The simultaneous measurement of infrasonic acoustic particle velocity and acoustic pressure in the ocean by freely drifting Swallow floats

The Marine Physical Lab´s Swallow floats, which are capable of simultaneously measuring the ocean´s infrasonic (1-20 Hz) acoustic particle velocity and infrasonic acoustic pressure, are described. The floats are independent, freely drifting, neutrally buoyant sensor systems that can be ballasted to any desired depth (within ±100 m) in the ocean. Calibration measurements of the infrasonic sensors and electronics agree to within ±1 dB in amplitude and ±3° in phase with the theoretically predicted response curves. Electronic self-noise, quantization noise, and effects of signal clipping are significantly below ambient ocean noise levels, except in unusual circumstances. Rotational resonances, primarily float rocking below 0.5 Hz, contaminate the geophone data at the lowest end of the infrasonic band. From data collected in a deep-ocean deployment, the equivalent pressure autospectrum calculated from the particle velocity data agrees to within 1 dB of the actual pressure autospectrum in the frequency band from 1-20 Hz, thereby verifying the high quality of the infrasonic acoustic data