Proceedings World Geothermal Congress 2015

Melbourne, Australia, 19-25 April 2015

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Effective Field Geological Mapping Techniques for Sumatran Geothermal Fields, Indonesia

Lucas D. Setijadji

Department of Geological Engineering, Gadjah Mada University, 2 Grafika Bulaksumur, Yogyakarta 55281 Indonesia

E-mail address: lucasdonny@ugm.ac.id; lucas_donny@yahoo.com

Keywords: geothermal, Sumatra, geology, field mapping

ABSTRACT

Sumatra island is considered to be a potential location to host high-enthalpy geothermal fields in Indonesia. However, despite

recent efforts to accelerate the development of new geothermal fields in this area, the results are not so good. There are some

success stories, such as Sarulla. However, some other projects are now struggling to discover economically feasible reservoirs. It is

possible that the complex geological setting of the Sumatran geothermal fields, due to the influence of the Sumatran Great Fault,

make these fields significantly different from the Java island ones, where a majority of existing producing fields have been

developed in Indonesia. While many geothermal companies rely heavily on geophysical surveys, especially MT, this paper will

discuss the importance of a good geological mapping program at the initial stage of geothermal exploration. Particularly, the focus

will be on the important aspects of mapping techniques that are considered critical, based on the experience in several projects in

Sumatra.

Before the real field mapping program, the prefield stage is important to generate tentative lithological, structural, alteration, and

thermal anomaly maps that can be used for an efficient field work campaign. Public-domain data such as topography (DEM) and

geological maps are not adequate for this purpose. High-resolution DEM data at several meters ground resolution (typically radar)

are needed to be used as base maps for appropriate desk studies on lithological units and structural aspects of the study area. The

identification of monogenic volcanic centres (many are only several hundreds of meters in diameter) is needed as volcanism in the

Sumatra island is present not only as a stratovolcano but also as many monogenic volcanic centres controlled by structures.

Multispectral thematic remote sensing such as Landsat TM have been used in some cases to detect the distribution of clay and

oxide minerals. Surface thermal anomalies can be detected mainly by the night thermal IR ASTER image.

At the field survey stage, efficient field observations should focus on the different facies of volcanic deposits (coherent versus

fragmental), volcanic stratigraphy of different volcanic units, structural geology, and alteration aspects of different units. As there

are many Holocene ash fall deposits in Sumatra that covers the majority of Pleistocene volcanic units, one must observe the

underlying rocks. The youngest ash and other volcanic products need to be mapped to evaluate the current activity of the volcano.

Hydrothermal alteration zones in Sumatra are not always related with currently active geothermal systems, so the identification and

study of the nature of alteration zones, including the observation of quartz vein-float, is required to elucidate which alteration zones

are directly related with the active geothermal system. Structural geology data is typically difficult to interpret at the initial

exploration program, but observations on the distribution of thermal manifestations, dykes, and epithermal veins may serve as a

good indication of the existence of extensional structures. Reliable geological information should then be incorporated in the

integration of other data, especially geochemical and geophysical data, in order to improve the targeting of exploration drilling

sites.

1. INTRODUCTION

Indonesia has the highest geothermal potential in the world, with geothermal fields that are scattered in many islands especially in

Java, Sumatra, Sulawesi, Maluku islands and Nusa Tenggara islands. Although Java is the center of geothermal development in the

country today, the highest potential for high-enthalpy geothermal fields is considered to be located in the Sumatra island. However,

despite recent efforts to accelerate the development of new geothermal fields here, the results have been poor. There are some

success stories, such as Sarulla, but other projects are now struggling to discover economical reservoirs. It seems that the complex

geological setting of Sumatran geothermal fields, due to the influence of the Sumatran Great Fault, make them significantly

different from the Java island cases.

Many geothermal companies working in this island, as well as in other places in the country, rely much on geophysical survey,

especially MT. It also seems to be influenced by the regulation of Indonesia government that each geothermal project must be

evaluated of its resource potency even at the preliminary stage of project. In this case, MT-derived anomalies are the main standard

for such evaluations. As a result, almost all the projects make detailed geophysical surveys from the beginning of the project.

