CONICET | Buscador de Institutos y Recursos Humanos

GPR antennae have limited directivities, with wide cones of illumination. As a consequence, an important fraction of the emitted energy is lost outside the emitter-reflector-receiver path, thus reducing the possibilities of detection, especially in cases of low signal-to-noise relationship (i.e. deep targets, high absorptions or low contrasts). This energy loss reduces the effectiveness of the GPR methodology when using single emitting and transmitting antennae and a fixed distance between them (fixed offset). The detection of signals from buried targets can be improved by using variable offset procedures. A way to do this is by increasing the directivity of the emitted fields, and by concentrating the available energy on the targets of interest. This directivity can be obtained using a set of closely spaced emitting antennae, forming an array. In this case, the phase, distance and amplitude relations among the antennae should be carefully selected in order to adequately narrow the transmitted fields and to direct them towards the target. Similar results can be attained with a single emitter, by consecutively placing it at the positions that the real array components would be, and then by synthesizing the complete transmitted field from the superposition of the individual records. In this approximation, the acquisition procedures are simpler, without a coupling among the antennae. In this work we present the synthetic emitted-field (SEF) method and apply it to investigate a case in which the single offset surveys had given imprecise or negative results. We theoretically describe the fields produced by dipole-type phased-array transmitters, and analyze their dependence on the most important parameters: the number of dipoles, the distance between them, their relative phases and the distance from the array to the evaluation point. We present two characteristic cases, the reflections at a small diffractor and at a smooth extensive interface, together with a third example that combines both cases. Finally, we apply it to determine the width and depth of adobe walls at an archaeological site, which could not be detected from the usual fixed-offset sounding.

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