2D Forward modeling of Dual-Coil Frequency-Domain EM data from resistive structures

MARTINELLI, PATRICIA; OSELLA, A.,; LASCANO, EUGENIA

Hyderabad

Congreso; 17th Workshop on electromagnetic induction in the Earth; 2004

Within the last three years we have been performing electromagnetic and geo-electric studies to characterize the archaeological remains of an Spanish fortress situated on the Atlantic coast in Patagonia (Argentina). The site has an area of  10000 m2 defined by the presence of elevations of the terrain that correspond to buried archaeological structures. Previous excavations showed that these structures were mostly adobe walls; also, tiles probably due to roof collapse were found within the zone of interest. The geoelectrical method is very reliable to map electrical resistivity contrasts and provides high-resolution electrical tomographies, even when mapping 3D structures. But, it is not so practical when the surveyed area is too large compared with the lateral resolution required for shallow prospections. In these cases a better strategy is to apply an alternative method, like GPR or Moving Coils, to delimit anomalous zones, and then the geoelectrical method to get a local  electrical image of the subsoil. In the above-mentioned field-work, we used a Frequency-domain Electromagnetic Induction system. One of the main advantages of these systems is that the process does not require direct contact with the ground and then are much faster than other electromagnetic methods. These equipments are mainly used to detect conductive buried structures since they are very sensitive to the presence of electrical anomalies. The data are usually interpreted directly from the In-Phase and Quadrature components; notwithstanding, much more information can be obtained if a quantitative analysis of data is added to the first qualitative result. Then, modeling methods are important tools to improve the interpretation of data. Moreover, they could be used when planning a field work; they could provide information to know beforehand the sensitivity of the method to detect the expected structures and also to find the best geometrical configuration.             In the present paper, we present a method to model data from a Dual-Coil Frequency-Domain EM system, assuming 2D structures. This method is an alternative to finite element and finite difference techniques and is especially suitable to model multi-layered structures, with irregular boundaries. We apply this method to numerically simulate the In-phase and Quadrature responses of different structural features expected at the archaeological site. We use information from excavations as well as geophysical data as a starting point for modeling the structures and studied the characteristics of the responses for different orientations of the dipole source and receiver. We analyze the resolution to detect different distribution and localization of buried walls and tiles.  Finally, we correlate the theoretical results to the data in order to obtain a better interpretation of the anomalies and improve the resolution of future geophysical  surveys.