Mapping of archaeological structure along east-coast of Aceh Besar District, Indonesia based on total magnetic field anomalies

The coast of Aceh along the northern tip of Sumatra had an important role since the beginning of the first millennium AD. The area was used as a transit point for the maritime silk road of the gate of the Malacca Strait. Many old settlements and ancient fortresses were found on the area. One of them is Indrapatra fortress. At present, the Indrapatra fortress has been slowly damaged by weathering and was also hit by disasters. Parts of the main building is still intact, while the other parts have been damaged and buried by sand deposits. In order to preserve the building, some actions need to be taken such as re-mapping the existence buildings and the buried parts as well. For this reason, a geomagnetic survey has been conducted to delineate the remains of the building at the subsurface. Data of total magnetic field intensity were acquired in an area of 31,255 m(2) with a grid of 3 meters for each station so that the measurement covered the whole area of the target site. Total magnetic field anomalies were calculated using diurnal and International Geomagnetic Reference Field corrections. The obtained anomalies represent magnetic objects (archaeological structures) which buried beneath the surface. To obtain a more detailed form of anomalies several transformations, such as analytic signals, tilt derivatives, and horizontal derivatives were applied. The results of the transformations show several circular and elongated anomalies. The circular forms of anomalies are considered as buried wells, and the elongated forms are considered as channels which presumed connected the buildings in the area. The data showed that there are also foundations of the buildings made of stone structures cemented with lime plats.