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    APPLICATION OF MICRO SEISMIC METHODSAPPLICATION OF MICRO SEISMIC METHODS

    TO GEOTHERMAL EXPLORATIONTO GEOTHERMAL EXPLORATION

    Silas M. Simiyu

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    BASIC REQUIREMENTS FOR A

    GEOTHERMAL SYSTEM

    1. Heat Source

    Magma Body related to volcanic activityDeep circulating fluids along Deep faults

    May be a Radioactive plutons

    2. ReservoirFluid availability and recharge

    Pathways/conduits along which fluids will

    circulateStatus of the reservoir

    Liquid/water dominated

    Steam/Vapour Dominated

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    SEISMIC WAVES

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    TYPE AND PROPERTIES OF SEISMIC

    WAVES

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    P and S Wave Properties

    Compressional (P) WaveCompressional (P) Wave

    Motion through compression & dilatationalMotion through compression & dilatationaltransfer of energytransfer of energy

    Propagates through solid and liquid mediaPropagates through solid and liquid media

    Dependent on the density and ElasticDependent on the density and Elastic ModuliModuli

    Shear (S) WaveShear (S) Wave

    Motion through pure shear strain.Motion through pure shear strain.

    Propagation is only through solid mediaPropagation is only through solid media

    Dependent on the density and ElasticDependent on the density and Elastic ModuliModuli

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    MAGNITUDE DETERMINATION

    -10

    -5

    0

    5

    10-10

    -5

    0

    5

    10

    P SCoda Length

    Vertical

    Radial

    Transverse

    0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

    Time [Seconds]

    0

    510

    15

    20

    25

    Magnitude determination

    Mc= 2log[C] + 0.00325d-0.97,

    d = epicentral distance [km],

    C = duration earthquake coda [sec].

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    Effect of Media on P and S Wave

    Transmission velocity 1

    Normal

    Low

    High

    Low

    Low

    Poissons Ratio

    () -MaterialStrength

    HighVery lowLowDuctile material

    Very Low

    (~1.4)

    NormalLowRocks (Steam

    filled pores)

    High (~2.1)V. LowNormalRocks (water

    filled Pores)

    Normal (~1.71)V. HighV. HighSolid Rocks

    Infinite (Very

    High)

    0V. LowLiquid (Water,

    Magma)

    Vp/VsVsVpMedia

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    Effect of Media on P and S Wave

    2Transmission velocity 2

    Very low

    Decrease

    Decrease

    Decrease

    Increase

    Poissons

    Ratio ()

    DecreaseIncreaseDecreasePore Pressure(Less Fluid)

    NormalDrasticDecrease

    DecreaseRocks -Brittle toDuctile transition

    IncreaseTends to ZeroDecreaseLiquefaction

    DecreaseIncreaseDecreasePhase Change

    (Liquid to vapour)

    IncreaseDecreaseIncreasePore Pressure

    (More fluid)

    Vp/VsVsVpMedia

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    Seismic Methods

    Active SourceActive SourceArtificial Sources (Dynamite,Artificial Sources (Dynamite, AirgunAirgun, weight drop), weight drop)

    Reflection MethodReflection Method Refraction MethodRefraction Method

    Shallow Geotechnical StudiesShallow Geotechnical Studies

    Passive SourcePassive SourceNatural Seismicity (MicroNatural Seismicity (Micro--Earthquakes)Earthquakes)

    Event distribution with depthEvent distribution with depth

    Wave velocity, attenuation and SWave velocity, attenuation and S--wave shadow zoneswave shadow zones

    Active faults and permeable zonesActive faults and permeable zones

    SS--wave shadows can indicate partial melt and magma chamberswave shadows can indicate partial melt and magma chambers

    Ground noise generated by the geothermal system (from boilingGround noise generated by the geothermal system (from boiling

    and/or convective flow)and/or convective flow)

    Hydraulic fracturing and tensile cracking of cooling intrusionsHydraulic fracturing and tensile cracking of cooling intrusions

    VolcanoVolcano--Seismic Hazards analysisSeismic Hazards analysis

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    ACTIVE SOURCE METHODS

    Refraction Seismology

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    Active Source

    Kenya Rift Axis Refraction Profile

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    Active Source

    Kenya Rift Axis Refraction

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    0 20 40 60 80 100 120 140 160 180 200 220

    Distance (km)

    -15

    -10

    -5

    0

    Depth

    2

    3

    4

    5

    6

    7

    Vp (km/s)SUSWAOLKARIAMENENGAI

    Figure 1.2. Seismic Velocity model along the Rift axis showing high velocity zones beneathMenengai, Olkaria and Suswa Volcanic centers based on the KRISP 1985-1990axial model by Simiyu and Keller, 2000.

