Relationships of sound pressure and particle velocity during pile driving in a flooded dock

Underwater sound is characterized by two different components, directional particle motion and scalar pressure waves. Here, we studied sound pressure and particle motion during experimental pile driving in a confined industrial-sized shipbuilding dock. The pile driving noise was generated by a 200 kg hammer striking a 7.5m steel pile. Noise data were collected using a hydrophone and a 3-axis accelerometer along 27 equally spaced locations. The results show that the relationship between the two components is approximately linear, as theory suggests, but the recorded values of particle velocity are generally larger than expected, particularly for the z-axis velocity which is shown to have a magnitude of 1 to 10 times (average 3.5) that of the theoretical velocity for a plane wave at the same sound pressure.Moreover, sound pressure and particle motion showed a different frequency distribution. For sound pressure, a shallow water cut-off frequency below approximately 400 Hz was observed in the power spectrum, which was not observed for particle velocity. This could be due to ground roll waves, but also wind induced waves and vibration on the cable could cause an increase in the low frequency vertical velocities.