INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF GEOPHYSICS AND ENGINEERING

J. Geophys. Eng. 3 (2006) 114–121 doi:10.1088/1742-2132/3/2/002

Exploration of a geothermal reservoirusing geoelectrical resistivity inversion:case study at Hammam Mousa, Sinai,EgyptGad El-Qady

National Research Institute of Astronomy and Geophysics (NRIAG), 11722 Helwan, Cairo, Egypt

E-mail: gadosan@yahoo.com

Received 13 September 2005Accepted for publication 21 February 2006Published 21 March 2006Online at stacks.iop.org/JGE/3/114

AbstractGeoelectrical resistivity is a pioneer geophysical technique used in geothermal exploration.With the advent of computing technology, it has become convenient to apply sophisticateddata analysis and inversion to geoelectrical resistivity field data. In this work, a geoelectricalresistivity survey was conducted in the Hammam Mousa area to explore the geothermalresources and groundwater aquifer. The survey comprises 19 vertical electrical soundings(VES) using the well-known Schlumberger array with AB/2 up to 1000 m. Interpretation ofone-dimensional (1D) inversion gave a layered-earth resistivity model using a nonlinearleast-squares method. However, some resistivity sections of the 1D inversion were not fullyresolved for the complicated geologic structure. Therefore, we carried out a two-dimensional(2D) inversion based on the ABIC least-squares method for the same data set. The generaldistribution of resistivity shows a very low value near Hammam Mousa Hot Spring. The 2Dresistivity cross section clearly elaborates the subsurface structure in the spring area and itelucidates and gives an explanation for the hot water source in the area. It is concluded that thehydrothermal system in the Hammam Mousa area is adequately delineated from the 1D and2D inversions of vertical electric sounding data using a Schlumberger electrode array.Accordingly, a proposal for geothermal drilling in the study area is recommended.

Keywords: geothermal, resistivity, 2D inversion, Hammam Mousa, Sinai

1. Introduction

Geophysical, mainly geoelectrical, methods are frequentlyemployed in exploration for geothermal resources.Geoelectrical methods, in particular, have been employedin the study of most geothermal fields. Reviews as well assome characteristic examples are discussed by Thanassoulas(1991). High temperatures and hot thermal fluid circulationin geothermal systems have a severe impact on the electricalproperties of geologic formations encountered in the areas.The use of these methods is justified from the fact that theelectrical conductivity of ionic conductors greatly increaseswith temperature. The conductivity of the host rock of

the geothermal field increases due to wall-rock alterationand hydrothermal mineral deposition in fracture zones. Onthe other hand, thermoelectric and electrokinetic couplingmechanisms generate self-potential anomalies of several tensof millivolts over geothermal fluid flow paths (fracture zonesin the basement rock). A large decrease in resistivity of therocks is observed due to saline hot waters that circulate in thepermeable paths. Consequently, geothermal and geoelectricalmethods are probably the most useful and widely usedin geothermal research. Several case studies for locatinggeothermal aquifers using dc resistivity are discussed, forexample Flovenz and Georgsson (1982), Thanassoulas et al(1987), Majumdar et al (2000) and El-Qady et al (2000).

1742-2132/06/020114+08$30.00 © 2006 Nanjing Institute of Geophysical Prospecting Printed in the UK 114

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Gulf

ofSuez

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VES’s stations in N-S spread direction.VES’s stations in E-W spread direction.

Shallow borehole

Figure 1. Location map for the study area, VES stations and surroundings. Topographic contours in metres.

The Sinai Peninsula is both a bridge and barrier betweenthe Asian and African continents. The traditional Sinaieconomy, based on fishing and trading in small coastal townsand nomadic herding by the Bedouin of the interior, is inprofound change. Recently, new projects to settle the Bedouinhave begun, and development is transforming several coastalareas. Tectonically, the Sinai area is considered as an unstableshelf due to frequent earthquake activity and its geologicsetting, which is controlled by tectonic activity at the RedSea, Gulf of Suez and Gulf of Aqaba (Said 1962). Thistectonic activity has been accompanied by thermal activityrepresented by a cluster of thermal surface manifestationsalong the eastern shore of the Gulf of Suez. Among thesethermal manifestations, Hammam Mousa (Moses’s Bath) isone of the best known hot springs along the Gulf of Suez, atEl-Tor City, the capital of south Sinai (figure 1).

Consequently, the main goal of this study is to investigatethe geothermal field and groundwater aquifer at HammamMousa Hot Spring using a geoelectrical resistivity survey. Inthis study, we have carried out one-dimensional (1D) and two-dimensional (2D) inversion based on the least-squares methodfor a Schlumberger VES data set measured in the HammamMousa area.