    Active Source

    Kenya Rift Axis Seismic Refraction Results

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    Active Source -Seismic Refraction

    Olkaria Geothermal Field

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    Active Source -Seismic Refraction Results

    Olkaria Geothermal Field

    Vp

    NEF

    OWF

    OCF

    OWF OCF NEF1.9

    3.3

    3.6

    4.2

    4.5

    6.4

    2.8

    2.2

    3.5

    3.5

    4.3

    4.3

    6.5

    6.3

    1.9

    2.2

    3.1

    3.8

    4.3

    1 2 3 4 5 6 7

    Velocity (Vp)

    0

    2

    4

    6

    Depth(km)

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    Active Source Seismic Refraction Results

    Olkaria Geothermal Field

    192000 194000 196000 198000 200000 202000 204000

    Grid Eastings

    -6.0

    -5.0

    -4.0

    -3.0

    -2.0

    -1.0

    0.0

    Depth(km)

    2.0

    2.7

    3.1

    3.5

    4.5

    5.2

    5.6

    OWF OCF NEF

    SEISMIC SECTION ACROSS OLKARIA WEST, CENTRAL AND NORTH EAST

    Velocity (km/s)

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    PASSIVE SOURCE METHODS

    Ground Noise Monitoring

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    MNA

    MNWMNE

    MCA

    MSWMSE

    Figure 2.1. Background Seismic Noise in Menengai Area: MNA, MSE, MNE, MCA, MSW and MNW

    Background Noise Monitoring

    Menengai Geothermal Field

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    APPLICATION OF WAVE PROPERTIES

    Determine Presence of Heat SourcesDetermine Presence of Heat Sources

    Seismicity Distribution With DepthSeismicity Distribution With Depth Mapping of Shear Wave Attenuating BodiesMapping of Shear Wave Attenuating Bodies

    Reflected ArrivalsReflected Arrivals

    Map Zones Of Reservoir RechargeMap Zones Of Reservoir Recharge

    Seismic Wave Spectral AnalysisSeismic Wave Spectral Analysis

    Seismicity Patterns & Velocity Ratio AnalysisSeismicity Patterns & Velocity Ratio Analysis

    Evaluate Existing Reservoir PropertiesEvaluate Existing Reservoir Properties

    Analysis of P & S wave Seismic Velocity RatiosAnalysis of P & S wave Seismic Velocity Ratios

    Analysis of Poisson RatiosAnalysis of Poisson Ratios

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    SCHEMATIC RIFT VALLEY GEOTHERMALSCHEMATIC RIFT VALLEY GEOTHERMAL

    MODELMODEL

    Lake

    Brittle Brittle

    DuctileDuctile

    Heat Source

    Reservoir Capping Rocks

    Reservoir

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    0 5 10 15 20 25 30 35 40 45 50

    Kilometers

    -14

    -12

    -10

    -8

    -6

    -4

    -2

    0

    Depth

    MODEL OF A GEOTHERMAL SYSTEM AND SEISMICITY

    DISTRIBUTION WITH DEPTH

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    HEAT SOURCE -OLKARIA

    Event Distribution with Depth

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    23188000 192000 196000 200000 204000 208000 -8

    -7

    -6

    -5

    -4

    -3

    -2-1

    0

    Depth[km]

    OWF OCF NEF-DOMES

    Horizontal error

    0 2000 4000 M

    [b]

