Title: Three-Dimensional Inversion of Magnetotelluric Data: Geological/Geothermal Interpretation of Asal Geothermal Field, Djibouti

Abstract

This research study is aiming at becoming acquainted with the resistivity method and different ways
of performing interpretation of MT data for deep lying subsurface investigations. The MT data
collected from Asal geothermal area were used; the comparison between the results of 1-D inversion
carried out previously and 3-D inversion is performed. A total of 105 MT soundings were considered
in this research project and the same number of corresponding TEM soundings collected at nearby
sites. To allow the static shift correction in the 1-D inversion, the MT data were jointly inverted
with TEM data. Shift correction was then applied to the two polarizations for 3-D inversion. The
modern computing systems have made 3-D modelling of MT data achievable and it is now becoming
common and useful for detailed subsurface surveys in geothermal industries as well as in other
fields including ground water, oil and natural gas and mineral exploration.
The WSINV3DMT code was used to perform 3-D inversion of the static shift corrected off-diagonal
impedance tensor elements. Three different initial models were considered in order to test the
inversion robustness and 31 periods evenly distributed on logarithmic scale from 0.003 to 300 s
were used. The first initial model was compiled from the joint 1-D inversion of TEM and MT
soundings; the second was a homogeneous earth of resistivity 10 Ωm and the third initial model was
a homogeneous earth with a resistivity 50 Ωm. The RMS in all three different initial models was not
of big difference with values of 1.44, 1.48 and 1.87, respectively. The final models gave similar
resistivity structures underneath Asal rift and are presented here as iso-depth resistivity maps
and cross-sections.

The result of the interpretation shows four main resistivity structures below the geothermal area:
A shallow lying thin high resistivity layer followed by low resistivity (conductive cap). Below
there is a high resistivity layer (resistive core) underlain by a deep lying conductor. Lithology
based on well data shows that the shallow thin high resistivity layer corresponds to dry basaltic
rocks covering the surface, the conductive layer reflects saline fluids but correlates also with
low temperature alteration (smectite and zeolites), the deep resistive core correlates with the
high temperature alteration minerals (chlorite and epidote) whereas the deep seated conductive body
is most likely connected to the heat source of the Asal geothermal system.

At sea level, high resistivity dominates the northeast part of Asal rift towards Lake Asal in the
vicinity of Ardoukoba volcano and the southeast part of the rift around Baddikoma region. An
updoming conductive cap intersects the high resistivity, running NE-SW and reflects presumably
alteration within the geothermal system. It covers the central part of the Asal rift including the
Fiale explosion crater (Lava Lake). This is the same area where the fumaroles and hot springs in
the Asal rift are
located.