However, this marginalizes the contribution of geological surveys. In this contribution, the author will discuss the importance of

geological mapping programs at the initial stage of geothermal exploration. In particular, the focus is on the important aspects of

mapping techniques that are considered critical, based on experience in several projects in Sumatra.

2. GEOLOGICAL SETTING OF SUMATRA ISLAND

The Sumatra island is the westernmost of five major islands in the Indonesian archipelago, measuring about 1,730 km long in a

NW-SE orientation. This island was built as an active continental margin subduction zone as part of the Cenozoic Sunda arc that

extends from NW tip of Sumatra, Java, Bali, Lombok and Sumbawa islands.

Sumatra was built up by the amalgamation of several continental fragments derived from Gondwanaland since the Late Paleozoic

or Early Mesozoic periods that formed the Sundaland which is the SE margin of Eurasia continent (e.g. Metcalfe, 2006). Since the

Late Mesozoic era, the Sundaland was already a stable craton, which was bound by a subduction zone at the west of Sumatra.

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Subduction formed a convergent tectonic margin between the Indian-Australian plates and the Sundaland (Hamilton, 1979).

Subduction takes place along the Java/Sunda trench that reaches the maximum depth of 6.75 km. The oceanic crust is being

subducted northward, more or less perpendicular, to the Sunda volcanic arc at a rate of 6 to 7 cm/yr (Hamilton, 1979; Simandjuntak

and Barber, 1996). The Benioff seismic zone, representation of the subducted slab, extends to the depth of more than 600 km in

Java but only 200 km in the case of Sumatra (Figure 1). However, tomographic imaging studies suggest that the lithospheric slab

penetrates to a depth of at least 1500 kilometers in all sections (Widiyantoro and Van der Hilst, 1996).

Sumatra is now a part of the Sunda-Banda volcanic arc that extends approximately 3,700 km long, from the northern tip of Sumatra

island through Java to east of Damar island (Hamilton, 1979). This long arc is divided into the Sunda arc (Sumatra-Java-Bali-

Lombok-Sumbawa) and Banda arc for the islands east of Sumbawa. (Fig.1). The basement crust thins eastward, from

approximately 30 km beneath Sumatra to 15 km beneath the Flores Sea (Ben Avraham and Emery, 1973).

Figure 1: Tectonic setting of Sunda Arc, Indonesia (modified after Setijadji, 2005).

3. SUMATRA VS JAVA ARC SEGMENTS

Although the Sumatra and Java islands are part of the same Sunda arc, it is important to highlight the significant differences in

terms of volcanic geology between these islands that affect the different styles of geothermal fields. There are several substantial

differences. Sumatra has a continental basement and its subduction style is an active continental margin. Meanwhile, Java is

underlain by different basement compositions, from a continental basement in West and Central Java, to an island arc crust in East

Java (Fig. 1). In addition, different from other parts of the Sunda Arc, the subduction beneath Sumatra is oblique, at the rate of 50 to

70 mm/yr (Natawidjaja and Triyoso, 2007; Figure 2). As the direct result of this oblique subduction, there is a presence of a great

Sumatran Fault Zone (SFZ) that crosscut the middle of the island entirely. It is an active, northwest-trending, right-lateral strike-slip

fault that accommodates the arc-parallel component of oblique subduction along the Sunda Trench (Figure 2).

The Sumatran fault zone (SFZ) traverses the back-bone of Sumatra, within or near the active volcanic arc (Sieh and Natawidjaja,

2000). Older belts of subduction-related plutonic rocks on Sumatra all have a roughly northwest trend subparallel to the present

magmatic arc, suggesting that the configuration of this part of the subduction system has remained almost the same throughout its

history. The Sumatra Fault Zone is clearly identified by the cluster of shallow earthquakes, and it separates older rocks (mainly

Tertiary) of volcanics and plutonics in the southwest, from the younger ones (Quaternary) in the northeast. In southern and central

part of Sumatera, all of Quartenary volcanoes are located within 50 km of the fault zone (Barber et al., 2005).