    Vertical error

    DM1KM5OS1

    KR6 DM4WE2 DM5

    KM6EP4

    NE2

    KM8 CE1NE3 DM2

    KM2KR2

    KM7

    DM3

    188000 192000 196000 200000 204000 208000

    9896

    000

    9900000

    9904000

    9908000

    GridNorthin

    gs[m]

    0 2000 4000M

    [a]

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    Event Distribution &

    with Depth at OlkariaHeat Source and Seismic Anisotropy

    0 1 2 3 4 5 6 7 8 9 10 Depth [km]

    0

    100

    200

    Events

    Figure 3.4A. Histogram of earthquake frequency distribution with depth

    0 1 2 3 4 5 6 7 8 9 10 Depth [km]

    0

    100

    200

    CrustStrength

    Figure 3.4B. Plot of crustal strength changes with depth showing theBrittle, Brittle-Ductile transition, Ductile Zone

    BRITTLE ZONE TRANSITIONZONE

    DUCTILE ZONE 0.10 0.15 0.20 0.25

    POISSON'S RATIO

    0

    1

    2

    3

    4

    5

    6

    DEPTH(KM)

    N[39-71]E AZIMUTH

    0.10 0.15 0.20 0.25

    POISSON'S RATIO

    0

    1

    2

    3

    4

    5

    6

    DEPTH(KM

    )

    N[20-68]W AZIMUTH

    Heat Source Depth Seismic Anisotropy

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    190000 192000 194000 196000 198000 200000 202000 204000 206000

    Grid Eastings

    Hypocenter Variations at Olkaria

    9896000

    9898000

    9900000

    9902000

    9904000

    9906000

    G

    ridNorthing

    Olkaria production wells.

    -7.5

    -6.5

    -5.5

    -4.5

    -3.5

    -2.5

    -1.5

    Depth

    EVENTS HYPOCENTER VARIATIONEVENTS HYPOCENTER VARIATION

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    Active Source Seismic Refraction Results

    Olkaria Geothermal Field

    191000 193000 195000 197000 199000 201000 203000

    Grid Eastings [m]

    989900

    0

    9901000

    9903000

    9905000

    GridNorthin

    gs[m]

    5

    10

    14

    17

    18

    20

    22

    24

    26

    28

    30

    32

    34

    36

    3840

    DOMES

    EPF

    NEFOCF

    OSF

    OWF

    Resistivity [ Ohm-m]

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    1

    2

    3

    4

    16

    2021

    22

    DM1

    DM4

    DM5

    NE3

    OS1

    SM6

    KM11

    KM12

    KM8

    OR1

    OR2

    OR3

    OR4

    180000 185000 190000 195000 200000 205000 210000 215000 220000

    Grid Eastings

    9885000

    9890000

    9895000

    9900000

    9905000

    9910000

    9915000

    GridNorthings

    LONGONOTCALDERA

    OLKARIA

    LAKENAIVASHA

    Recording stationEvent EpicenterNAIRAGIEENGARE

    Shear Wave attenuation

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    SHEAR WAVE ATTENUATION:SHEAR WAVE ATTENUATION:

    RAYPATH SECTION ACROSS OLKARIARAYPATH SECTION ACROSS OLKARIA

    KM5WE2

    EVENT 2

    EVENT 13

    KM11 KM12 DM5EP4 NE5 KM2

    EVENT 8

    OR2

    EVENT 18

    178000 182000 186000 190000 194000 198000 202000 206000 210000

    Grid Eastings

    -20

    -15

    -10

    -5

    0

    Depth

    [Km]

    OHC GFC

    Normal raypath

    Attenuated raypath

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    Possible Seismic S-wave attenuating Bodies [Semi-Fluidized]

    EPFNEF

    DOMESOWF OW-01

    3D PLOT OF DEPTH TO ATTENUATING HEAT3D PLOT OF DEPTH TO ATTENUATING HEAT

    SOURCES BENEATH OLKARIA AREASOURCES BENEATH OLKARIA AREA

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    HEAT SOURCE -SUSWA

    Event Distribution with Depth

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    SEISMIC EVENTS DISTRIBUTIONSEISMIC EVENTS DISTRIBUTION

    SU01

    SU02

    SU03

    SU04

    SU05

    SU06

    SU07

    SU08

    SU09

    SU10

    SU11

    SU12

    SU13

    SU14

    SU15

    SU16

    196000 200000 204000 208000 212000

    9864000

    9868000

    9872000

    9876000

    9880000

    GridNo

    rthings[m]

