Detection of the Possible Buried Archeological Targets Using the Geophysical Methods of Ground Penetrating Radar (GPR) and Self Potential (SP), Kom Ombo Temple, Aswan Governorate, Egypt

Kom Ombo temple is one of temples which were belted over high plateau close to the River Nile, near to Aswan in Egypt in the Greek-Roman period. The expected archaeological remains in the selected area are the hidden tunnels of the mummified crocodiles. The aim of the present work is to detect any of these tunnels by the application of the (GPR) and (SP) methods. The interpretation of the 10 GPR profiles revealed some locations of possible hidden tunnels. These locations show different contrasts and high amplitudes of the reflected signals, compared to the enclosing soil; also the scattering of the signals is higher than the bed layer in these locations, which may reveal the possible buried mummified crocodile tunnels in the study area. The depths of the possible targets range from 2.0 m to 2.5 m. The SP electric map shows that the study area possesses a range of about 135 mV of the potential differences between the measured stations. The positive response of the SP data is mainly concentrated at the central part of the study area. The relatively weak, negative SP anomalies may be related to moisture in the soil. The positive SP anomalies on the SP electric map display possible significant correlation between them and the inferred tunnel locations from the GPR data. The calculated depths from the SP profiles show significant agreement with that estimated from GPR data depths, which indicate that the SP electric method can be used as a successful tool in detecting buried archaeological remains in support of GPR.


Introduction
Kom Ombo city is located at about 40 km to the north of Aswan city, in adjacent to the River Nile ( Figure 1). Kom Ombo temple is located to the south of Kom Ombo city at the intersection of latitudes 24˚27'8''N and longitudes 32˚55'42''E ( Figure 1 and Figure 2).  Kom Ombo temple is one of the Greek-Roman temples which were built over high plateau close to the River Nile in Egypt in the Greek-Roman period. The temple was dedicated for worshiping the God Sobek, the crocodile headed God. In that period, they mummified the crocodiles and hid them in tunnels under the temples. Many excavations were carried out to discover the archaeological remains in the temple. In some of these excavations, they discovered some mummified crocodiles, still found in the museum inside the temple. These tunnels were discovered in different depths. Other tunnels are still buried under the temple [1].
The expected archaeological remains in the selected area are the hidden tunnels of the mummified crocodiles, depending to the reports of the previous excavations which were conducted close to the study area [2]. This area was selected because it was not excavated till now.
The sediments succession in the study area is belong to the Nile valley succession sediments of the late Cretaceous to early Eocene which is dry mud and sandy mud overlying the wet sand layer [3].
The aim of the present work is to detect any possible buried tunnel of the mummified crocodiles through the application of GPR and SP methods.
According to the previous excavations, the bed layer sequence in the selected area is from top to bottom: dry mud, sandy mud and wet sand. All the discovered mummified crocodiles were buried in wooden coffins.

GPR Basic Concept
Ground-penetrating Radar (GPR) is a near-surface geophysical technique that allows archaeologists to discover and map buried archaeological features for landscape analysis in ways not possible using traditional field methods. The method consist of measuring the elapsed time between when pulses of radar energy are transmitted from a surface antenna, reflected from buried discontinuities, and then received back at the surface. When the distribution and orientation of those subsurface reflections can be related to certain aspects of archaeological sites such as the presence of architecture, use areas or other associated cultural features, high definition three-dimensional maps and images of buried archaeological remains can be produced [4] [5] [6].
Most of the possible archaeological targets are buried in clay soil. It is important to understand that the depth of penetration is highly dependent on the clay content of the medium, with clay significantly attenuating electromagnetic waves and limiting the investigation depth [7].

The GPR Instrument Used in Present Study
In the present study, MALA GPR system was used connected to 100 MHz antenna ( Figure 3). It provides a detailed look at what's beneath the surface. The system offers leading-edge GPR technology, with full digital control of all setup parameters and multi-channel color display. Geomaterials

GPR Data Collection and Processing
GPR data acquisitions are very similar to the method used in seismic reflection.
However, GPR has much higher resolution and it is sensitive to changes in electromagnetic rather than caustic properties. GPR reflections are caused by electromagnetic waves encountering media that have different electrical properties namely; boundaries consisting dielectric constant contrasts. Reflection is approximately proportional to the difference of the dielectric constants at the boundary [8].
In the selected area, 10 GPR profiles were conducted with a line separation of 1 m, using antenna of 100 MHz. The GPR profiles are processed using the software program (Reflex W, 2D/3D). The applied parameters for filtering the raw data were as following; Static correction, cross-correlation, deconvolution, Fk migration, back ground removal and remove range.

