Based on geological surveys the area is mostly covered by the Quaternary andesitic volcanic rock, while those rocks are uncomfortably underlain by Tertiary basement rocks of Kiro and Nangapanda formations. Volcanic products include lavas, pyroclastics and lahars. From thin section in petrography observation shows that the volcanic rocks are andesine pyroxene composition. The volcanic products can be divided into two groups; the old volcanic and the young volcanic. The crop outs were founded at the northern and the southern part of the area with unknown eruption centers and forms a high dissected terrain. Also those were the forms cones at the central and eastern part of the area along the NNW-SSE trending volcanic lineament.
The couple of NE-SW trending normal faults in this area recognized as Wolo Kebingin fault in the north and Mauraja fault in the south, these faults possibly control occurring thermal features in the area. There is also a major volcanic center known as Watuwawer caldera of about 2,5 km2 in diameter, located in the central part of the study area (Figure 4.1).
Figure 4.1: Geological Map of Atadai Geothermal Area (Sundhoro, et al., 2003)
Geochemical
Thermal features in Atadai geother¬mal area occur over an area about 25 km2 within the Watuwawer caldera, eastern and western flank of the volcanic lineaments. There are at least 9 locations of thermal features which are in an elevation between 170 m to 500 m above sea level. These include hot springs, fumaroles, steaming grounds/ hot grounds, and altered rocks. And then the flow rates of these hot springs are between 1 to 50 liter/minute. The temperature of six hot springs are up to 42 oC, however the temperature of three fumaroles and hot ground are up to 98 oC (Table. 4.1).
No Location Manifestations Temp. (oC) pH Type of Fluid Remarks
1. Watuwawer (Koru Matek) Fumaroles, hot ground and altered rock 96 2-3 Sulphate Up Flow
2. Lowokebingin (Wai Kating) Fumaroles, hot ground and altered rock 98 2-3 Sulphate Up Flow
3. Lowokoba (W. Teba) Hot ground and altered rock 96 3 Sulphate Up Flow
4. Wae Kerata Hot Spring 40 6.8 Bi -Carbon¬ate Out Flow
5. Wae Tupat Kecil Hot Spring 40 7.6 Bi -Carbon¬ate Out Flow
6. Wae Kowan Hot Spring 40 6.8 Bi -Carbon¬ate Out Flow
7. Wae Tupat Hot Spring 35 7.2 Bi -Carbon¬ate Out Flow
8. Wae Keti / Lewogeroma Hot Spring 38 7.2 Bi -Carbon¬ate Out Flow
9. Wae Teba Hot Spring 42 6.5 Bi -Carbon¬ate Out Flow
Table 4.1: The Thermal Features in Atadai, Lembata, East Nusa Tenggara
Following the example of the hot water have been taken in order to be known the nature of physical and its chemistry. From chemical analysis result of example can be determinated hot water type in investigation area (Figure 4.2). According to diagram Cl-, S042= and HCO3 water in investigation area can become 2 system that is:
1. Bicarbonate system
2. Sulphate system
Figure 4.2: Types of Water in Atadai Geothermal Area (suparman, et al., 1997)
The first system and the second ones show out flow system and the up flow system. Both of the hot water system supposed have been contaminated by meteoric water. Quantitatively, temperature of under surface was determined with a few method such as: silica method, method Na-K, and method of gas (methane and ammonia). The Result of temperature measurement of under surface with the silica method ( SiO2) show the temperature between 129-147 oC, while using method Na/K relative more higher (> 300oC), with the method of gas show the temperature between 178-221oC.
Considering that the geothermometer Na/K higher level than the other, water type dominant is bicarbonate with the surface temperature 40oC. It is included in enthalpy intermediate (125-225oC). Hence temperature of under surface was used from geothermometer SiO2 as minimum temperature (129oC) and maximum temperature from geothermometer gas ( 221oC).
There are also surface alteration of advanced argillic type. Clay minerals present in both the active thermal features area of Watuwawer and Lowokebingin. Minerals are predominantly: Kaolin, Halloysite, Smectite and Alunite. However Phyrophillite and Dickite are also present, which probably represent the high temperature of clay minerals in the acidic condition.
The high contour values of soil mercury is concentrated around Lowo Kebingin, Lowokoba and Watuwawer hot springs, where the maximum concentration of Hg are up to 2566, 564 and 330 ppb. The high values of soil gas CO2 is 6,76 %, It is also concentrated around the active geothermal manifestation of Watuwawer (Figure 4.3 and 4.4).
Figure 4.3: Soil Mercury in Atadai Geothermal Area (Suparman, et al., 1997)
Figure 4.4: Distribution of CO2 in Atadai Geothermal Area (Suparman, et al., 1997)
Geophysical
To get more interpretation about depth investigation then Volcanology Department of Indonesia used Schlumberger geoelectrical method. From this method gained more information of lateral and vertical distributions using resistivity data. Resistivity data indicated that low resistivity zones which represent argillic alterations about 15 km. The lateral of the low resistivity boundaries (below 10 Ohm-m) are related with the active thermal features areas, such as: Watuwawer and Lowokebingin. The resistivity boundaries which represent a prospect area covers at least 7 km2 area. The vertical sounding data shows that the thickness of conductive layers which represent the clay cap of the Atadai geothermal system are estimated between 500 m to 600 m depth of the bottom surface. The models of geothermal system have been made by geophysical team member of Volcanology Department of Indonesia on period 1997-2000 (Figure 4.2 and 4.3).
Figure 4.5: Apparent Resistivity Map of Atadai Geothermal Area (Sundhoro, et al., 2003)
Figure 4.6: Apparent Resistivity Cross Section of Atadai (Sundhoro, et al., 2003)
The tentative model of the Atadai geothermal area is shown in Figure 4.4. It is looks that in the active geothermal manifestations strictly an up-flow system with elevation are between 300 to 500 m above sea level. Whereas out flow geothermal system is manifested by the hot springs, which are situated in the flank of caldera Watuwawer with the elevation are between 170 to 250 m above sea level.
Figure 4.7: Tentative Geothermal Model of Atadai Geothermal Area (Sundhoro, et al., 2003)
Prospects Areas
The prospect area in Atadai is decided from the result of the geological, geochemical and geophysical surveys, such as: thermal features of hot springs, fumaroles, steaming grounds/ hot grounds, altered rocks, the resistivity boundaries, the high contour values of soil mercury and soil gas CO2, and also the structure geology of Watuwawer caldera. The structure of the cones at the central and eastern part of area, the NNW-SSE trend volcanic lineament, and the couple of NE-SW trending normal faults of Lowo Kebingin fault in the north and Mauraja fault in the south are dominantly controlling the prospect area (Figure 4.5).
Base on the prospect area of about 7-8 km2, than the estimated of the potential energy in Atadai geothermal area is about 22 Mwe. The estimation of geothermal energy in Atadai area are:
1. Watuwawer = 21.347 MWe
2. Wolokebingin = 1.187 MWe
Figure 4.8: The Prospect Area of Atadai Geothermal Area (Sundhoro, et al., 2003)
Based on those data, concluded that the prospect area in Atadai covers into Watuwawer and Lowo Kebingin, which both of the areas are situated in the nearby of the surface manifestations. Comparing between these prospect area indicate that Watuwawer is the most attractively exposure up of thermal features rather than in the Lowo Kebingin. The surface terrain of both prospect area are very different, Lowokebingin lies in the medium to the high steeply valley, whereas Watuwawer lies in the flat area. It forms a high dissected terrain on the floor of the Watuwawer caldera and it is also boundaried by caldera rim.
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