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Istanbul International Geophysical Conference and Oil & Gas Exhibition, Istanbul,Turkey, 17-19 September 2012.

Magnetic studies for geothermal exploration in Mahallat, Iran

M. Mohammadzadeh-Moghaddam*, M.Sc. student, Institute of geophysics, University of Tehran, Iran.

B. Oskooi, Assistant professor, Institute of geophysics, University of Tehran, Iran.M. Mirzaei, Associate professor, Department of physics, Faculty of sciences, Arak University, Iran.S. Jazayeri Jouneghani, M. sc. Institute of geophysics, University of Tehran, Iran.

Summary

Ground magnetic survey was carried out in Mahallat areain center of Iran as the primary part of explorations and

development of geothermal energy investigation program

conducted by Arak and Tehran universities. The data hadbeen gathered on more than 4000 stations over 250square kilometers of ground surface. Magnetic survey

was applied for reconnaissance of high potentialgeothermal resources. The aspects of geothermal activityin the area can easily be found in the form of hot springs,hydrothermal deposits, thermal alteration, vast travertineoutcrops and youngvolcanic rocks. This study shows that

the deep source of geothermal activities had created abroad magnetic anomaly whithin the depths of more than

1200 meter below the surface. Based on the geology ofthe region, it seems that deep hot igneous rocks form the

heating source of geothermal system of the region. Thewater moves down into the earth through cracks andfaults and gets hot by moving across hot igneous rocksand again through the same way comes up and reaches to

the surface and forms the hot springs of the region.

The gravimetry, resistivity and MT surveys and alsoexploratory drillings over in the area are suggested for thefuture assessment of geothermal energy in the region on

an industrial and scientific scale.

1. Introduction

The magnetic method has come into use for identifyingand locating masses of hot igneous rocks that haverelatively high concentrations of magnetite. The magneticrocks include basalt and gabbro, granite, granodiorite and

rhyolite have only moderately high magneticsusceptibilities. The magnetic method is useful inmapping near-surface volcanic rocks that are kind ofinterest rocks in geothermal exploration (Zhdanov, 1994).

In the USA, for example, a ground magnetic survey overthe Coso volcanic field in California recorded low

anomalies associated with the geothermal prospect(Roquemore, 1984).It is known that an area may have a geothermal power

deposition only if the following four main factors occurat the same place simultaneously:

1. A natural heat source of great output such as coolingmagma,

2. An adequate water supply,3. An aquifer or permeable reservoir,4. An impermeable cap rock.

In Iran, there are many areas with such properties

mentioned above.Mahallat is one of the most important

examples of such regions which was identified by therenewable energy organization of Iran in 1998. This is atoo broad area (about 51000 km2), extending across some

parts of Qom, Markazi and Isfahan provinces, which has

been shown in figure 1 in the red rhombic shape. As it isshown in the figure, it is a very broad area and thereforeit is not exactly clear where does the geothermal potentialsource exists. However, the areas in vicinity of the hot

springs of Mahallat have the following characteristics:

hot springs, young volcanic rocks, altered zones, widetravertine outcrop and intrusive masses. Based on thesefacts, it seems that the area has a high geothermic

potential and conducting magnetic researches is verysignificant for exploring and obtaining more knowledgefor the area (Mohammadzadeh-Moghaddam, 2011).

In this study, the Euler deconvolution method has beenapplied in order to estimate the depth of magneticanomalies. We also, use of Mag3D software to create theexisting magnetic model.

Figure 1. Distribution map of potential areas for geothermal

resources in Iran (renewable energy organization of Iran, 1998).2. Methods

2.1 Magnetometry

The magnetometric study of igneous intrusions isthoroughly linked to the definition of the vectorcomponents of the magnetic field (Blakely, 1995):

MT = MI + MR (1)

being MT the vector of total magnetization, MI the vectorof induced magnetization and MRthe vector of remanent

magnetization. The importance of this definition comesfrom the magnetic processing techniques dependence on

the direction of the field.In order to locating the potential geothermal areasaccurately, a ground magnetic survey across 10 long

profile of about 16 km with a station spacing of 40 meter

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and profile spacing of 1/5 km was conducted in areasaround hot springs of Mahallat. This project wasconducted in 2011 by University of Tehran and Arak

University. Finally more than 4000 magnetic stationswere collected. The investigated area in this project isabout 250 square kilometers (the black colored region inFigure 1).

2.2 Reduction of Magnetic Anomalies

The gridded map of the observed magnetic field of theMahallat geothermal area reduced for diurnal variationover the Mahallat geothermal area is shown in Figure 2.

To obtain magnetic anomalies associated with localmagnetic variations of rocks, it is necessary to remove the

normal geomagnetic field of the Mahallat geothermalarea from the data. The normal geomagnetic field used

for the reduction was the International Geomagnetic

Reference Field (IGRF) computed using the programGEOMAG (National Geophysical Data Center, 2010) for

2011 data collecting. After the reduction of magneticanomalies a reduced to the pole method was used. The

mathematic procedure is carried on a grid written from acontour map, similar to that of contouring procedures,from observed values (not a grid). The anomaly depends

on its magnetic latitude and the corresponding variation

of the dip angle of the magnetization vector in the body.The resulting magnetic anomaly is represented in Figure2. The magnetic anomalies are shown by color shaded

grid.

