Report
Project no.: 590112

Resistivity survey in Alidgeothermal area, Eritrea

Hjálmar Eysteinsson
Andemariam TeklesenbetGuðni Karl Rosenkjær
Ragna Karlsdóttir

Prepared for ICEIDA (ÞSSÍ)ÍSOR-2009/016

February 2009

Key page

Report no.

DateÍSOR-2009/016

February 2009

Distribution

Open

ClosedReport name / Main and subheadings

Number of copies

Resistivity survey in Alid geothermal area, Eritrea

10
Number of pages

42 + Appendices
Authors

Project manager

Hjálmar Eysteinsson, Andemariam Teklesenbet, Guðni Karl
Rosenkjær and Ragna Karlsdóttir

Hjálmar Eysteinsson

Classification of report

Project no.

590112
Prepared for

ICEIDA (Þróunarsamvinnustofnun Íslands)
Cooperators

Abstract

A geophysical survey of the Mt. Alid geothermal area in Eritrea was performed in late 2008. The
aim of the survey was to delineate if possible the geothermal reservoir by the use of TEM and MT
soundings. The original plan was to cover the area of Mt. Alid and its nearest vicinity. The terrain
however was very difficult for vehicles and bikes used for transportation of the equipment andinaccessible at times. Within given timeframe the area to the west and southwest of the mountain
was covered as well as the top of Mt. Alid.
The resistivity survey only reveals a part of the geothermal reservoir, due to lack of soundings.
A SW-NE lineament is detected at a depth interval 500 metres to 2 km, interpreted as a transform
fault intersecting the geothermal reservoir and most likely controls the main upflow into thereservoir.
At the surface at the southwest flanks of Mt. Alid, an area with exceptional vegetation incomparison to the barren dry volcanic landscape around, indicates moisture and water in the
ground that could be explained by steam upflow from the geothermal reservoir.

Key words

Eritrea, Mt. Alid, resistivity survey, geothermal
reservoir, fault, upflow zone

ISBN-number

Project manager’s signature

Reviewed by

KÁ, BS

Table of contents
1

Introduction.......................................................................................................................... 7

2

Geological settings for Mt. Alid geothermal field ........................................................ 8

3

Field survey .......................................................................................................................... 9

4

Data acquisition................................................................................................................. 12
4.1
4.2
4.3
4.4

5

The survey area .......................................................................................................... 13
TEM data ..................................................................................................................... 13
MT data ....................................................................................................................... 14
Telluric shift in MT soundings ................................................................................. 20

Result of joined inversion of MT and TEM data ........................................................ 22
5.1 Resistivity maps ......................................................................................................... 22
5.2 Resistivity cross sections ........................................................................................... 26

6

Conclusions and recommendations............................................................................... 37

7

References ........................................................................................................................... 42

Appendices ...................................................................................................................................
1.
2.
3.
4.
5.
6.

TEM and MT soundings, coordinates and elevation ................................................
TEM data .........................................................................................................................
MT data Z ........................................................................................................................
MT data apparent resitivity and phase .......................................................................
1‐D interpretation models for joint TEM/MT inversion ...........................................
Resistivity maps .............................................................................................................

List of figures
Figure 1. The location of Mt. Alid geothermal field. ................................................................... 7
Figure 2. The east African rift system ......................................................................................... 8
Figure 3. The camp at Buya ........................................................................................................ 9
Figure 4. Original plan for the MT/TEM soundings ............................................................... 10
Figure 5. The sixwheel‐bikes used for transportation of the equipment in the field .................. 10
Figure 6. Location of TEM and MT soundings ........................................................................ 11
Figure 7. Preparing for the tour to Mount Alid ....................................................................... 12
Figure 8. TEM survey on Mount Alid ..................................................................................... 13
Figure 9. Performing MT measurements .................................................................................. 14
Figure 10. Impedance elements for MT site 006 with the amplitude on the left and thephase on the right ............................................................................................................... 16
Figure 11. Rose diagrams of strike directions for all frequencies of all the MT sites ................ 17
Figure 12. Rose diagrams of strike directions for periods greater than 10 seconds, for all theMT sites .............................................................................................................................. 18‐5‐

