Source and receiver acquisition geometry in the boreholes and on the surface of Gallery 18 (Ga18). Receiver locations: black dots on the surface of Gallery 18 and in borehole BDR-D1; source points (SP): red dots in boreholes BDR-D1 and BDR-D2. Numbers (1, 141, 180) indicate trace numbers of one shot gather shown in Fig. 3 (SP 46). Stars show shot positions of gathers shown in Fig. 3.