The fault zone is highly segmented (Figure 2), in which most of the principal segment boundaries are dilational in nature (Sieh and

Natawidjaja, 1998, 2000). A series of grabens (e.g., Katili, 1970; Simandjuntak and Barber, 1996) have formed at these extensional

segment boundaries, and volcanic edifices are situated within some of them (Bellier and Sebrier, 1994; Sieh and Natawidjaja,

2000). The slip rates along the fault segments are different in the south and north of Equator, as determined from the observation of

river channels and offset along the Sumatran fault which had incised many thick pyroclastic deposits. It is interpreted that dextral

slip rates increase northward but not uniformly. In the south of Equator, slip rates are approximately 11mm/yr. In contrast, north of

Equator, these increase to 27 mm/yr at 2oN (Sieh and Natawidjaja, 2000).

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Figure 2: Regional geological-structural setting of Sumatra island, including the great Sumatran Fault Zone (SFZ) and the

rate of plate movement along its subduction boundary (Natawidjaja and Triyoso, 2007).

4. SUMATRAN GEOTHERMAL FIELDS

Sumatran geothermal fields are scattered from the Aceh (the northern tip) to Lampung (the southern tip of the island), with a total

of 84 prospects are already identified in 2004 by the Ministry of Energy and Mineral Resources (Figure 3). The majority of

geothermal prospects, especially those with high-enthalpy potentials have common characteristics as follows. Thermal

manifestations are associated with active or inactive, young stratovolcanoes, including ones that are partly and deeply eroded. At

least one solfatara field, either small or large, can usually be found with traces of magmatic gases. Significant discharges of acid

sulfate-chloride or acid chloride-sulfate waters (hot acid springs) can be found on the flanks of the stratovolcano. Partially

neutralized thermal waters with chemical affinity to the acid flank discharges occur in the foothill region. At still lower levels, hot

springs with a neutral or slightly alkaline NaCl type of thermal water can sometimes be found. Warm springs discharging

bicarbonate-type waters in the foothill region can also be recognized. Finally, acid surface alteration typically occurs.

The SFZ is close to the axis of the Quaternary magmatic arc on Sumatra, and the concentration of thermal manifestations such as

hot and warm springs, fumaroles, and altered rock are commonly found in the vicinity of the fault zone suggests its potential as an

important source of geothermal energy (Hochstein and Sudarman, 1993).

5. CHALLENGES IN GEOLOGICAL SURVEY

Sumatran geothermal fields are relatively more challenging to be geologically studied for several following reasons.

5.1 Accessibility

Most of the geothermal prospects are located in the protected forests. As a result, they are located at very remote areas with very

limited access. Working in protected forests also requires a special license from authorities and visiting national forests is typically

forbidden for foreigners. Remote sensing is expected to be effective to help the survey at the initial stage, but the dense vegetation

coverage influences the effectiveness of remote sensing techniques.

5.2 Complexity of Volcanic Geology

Based on the experience of working in several projects, the author considers that the volcanic geology of many Sumatran

geothermal projects to be more complex than the Java case. Many Indonesian geoscientists are not well trained or educated in

volcanic geology. Moreover, those that have training are typically trained in Java island, which is the center of geoscientific

education in Indonesia. In the field, many volcanic manifestations in Sumatra are not commonly found in Java, so that geologists

may face many difficulties in recognizing important features that are critical to the determination of the locations of potential heat

sources, permeability zones, and active versus fossil hydrothermal alterations. The difficulties in volcanic geology and related

hydrothermal system studies are manifested by the phenomenon that many of such works are contracted to other parties including

universities.

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Figure 3: Location of geothermal prospects in Sumatra, based on the map by Ministry of Energy and Mineral Resources

2004.