Figure 13. Apparent resistivity and phase for two sites ........................................................... 19
Figure 14. An example of a joint inversion result of TEM‐ and MT data ................................ 20
Figure 15. Histogram of shift factors applied to the MT apparent resistivity curves. .............. 21
Figure 16. Fixing one of the bikes in a difficult terrain ............................................................. 22
Figure 17. Resistivity map at 100 metres below sea level. ........................................................ 23
Figure 18. Resistivity map at 500 metres below sea level. ........................................................ 23
Figure 19. Resistivity map at 1000 metres below sea level. ...................................................... 24
Figure 20. Resistivity map at 3500 metres below sea level. ...................................................... 25
Figure 21. Location of cross sections. ........................................................................................ 26
Figure 22. A resistivity cross section through the area, with NW‐SE direction. ..................... 27
Figure 23. Cross section WE1640. ............................................................................................ 29
Figure 24. Cross section WE1641. ............................................................................................ 29
Figure 25. Cross section WE1642. ............................................................................................ 30
Figure 26. Cross section WE1643. ............................................................................................ 30
Figure 27. Cross section WE1644. ............................................................................................ 31
Figure 28. Cross section WE1645. ............................................................................................ 31
Figure 29. Cross section WE1646. ............................................................................................ 32
Figure 30. Cross section WE1647. ............................................................................................ 32
Figure 31. Cross section SN593. ............................................................................................... 33
Figure 32. Cross section SN594. ............................................................................................... 33
Figure 33. Cross section SN595. ............................................................................................... 34
Figure 34. Cross section SN596. ............................................................................................... 34
Figure 35. Cross section SN597. ............................................................................................... 35
Figure 36. Cross section SN598. ............................................................................................... 35
Figure 37. Cross section SN599. ............................................................................................... 36
Figure 38. Conclusions ............................................................................................................. 37
Figure 39. An oases in the middle of the dry volcanic landscape .............................................. 38
Figure 40. An aerial photo of Mt. Alid and part of the Danakil depression from Google‐
earth. ................................................................................................................................... 40‐6‐

1 Introduction
In October 2008 the Iceland GeoSurvey (ISOR) signed an agreement with Icelandic
Developing Agency (ICEDA), concerning ISORs implementation of a geophysicalsurvey in the Mount‐Alid geothermal region in Eritrea (Figure 1). The survey should
consist of magneto‐telluric (MT) and central loop transient electromagnetic (TEM)soundings at and in the nearest vicinity of Mt Alid. Iceland Geosurvey made acooperation agreement with the Geological Survey of Eritrea (GSE, under the Eritrean
ministry of energy and mines) concerning this survey. The field survey was led by Dr.
Hjálmar Eysteinsson, geophysicist from ISOR, and Andemariam Teklesenbet,
geophysicist from GSE. The Geological Survey of Eritrea provided geoscientists toparticipate in the survey and make necessary preparatory arrangements. The
geophysical exploration was planned to last for 30 working days in the field and theamount of 100 soundings of both MT and TEM was expected to be measured.

Figure 1. The location of Mt. Alid geothermal field.

‐7‐

2 Geological settings for Mt. Alid geothermal field
Alid volcanic center, Eritrea, is located along the axis of the Danakil depression, thegraben trace of a crustal spreading center that radiates NNW from a plate‐tectonic
triple junction situated within complexly rifted and faulted basaltic lowland called theAfar triangle (Figure 2). The Danakil depression is a subaerial segment of the spreading
system that is opening to form the Red Sea. Crustal spreading along the axis of the Red
Sea is transferred to spreading along the Danakil segment in a right stepping enechelon pattern (Barberi and Varet, 1977). The northern Danakil depression lies near or
below sea level for the most of its extent. It is surrounded by the Danakil horst to theeast and the Eritrean plateau to the west.
The Alid mountain rises up to about 700 metres above the flat plains of the Danakil
depression, to a summit of 904m. The mountain is elongated with major axis of 7 km in
ENE‐WSW direction, perpendicular to the graben, and a minor axis is about 5 km longparallel to the graben. MT. Alid is a structural dome with rocks subdivided from oldest
to youngest into: 1) Precambrian basement; 2) Interlayered sedimentary deposits andlava‐flows; and 3) Basaltic and rhyolitic lava‐flows (Clynne et al., 2005).
Geological and geochemical studies imply that a high temperature geothermal system
underlies the Alid volcanic center. Fumaroles and boiling pools on the northern part of
Mt. Alid suggest that an active hydrothermal system underlies that part of themountain. Geothermometers indicate temperatures exceeding 250 °C in the reservoir(Lowenstern et al., 1999; Clynne et al., 2005).

Figure 2. The east African rift system, showing some of the historically active volcanoes (redtriangles) and the Afar Triangle (shaded, center) — a triple junction where three plates
are pulling away from one another: the Arabian Plate, and the two parts of the African
Plate (the Nubian and the Somalian) splitting along the East African Rift Zone..

‐8‐

3 Field survey
Prior to the fieldwork period, Mr. Þorleifur Finnson went to Djibouti on behalf of ISOR
to take care of shipping of four six‐wheel bikes and other equipment from Djibouti to
Eritrea. The bikes had been used in Djibouti in a similar survey by ISOR. Prior attempts
to get the bikes shipped to Eritrea had not been successful. He also made a trip to
Eritrea for the preparations for the survey, i.e. finished the contract with the GSE,
opened a bank account, visited the survey area and the proposed camping area,
arranged custom clearance of the bikes etc. Due to financial crises in Iceland at thistime no money could be transferred to Eritrea while Þorleifur was there and therefore
he could not arrange the setup of the base camp.
Two geophysicist from Iceland GeoSurvey arrived to Asmara, the capital of Eritrea, inthe evening of November 10th. It took more than a week to prepare for the fieldwork.
Custom clearance of all the equipment took a long time, and permission for using asatellite phone in the field was extremely difficult to get. Getting all necessary
appliances for the field (refrigerator, freezer, furniture etc.) and food was also time
consuming. On the 19th of November the survey crew left Asmara. All the gear had
been loaded on one truck and the two pickups to be used in the field. Another truck
was loaded in Massawa with the equipment and the four 6weel bikes that came fromDjibouti. The group arrived in the Alid field, to the village of Buya well after midnight(Figure 3).

