40th Anniversary - orkustofnun.is · Korosi–1446masl Korosi‐Paka‐Silalivolcanoes The volcanic zone is intersected

Mapping geothermal and structural features of the Northern Kenya Rift with MT and gravity measurements

Charles M. LichoroPhD StudentUNU‐GTP

40th Anniversary

1. Introduction

2. Motivation of current study

3. Demonstrate results from resistivity; Mainly MT and TEM

4. Gravity modelling of the rift zone

5. Joint Interpretation of resistivity and gravity

6. Conclusion

Outline

April 2018Charles Muturia ‐ 40th Anniversary of UNU‐GTP

Location setting


Kenya Geology

• Craton ‐ west

• Reworked craton ‐ central zone

• Mozambique belt largely underlie the Rift

• Rift tectonics is influenced by underlying rocks and older structures


Volcanic zone is next frontier for geothermal development

Elevations:Silali – 1528 maslPaka – 1697 maslKorosi – 1446 masl

Korosi‐Paka‐Silali volcanoes

The volcanic zone is intersected by a 10 km wide fissure zone that strike in NE‐SW direction

Study area

April 2018

Previous geophysical workNumerous studies within the KR have been dedicated to deeper lithospheric investigations

Refraction seismics and gravity studies have revealed high velocity and high density material within the rift zone due to significant amounts of mafic intrusive activity and crustal thinning

Keller et al., 1994a

Study area


Motivation?

1. Puzzle of the thick pile of low resistivity adjacent to most of the volcanoes in the Kenya rift

Has led some people to misinterprete it as a geothermal resource!

2. The presently deep high resistivity slab SW of Silali volcano


ResistivityMagnetotelluric (MT) Transient Electromagnetic (TEM)

m a.s.l.400 -

200 -

- 200 -

- 400 -

- 600 -

0 -

0 500 1000 1500 2000 mResistivityTemperature°C Alteration

Unaltered rocksSmectite - zeolite zoneMixed layered clay zoneChlorite zoneChlorite-epidote zone

> 25 m10 - 25 m2 - 10 m low resistivity capHigh resistivity core

250

200

200

150

100

50

NJ-1

1

NG-7

NG-1

0

Nesjavellir

Typical resistivity structure


MT/TEM Locations

Both MT and TEM done on the same spot

314 MT/TEM pairs

MT Static shift correction

April 2018

1D Resistivity @ 900 masl, sea level & 4000 mbsl

April 2018

Resistivity cross-sections

April 2018

Clay cap geometry

Mapping of base of clay cap(hydrothermal alteration)

~ Reservoir depth

* Can help in setting casing depthFumarolesSurface alteration

April 2018

Discussion Resistivity

~ Reservoir depth ??Structural mapping


Gravity


Gravity data ~ 515 points

Legend

FumarolesGravity stations

FaultsCaldera

Gravity measurements


Near-surface density estimation

Parasnis ~ 2.19 g/cm3

Line corresponding to density of 2.19 g/cm3 gives the best fit


Bouguer and residual anomalyRegional gravityBouguer gravity Residual gravity


2.5D gravity model across Paka

FumarolesGravity stations


Correlation of gravity and resistivity structureResistivityGravity Structural mapping


SummaryGood correspondence between the high gravity and the high resistivity alignment within the rift zone

and the low gravity with the low resistivity on the flanks

The joint interpretation of low resistivity and low gravity supports the conclusion that a thick zone of volcanclastics flanks the volcanoes and fissure eruption zones

This study shows that if resistivity and gravity are interpreted together, the independent constraints provided by resistivity pattern can improve confidence in relative shallow gravity models

The mapped fissure swarm agrees with both gravity and resistivity south of Paka volcano but northwards between Paka and Silali volcanoes there is inconsistency where fissure swarm trends NE but both gravity and resistivity maps trend west of the currently active fissure zone

The more westerly trend is not discernible from surface geological mapping and has been interpreted as a buried fissure eruption west of the current fissure trend


Acknowledgements

1. Geothermal Development company Ltd (GDC) for allowing use of the data & study leave

2. Geophysics staff at GDC for assistance in data collection

3. United Nations University - Geothermal Training programme (UNU-GTP) for the fellowship grant



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40th Anniversary - orkustofnun.is · Korosi–1446masl Korosi‐Paka‐Silalivolcanoes The volcanic zone is intersected