Working in Sumatra geothermal fields must include the anticipation of the presence of several volcanic eruption centers, rather than

a single center, as the influence of faults is very intense. Preliminary information on such different volcanic centers are difficult to

be found from published geological maps since such maps typically have a scale at 1:250,000 in which all Quaternary volcanics are

mapped as single unit. In several cases, the intensive geological mapping program using a volcanic stratigraphy mapping approach

revealed many young volcanic edifices. Different styles of volcanoes are present, such as, monogenic, stratovolcano, caldera, and

composite volcanoes. In general, there are two main styles of Sumatran volcanoes based on their setting relative to the Sumatra

Fault Zone: stratovolcanoes (similar to the Java case) and complex of monogenic volcanoes. Stratovolcanoes are typically located

at a distance of around 10 km to 20 km from the Sumatra Fault Zone, while the monogenic volcanic complexes are typically found

at pull-apart tectonic-volcanic depression settings, located at the proximity of the Sumatra Fault Zone.

Magmatic eruption centers can be present in many ways: from central eruption crater, caldera, to lava dome and dike. For Sumatran

lava domes and dikes, many of them represent themselves as glassy obsidian and its derivative products rather than porphyritic

textures, probably due to the rapid movement of magma extrusion related to the active Sumatra Fault Zone. Acid volcanic rocks

produced by explosive eruptions, especially welded ignimbrite, are also more common in the Sumatran compared to the Java case.

These rocks can cover the majority of survey areas and restrict the observation of underlying rock units.

Hydrothermal systems are believed to be composed of multiple stages rather than a single event. Recognizing how many

hydrothermal systems are present in the survey area is truly challenging. It is the regulation by the Indonesian government that each

geothermal concession area or WKP must be composed of a single geothermal field only. This regulation has forced companies, as

operators of certain geothermal prospect areas, to determine the nature of geothermal system, such as the boundary of reservoir,

from the early stages of exploration. This is very difficult to determine. Companies typically apply MT survey to determine the

boundary of geothermal fields, compared with information on the geochemistry of hot springs. However, they may not be properly

matched with geological information on styles of alteration and thermal manifestations. Several geothermal fields in Sumatra are

located at the proximity with active epithermal gold mines that indicate the presence of overlapping fossil geothermal systems that

are now producing epithermal gold deposits and an active system that is being targeted for geothermal exploration. Even in the

active thermal manifestations, especially those of neutral pH hot springs, it is common to find quartz veins already formed. This

phenomenon should be carefully examined, either active hot spring discharge and quartz veining are coexistent or they come from

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two overlapping events (i.e. active versus fossil system). Detailed mapping of hydrothermal alteration zones can help in elucidating

the problem. Fossil systems are typically already eroded that may be well represented by the presence of high temperature

secondary minerals. However, since the Sumatra Fault Zone is a very active fault, it is also anticipated that high temperature

secondary minerals can be exposed locally due to fault’s movement, while the whole alteration system is still an active and largely

not eroded. Detailed and careful field observations, combined with proper laboratory analysis, are required to make solid

conclusion.

Structural geology data, such as joints and faults, are more commonly found in Sumatra geothermal fields compared with the case

of Java. The logical reason is that the Sumatra Fault System is very active such that most of volcanic units, even the historical ones,

may be affected by such structures. The geological mapping program at the initial stage of geothermal exploration surveys should

consider structural geology as important geological data to make preliminary interpretations on the zones of permeability related to

extensional joints and faults. In this case, it is not enough to make structural geology interpretations based on the regional setting of

the Sumatra Fault Zone, as this regional strike-slip fault is in fact highly segmented with at least 20 segments. Most of the principal

segment boundaries are dilational in nature (Sieh and Natawidjaja, 2000) and become the sites of a series of grabens with volcanic

edifices situated within some of them. The slip rates along the fault segments are also different in the south and north. In general it

is interpreted that dextral slip rates increase northward, but not uniformly.

5.3 Complexity in Geohazards Risk

Because of the complexity of volcanic geology, geothermal fields in Sumatra also hold more complex geohazards potential for

future development, compared with Java’s case. For example, recent eruptions of the Sinabung volcano, after its dormancy for

more than 400 years, affected the development plan of the nearby Sibayak geothermal field. Ideal geothermal fields to be developed

in Indonesia are considered to be the ones that are located within young volcanoes but do not show historical eruptions. Information

of historical eruptions of Sumatran volcanoes are far from adequate, each geothermal project should collect their own evidence to

evaluate the history of volcanic eruptions in the proximity of the survey area. Some major hydrothermal eruptions are recorded in

the literature, such as the eruption in the Suoh valley, south Sumatra, in the 1930s and 1990s. However, much more are to be found

by direct field surveys. Tephra deposits must be carefully examined, especially if they buried human artifacts, which is an

indication of a historical event. Deposits surrounding the lakes must be examined to clarify the nature of lakes to determine whether

they are sites of recent volcanic eruptions or not.