2. Geologic context

The geology of the Sinai Peninsula is complicated andrepresents almost all of geologic time. Many authors havestudied the geology of Sinai (for example, Said 1962, 1990,

Hume 1912, 1965, Omara 1956, 1965, Ghorab and Marzouk1967, Beadnell 1927). The Hammam Mousa area is locatedon the eastern shore of the Gulf of Suez (figure 1). The surfacearea near the hot spring is composed of sabkha deposits.Far to the east, alluvial deposits dominate and occupy thesurface of the El-Qaa plain. Figure 2 shows the surfacegeology of the study area and its surroundings. The subsurfacegeologic section is represented by Cretaceous (Campanian toCenomanian) up to Miocene rocks (Said 1962). During theEarly Cretaceous, the study area was a shallow sea, and sandysediment of the Nubian facies is represented. This is overlainby Cenomanian beds, which are brownish and varicolouredmarls, with a clastic content of sand and shale (Kostandi 1959,Said 1961). Above the Cenomanian beds rest lower Turonianbeds represented by the Rudayes Formation of soft marl andshale (Beadnell 1927). Lower Eocene beds of limestone withflint and marls of Thebes formation rest with unconformityabove the Rudayes Formation. This is followed by the EsnaFormation of the upper Paleocene. The Esna Formation iscomposed of laminated green and grey shale (Said 1962).At the top, Quaternary sediments are represented by clasticsknown as the El-Tor Group. They consist mainly of gravel,sand, clay and silt.

During the early Tertiary Period (Oligocene to Miocene),at the opening of the Red Sea Rift, some volcanic activitytook place. In western and central Sinai, there are manybasaltic bodies mostly of doleritic dykes, sills, plugs and flows(Meneisy 1990). Also, to this episode of deformation belongsthe great syncline area of the El-Qaa Plain that lies to the east

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Figure 2. Geologic map for the study area and its surroundings (modified after Geological Survey of Egypt 1994).

of our study area. Similarly, a series of anticlines and synclineslies on both sides of the Gulf of Suez. The major structuralfeatures are well-defined NNW trending fault blocks, which tiltstrongly eastward on their west side (El-Shinnawi and Sultan1973, Said 1962).

3. Geothermal regime

At a distance of 100 km from Sharm El-Sheikh, Moses’sBath (Hammam Mousa) is just 3 km from the centre ofEl Tor City. Basically, it has five springs categorized intotwo groups. The first group issues into a bathhouse witha temperature of 33 ◦C, and the second, nearby, flows intoa trench with a temperature of 31 ◦C. For a long time,the springs’ water has been considered highly effective intreating skin diseases and healing wounds especially resultingfrom diabetes. Additionally, it helps in reducing tension andincreasing relaxation; so it has been used for tourist purposesfor many years.

Compositionally, the Hammam Mousa thermal watersseem to be Tiberias–Faraun waters that dissolved additionalsalts from the Neogene rocks, being enriched in SO4 andHCO3 along with Mg and Ca. The relative abundances ofthe dissolved ions in the water of the two spring groups arealmost identical, the bathhouse springs being a little saltier(Mazor et al 1973, Sturchio et al 1996).

The heat source for these springs is probably derivedfrom high heat flow and deep circulation controlled by faults

associated with the opening of the Red Sea and Gulf of SuezRifts. The spring’s water is presumably a mixture of brine andwater contained in the Nubian sandstone aquifer that infiltratedoutcrops on the highland of Sinai and emerged along the faultlines bordering the Sinai Peninsula. However, the springs issuefrom Neogene rocks (Magaritz and Issar 1973).

4. Geophysical exploration

The geophysical survey described in this work was carriedout by dc resistivity sounding using a Schlumberger array.Nineteen VES stations were measured (figure 1) using anelectrode spacing starting from AB/2 = 2 up to 1000 m, insuccessive steps. The VES sites were chosen according to theaccessibility and applicability of the Schlumberger method.With (2D) processing in mind, the Schlumberger line wasoriented parallel to the profile direction. Apparent resistivityslice maps at different AB/2 values are illustrated in figure 3.The apparent resistivity has relatively high values in theshallow parts (AB/2 = 4 and 20 metres respectively) as wellas in deeper parts (AB/2 = 1000 m). This can be due to theeffect of a surface layer of gravel at the top and the geoelectricalbasement below. Relatively low resistivity values characterizethe central and southwestern parts of the study area. This canbe due to either the effect of geothermal water circulation orthe effect of seawater intrusion into the area. The field curvesof all the stations have been inverted one-dimensionally, aswill be explained in the following section.

