Estimating the speed of poroelastic interface waves using ambient noise

Abstract

Pairs of hydrophones were buried at mid-tide height in a 1:10 sloped mixed gravel and coarse sand beach and used to make ambient noise recordings over a period of three weeks in Advocate Harbour, Nova Scotia, in the Bay of Fundy. The pairs were arranged in vertical and horizontal configurations and recorded pressure time series, power spectral density, and vertical and horizontal coherence measurements of the noise field in the seabed. A nearby suite of oceanographic instruments measured the water level, ocean wave properties, bed dynamics and weather during the experiment. The measured noise between 1 Hz and 1 kHz was dominated by poroelastic interface waves generated by plunging surf on the beach face. The speed of the compressional component of the interface wave was estimated by cross-correlating the noise recorded on the across-shore oriented pair of sensors while the unconsolidated sediment was water-saturated as well as drained. Additionally, the increasing and decreasing overburden pressure due to the rising and falling 10-m tide was found to drive a respective increase and decrease in the poroelastic interface wave speed. A buried acoustic source was used to directly measure the compressional wave speed in the seabed on the across-shore array.