Understanding soil transmission paths of offshore pile driving noise - Seismic waves and their implications

Pile driving is a common technique to install the foundations of offshore structures, like monopiles for wind turbines or jacket piles for oil and gas platforms. It is accompanied by significant hydroacoustic noise immissions, which has led to an increased awareness of the approving authorities and tightening regulations in some countries over the past years. Depending on the governing law, these include sound monitoring measures, seasonally restricted construction phases as well as limiting values for construction-related sound levels. In many cases, the construction work takes place in shallow water environments, where the soil has a major impact on the resulting wave field. This is mainly due to the occurrence of multiple reflections, the excitation of head waves, and the possibility of energy tunneling sound mitigation systems through the soil.The pile that incrementally penetrates the soil with each hammer blow acts like a very characteristic line source, which excites seismic surface as well as body waves. Measuring these seismic arrivals enables to gain further information about the local soil characteristics. This work is based on measurement data that has been collected within the frame of three construction-accompanying offshore measurement campaigns and focuses on penetration depth-dependent frequency content of hydroacoustic/seismic signals.