Amplitude versus incidence angle (AVA) analysis is a multi – offset attribute analysis technique which involves relating amplitude variations with increasing angle of incidence (or practically, offsets), in order to characterize interfaces and layers. The GPR AVA technique has its origin from seismic reflection processing and assumes isotropic source radiation (and reception) for GPR and also, far – field conditions. GPR antennae radiate EM energy as waves travelling in a hemispherical wave front into the ground and, typically show a strong directionality in radiation and reception with respect to amplitude and/ or frequency of the signal when used for surface surveys. The antennae patterns depend on the properties of the media in which the GPR signal is radiated into and must, therefore be accounted for in AVA analysis. This paper reports the results of numerical simulations and field measurement of antennae patterns. The field site is Threshfield quarry, Yorkshire, United Kingdom, extracting the Carboniferous Limestone. A series of transillumination surveys, across limestone boulders of different thicknesses were done to enable in – situ determination of the antennae patterns. In addition to these and for comparison purposes, numerical simulations using GprMax software were also done. Both numerical and field data were processed using Reflex-Win Version 3.5.1 software (Sandmeier, 1997-2004). Processing comprises: de-wow (for field data), band – pass Butterworth filtering (250-750MHz), geometric spreading and envelope, or Hilbert transform. The resulting antennae patterns measured in the field agree broadly with numerically derived patterns, in that the basic shape is the same.
Keywords: AVA, Amplitude, Antennae Patterns, GPR