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    196000 200000 204000 208000 212000

    Grid Eastings [m]

    -10000

    -8000

    -6000

    -4000

    -2000

    0

    Depth(m)

    SEISMIC SECTION SHOWING EVENTSEISMIC SECTION SHOWING EVENT

    DISTRIBUTION ACROSS SUSWA CALDERADISTRIBUTION ACROSS SUSWA CALDERA

    -----------Outer Caldera-----------

    -Inner Caldera-

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    196000 200000 204000 208000 212000

    Grid Eastings [m]

    9864000

    9868000

    9872000

    9876000

    9880000

    GridNorthings[m]

    [a] 5001500 m

    196000 200000 204000 208000 212000

    Grid Eastings [m]

    [b] 15002500 m

    196000 200000 204000 208000 212000

    Grid Eastings [m]

    [d] dept >3500 m

    196000 200000 204000 208000 212000

    Grid Eastings [m]

    9864000

    9868000

    9872000

    9876000

    9880000

    GridNorthings[m]

    [c] 25003500 m

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    800

    1100

    1400

    2000

    2600

    3200

    196000 200000 204000 208000 212000

    9864000

    9868000

    9872000

    9876000

    9880000

    SU01

    SU02

    SU03

    SU04

    SU05

    SU06

    SU07

    SU08

    SU09

    SU10

    SU11

    SU12

    SU13

    SU14

    SU15

    SU16

    Average EventDepth

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    196000 200000 204000 208000 212000

    Grid Eastings

    -15

    -11

    -7

    -3

    1

    Dept

    h[km]

    Outer Caldera

    Inner Caldera

    HEAT

    SOURCE

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    HEAT SOURCE -MENENGAI

    Event Distribution with Depth

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    38MN01MN02

    MN03

    MN04

    MN05

    MN06

    MN07MN08

    MN09MN10

    MN11

    MN12

    MN13

    MN14

    MN15

    160000 165000 170000 175000 180000 185000

    Grid Eastings (m)

    9965000

    9970000

    9975000

    9980000

    9985000

    9990000

    GridNorthings Kabarak

    Majani Mingi

    Rongai

    NAKURUURBAN AREA

    Lake NakuruNational Park

    BahatiSettlement

    Bahati

    ResidentialArea

    LakeNakuru

    ToEldoret

    ToBarin

    go

    ToNy

    ahuru

    ru

    ToNairobi

    MENENGAI

    CALDERA

    -8

    -7

    -5

    -4

    -3

    -2

    SEISMIC SECTION SHOWING EVENTSEISMIC SECTION SHOWING EVENT

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    Men. CalderaNorthWest SouthEast

    165000 167500 170000 172500 175000 177500 180000

    Distance (M)

    -8

    -6

    -4

    -2

    0

    D

    epth(Km)

    Olbanita

    SEISMIC SECTION SHOWING EVENTSEISMIC SECTION SHOWING EVENT

    DISTRIBUTION ACROSS MENENGAI CALDERADISTRIBUTION ACROSS MENENGAI CALDERA

    SEISMIC RAYPATHS ACROSS MENENGAISEISMIC RAYPATHS ACROSS MENENGAI

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    SEISMIC RAYPATHS ACROSS MENENGAISEISMIC RAYPATHS ACROSS MENENGAI

    CALDERACALDERA

    Bahati

    MN01MN02

    MN03

    MN04

    MN05

    MN06

    MN07

    MN08

    MN09MN10

    MN11

    MN12

    MN13

    MN14

    MN15

    160000 165000 170000 175000 180000 185000

    Grid Eastings (m)

    9965000

    9970000

    9975000

    9980000

    9985000

    9990000

    GridNor

    things

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    RESERVOIR CHARACTERIZATION

    Vp/Vs Velocity Ratios

    Olkaria

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    9903000

    9

    904000

    9905000

    GridNo

    rthings[m]

