Optimal seismic-data acquisition in very shallow waters: Surveys in the Venice lagoon

Acquiring large amounts of data in very shallow waters of the immediate subtidal zone using a multichannel technique is unaffordable because moveout correction and a standard stacking procedure are required. The conventional inline longitudinal deployment of source and streamer leads to a phase difference along the hydrophone array for nonvertical arrivals. The array in this system is hard wired, so these phase differences cannot be accounted for and the summed output is attenuated as a result.We propose a transverse configuration, whereby the variation in phase along the array is smaller. The summed output improves because pairs of hydrophones with equivalent absolute offset are stacked in phase as a result of the symmetric configuration; however, all of the hydrophones are out of phase in the conventional geometry. Even at depths of 40 m, a better image is obtained. The technique has been used during surveys in theVenice lagoon, where the water depth ranges from 40 cm to 2 m.