Figure 3. The camp at Buya (photo: Hjálmar Eysteinsson).

The next day the camp was set up, the two trucks were unloaded, tents where set up,
kitchen made clear, bikes where made ready etc. The day after, on the 21st ofNovember, the first TEM soundings where done, and the MT equipment calibrated
and an MT base station installed (for remote reference). The survey continued until 14thof December, when the last soundings where done and the equipments where collected
and the MT instruments calibrated.
‐9‐

Figure 4. Original plan for the MT/TEM soundings. (Scale is UTM coordinates, in km,
zone37).

Figure 5. The sixwheel‐bikes used for transportation of the equipment in the field (photo:
Hjálmar Eysteinsson).

‐ 10 ‐

Four units of MT equipments were used in the survey, where one served as a base orreference station measuring a continuous MT signal and used for reference signal to
give better data quality, a method named remote reference. One set of TEM equipment
was in use.

Figure 6. Location of TEM (open diamonds, with the name above) and MT soundings (filledpoints with the name below). Roads and tracks used are shown as black dashed lines andgray lines respectively. Coordinates are in UTM, km units.

‐ 11 ‐

On the 15th of December the field crew left Buya. One truck carried the bikes to
Massawa, and another carried the rest of the field gear to Asmara. After some delay in
Massawa the bikes where finally put into a container to be shipped back to Djibouti.
The convoy arrived to Asmara late in the evening. On the 16th we finished all the
payments that needed to be done and closed the bank account. The ISOR personnel left
for Iceland on the 17th of December.
As said before, the field survey was led by Dr. Hjálmar Eysteinsson (geophysicist) from
Iceland GeoSurvey (ISOR), and Andemariam Teklesenbet (geophysicist) from EGS.
Other participants where Guðni Karl Rosenkjær (geophysicist) from ISOR, Yonas
Estianos (EGS geologist) Mussie Beraki (EGS geologist), Debesay Habte (EGS, fieldassistant), Rigat (cook), Azieb Hagos (cook), Birhane Sultan (driver) and Salihadin(driver). Many local assistant also took part in the project.

4 Data acquisition
During the data acquisition two groups worked in the field, one for each method of
measurements. The project leader from Iceland GeoSurvey performed data quality
check and a preliminary interpretation of most of the data at base camp as data wascollected. During the field work period of 24 days, a total of 52 MT soundings where
measured (some of which were repeated due to bad data quality) and 67 TEM
soundings (figure 6). This makes 2.2 MT and 2.8 TEM soundings a day on the average.
Some days, or part of days, were not productive due to instrument failure. The
locations of the soundings are listed in Table 1 in Appendix 1.

Figure 7. Preparing for the tour to Mount Alid (photo: Hjálmar Eysteinsson).

‐ 12 ‐

4.1 The survey area
The original plan consisted of 73 MT and TEM soundings shown on Figure 4. As
shown, the plan was to carry out the survey on the top of mountain Alid and the area,
2–3 km away from the mountain. This was quite an optimistic plan, considering thetime available and the difficult terrain. Some of the area is covered with inaccessible
lavaflows. Some time was spent on finding trails into or around the lavafields but
without luck. The bikes came to a good use in the field. It was however only possible todo the TEM measurements on the top of Mount Alid by carrying the equipment on theback of camels (Figure 7).

Figure 8. TEM survey on Mount Alid. Note the steam vent in the back (photo: Andemariam
Teklesenbet).

4.2 TEM data
In the TEM soundings electrical current is transmitted into a big loop of wire (300x300m) laid on the ground. This current produces a magnetic field. The current is abruptly
turned off and the decaying e magnetic field causes induction currents in the ground.
The strength of the induced currents is dependent on the resistivity structure below thesurvey site. The ground response is measured by a small coil in the center of the bigtransmitting loop. The time decaying signal is measured from 88 μs to 70 ms from thetime of turning of the current.
The TEM instruments used is from Geonics Ltd. in Canada. The TEM soundings cansurvey the ground resistivity down to about ½–1 km, depending on the resistivity. For
low subsurface resistivity the depth of penetration can be as low as a few hundred
meters, but in higher resistivity surroundings it is possible to explore the resistivity
down to about 1 km in favorable conditions. For the conditions in the Alid area and the
instrument setup, the depth of penetration of the TEM soundings was generally‐ 13 ‐

around 500 meters. One TEM sounding takes a few hours to perform, from the start ofsetup to the end of the measuring time.
All the TEM data was interpreted in terms of 1‐D models (resistivity only varies withdepth) using a program developed at ISOR. The TEM data and their interpretation are
shown in Appendix 2.

Figure 9. Performing MT measurements (photo: Hjálmar Eysteinsson).

