1 Introduction

The study of the ${\rm GeV}-{\rm TeV}$ component of gamma-ray bursts is of great importance to understand the acceleration mechanisms and the sources physical conditions. The detection of GeV gamma-rays by EGRET during some intense GRBs [(Catelli et al. 1997)] suggests the possibility that a high energy component could be present in all events. Furthermore several models predict GeV and TeV emission, sometimes correlated with UHECRs production (see [Baring 1997], for a review). Due to the low fluxes and the small sensitive areas of satellite experiments, gamma-rays of energy larger than a few tens of GeV, must be detected by ground based experiments located at mountain altitude measuring the secondary particles generated by gamma-rays in the atmosphere. At energies E<10 TeV the number of particles reaching the ground is to small to reconstruct the shower parameters using a standard air shower array, made of several detectors spread over large areas. On the contrary, a detector consisting of a full coverage layer of counters, providing a high granularity sampling of all particle showers, can successfully measure arrival direction and primary energy of small showers, allowing the study of the unexplored range of gamma energies between 20 GeV and 300 GeV [(Abbrescia et al. 1996)].