3D Analyses of the GPR Data
Three-dimensional interpretations of the ground-penetrating radar (GPR) data were applied to identify burials and other cultural features. The recent development of sophisticated software has enabled signal enhancement and improved pattern recognition on radar records [9].
Time slicing was performed for the GPR results to illustrate the presence of possible buried tunnels in the study area in a 3D form as shown in Figure 8 and
The SP data is generated from the measurements of naturally occurring electric potentials across two electrodes placed on the earth's surface and the potentials measured during these surveys are small, generally less than 100 millivolts, and may be positive or negative to locate anomalies of interest [20].

SP Data Interpretation
In this study, the SP method is applied to measure the response of the possible archaeological targets and integrate these results with the GPR data. The main idea of using the SP method in this study is the possibility that there are spaces inside the tunnels, as well as the presence of different materials inside the tunnels that may be different in composition from the dusty environment around which enable to measure these differences in voltage and identify the zones of the sub-surface tunnels.
Ten SP profiles were carried out over the GPR profiles with station separation 1 m. The SP survey was applied with the GPR technique to confirm the response of the GPR method in detecting the buried targets in the archaeological prospecting.
The SP electric filled-colour map ( Figure 10) shows that the study area possesses a range of about 135 mV of the potential differences between the measured stations. The positive response of the SP data is mainly concentrated at the central part of the study area. In addition, a number of relatively weak, negative SP anomalies are distributed through the study area. These anomalies may be related to moisture in the soil. A close correspondence exists between the positive SP anomalies on the SP map display a significant correlation between the SP anomalies and the deduced tunnel locations from the GPR data ( Figure 11).
These positive values can be due to two reasons; the first reason may be the woody material that constitutes coffins, which is less in the humidity than the soil surrounding it, and therefore showed a higher effect. The second reason may be due to the presence of voids in the coffins filled with air. The presence of these voids reduces the electrokinetic process and gives rise to an SP voltage positive above it.
The quantitative interpretation of the SP data depends usually on the transformation of the SP anomalies to a physical model of simple geometric shape.
The model parameters include shape, depth and polarization angle that use  several graphical methods. In the present study, two SP anomaly profiles, representing two anomalies, were selected for the quantitative interpretations ( Figure 12 and Figure 13). The depths of the source anomalies were evaluated using the method of characteristic curves [22] where the field profiles can be easily and accurately interpreted in a very short time. Figure 12 and Figure 13 illustrate the possible buried tunnels as interpreted from GPR data and their corresponding SP anomalies. As shown on the two figures, there are obvious changes in the voltages related to the buried objects. The calculated depths for these two anomalies were 2.8 m and 2.3 m, respectively.
Comparison of these calculated depths with GPR measured depths shows very well agreement, which indicate that the SP method can be used as a successful tool in detecting buried tunnels with GPR.

Conclusions
The aim of the present work is to recognize and detect the possible buried tunnels of the mummified crocodiles by the application of the GPR and SP methods. The expected archaeological remains in the study area are the hidden tunnels of the mummified crocodiles, depending to the reports of the previous excavations which were done close to the study area. This area was selected, because it is not excavated till now.  show different contrasts and high amplitudes of the reflected signals, compared to the soil. Also the scattering of the signals is higher than the bed layer in these locations, which may reveals the possible buried mummified crocodile tunnels in the study area. The depths of the possible targets range from 2 to 2.5 m.
The SP electric map shows that the study area possesses a range of about 135 mV of the potential differences between the measured stations. The positive responses of the SP data are mainly concentrated at the central part of the study area. The relatively weak, negative SP anomalies may be related to moisture in the soil. A close correspondence exists between the positive SP anomalies on the SP map, which may display a significant correlation between the SP anomalies and the inferred tunnel locations from the GPR data. The positive values may be due to two reasons: 1) the woody material that constitutes coffins, which is less in the humidity than the surrounding soil, and therefore shows a higher effect.
2) The presence of voids in the coffins filled with air. The presence of these voids reduces the electrokinetic process and gives rise to a positive SP voltage above it.
The quantitative interpretation of the SP data revealed that the calculated depths for the two anomalies were 2.8 m and 2.3 m, respectively.
Comparison of the calculated SP depths with that of the GPR depths shows a very well agreement between both, which indicate that the SP method can be used as a successful tool in detecting buried archaeological remains with GPR. It is recommended to use 2D resistivity method, to ensure the accuracy of the results obtained in this study.