Harnessing the ghost

Abstract

A previous field experiment demonstrated that the interface between an air‐water mixture of low fractional air volume (say, 0.005) and air‐free water is an effective reflector of acoustic waves, suggesting that acoustic waves in water might be beamed by placing an energy source at the focus of a paraboloidal interface between an air‐water mixture and air‐free water. When this interface is concave downward, wave energy traveling upward and outward from the source at the focus is reflected downward and the surface reflection (ghost) is eliminated. Accordingly, a paraboloid reflector consisting of perforated circular air pipes was constructed and tested with a small water gun at its focus. The structure, suspended from its apex, had a height of 4 ft (1.2 m), a base diameter of 8 ft (2.4 m), and a focus 1 ft (0.3 m) below the apex. The wave field, measured with an air‐bubble stream emanating from the perforated pipes, displays a beam with a nearly constant half‐width of 22 ft (6.7 m) created by the paraboloid reflector. Signal amplitudes directly below the paraboloid on an extension of its axis are as much as 9 dB above, and off‐axis amplitudes are as much as 17 dB below, signal amplitudes for no air‐bubble stream. The source signal’s amplitude spectrum, extending to about 1.5 kHz, is skewed toward higher frequencies by the paraboloid reflector, resulting in a dominant peak frequency of about 1210 Hz. The ghost is not evident. Theoretical wave fields of beamed monofrequency source signals demonstrate that the paraboloid reflector behaves as a high‐cut filter, the band‐pass narrowing as axial distance to the paraboloid decreases and off‐axis distance increases. Axial rates of decrease of the beamed experimental signal with increasing depth, at depths of 20 ft (6.1 m) and greater below the source, compare favorably to those of beamed theoretical continuous sinusoidal signals which are nearly equal at all frequencies within the experimental signal’s frequency band. Additional research is required to determine the complete effect of the paraboloid reflector on water‐gun signal characteristics. The small prototype reflector used in this experiment is not suitable for typical seismic exploration efforts because it produces excessively high‐frequency content. A useful paraboloid reflector may have to be several times the size of this prototype.