The detection of cracks in mechanical engineering is mainly based on ultra-sonic testing and Foucault currents. But even if they are efficient tools, this technology requires an important handling and is limited to the detection of cracks which are close to the source. Recently, several searchers have discussed the possibility of using waves as Lamb waves, for thin plates and shells, but also Love waves for bimaterials. In both cases the structure works as a wave guide and enables a long range propagation which is a promising possibility for detecting a crack quite far from the source. In this paper, we discuss the observability property of a crack inside an open set using Love waves. But there is a condition which connects the space representation of the excitation and the geometry of the structure containing the crack. Introduction The aim of this paper is to obtain a criterion of detection of a crack using local waves for bimaterial modelled by a transmission problem. The crack is located inside the material along the interface of transmission and the local waves that we consider are Love waves which are located in the low velocity side of the set occupied by the material. The detection of cracks in mechanical engineering is mainly based on ultrasonic testing and Foucault currents. But even if they are efficient tools, this technology requires an important handling and is limited to the detection of cracks which are close to the source. Recently, several searchers have discussed the possibility of using waves as Lamb waves, for thin plates and shells, but also Love waves for bimaterials (see [11], [13], [19] and [22]). In both cases, the structure works as a wave guide and enables a long range propagation which is a promising possibility for detecting a crack quite far from the source. In this paper, we discuss the observability property of a crack inside an open set using Love waves. In order to be able to detect cracks using Love waves, we state in the paper a condition which connects the space representation of the excitation and the geometry of the structure containing the crack. Our plan is the following : In section 1, we present the mathematical problem, its notations and the mathematical tools used in the paper such as the time-Fourier transform of solutions of the involved partial differential equations. We state some classical existence results.