The four diverging arms of Sulawesi record a complex geologic history of the tectonic processes of active plate boundaries. The four arms of Sulawesi are named as shown in Figure 1.
Lake Matano and Towuti are tectonic in origin and is hosted by a cryptodepression with a graben structure. More than 590-m deep, Lake Matano is among the 10 deepest lakes in the world. The lake is 28-km long, 8-km wide, and has a relatively small surface area (164 km2). It has been suggested, based on geological and biological information, that Lake Matano is between one and four million years old. Ancient lakes, such as Lake Matano, have relatively stable physical characteristics and are therefore conducive to the development of high degrees of species endemism. Lake Towuti is the largest lake in Sulawesi Island, Indonesia and the second largest lake in Indonesia after Lake Toba in North Sumatera. The Lake Towuti is about 562 km2, located 293 m above sea level, but the depth is only about 203 m. Both Lake Matano & Lake Towuti are situated in the Malili Basin Area.
It has been proposed that the low levels of primary production in Lake Matano in general, result from a combination of both nutrient limitation and metal toxicity. Iron and manganese are abundant in the soils of the Malili catchment area, and thus Fe and Mn redox cycling may be especially important in regulating the chemical composition of Lake Matano.
In this study we employ Landsat 8 imagery of Lake Matano and Lake Towuti to explore the unidentified objects beneath the lakes. Using only RGB (Red-Green-Blue) band combination will not yields any result in order to explore the unidentified objects beneath the lakes (see natural color image below). Therefore we need to employ image transformation techniques.
This study employ MNF (Minimum Noise Fraction) to reduce data dimensionality and then employ Image Fusion to highlighted the unidentified objects. The algorithm used in this study:
1. Layers stacking (band 1-7 of Landsat 8)
2. MNF transformation
3. Image Fusion (with band 8 – panchromatic)
4. Visualization using linear 2%
5. Superimposed the image with available geological data (fault, contour) & earthquake events in the study area.
The unidentified object beneath Lake Matano is situated between 100 m to 200 m below as shown in the following figure. As explained above, if we only employ normal RGB band combination we will not be able to see the unidentified object beneath the lake. The image transformation technique gives opportunity to explore more details what objects are hidden under water.
While the unidentified object beneath Lake Matano is to small and difficult to interpret, the object beneath Lake Towuti is bigger and relatively easier to interpret the object. From the structure, it seems that the unidentified object probably was the land and due to the tectonic process the object then submerged under water. The structure also looks like ancient local fault. However, all of the statement here are not verified in the field yet. We need more data derived from extensive field work and more research to verify it.
Unfortunately, we do not have any contour data of Lake Towuti (Figure B), therefore we could not visualize the map as Lake Matano above (Figure A).