Nearfield noise measurements from an Arctic pressure ridge

Abstract

This paper presents initial analysis results of ambient noise measurements from an active ice pressure ridge over a 3-day period in April 1979. The site was a research ice camp 180 miles from the pole in the Eurasian Basin of the Arctic Ocean. The ridge was instrumented with calibrated hydrophones paired in both the horizontal and vertical directions. Their distance from the ridge was about 100 m and their outputs were tape recorded. Dimensional surveys of the active portions of the ridge were made at the surface. Source level linear density is deduced from ambient noise measurements near the pressure ridge by assuming the ridge to be a line source of spatially incoherent point sources. Measured Arctic transmission loss data are used to deduce an active pressure ridge average separation compatable with previously measured omnidirectional ambient noise data statistics for the month of April. The effects of surface/image interference are present in the data and allow a preliminary estimate of acoustic source depth of between 14–18 m. The ridge length was approximately 500 m and was formed when a 1 m-thick refrozen lead was pushed on top of a thicker 4-m ice floe. As such, this ridge was, most likely only one ‘‘subridge’’ along the length of the boundaries of the two interacting floes. Recent SEASAT SAR data from October 1978 have shown that ice pans (defined as groups of separate ice floes that move together) can vary from 10 to 100 km in linear dimension and, unless pan size changes significantly from October to April, extremely long ridges (composed of smaller discrete ridges such as the one analyzed in this study) may exist between pressured boundaries of adjacent pans. A uniform spacing of ridges was assumed for noise model calculations and, with an active ridge spacing of 37 km, model noise level estimates and measured 50 percentile noise levels of April were in good agreement. An average ridge was assumed to be 2 km in length consisting of two active 500-m subridges. It is not known whether this ridge was typical in level, length, etc., since these were the first-known quantitative acoustical measurements of ambient noise from a nearby pressure ridge.