Characterization of Infrasonic Signals Using Time of Flight Measurements

Abstract:

This report is motivated by a NASA mission concept for studying seismic activity near the surface of Venus, using sensors located on a balloon platform in the more thermally hospitable cloud levels of Venus’ atmosphere. The high atmospheric density at the Venusian surface allows for seismic tremors to couple to the atmosphere and propagate through the air as faint infrasonic waves. We propose a method of detecting and characterizing these infrasonic signals using an acoustic anemometer. By measuring the time of flight of an ultrasonic signal across a fixed gap and studying the variations in the travel time, it is possible to detect the oscillatory air motions, and thus the pressure waves emitted from an infrasonic source.

We used an off-the-shelf subwoofer to generate the low frequency sine waves and our general-purpose ultrasonics units (VN Instruments Inc. SIA7) to conduct the time of flight measurements. Each SIA7 unit measured the time of flight of an ultrasonic chirp between two transducers placed one meter apart down to ~single nanosecond accuracy. The infrasonic signal produced an AC oscillation (with frequency matching that of the infrasonic source) in the time of flight data. The amplitude of the oscillation was dependent on the orientation of transducers relative to the infrasonic source. Off-axis measurements (transducers perpendicular to the incident infrasonic wave) resulted in a muted AC signal, demonstrating that the system has directional sensitivity. Another crucial finding was that the transducers were sensitive to sound pressure levels on the order of millipascals at Earth ambient pressure (i.e., 10^5 Pa), which is similar to the pressure levels proposed for the Venus seismology mission.

In other directions, work is being done using multiple SIA7 units and a subwoofer sound source to demonstrate long distance infrasound detection and pulse echo style infrasound imaging of nearby buildings and structures.

The feasibility of using ultrasonic time-of-flight measurements for directional measurement of infrasonic signals and detection of low-amplitude sound pressure waves is very promising. An infrasound detecting system for Venus (or Earth), capable of resolving amplitude (down to ~1 mPa), frequency (<~20Hz) and propagation direction could be produced with mass <1kg and power <~0.5W.