6. PLAN FOR EFFECTIVE GEOLOGICAL MAPPING

Considering the complexity of the volcanic geology of Sumatran geothermal fields and the requirement to collect precise geological

data to enable proper interpretation from the early stage of geothermal surveys, here are some suggestions that may help improve

the planning and execution of geological field mapping programs. Effective field geological mapping programs, especially at the

early stage of a geothermal survey program, should focus on several main targets. These targets include the distribution of surface

rocks, understanding volcanism history, location of young volcanic eruptive centers, geological structures (faults and fractures), and

the distribution and nature of hydrothermal alterations and thermal manifestations. These data are critical in making proper

geological contributions for early assessments of the potential of the studied geothermal prospect.

Well-designed geological mapping typically consists of three stages of activities: prefield, field work, and postfield work stage that

consists of laboratory analysis and reporting (Figure 4).

Figure 4: Typical workflow of geological mapping program and its contribution for geothermal exploration.

Remote Sensing

Landsat/ASTER

VNIR SWIR TIR

DEM/DTM

Tentative Lineament map

Tentative Volcanic Units

Map

Tentative Alteration (Mineralogical) Map

Tentative Thermal Anomaly Map

Structural mapping

Geological mapping

Volcanic Facies mapping

Alteration mapping

Pre-Work

FieldworksThermal Manifestation

mapping

Geological Structure Map Alteration MapGeological Map

Evolution of Geology

Conceptual Model of Geothermal System

Tentative Geological Map

Volcanic Facies Map

Lab. Analysis

Rock samples

Geological Map, Reports

XRDPetrography

Radiometric Dating: K-Ar, Ar-Ar, Carbon dating

Data Analysis and Reporting

GIS Database

Geochemical, Geophysical data

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6.1 Prefield Stage

The prefield stage is aimed to generate tentative lithological, structural, alteration and thermal anomaly maps that can be used for

efficient field work campaigns. This stage consists of the collection and integration of previous works in the survey area, combined

with the interpretation of remote sensing images. Moreover, this stage must generate more detailed tentative geological maps,

typically at a scale of 1:40,000 to 1:50,000, based on the reinterpretation of the published regional 1:250,000 geological maps.

Additionally, the prefield stage must generate a plan map for the planned field survey tracks and observation points. Since many of

Sumatran geothermal fields are located in remote areas and protected forests, effective field data collection can only be achieved

from good planning on proposed survey tracks and locations of critical observation sites.

Recently, high-resolution digital elevation model (DEM) data have become one of the best sources for prefield studies because they

can be digitally and manually processed and interpreted with regard to lithology and structural geology information. Unfortunately,

in the Indonesian case, for public-domain DEM data, such as topography maps on the scale of 1:50,000, raster ASTER DEM, and

SRTM DEM are not adequate for this purpose. High-resolution DEM data at several meters ground resolution (typically radar) are

needed to construct a base map for appropriate desk studies on lithological units and structural aspects of the study area. For

example, the identification of monogenic volcanic centres (many of them are only several hundreds of meters in diameter) is

needed, as volcanism in Sumatra is not only present as stratovolcanoes but also many monogenic volcanic centres controlled by

structures. An example of the benefit of using high resolution DEM image is shown in Figure 5. Recently, Lidar has been

introduced in Indonesia’s geothermal projects and this technology seems to have a high potential of being used in geothermal

applications.

Figure 5: Comparison of accuracy on volcanic features recognition between high resolution DEM data (left, 5m resolution)

and public-domain SRTM data (right, 30m resolution) at one Sumatran active volcano.

