[Paper Review] Acoustic detection of high energy neutrinos in ice: Status and results from the South Pole Acoustic Test Setup

F. Descamps|arXiv (Cornell University)|Aug 22, 2009

Astrophysics and Cosmic Phenomena1 references3 citations

TL;DR

This paper presents results from the South Pole Acoustic Test Setup (SPATS), evaluating the acoustic properties of Antarctic ice for future ultra-high-energy neutrino detection. It measures sound speed, attenuation length, and background noise, finding a pressure wave speed of 3878 ± 12 m/s at 375 m depth with no significant gradient, and preliminary attenuation lengths in the 200–350 m range, supporting the feasibility of large-scale acoustic neutrino telescopes in ice.

ABSTRACT

The feasibility and specific design of an acoustic neutrino detection array at the South Pole depend on the acoustic properties of the ice. The South Pole Acoustic Test Setup (SPATS) has been built to evaluate the acoustic characteristics of the ice in the 1 to 100 kHz frequency range. The most recent results of SPATS are presented.

Motivation & Objective

• To assess the feasibility of using acoustic detection for ultra-high-energy neutrinos in Antarctic ice.
• To measure key acoustic properties—sound speed, attenuation length, background noise, and transient rates—critical for designing a large-scale acoustic neutrino telescope.
• To evaluate the ice's acoustic transparency and signal propagation characteristics in the 1–100 kHz frequency range.
• To determine whether the ice at the South Pole supports long-range acoustic signal transmission necessary for a km³-scale neutrino detector.

Proposed method

• Deployment of four vertical SPATS strings with 7 acoustic stages each, forming a trapezoidal array at depths from 80 to 500 m in IceCube holes.
• Use of retrievable pingers and frozen-in SPATS transmitters to generate broadband acoustic pulses at 1–100 kHz, synchronized with GPS for precise time-of-flight measurements.
• Measurement of pressure and shear wave travel times to determine sound speed profiles, with linear fits applied to data from 250–500 m depth.
• Analysis of signal amplitude decay with distance to estimate attenuation length, using both time-domain and frequency-domain energy extraction methods.
• Application of amplitude ratio techniques between transmitter-sensor pairs to minimize calibration uncertainties and estimate attenuation length independently of absolute sensitivity.
• Background noise analysis using averaged noise waveforms and comparison with laboratory-calibrated sensitivities to estimate absolute noise levels below 200 m.

Experimental results

Research questions

• RQ1 What is the vertical sound speed profile of the South Pole ice cap between 250 and 500 m depth, and does it exhibit significant gradients that would cause signal refraction?
• RQ2 What is the acoustic attenuation length of the ice in the 1–100 kHz range, and is it dominated by absorption or scattering?
• RQ3 What is the level and spectral shape of continuous and transient background noise in the ice at depths below 200 m?
• RQ4 How do signal directivity and sensor response affect the detection efficiency and angular resolution of acoustic neutrino signals?
• RQ5 Can the acoustic properties of the ice support the design of a km³-scale acoustic neutrino telescope?

Key findings

• The pressure wave speed at 375 m depth is measured as 3878 ± 12 m/s, with a sound speed gradient of (0.09 ± 0.13) m/s per meter, consistent with zero.
• The shear wave speed at 375 m depth is measured as 1975.8 ± 8.0 m/s, with a gradient of (0.067 ± 0.806) m/s per meter, also consistent with zero.
• The first direct measurement of shear wave speed in South Pole ice confirms it is approximately half the pressure wave speed, as expected from theory.
• The continuous background noise level is estimated to be below 10 mPa integrated over 10–50 kHz, based on extrapolated laboratory calibrations.
• Preliminary attenuation length measurements from pinger and inter-string transmitter experiments favor values in the 200–350 m range, though absorption- or scatter-dominated behavior remains undetermined.
• In-ice transient rates are low, and most transients can be correlated with known anthropogenic sources, reducing false alarm risk for neutrino detection.