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Figure 3. Horizontal slice maps of apparent resistivity (� m) at different AB/2. AB/2 = 4, 20, 120, 400, 800 and 1000, for images (a)through ( f ), respectively.

4.1. One-dimensional interpretation

A 1D interpretation using the least-squares method hasbeen conducted (Zohdy 1989). A general overview of theinterpreted VES curves reveals that the number of interpretedlayers varies from five to eight layers through the study area.The true resistivity of these layers varies from 2.0 to 500 � m,while the thickness varies from 0.9 to 177 m. Figure 4(a)illustrates a correlation between the 1D interpretation ofVES 9 and a lithologic column for a shallow borehole nearby.The true resistivity starts relatively higher due to the surfaceclastics layer and erosion effects. Then, it decreases graduallyfor the 2nd, 3rd and 4th layers, emphasizing the effect ofgroundwater in the shallow water-bearing layer at a depthof 35 m. Figure 4(b) shows the apparent resistivity and thecalculated 1D model for VES 17. The calculated 1D modelcorrelates well with the observed curve. However, at the endof the curve (at large offsets) the curve starts to deviate, as faras the depth increase. This deviation can be due to the effect ofdeep-seated three-dimensional (3D) structure. Although thededuced information from the 1D interpretation was correlatedwith the geologic studies and surface thermal manifestations,

it is still not fully understood for the 3D geologic structure. Toget a realistic solution, we have applied 2D inversion for thesame data set, as discussed in the following section.

4.2. Two-dimensional interpretation

In this work, we present a 2D inversion for the same data setusing Uchida’s algorithm (El-Qady et al 1999). This algorithmis based on the ABIC (Akaike Bayesian information criterion)to converge to optimum smoothness using a finite elementcalculation mesh (Akaike 1980). The algorithm considers a2D Earth model, whose resistivity varies along the x and z axeswhile it does not change along the y axis. In as much as thecurrent is injected at a point on the surface, however, it flowsthree-dimensionally in the Earth; the response in a 2D Earthis given by Poisson’s equation as

−∇ [σ(x, z)∇V (x, y, z)] = I (x, y, z), (1)

where σ(x, z) is the conductivity, V (x, y, z) is the electricpotential, and I (x, y, z) represents the source current intensity.By applying the Fourier transform to equation (1) with respect

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(b)

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Figure 4. Correlation of VES interpretation with the boreholeinformation. (a) Inverted 1D model of VES 9 with the lithologiclog. (b) Apparent and calculated 1D resistivity model of VES 17.

to the y coordinate, we obtain

−∇[σ(x, z)∇V (x, ky, z)] + k2yσ (x, z)V (x, ky, z)

= I (x, ky, z), (2)

where ˆ is the Fourier transform and ky is the Fourier transformvariable. Then, applying the inverse Fourier transform,

�V (x, 0, z) = (1/π)

∫ ∞

0V (x, ky, z) dky, (3)

and the apparent resistivity for Schlumberger can be calculatedas

ρa = G�V

I, (4)

where G is a geometrical factor that depends on the electrodearrangement and their spacing, and �V is the calculatedpotential difference between the receiving electrodes M and N.A detailed explanation of the finite-element discretization ofequation (2) is given in Sasaki (1981). Further details about thealgorithm and convergence criteria can be found in El-Qadyet al (1999).

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4.2.1. Inversion results. For the least-squares inversion withsmoothness regularization, we seek a model that minimizesboth the data misfit and model roughness. From a statisticalpoint of view, ABIC works as an index to determine themaximum likelihood of the model. This means that a smallerABIC indicates a larger likelihood and higher entropy, hencegives a best-fit model. This also means that the optimumsmoothness is judged by minimizing ABIC, which makesthe convergence; however, the selection of the optimumsmoothness is objective.

Figure 5 shows the RMS misfit and the smoothing factor(α) as a function of the iteration number for some profiles. Infigure 5(a), for line 1, the RMS clearly attains a minimum afterthe second iteration, while the smoothing factor (α) attains it atthe third iteration. For line 3, α attains a minimum at the fifthiteration (figure 5(b)), and it does so at the third iteration forline 6 (figure 5(c)), and fifth iteration for line 7 (figure 5(d)).

In addition, to establish the degree of fitting for eachsounding, the measured field resistivity curves have beencorrelated with the calculated model response. Figure 6 showsthe correlation between the observed data and the calculatedresponse from a 2D inversion for the same station. Generally,for most VES data, the fitting is acceptable, whereas there is amisfit at small and large offsets of the curve. This may resultfrom the effects of the arid conditions at the surface and fromdeep-seated 3D structures, respectively. Figure 6(a) illustratesthe correlation of the calculated curve for VES 7 from the2D inversion of the same VES on two different profiles. Thedifference is due to the inversion conditions for each profile

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Figure 6. Correlation between the observed data and the calculated response of 2D inversion for some stations.

