastern Branch of the East Africa Rift and implications for magmatically 7.0 Conclusions active continental rifts. Journal of African Earth Sciences, 8(2/3/4), 434-453. • The study has demonstrated that joint Klein, F. (2002). User's guide to Hypoinverse-2000, a interpretation of MEQ, S-wave polarization Fortran program to solve for earthquake locations and splitting magnitude, resistivity, and and magnitudes: United States Geological Survey. magnetotelluric (MT) strike directions can be McCausland, W.A., A. Stroujkova, P.E. Malin, S.M. used to identify fracture zones and heat sources Simiyu, and E. Shalev, (2000). Discrimination for the geothermal systems. between site effects and source processes for Mt • Strong evidence exists for correlation between Longonot, Kenya. In Eos Trans. AGU, 81 (48), MT strike direction and anisotropy and MEQ Fall Meeting Supplement, Abstract S12B-09. S-wave splitting at sites close to fluid-filled Onacha, S.A (1998). Transient Electromagnetic fracture zones. studies of the Longonot geothermal prospect, • Resistivity is lowest within the core of the unpublished KenGen report. fracture zone and partially molten magma Onacha, S.A. (2006). Hydrothermal fault zone chamber. mapping using seismic and electrical • Inappropriate deployment of MEQ can lead to measurements. Ph.D Thesis, Duke University the wrong conclusion that hydrothermal pp225 systems are not seismically active, for instance Pellerin, L., & Hohmann, G. W. (1990). Transient at Mt. Longonot and Krafla. Electromagnetic Inversion - a Remedy for • Analysis of all earthquake events recorded in Magnetotelluric Static Shifts. Geophysics, 55(9), Longonot during the 2004 deployment show S- 1242-1250. wave splitting at sites close to the interpreted Rodi, W., & Mackie, R. L. (2001). Nonlinear fracture zone. conjugate gradients algorithm for 2-D ACKNOWLEDGEMENTS magnetotelluric inversion. Geophysics, 66(1), 174- We are grateful to Duke University, Kenya 187. Electricity Generating Company (KenGen), DOE, the Scott, S., and Skilling, I. (1999). The role of Icelandic National Power Company (Landsvirkjun), tephrachronogy in recognizing synchronous and Icelandic Geosurvey (ISOR) for supporting this caldera-forming events at the Quaternary work. volcanoes Longonot and Suswa, South Kenya Rift. Geological Society Special Publications, 161, 47- References 67. Arnason, K. (2006). A Programme for 1-D inversion Simiyu, S.M. and G.R. Keller, 2000. Seismic of central loop TEM and MT data. In S., O. (Ed.) monitoring of the Olkaria Geothermal area, Kenya (pp. 9). Reykjavik: ISOR, Iceland Geosurvey. Rift Valley. Journal of Volcanology and Baker, H., and Wohlenberg, J. (1971). Structure and Geothermal Research, 95, pp.197-208. Evolution of the Kenya Rift Valley. Nature, 229, Shalev, E., P. Malin, S. Simiyu, W. McCausland, and 538-542. M. Lichoro, 2000. The “SIGAR 2000” Clarke, M., Woodhall, D., Allen, D., and Darling, W. microearthquake study of Longonot and Suswa (1990). Geological, volcanological and volcanoes, Kenya. In Eos Trans. AGU, 81 (48), hydrogeological controls on the occurrence of Fall Meeting Supplement, Abstract S12B-08. geothermal activity in the area surrounding Lake