Typically, one can apply several methods of digital processing on original DEM data using GIS software, such as ArcGIS spatial

analysis tools. The purposes of DEM analysis are to identify two main targets: lithological unit interpretation (delineation) and

structural geology analysis through lineament interpretation. Lineaments are an important aspect in interpreting geological

structures using remote sensing digital images. The lineament is the aligned and elongated geometry of an object which is oriented

systematically in a specific direction. Usually, the lineament is interpreted by following river or valley alignments which are

controlled by strong erosion. For this purpose, several derivative thematic images can be generated from the original DEM map

through some spatial analysis, such as:

1. Hillshade analysis, from this process one generates 3D-like images that enhance morphology, especially around steep slopes.

2. Lithological units interpretation, done manually (free hand), which considers the texture, shape and spatial association of

features observable from the hillshade images.

3. Manual lineaments interpretation by free hand delineation following valleys or rivers which have relatively straight patterns

at specific lengths. Tectonic lineaments typically have distinguished features, including a systematic interval and

repetitiveness.

4. If available, a digital lineaments extraction can also be done using some specific software programs. This method of

extraction is good for statistical analysis but it cannot be used for distinguishing tectonic versus nontectonic structures. Hence,

a careful interpretation of the results is required.

5. On the large lineaments dataset generated by the DEM image, statistical analyses can be applied to generate conclusions on the

dominant force (maxima) in the structural data cluster and its spatial direction. It is achieved by making a rose diagram for

strike or bearing data.

The use of multispectral thematic remote sensing images such as Landsat TM, SPOT and ASTER should be treated carefully, as

there are several fundamental restrictions attached. Firstly, there is the fact that the majority of geothermal fields in Sumatra are

located within dense tropical forest that limit the spectral information collected directly from the ground. Secondly, sensors from

several of the most commonly available images, especially Landsat and ASTER, are currently not working properly, so that one

Central crater

Crater lake

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needs to carefully select the scenes that are free of sensor errors. With several precautions, thematic mapping sensors can still

provide important information on the studied geothermal fields. It is already proven in some cases, that some remote sensing data

can be utilized to detect the distribution of clay and oxide minerals by a combination of the VNIR and SWIR bands of Landsat TM,

SPOT, and ASTER. Surface thermal anomalies are mainly detected using the night thermal IR ASTER image (Figure 6). On

processing remote sensing images, one typically starts with the initial processing that includes geometric and radiometric

corrections, radiance calibration, dark pixel correction, cloud masking, and land cover classification. Then, mineral groups (i.e. iron

oxide and clay groups) mapping can be done based on suitable band ratios. The resulting mineral group maps, such as kaolin, silica-

illite and hematite groups can then displayed as RGB map. After this, surface temperatures are calculated using the digital number

(DN) of TIR bands analyzed with a certain algorithm. Such analysis can be applied on Landsat TIR bands, but in the author’s

experience, it is better to derive it from the night TIR ASTER bands.

Figure 6: Hydrothermal alteration map (left, derived from Landsat TM) and surface thermal anomaly map (right, derived

from night TIR band ASTER) at one Sumatran active volcano.

In summary, on the prefield stage, the analysis methods consist of following major steps:

1. Rectification of many images, such as remote sensing images and geological map;

2. Digitizing of geological maps that consist of lithological and structural features;

3. Image processing of satellite images (Landsat, SPOT, and/or ASTER) for thermal and alteration manifestations utilizing

VNIR, SWIR, and TIR bands;

4. DEM/DTM image processing for lithological and geological structures interpretation;

5. Integration of data in GIS for a final interpretation on the lithology, structural geology, hydrothermal alterations, and thermal

manifestations;

6. Generating tentative maps on lithological units, geological structures, hydrothermal alteration zones, thermal anomalies, and

proposed tracts for the field investigation.

6.2 Field Work Stage

During the field work stage, efficient field work should follow the planned tracks and observation points derived from the prefield

stage. On all the observation sites, observations should focus on the different facies of volcanic deposits (coherent versus

fragmental), volcanic stratigraphy of different volcanic units, structural geology, and alteration aspects of different units. Volcanic

rock facies classification can follow the schema by McPhie et al., 1993 (Figure 7).