4.3 MT data
In the MT method, the natural fluctuations of the earth’s magnetic field are used as
signal source. Those fluctuations induce currents in the ground and are measured on
the surface with two horizontal and orthogonal electrical dipoles (Ex and Ey) and themagnetic field is measured in three orthogonal directions (Hx, Hy and Hz). It is
customary to set the x direction to the magnetic north direction. For simple earth (i.e.
homogenous or layered) the electrical field is coherent with its orthogonal source
magnetic field (i.e. Ex correlates with Hy, and Ey with Hx) and this relation depends
on the earth’s resistivity structure.
The MT instruments used here are from Phoenix Ltd. in Canada (MTU type), and canmeasure the MT signals in the frequency range from 320 Hz up to DC. Four MT units
where used in this survey. One 5 component (i.e. Ex, Ey, Hx, Hy and Hz) station was
kept running continuously at the same location some 10 km south of the survey area
and used for remote reference data processing. This is a standard method to get better
quality data from the processing. The other three sites where deployed each day andpicked up the next day. This gives about 20 hours of continuous time series, and MT
data in the range from 320 Hz (0.003 sec) to about 1000 seconds. The short‐period MT
data (high frequency) is mainly dependent on the shallow structures due to their short
depth of penetration, whereas the long period data are mainly dependent on the‐ 14 ‐

deeper structures. The MT method has the greatest exploration depth of all resistivity
methods (some tens or hundreds of kilometers) and is practically the only method forstudying deep resistivity structures. In this survey, the exploration depth of the MTsoundings was around 10 km, but varied quite much depending on data quality andthe resistivity structure.
Three of the MT units are 5 components and one is a two component unit (i.e.
measures only Ex and Ey). The two component unit is always set up close to a nearby 5
component site (usually around 1km), and the magnetic field at that site is used foranalyzing the result at the two component site. This is of course only valid if themagnetic field is homogeneous locally, an assumption which is usually valid for shortdistances. In the middle of the field work period, one of the 5 component MT unit
broke down in such a way that it was not possible to measure the magnetic field, andtherefore it was used thereafter as a two component unit. This made more restriction
on where the three MT units were located.
The quality of the MT data was not always satisfactory and therefore many sites had tobe re‐measured. The main reason for this was low signal strength. The source for theMT signal are mainly thunderstorms in the equatorial belt of the earth (high frequency)
and solar storms interacting with the earth’s magnetic field (low frequency). During
the survey time the solar activity was at the minimum level in it´s 11 years activity
period, so low signals where expected. In the beginning the electrode dipole length
was kept 50 meters but was increased using all the extra wire at hand up to 125 meters.
That usually gave better data quality. The electrodes where also refilled with led‐
chlorite mixture.
Table 2 in Appendix 1, lists the MT soundings, the date of performance, the serial
number of the MT unit used and the serial number of the MT unit used for magnetic
field in case of a two component site. The electrical dipole length is also shown. For
some sites a sounding was done more than once (up to 4 times), these are the instances
when data quality was too poor and the soundings had to be repeated.

The measured MT time series are Fourier transformed into the frequency domain andthe “best” solution is found that describes the relation between the electrical andmagnetic fields through the following equation:or in matrix notation:

where and are electrical and magnetic vectors (in the frequency domain) and Z isthe complex impedance tensor which contains the information on the subsurface
resistivity structure. Programs from Phoenix Ltd. SSMT2000, were used to process thetime‐series signal using a robust processing method technique. The result was edited in‐ 15 ‐

a MTEDITOR program by Phoenix, and the output was run through a program callededi2edi made by ISOR, which calculates various MT parameters and produces theresults in the standard EDI file format. The processed MT data in terms of magnitude
and phases of impedance elements are given in Appendix 3.

Figure 10. Impedance elements for MT site 006 with the amplitude on the left and the phase on
the right. The diagonal elements of the impedance are shown by light red and blue.

The values of the impedance elements are dependent on the resistivity structures
below and around the site. For a homogeneous and 1‐D earth (resistivity varies onlywith depth)=
and==0. For a 2‐D earth, i.e. resistivity varies with depth
and in one horizontal direction, it is possible to rotate the coordinate system such that==0, but. For a 3‐D earth all the impedance elements are different. An
example of the impedance results for one of the MT sites is shown in Figure 10. As seen
are not zero, but they
there the diagonal elements of the impedance tensor (Z and Z
are more than an order of magnitude lower than the off diagonal elements up to about
1 second, and the main impedance elements (Z and Z are about the same
indicating 1‐D resistivity structure for the shallow part. At greater depths, i.e. forperiods greater than 10 seconds, all the impedance elements are in the same order ofmagnitude showing a high degree of three‐dimensionality in the resistivity structures
below this site.
From the impedances the apparent resistivity and phase is calculated according to
0.2

.;

=arg(

0.2

.;

=arg(

.

Graphs of the apparent resistivity and phases for all the MT soundings are given in
Appendix 4. As mentioned earlier the MT impedance is dependent on the orientationof the horizontal electrical and magnetic fields. The rotation that minimizes the
diagonal elements in the impedance tensor is called the electrical strike direction, andis also plotted for each site in Appendix 4. The MT parameters in a rotated coordinate
system (one fixed rotation angle for all frequencies) are also shown along with the‐ 16 ‐

result of the tipper for those sites where vertical magnetic field was measured (therelation between the vertical‐ and the horizontal magnetic fields). A rose‐diagram
rotation plot for the strike direction is shown in Figures 11 for all frequencies and on
Figure 12 only for periods greater 10 seconds (the data representing the deeper parts).
The figures clearly demonstrate that the main geo‐electrical strike is parallel to the rift
direction. The rift strike is better represented by the long periods compared to the shortperiods which are more dependent on the local resistivity close to each site.

