GEOPHYSICAL FORENSIC FOR ARCHAEOLOGICAL EXPLORATION IN MUAROJAMBI, INDONESIA

In July 2011, archaeological exploration tried to apply the physics method for the first time in Muarojambi, Indonesia. We combined physics with geosciences and called it geophysical forensic. Our method is known as Ground Penetrating Radar (GPR). GPR used high-frequency electromagnetic (EM) waves between 10-3000 MHz to imaging subsurface based on dielectric permittivity’s physical parameters. Changes in the electrical properties, rock magnetism, and water content of the material under the surface will provide a response recorded on the radargram as a function of distance to time (two-way travel time). Data processing performs to reduce the noise recorded when collecting data. We have successfully obtained four GPR lines; three lines gathered near Gumpung Temple and one line at Telago Rajo Pool. The GPR method succeeded in giving a subsurface image and possibility of the archaeological objects near the Gumpung Temple and Telago Rajo Pool.


INTRODUCTION
Muarojambi has many unexposed archaeological objects. Muarojambi, as an archaeological site, has a high historical and cultural value related to the Srivijaya Kingdom. Srivijaya is the most maritime power in Southeast Asia during the ancient period [1]. Since 1820 many archaeological objects founded. Only ten objects have been given names and identified in a scientific publication [2]. More than 82 objects still unpublished. It was costly to do archaeological exploration with a conventional method like excavation. We need a fast and accurate method to support archaeological exploration. FIGURE 1 shows the distribution of archaeological objects in Muarojambi. The application of physics to support archaeological exploration is still limited in Indonesia. This study intends to reveal archaeological objects below the surface of the land using geophysical forensic. The combination of physics and analysis of geology, geophysics/earth physics, and archaeology supports the Muarojambi temple complex.
Our research objective is to get the possibility of archaeological objects in Gumpung Temple and Telago Rajo Pool. A previous study [6] presented the possibility of the archaeological object in Kedaton Temple, Muarojambi.

METHODS AND MATERIALS
Muarojambi is located in the lowlands area, not exceeding 20 meters above sea level. Although located 100 kilometers from the present coastline, the existence of the Batanghari River and its connected branches allows this area reached using water transportation. This area is far from geological hazards like earthquakes, tsunami, and volcanic eruptions. FIGURE 2 shows the tectonic map of Sumatra, plotted using Generic Mapping Tool [3]. The study area is located in Marosebo District's Muarojambi Regency, indicated with a blue rectangular block in FIGURE 2. FIGURE 3 shows a geological map of Muarojambi modified from [4]. Geophysical forensic is the application of non-destructive geophysical methods to study, discover, and map invisible objects buried or hidden either in layers of soil, rock, underwater, on walls [7]. Geophysical forensic is about creating a more efficient investigation. It can covers large areas rapidly [8]. Geophysical forensic can detect various phases of burial over different periods [9]. Geophysical forensic give recommendation for archaeological excavation [10]. The Ground Penetrating Radar (GPR) method is nearly the same as the seismic reflection method. FIGURE 4a shows the basic principle of GPR [11]. If the mechanical waves are used in seismic, the GPR method uses high-frequency electromagnetic (EM) waves from 10 to 3000 MHz [11]. The EM wave is transmitted by transmitter antennas, then the energy is detected by the receiver antenna. The transmitter antenna is often joined with the receiver antenna and called a transducer. We use the GSSI SIR-20 GPR System with a 200 MHz antenna for data acquisition. Basic processing like editing, move start time, gain recovery, and stacking doing in RADAN 6. Filtering and visualizing doing with MATGPR [12].

RESULT AND DISCUSSION
The Gumpung Temple condition was clear from noise. FIGURE 5 shows an image from UAV (drone). Our GPR line position is near the main temple. It was flat terrain condition. We can not see the anomaly in the surface.  We have successfully imaging three GPR in Gumpung Temple and one GPR in Telaga Rajo Pool. We compare each GPR imaging with four view modes: grayscale, wiggle, blue-red, and jet view.

CONCLUSION
We have conducted four GPR lines that show the possibility of archaeological objects. All objects show an anomaly recorded in four view modes. These objects can be confirmed and validated using the hand auger, shallow drilling, or trenching before going to an excavation. GPR has successfully to give the possibility of hidden objects in the subsurface.