Acoustic Signal Propagation and Ice Noise in the Arctic Basin
By Bruce Rule - Feb 22, 2014
There is a significant “volume” of misinformation about the propagation of low frequency acoustic energy within and across the Arctic Basin. The following abstract, from http://anchor.apl.washington.edu/papers/TAP.pdf provides useful information in this subject area. Paragraphs further below discuss ice noise.
Abstract—In April 1994, coherent acoustic transmissions were propagated across the entire Arctic basin for the first time. This experiment, known as the Transarctic Acoustic Propagation Experiment (TAP), was designed to determine the feasibility of using these signals to monitor changes in Arctic Ocean temperature and changes in sea ice thickness and concentration. CW and maximal length sequences (MLS) were transmitted from the source camp located north of the Svalbard Archipelago 1000 km to a vertical line array in the Lincoln Sea and 2600 km to a two-dimensional horizontal array and a vertical array in the Beaufort Sea. TAP demonstrated that the 19.6-Hz 195-dB (251-W) signals propagated with both sufficiently low loss and high phase stability to support the coherent pulse compression processing of the MLS and the phase detection of the CW signals.
This research establishes that signals in the area of the acoustic spectrum of interest to the IUSS community can propagate to bathymetry-limited ranges in the Arctic Basin and, if high phase stability is not a requirement, levels significantly less than 195-dB can be detected at extended ranges.
All well and good if the detectability of a known (test) frequency is the objective. If; however, the objective is surveillance to identify signals for which the frequencies are not precisely known, the problem enters another arena.
The acoustic environment beneath and near sea-ice covered areas can be – and usually is - a menagerie of signals: from extremely stable low frequency sources with durations measured in hours to highly variable "diffuse" sources with harmonic structure. All such signals can be detected beyond the limits of the ice-covered areas. These signals are produced by the movement of areas of ice-cover against contiguous areas: a long term grinding effect - to use an unscientific descriptor.
The purpose of this posting is to alert personnel who may be tasked with analysis of acoustic data collected within (beneath) or near ice-covered areas that such data can include an almost bewildering variety of high-amplitude acoustic signals with characteristics not previously observed in open-ocean areas remote from ice-cover. Some of these signals can be misclassified as "signals of interest."
Bottom line: “thresholds” established elsewhere apply under-ice, i.e., do not expect extraordinary detection threshold anomalies, and remember Occam's Razor, “the principle that states one should not make more assumptions than the minimum needed. This principle is often called the principle of parsimony. It underlies all scientific modelling and theory building. It admonishes us to choose - from among a set of otherwise equivalent models of a given phenomenon - the simplest one.”
More than 20 years ago, the writer destroyed a "sand castle" in this subject area which seriously violated Occam's Razor. This posting is all the writer can do now - primarily for active-duty analysts - to prevent another such structure from being built based on the same or other equally unreasonable assumptions.