Figure 11. Rose diagrams of strike directions for all frequencies of all the MT sites. The
preferred rotation for each site is shown by a red line through each site location.

‐ 17 ‐

Figure 12. Rose diagrams of strike directions for periods greater than 10 seconds, for all theMT sites. The preferred rotation for each site is shown by a red line through each site.

It is clear from the MT data that the resistivity structures of the Alid area are highly
three dimensional. A measure of the three‐dimensionality is the so called skew
parameter which is the ratio between the magnitude of the sum of the diagonal
elements and the magnitude of the difference of the off diagonal elements. Thisparameter is plotted in Appendix 4 for all the soundings. This parameter is usually lessthan 0.1 for periods less than 1 second , but increases dramatically for periods greaterthan 1–10 seconds, to values even greater than 1. Thus it is clear that for a fullunderstanding of the MT parameters a 3‐D interpretation of the data is called for. Thisis however out of the scope of this work since 3‐D modeling is very time consuming. A
1‐D modeling (horizontal layered earth) of the MT data is quite straightforward and is
the only interpretation of the data used in this report. The question is whichparameters should be inverted for? In a 3‐D data inversion all the impedance elements‐ 18 ‐

are inverted for, where as in a 2‐D inversion both the xy and yx pairs are inverted for.
But in a 1‐D inversion one could invert for either xy or yx parameter or what is
becoming more customary nowadays is to invert for some rotational invariant
parameter and therefore one has not to deal with the question of rotation. There exist
three such invariants, namely:2

All these parameters give the same values for a 1‐D earth response, for 2‐D the det(determinant) and the gm (geometric mean) reduce to the same value but the av(arithmetic mean or effective) is different. For 3‐D responses, all these parameters aredifferent. A plot of these parameters in terms of apparent resistivity and phase isshown in Figure 13 for two sites. For site 080 there is a little difference between thethree invariants, but the average (ave) is slightly different from the others which is inagreement for a 2‐D case as discussed above. For site 006 in Figure 13 the det value isquite different than the others, showing a high degree of 3‐D
It is not known which of the three invariants, if any, is best suited for 1D inversion.
However several scientists have suggested that the determinant invariant is the one touse, based on the comparison of model responses for 2‐ and 3‐D models (Park andLivelybrook, 1989; Rangabayaki, 1984; Ingham, 1988). Therefore we have chosen to use
the determinant invariant impedance for the 1‐D inversion.

Figure 13. Apparent resistivity and phase for two sites. The different colours refer to different
parameters determined form the impedance.
‐ 19 ‐

4.4 Telluric shift in MT soundings
The MT method, like all resistivity methods where the electric field is measured on thesurface, suffers a problem called the telluric shift problem. The reason for this is local
resistivity inhomogeneity which distorts the electric field, independent of period. The
result of this is that the apparent resistivity values have an unknown multiplier (shiftof logarithmic values). The TEM soundings do not suffer this problem because theymeasure magnetic induction in a receiver coil. By interpreting together TEM and MT
soundings made at the same (or nearly the same) location, the TEM data can be used todetermine the unknown multiplier of the MT apparent resistivity.
Due to this reason the TEM soundings are essential in areas where telluric shifts in MT
soundings can be expected, such as in a volcanic area as in the Alid area. This is thereason for setting up the TEM site at the same, or close by the location of the MT sites.
Table 1 list which TEM sounding is used for static shift correction for each MT
sounding. The maximum distance between a MT site and a corresponding TEM site is
260 m (site 047).
The shift parameter is found through a joint inversion of the TEM and the MT data
where the shift value for the MT apparent resistivity data is one of the inversion
parameters. The inversion program used is developed at ISOR and named “temtd”. An
example of an inversion results is shown in Figure 14. The MT data points are shownby blue dots (apparent resistivity and phase) and the corresponding TEM data isshown by red apparent resistivity points. The 1‐D model is shown to the right and its
TEM and MT response is shown by red and blue lines respectively. The model
response fits the data quite well.

Figure 14. An example of a joint inversion result of TEM (red points) and MT data (bluepoints). The red and blue solid curves show the TEM and MT responses respectively ofthe model shown to the right. Inversion results of all the soundings are given in
Appendix 5.

‐ 20 ‐

The distribution of all the shift parameters in the MT soundings is shown in Figure 15.
If the telluric shift in the MT data is random, one would expect a mean value of 1, but
as is shown in Figure 15, the mean value is about 0.7, with minimum and maximum
values of 0.1 and 1.7 respectively. A shift of 0.1 means that the apparent resistivity data
have been shifted down by an order of magnitude due to local resistivity
inhomogeneities. Uncorrected interpretation of that sounding would have yielded amodel with a resistivity about and order of magnitude lower and depths of layers
about 3 times to shallow. A mean shift less than one is quite common in a volcanic
environment. In Iceland similar telluric shift mean values are found as in the Alid area.