In each observation point, the appropriate data are collected, which consist of locational information, lithology, structural geology,

and alteration-thermal manifestation aspects. If necessary, certain checklist forms can be prepared to make a standardized input data

list from all observation points.

In selected observation points, rock samples of enough quantities should be collected for further analysis with different purposes.

Fresh rock samples are typically collected for petrography analysis. For lithological unit classification, XRD for analysis on

secondary minerals detection, and selected samples for radiometric dating are used. Meanwhile, hydrothermally altered rocks are

sampled for petrography and XRD analysis for defining hydrothermal alteration styles. Travertine, silica sinter, and silica veins are

also collected for petrography and XRD analysis for the interpretation of the environment of deposition. Charcoal fragments can

also be collected for carbon dating.

As there are many Holocene ash fall deposits in Sumatra that covers the majority of Pleistocene volcanic units, one must observe

the underlying rocks to check if the surface is covered by tephra. In addition, the youngest ash and other volcanic products need to

be mapped to evaluate the current activity of the volcano.

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Hydrothermal alteration zones in Sumatra are not always related with currently active geothermal systems. Thus, the identification

and study of the nature of alteration zones, including the observation of quartz vein-floats, is required to elucidate which alteration

zones are directly related with an active geothermal system. The results of field checking on altered grounds suggest that, in several

geothermal prospect areas, one can differentiate zones of altered grounds despite the fact that most areas are covered by rain forests.

Furthermore, one can differentiate acid alterations (kaolin-rich), neutral (quartz-illite), and intermediate (iron-oxide) using the

Landsat TM images. ASTER images also has a potential of achieving this, but it is difficult to find free images. Surface temperature

values extracted from the TIR band of Landsat have different distributions from that of ASTER, especially from the night data. It is

evaluated, based on the conceptual model and ground checking of results that, night TIR image of ASTER is more accurate. The

results suggest that remote sensing interpretation can contribute significantly in the early stage of geothermal exploration, despite

the presence of large vegetation coverages in Indonesia.

Structural geology data is typically difficult to interpret at the initial exploration program, but observations on the distribution of

thermal manifestations, dykes and epithermal veins may serve as a good indication of the existence of extensional structures. Field

geological structure data typically consist of field measurements on shear joints, extensional joints, faults planes, and striations.

These data can then be analyzed to determine the styles of geological structures, direction of the compressive field, and the

geological structure history in combination with secondary data derived from DTM interpretation.

Figure 7: Classification of volcanic rocks by McPhie et al. (1993).

6.3 PostField Work Stage

The postfield work stage typically consist of following steps:

1. Laboratory activities that typically consist of petrography and XRD analysis. Petrography is used to clarify the different

lithological units, mineralogical compositions of volcanic rocks, and alteration degrees of rocks. Meanwhile, XRD analysis is

done to clarify the secondary minerals present. XRD analysis is typically conducted on air dried bulk rocks, clay minerals

separate, and ethylene glycol-treated clay samples. Selected rocks are analyzed for radiometric dating, such as fresh lavas for

K-Ar or Ar-Ar dating, and charcoal deposits for carbon dating.

2. Structural geology analysis is not often performed in preliminary surveys, but it is an important part of the geological

mapping program. In this stage, a local-scale structural analysis is done for determining structural patterns that play important

roles in the development of the geothermal system by providing permeable zones in the working area. Local structural

analysis uses mainly field measurements data that include joints, bedding orientation, and offset lithology. Statistical analysis

can be used in the large structure dataset, such as field joints data. The purpose of this analysis is to get the dominant force of

the geological structure data cluster by making a rose diagram for the strike or bearing data. Then, kinematic analysis can be

performed for the fault data to determine the relative offset in the lithology and also to interpret the tectonic regime which

controls the development geological structures. The results of all analyses are then synthesized in order to derive the best

conclusions for the type of deformation processes occurring in the studied area. In addition, the finalization of structural

geology map through classification of structures into faults, lineaments and other structural features is likewise included.