    OW-703OW-704

    OW-705

    OW-709

    OW-710

    OW-711

    OW-713

    OW-714

    OW-718

    OW-722

    OW-723

    OW-724

    OW-726OW-727

    OW-707

    OW-716

    OW-30OW-32

    78

    82

    87

    90

    92

    93

    95

    98

    Bars

    9903000

    9904000

    9905000

    GridNorthings

    [m]

    OW-703OW-704

    OW-705

    OW-709

    OW-710

    OW-711

    OW-713

    OW-714

    OW-718

    OW-722

    OW-723

    OW-724

    OW-726OW-727

    OW-707

    OW-716

    OW-30OW-32

    1.63

    1.64

    1.66

    1.67

    1.69

    1.73

    1.75

    1.78

    Pressure at 500 m.a.s.lPressure at 500 m.a.s.l

    Velocity Ratio DistributionVelocity Ratio Distribution

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    9864000

    9869000

    9874000

    9879000

    15

    35

    55

    75

    95A RESISTIVITY (Ohm-m)

    9864000

    9869000

    9874000

    9879000

    -200

    -100

    0

    100

    200

    B AEROMAGNETIC (Gammas)

    9864000

    9869000

    9874000

    9879000

    -140

    -100

    -60

    -20

    20

    60

    100

    140C RESIDUAL GRAVITY (gu)

    1.9

    2.0

    2.1

    2.2

    2.3

    2.4

    [a] Vp 1000

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    3.5

    3.8

    4.1

    4.4

    4.7

    5.0

    [c] Vp 3000

    2.02

    2.03

    2.04

    2.05[f] Vp/Vs 3000

    1.84

    1.86

    1.88

    1.90

    1.92

    1.94

    [e] Vp/Vs 2000

    1.60

    1.65

    1.70

    1.75

    1.80

    [d] Vp/Vs 1000

    1.9

    2.0

    2.1

    2.2

    2.3

    2.4

    [a] Vp 1000

    2.5

    2.8

    3.1

    3.4

    3.7

    [b] Vp 2000

    RESERVOIR COMPARISSON: Vp/Vs RATIOSRESERVOIR COMPARISSON: Vp/Vs RATIOS

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    RESERVOIR COMPARISSON: Vp/Vs RATIOSRESERVOIR COMPARISSON: Vp/Vs RATIOS

    MENENGAI OLK-NEAST

    OLK-EAST SUSWA

    OLK-CENTRALOLK-SOUTH

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    196000 200000 204000 208000 212000

    Grid Eastings[m]

    9864000

    9868000

    9872000

    9876000

    9880000

    GridN

    orthings[m]

    1.58

    1.62

    1.66

    1.70

    1.74

    1.78

    1.82

    Vp/Vs Ratio

    HOT

    UPFLOW

    COLD

    INFLOW

    Outflo

    w

    Outflow

    Inner and outerCaldera walls

    Site 1

    Site 2

    Site 3

    COL

    DIN

    FLO

    W

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    160000 165000 170000 175000 180000 1850009965000

    9970000

    9975000

    9980000

    9985000

    9990000

    Rongai

    ToEldoret

    ToBaringo

    ToNyahu

    ruru

    ToNairobi

    1.5

    1.7

    1.7

    2.0

    Vp/Vs Ratio

    Figure 5.1. Average Vp/Vs rartion variations in the Menengai study area showing proposedtest drill sites [Stars], interpreted upflow zones and fluid recharge for the system

    MN01

    MN02

    MN03

    Cold Water

    ColdWa

    ter

    Cold

    Water

    ColdW

    ater

    ColdW

    ater

    UPFLOW

    UPFLOW

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    VOLCANO-SEISMIC HAZARDS ANALYSIS

    THETHE B VALUEB VALUE RELATIONSHIPRELATIONSHIP

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    THETHE B VALUEB VALUE RELATIONSHIPRELATIONSHIP

    0.0 1.0 2.0 3.0

    MAGNITUDE

    0.0

    1.0

    2.0

    3.0

    4.0

    LOG

    (N)

    Menengai =1.21

    Suswa =1.09

    Olkaria =1.05

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    THANK YOU