The stress state and the geometrical structures in rock mass are adjusted during unloading. Since it is difficult to describe the process of stress adjustment

the geometrical structures are used in the present paper to investigate the process of rock mass unloading

and the elastic wave velocity is studied by considering the interaction of geometrical structure and elastic wave. A double-crack structure is employed to make approximate analysis of the elastic wave velocity in jointed rock mass. In the model

the analysis method of interaction within the double-crack structure is used to partly consider multiple scattering between the joints of rock mass

and the linear superposition method between double-crack structures is adopted to analyze the localization effect for defects in rocks. The influence of unloading on the rock bridge is described by the opening displacement and the opening rate of joints

and variation of elastic wave velocity of four different kinds of rocks corresponding to two dominant frequencies

25 kHz and 1 kHz

is studied under excavation unloading condition. The results show that in the process of unloading

relative acoustic velocity corresponding to 25 kHz and seismic velocity corresponding to 1 kHz are gradually decreased; however

the decreasing degree of acoustic velocity is less than that of seismic velocity. For example

acoustic velocity of highly unloaded rock mass reduces to be 80% of that of the original rock mass

and seismic wave velocity reduces to be 50% of that of the original rock mass. These conclusions are of good guidance for the inspection and evaluation of water conservancy project and railway tunneling construction