Figure 15. Histogram of shift factors applied to the MT apparent resistivity curves.

‐ 21 ‐

5 Result of joined inversion of MT and TEM data
The 1‐D TEM/MT joint inversion results (Appendix 5) are compiled into a 3‐D
resistivity structure and is presented by iso‐resistivity maps at different elevations
above and below the sea level, and by resistivity cross sections through the survey
area.
For each site a depth of exploration is estimated. At sites where only TEM sounding ispresent the depth of exploration is much less than the other sites, therefore those are
not used at depths greater than their exploration depth which is usually not more thanabout 500 meters. For MT sites where data quality is bad at long periods the depth of
exploration is as low as 2 km, but generally 6–10 km. In some instances where 3‐D
signal is strong in the long period MT data, a 1‐D model could not be generated thatfits both the apparent resistivity and phase, therefore the depth of penetration at thosesoundings is often dramatically reduces in their 1‐D modelling.

Figure 16. Fixing one of the bikes in a difficult terrain (photo: Andemariam Teklesenbet).

5.1 Resistivity maps
Resistivity maps were drawn for resistivity at 400 and 200 metres above sea level, at
sea level and 100, 200, 300, 500, 750, 1000, 1500, 2000, 2500 3000, 3500, 4000, 4500, 5000,
6000, 7000, 8000, 9000 and 10000 meters below sea level. All maps are presented inAppendix 6 but some of them will be shown and commented in the main text, in order
to explain the resistivity structure of the Mt. Alid area. The elevation of the survey area
is generally 100–200 metres above sea level (m a.sl.) and the soundings on top of Mt.
Alid are at around 600 m a.sl. (Table 1 in Appendix 1).

‐ 22 ‐

Figure 17. Resistivity map at 100 metres below sea level.
At 100 m b.sl. (Figure 17) a 1 Ωm low resistivity appears NW of Mt. Alid, which
expands at deeper levels but is not observed below Mt. Alid. At 500 m.b.sl. (Figure 18)
a high resistivity body appears to cut Mt. Alid in a WWS‐NEE direction perpendicular
to the rift system.

Figure 18. Resistivity map at 500 metres below sea level.
‐ 23 ‐

Figure 19. Resistivity map at 1000 metres below sea level.

At 1000 m.b.sl. (Figure 19) the low resistivity west and north of Mt. Alid has
disappeared and a clear SW‐NE resistivity boundary is observed under the southernpart of the mountain. This boundary extends down to about 2000 metres b.sl. Belowthat a low resistivity appears again to the northwest of the mountain, but south andsouthwest of it, the low resistivity continues to deeper levels. Again no low resistivity
is seen below the mountain.

‐ 24 ‐

Figure 20. Resistivity map at 3500 metres below sea level.

At 3500 m.b.sl. (Figure 20) a clear low resistivity NNW‐SSE body is seen west of themountain and connected to the broader low resistivity to the south. Note that at thisdepth the resistivity is lowest at the base of the Mt. Alid slopes, SW of it, where theresistivity gets as low as 0.3 Ωm. There appears to be a sharp resistivity boundary in
NNW‐SSE direction under the western part of the mountain that extends as deep as theresolution of the soundings. This boundary is better seen in the resistivity crosssections.
Below 3500 m b.sl. the resistivity structure is similar. The low resistivity body west of
the mountain widens, and a clear low resistivity region is observed at the base of themountain southwest of it (see Figures in Appendix 6).

‐ 25 ‐

5.2 Resistivity cross sections

Figure 21. Location of cross sections.

Figure 21 shows the location of the cross sections shown on Figures 22 through 37. On
all the figures no resistivity is shown below the estimated depth of resolution for eachsounding. The name of the WE and SN sections are derived from the north and east
UTM coordinates.
One 25 km long SE‐NW resistivity cross section is presented, as well as 8 cross sections
bearing west‐east direction and 7 cross sections bearing north‐south direction.

‐ 26 ‐

Figure 22. A resistivity cross section through the area, with NW‐SE direction.