Figure 2. Residual magnetic field of the study area.

2.3 Euler Method

3D form of Euler's equation can be defined (Reid, 1990)as

Where , and are the derivatives of the field in

the x, y, and zdirections, is the structural index value

that needs to be chosen according to a prior knowledge ofthe source geometry. By considering four or moreneighboring observations at a time (an operated window),

source location (x0,y0, and z0) and b can be computed bysolving a linear system of equations generated fromequation 2. Then by moving the operated window fromone location to the next over the anomaly, multiplesolutions for the same source are obtained.

The Euler method has been applied to the residual data

using a moving window of 400 m x 400 m. We haveassigned several structural index values and found that

structural index of 1.7 gives the best clustering solutions.Figure 3 shows the results of the Euler method from the

residual data. The Euler solutions (Figure 3) indicatesthat the main anomaly in the region has the depth of 1000meters below the surface in the western side to 2000

meters in the easthern side. The mean depth is 1500

meters, which extends toward the north east. Also thestructural index obtained by Euler method shows that themain anomaly in this area is like a cone and that extendedmore in depths. Based on the geological information of

the area, it seems to be caused by deep hot granodiorite

rocks into the earth. The vast outcrops can be confirmedby the large outcrops of granodiorite rocks in the northernpart of the region.

Figure 3. Euler solutions from the residual magnetic data of theMahllat geothermal area.

2.4 Three Dimensional Modeling

The three dimensional program is a compilation of

algorithms for inverting magnetic data gathered in threedimensions. This program has greater flexibility and

efficiency than the basic inversion algorithms. Inparticular it allows larger problems to be solved through

the use of wavelet transforms, and it allows geophysicalconstraints, in the form of upper and lower bounds on thedensity of each cell, to be included.

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Figure 4. Three dimensional body facing north-east.

Figure 5. Three dimensional body facing south-west.

Figure 6. Three dimensional body facing to south-west, cut at

Northing 3774300.

Figure 7. Three dimensional body facing to north-east, cut at

E: 466400, N: 3774300

2.4.1 Model Analysis

The data in Figure 1 show that positive magnetic

anomalies are dominant in the Mahallat geothermal area.Magnetic investigations over the Mahallat area have

shown that magnetic anomalies may be caused by highsusceptible rocks, such as diorite, granodiorite or basalticrocks (Mohammadzadeh-Moghaddam, 2011).

It seems that the content of magnetic materials of igneousrocks that cause this sharp anomaly, like magnetite, in

deep increase and have higher susceptibility than shallowrocks. Based on the information obtained from the region,it seems that the temperature of these igneous materials isas enough as to be considered as the geothermal system

heat source of Mahallat. In fact the water moves downinto the earth through cracks and faults and gets hot bymoving across hot igneous rocks and again through the

same way comes up and forms the hot springs of theregion.

Figure 4, shows the 3D model of the studied anomaly inthe area. The body faces north-east. It shows that the high

susceptible area (red color) should occur in the northern,

eastern and western part of study area. Figure 5, showsthe 3D model of the study area faces south-west. Figure6, shows the model facing north-west, cut at Northing

3774300. It shows that the high susceptible area (redcolored) spreads out over the central parts of study area.Figure 7, shows the model of the study area facing north-

east, E: 466400, N: 3774300. It shows the high

susceptible area (red color) surrounding the hot springs ofnorth-east of Mahallat city.

3. Conceptual model of study area

Based on the available information about the Mahallatgeothermal area, the conceptual model of exicting

geothermal source was obtained. We drawn a north-southschematic section from the geothermal area of the region

(Section A-A' in figure 2).Figure 8 shows the conceptual model of Mahallatgeothermal structures. As it can be seen, the intrusivemasses exist in deep detected by ground magnetic data

and based on the geological information of the region andplays the role of a heating source.

Travertine can be seen in different parts of Mahallat, but

in areas around hot springs, their thickness and mass ishigher. A lot of faults can be seen in this model, although

their slopes and directions are not clear yet. It should benoted that the maximum topography difference between

the lowest and the highest point in this section is about400 meter.

The presented Mahallat geothermal source conceptual

model is just an initial one and can become updated andmore accurate, by next geological, geochemical,

geophysical surveys and exploratory drillings.

4. Conclusion

According to figure 3, a wide anomaly can be seen

clearly in the region. Based on the Euler deconvoltiondepth estimation results, the major anomaly in the center

of the area has at least 1 km depth and the best structuralindex for it has been obtained to be 1.7.

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Also the 3D inversioned model of the data showes that

the susceptibility of this magnetic mass increases bymoving through the depth. Based on geological data and

also the schematic section in figure 8, the main source ofpositive magnetic anomaly in the region is related to anintrusive igneous mass which actually forms the source ofgeothermal system of the region.

The broad travertine's masses and its high thickness in theregion probably suggest that the geothermal system of theregion is very old. In other words, the geothermal source

in this system has been a big intrusive igneous masswhich is passing the last of its cooling stages and the

Mahallat hot springs are related to the last cooling stagesof magma in the region.

Acknolegments

The authors wish to thank Dr. L. Namaki for manyhelpful discussion and who contributed to the several

stages of the field and office work.

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Figure 8. Conceptual model of study area (Section A-A' in figure 1).

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