Measurement of sound speed vs. depth in South Pole ice for neutrino astronomy

TL;DR

This study precisely measured the sound speed of pressure and shear waves in South Pole ice between 80 and 500 meters depth, finding negligible variation with depth, which benefits acoustic neutrino detection.

Contribution

It provides the first detailed depth profile of acoustic wave speeds in South Pole ice, confirming minimal refraction at depths relevant for neutrino astronomy.

Findings

Sound speed is nearly constant between 200 and 500 m depth.

Negligible refraction of acoustic waves at depths >200 m.

Pressure wave speed from explosive signals agrees with sensor measurements.

Abstract

We have measured the speed of both pressure waves and shear waves as a function of depth between 80 and 500 m depth in South Pole ice with better than 1% precision. The measurements were made using the South Pole Acoustic Test Setup ({SPATS}), an array of transmitters and sensors deployed in the ice at South Pole Station in order to measure the acoustic properties relevant to acoustic detection of astrophysical neutrinos. The transmitters and sensors use piezoceramics operating at $\sim$5-25 kHz. Between 200 m and 500 m depth, the measured profile is consistent with zero variation of the sound speed with depth, resulting in zero refraction, for both pressure and shear waves. We also performed a complementary study featuring an explosive signal propagating from 50 to 2250 m depth, from which we determined a value for the pressure wave speed consistent with that determined with the sensors operating at shallower depths and higher frequencies. These results have encouraging implications for neutrino astronomy: The negligible refraction of acoustic waves deeper than 200 m indicates that good neutrino direction and energy reconstruction, as well as separation from background events, could be achieved.