‐ 27 ‐

The long SE‐NW resistivity cross section is shown on Figure 22. Its location is west of Mt.
Alid (see Fig. 20). The southernmost site is the base station. All soundings with the namestarting with ALI show the result from the TEM measurements only and therefore they havea shallow depth penetration. There was not enough time also to measure MT soundings at
those locations. The data clearly show a conductive shallow layer through the whole area ata few hundred metres depth. The sharp vertical resistivity boundary between sites 018 and
010, at the depth of 1–2 km b.sl. is very clear. South of this boundary the shallow low
resistive layer adjoins to the deeper low resistivity body, seen at 1.5–2.5. km b.sl.. The factthat this deeper low resistivity body is seen in the southern most sounding (000),
approximately 10 km away from the mountain, suggests that it exist over a very large area.
However, its conductivity is somewhat lower there, than in the vicinity of Mt. Alid.
The next 8 cross sections have a west‐east direction and they are shown on Figures 23
through 30. The southernmost cross section (Figure 23) shows clearly the extent of the large
low resistivity body to the south and south west of the mountain. Sections WE1641 andWE1642 , (Figures 24 and 25) show that the low resistivity area is bounded to the west.
Section WE1643 (Figure 26) has no soundings in the middle and therefore resistivity values
are not shown at depth there. This sections shows two low resistivity layers, one at shallow
depth (~200 m. b. sl.) and another at 2–3.5 km b. sl. It is clear that this section is located in ahighly 3‐D environment and therefore the deeper part is badly resolved with the 1‐Dinterpretation technique used here. Sections WE1644 and WE1645 (Figures 27 and 28) clearlyshow the low resistivity body west of Mt. Alid. There appears to be a very sharp verticalboundary running NS under the western part of the mountain. The next two sections,
WE1646 and WE1647 (Figures 29 and 30), show that the low resistivity body is not as clear to
the north, and is clearly bounded in the west.
The SN oriented sections are shown on Figures 31 trough 37. Some of those sections are very
short. All these sections show the vertical EW oriented resistivity boundary which runs
through the southern part of Mt. Alid. Sections SN594, SN595 and SN596 (Figures 32–34),
show that the NS oriented low resistivity body west of the mountain is not seen at thenorthernmost sites.

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Figure 23. Cross section WE1640.

Figure 24. Cross section WE1641.

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Figure 25. Cross section WE1642.

Figure 26. Cross section WE1643.

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Figure 27. Cross section WE1644.

Figure 28. Cross section WE1645.

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Figure 29. Cross section WE1646.

Figure 30. Cross section WE1647.

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Figure 31. Cross section SN593.

Figure 32. Cross section SN594.

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Figure 33. Cross section SN595.

Figure 34. Cross section SN596.

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Figure 35. Cross section SN597.

Figure 36. Cross section SN598.

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Figure 37. Cross section SN599.

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6 Conclusions and recommendations

Figure 38. The Yellow line shows the location of the vertical resistivity boundary between ½ and 2
km depth. The brown contour lines outline the low resistivity body west of Mt. Alid at about 2
km depth. Red dots are geothermal vents on Mt. Alid (data from Geological survey of Eritrea),
and the red polygon is the area of enhanced vegetation discussed in the text.

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The resistivity structures of the Alid area are characterized by the following main features:¾ A thin low resistivity layer. A shallow (at a few hundred metres depth) and thin(few hundred metres) low resistivity layer is seen in most of the soundings, except in
those soundings on the top of Mt. Alid.
¾ A SW‐NE Lineament. A conductive zone is seen down to about 6–7 km depth (andeven more in some places) in the south and southwest of Mt. Alid. This zone has a
sharp vertical boundary or a lineament in the depth interval from ½–2 km depth
shown by a yellow line on Figure 38. This boundary is best seen on the iso‐resistivity
map at 1 km b.sl. on Figure 19.
¾ A low resistivity body. Below the western part of Mt Alid and to the west of themountain there is a low resistivity body, approximately 3 km wide and with a NNW‐
SSE direction. It reaches the highest elevation at 2–3 km b.sl., and extending down toa depth of about 7 km. The location of this body is shown on Figure 38, marked witha brown contour line.
¾ Beneath most of Mt Alid there is a rather high resistivity, compared to thesurroundings, and no deep conductor, except in the westernmost sounding on themountain.

Figure 39. An oases in the middle of the dry volcanic landscape. Most likely the cause of geothermal
activity below the surface (photo: Hjálmar Eysteinsson).

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How can these resistivity structures relate to the geothermal system?
The thin low resistivity layer seen down to 1000 metres depth is interpreted as the old
conductive sea bed.
According to Lowenstern et al. (1999), geological and geochemical studies imply that a hightemperature geothermal system underlies the Alid volcanic center. The resistivity survey
does not cover enough area to compare the resistivity structure beneath Mt. Alid to thestructures surrounding it. The high resistivity beneath Mt. Alid does not stand out as ananomaly, as it is not bounded to the east and north‐east in lack of soundings. Until we see, if
the high resistivity represents a resistive body in low resistivity surroundings we can´tconclude that this high resistivity represents the geothermal heat source.
If this, however, were the case, i.e. the high resistivity under Mt. Alid represented the heatsource, one would expect it to be imbedded in low resistivity surroundings. From the resultsof this survey we see that the high resistivity has a very sharp SW‐NE boundary with the low
resistivity on the southern side. This boundary forms a clear lineament extending from thedepth 500m .b.sl. down to 2000m b.sl. From the depth of 3500 down as far as can be detected
there is a distinctive low resistivity body south‐west of Mt. Alid. These low resistivity bodies
to the south and west of the mountain could well be the part of a greater area surrounding
Mt. Alid, but as of now, we do not have information on the resistivity to the east andnortheast.
The lineament mentioned is sure to play a major role in the geothermal system. It most likely
represents a SW‐NE fault or a fault system that cuts under the southern part of the mountain.
According to Clynne et al. (2005), some fumaroles on the top of the mountain, form weak
N45°E alignments, the same direction as the lineament. The major axis of the mountain itself
has this direction. A geological map, in the paper of Clynne et al., shows a shift in the fissure
swarm through Mt Alid, where it intersects the NE‐SW lineament. This could possibly
indicate that the SW‐NE lineament represents a transform fault underlying Mt. Alid.
At the SW slopes of Mt. Alid, an area, the size of about 1 km2, shows quite a different
character in vegetation than the surrounding dry volcanic landscape (see Figure 38). The
growth of the vegetation is clearly influenced by water or moisture in the ground. It isreasonable to expect that this is due to steam, coming from the geothermal system below.
This area is shown on Figure 38, marked by a red polygon. The area coincides with the NE‐
SW lineament, marked as a yellow line on the figure. This supports the idea that thetransform fault or connected fault system takes part in, or controls the upflow into thegeothermal system.
With the information available on the resistivity structures, it is more difficult to explain the
deep seated low resistivity body below the western part and west of Mt. Alid (brown
contour lines on Figure 38). It is reasonable to assume that resistivity at such depths is
influenced by high temperatures. It is also interesting to observe that the vegetated area isright above the top of the low resistivity body. However, this distinctive low resistivity bodyhas to be seen in a greater context to be better understood. The main features are marked on
an aerial photo of the study area from Google‐Earth (http://earth.google.com) in figure 40.