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3. Integration of all geological data to generate lithological units, structural geology features, interpretation on volcanic history,

and alteration-thermal manifestation styles

4. Integration of geological, geochemical, and geophysical data to develop early an conceptual model of the potential geothermal

system

5. Generating digital GIS database from all data

7. CONCLUDING REMARKS

Sumatran geothermal fields are considered to be more difficult to be explored compared with the Java cases. Thus, good planning

and effective execution of field geological mapping are important for generating a good initial evaluation on the geothermal

development potential at the early stage of exploration. With such complexity, it is considered that the integration of geological,

geochemical, and geophysical data are required to fully understand Sumatran geothermal fields.

Effective field geological mapping program can only be achieved after intensive prefield work to provide tentative lithological,

structural, alteration, thermal anomaly, and planned survey tracks and observation points maps. Field work observation should

focus on the different facies of volcanic deposits, volcanic stratigraphy of different volcanic units, structural geology, alteration and

thermal manifestation aspects of the different units. Hydrothermal alteration zones in Sumatra are not always related with currently

active geothermal system, so that the field identification and laboratory study of the nature of alteration zones is required to

determine which alteration zones are directly related to the active geothermal system. Observations on the distribution of thermal

manifestations, dykes and epithermal veins may serve as a good indication of the existence of extensional and permeable structures.

Ideally, reliable geological information should then be incorporated in the integration with geochemical and geophysical data in

order to better define the system and target the sites of exploration drilling.

REFERENCES

Gasparon, M.: Quaternary Volcanicity, in A. J. Barber, M. J. Crow, and J. M. (eds) Sumatra: Geology, Resources and Tectonic

Evolution, Geological Society London, Memoirs 31, (2005), 120-130.

Gozzard, J.R.: Image Processing of ASTER Multispectral Data, Department of Industry and Resources, Geological Survey of

Western Australia, (2006).

Hamilton, W.B.: Tectonics of the Indonesian Region, Professional Paper 1078, U.S. Geol. Surv., Washington, DC, (1979), 345 p.

Hochstein, M. P. and Sudarman, S.: Geothermal Resources of Sumatra. Geothermics, 22, (1993), 181-200.

Ibrahim, R. F., Fauzi, A., and Suryadarma: The Progress of Geothermal Energy Resources Activities in Indonesia. Proceedings,

World Geothermal Congress 2005, Antalya, Turkey, (2005).

Katili, J. A.: Volcanism and plate tectonics in the Indonesian island arcs, Tectonophysics, 26, (1975), 165-188.

McPhie, J., Doyle, M., and Allen, R.: Volcanic textures : a guide to the interpretation of textures in volcanic rocks. University of

Tasmania, Centre for Ore Deposit and Exploration Studies, (1993), 198 p.

Natawidjaja, D. H. and Triyoso, W.: The Sumatran Fault Zone- From Source to Hazard. Journal of Earthquake and Tsunami, 1,

(2007), 21–47.

Neumann van Padang, M.: Indonesia. Catalog of Active Volcanoes of the World and Solfatara Fields, IAVCEI, Rome, 1, (1951), 1-

271.

Puspito, N. T. and Shimazaki, K.: Mantle Structure and Seismotectonics of the Sunda and Banda Arcs, Tectonophysics, 251,

(1995), 215-228.

Siebert, L. and Simkin, T.: Volcanoes of the World: an Illustrated Catalog of Holocene Volcanoes and their Eruptions. Smithsonian

Institution, Global Volcanism Program Digital Information Series, GVP-3, (http://www.volcano.si.edu/world/), (2002).

Simandjuntak, T.O. and Barber, A.J.: Contrasting Tectonic Styles in the Neogene Orogenic Belts of Indonesia. In Hall, R. and

Blundell, D.J. (Eds.), Tectonic Evolution of Southeast Asia, Geological Society Special Publication, 106, (1996), 185-201.

Tatsumi, Y. and Eggins, S.: Subduction zone magmatism. Blackwell Science, Ann Arbor, (1995), 211 p.

Van Bemellen, R. W.: The Geology of Indonesia, vol. 1A. Martinus Nijhoff, the Hague, (1949), 732 p.

Widiyantoro, S. and van der Hilst, R.: Structure and Evolution of Lithospheric Slab Beneath the Sunda Arc, Indonesia, Science,

271, (1996), 1566-1570.