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as well as neighbouring stations along the profile. Even so,both curves have a good correlation with each other and to theobserved curve. Accordingly, the best model for each profileis obtained and then displayed in 2D cross section form. Thiswill be explained in the following section.

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Figure 8. 2D geoelectrical cross section along line 8.

4.2.2. 2D cross section. According to the results obtainedthrough the inversion process, we could construct the 2Dgeoelectrical cross section for each profile. The cross sectionrepresents the model of iteration that minimizes ABIC andgets convergence. Figure 7 shows the 2D cross section of the

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Figure 9. Integrated 2D geoelectrical cross sections for the Hammam Mousa Hot Spring area.

inverted model after the fifth iteration for line 1. The initialmodel is assumed to be a 30 � m homogeneous Earth, and thetopography is incorporated into the modelling. The number ofthe observed data sets used for the inversion is 140, while thenumber of resistivity blocks is 66. The general feature of thisinverted section is a quite thick (up to 90 m) low resistive bodyin the northwestern part of the profile (VES 1). That might becorrelated with the effect of the thermal water circulation inthis place, where it issues on the surface at the hot spring. Thislow-resistivity layer extends along the shallower part of thesection, but with smaller thickness. This layer, which can becorrelated with the surface sabkha deposits, meanwhile mayreflect seawater intrusion in the area.

Figure 8 illustrates the 2D cross section along line 4.This line extends N–S across the area, and it showed moreinteresting geoelectrical features. There is relatively highresistivity in the shallow part of the section at VES 13. Thishigh resistivity might be due to the surface clastic depositsof the El-Tor Group that dominate in that area. At VES 18,

relatively low resistivity values exist. This could be due to thepresence of clay and silt deposits in this part. The presenceof different geoelectrical layers of variable resistivity valuesis mainly governed by the variation of lithology composed ofalternations of sand, silt and clay of the El-Tor Group and thePaleocene deposits. At a depth of nearly 100 m, a geoelectricallayer with resistivity values up to 200 � m was observed.This layer represents a groundwater-bearing formation in theshallow subsurface of the El-Tor area and south of the El-Qaa Plain. At the bottom of the section, there is uplift at theelectrical basement at VES 15 and 13. This suggests that afault system dissects this area.

The integrated 2D geoelectrical cross section for theHammam Mousa area (figure 9) provides valuable informationwhich enables us to configure the subsurface structure of thestudy area. The area seems to be affected by local structuresand major faults, which might have been reactivated afterthe Oligocene–Miocene rifting. The major faults take a

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direction parallel to the Gulf of Suez, while the minor ones areperpendicular.

5. Conclusion

The present work aimed to delineate and elucidate thegeothermal reservoir at Hammam Mousa Hot Spring usingABIC least-squares 1D and 2D inversion of Schlumbergerresistivity soundings measured in the area. In conclusion, theinversion procedure can reduce the misfit through iteration. Inaddition, the 2D calculated response was highly correlatedwith the observed data. The resulting 2D cross sectioncorrelates with the 1D inversion. However, the 2D crosssection elaborates the geologic structure further in the studyarea, which matches previously published geologic studies.

According to the 2D interpretation of this data set,a promising area for geothermal drilling is recommended,around the hot spring and its neighbourhood (VES 1, 2 and 3),where there is considerable aquifer thickness. The 2D crosssection clearly elucidates and gives an explanation for theorigin of the hot water source in the study area, which is adeep circulation of hot water on the fault system.

Although the results correlate well with the availablegeologic information, some of the 2D cross sections stillshow a rough spatial resistivity distribution of abrupt changesof resistivity between adjacent blocks. Hence anotherinterpretation technique (for example, 3D) is recommended toovercome this problem and give a more realistic solution. Inaddition, a detailed geophysical survey is recommended usingdifferent geophysical resistivity tools, such as magnetotelluricand electromagnetic methods, which can overcome theproblem of seawater intrusion and arid condition in this area.

Acknowledgments

The author would like to express his deepest and sincere thanksto the staff of the National Research Institute of Astronomyand Geophysics (NRIAG), Egypt, for the facilities requiredfor data acquisition in this work. Sincere thanks to all thestaff of the Exploration Geophysics Lab of Kyushu Universityfor their continuous guidance and support during this work.Sincere thanks to the anonymous reviewer and to Dr GeorgeMoore of Oregon State University for the valuable commentsthat enhanced the manuscript to the present form.

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