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Figure 40. An aerial photo of Mt. Alid and part of the Danakil depression from Google‐earth. The
main features from the resistivity survey are shown as: a yellow line (the transform fault) andthe brown contour lines (the deep seated low‐resistivity body). The red polygon outlines thevegetated area and red dots show geothermal manifestations.

Recommendations of further studies:
While surveying the area, the geophysicists came across steam vents/springs not present on
any maps. Some of them were found by the direction of the local people.
¾ A careful mapping of all geothermal surface manifestations is recommended withdirections from the local people. This could give invaluable information on thegeothermal activity in the area and the size of the geothermal field.
The original survey plan covered Mt. Alid and nearest vicinity. It was however not possible
to survey the area to the northeast and north of the mountain within the given timeframe.
The terrain also turned out to be difficult, at times inaccessible but some parts of the lava
could possibly be accessed with the aid of camels for the transportation of the equipment.
¾ It is recommended to explore if further resistivity surveying is possible to the NE of
the present survey area. It is also recommended to finish the MT survey in the long
NW‐SE profile where only TEM soundings are present.

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Quite much of the MT data in this study shows a high degree of three‐dimensionality.
Therefore a 3‐D modeling of the area is called for to fully explain the data. Such modelingmight well change the resistivity picture given in this report, how much is impossible to say
until the modeling has been done.
¾ It is recommended that the data should be interpreted with a 3‐D modeling program.

A resistivity profile across the rift depression, further away from the Alid area would givevaluable information on the resistivity differences between areas far away from geothermal
activity and close to it, i.e. what resistivity structure is normal and what caused bygeothermal activity.
¾ It is recommended to extend the soundings to a profile across Danakil depression

As tectonics play the major role in geothermal activity it is valuable to see if tectonic
movements take place in the Mt. Alid area, and map the seismically active structures.
¾ A seismicity survey is recommended.

A gravity Bouguer map would be a valuable tool to evaluate density difference in the upper
part of the crust. Intrusive bodies are likely to be heat source for the geothermal system andcould be identified with gravity mapping.
¾ A gravity survey is recommended, especially if a full resistivity survey turns out to be
impossible

Iceland GeoSurvey would like to extend its gratitude to the Geological Survey of Eritrea
for their cooperation in this project. This was a hard work, often challenging due to thedifficult terrain and the short time frame. Without their cooperation this project would nothave been possible to implement.

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7 References
Barberi F. and Varet Jacques (1977). Volcanism of Afar: Small‐scale plate tectonic implications.
Geological Society of America Bulletin. v 88. p 1251‐1266.
Clynne, M. A., Duffield, W. A., Fournier, R. O., Woldegiorgis, L., Janik, C. J., Kahsai, G.,
Lowenstern, J. B., Weldemariam, K., Smith, J. G. and Tesfai, T. (2005). Proceedings World
Geothermal Congress 2005, Antalya, Turkey. 24.–29. April 2005.
Ingham, M. R. (1988). The use of invariant impedances in magnetotelluric interpretation.
Geophysical Journal, 92, 165–169.
Lowenstern, J. B., Janik, C. J., Fournier, R. O., Tesfai, T., Duffield, W. A., Clynne, M. A.,
Smith, J. G., Woldegiorgis, L., Weldemariam, K. and Kahsai, G. (1999). A geochemicalreconnaissance of the Alid volcanic center and the geothermal system, Danakil depression, Eritrea.
Geothermics. p 161–187.
Park, S. K. and Livelybrook, D. W. (1989). Quantitative interpretation of rotationallyinvariant parameters in magnetotellurics. Geophysics, 54, 1483–1490.
Rangabayaki, R. P. (1984). An interpretive analyses of magnetotelluric data. Geophysics 49,
1730–1748.
Sternberg, B. K., Washburne, J. C. and Pellerin, L. (1988). Correction for the static shift inmagnetotelluric, using transient electromagnetic soundings. Geophysics, 53, 1459–1468.

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