INVESTIGADORES
OSELLA Ana Maria
congresos y reuniones científicas
Título:
Synthetic Emitted Field method to improve GPR signals
Autor/es:
CEDRINA, LORENA; BONOMO, NÉSTOR; OSELLA, A.,
Lugar:
Beijing
Reunión:
Congreso; 19th Workshop on electromagnetic induction in the Earth; 2008
Resumen:
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.