Seismoacoustic beamforming with a linear array of geophones on the seafloor

Abstract

Experiments were carried out in the Straits of Sicily, Malta Channel where the seafloor is a relatively flat silty sediment some 150 m deep. A linear array of ten vertically gimballed geophones with element separations of 5 m was laid out on the seafloor. Measurements were made on seismic fields created by explosive shots. Similar experiments were performed in Vestfiord, Norway, where the seafloor is mud over glacial till. The seismic waves of primary interest at these sites are the dispersive Scholte waves. Group velocity dispersion curves from several different shots were derived using the multiple‐filter method and show both sites to be isotropic. From these curves the phase velocity as a function of frequency was obtained. A beamforming algorithm was implemented that accounted for the change in seismic velocity with frequency. This enabled the array to be beamsteered on the shot signals providing array responses at various frequencies. The resulting directivities and sidelobe characteristics were in good qualitative agreement with array theory. Since seismic velocities were quite low (50–130 m/s), narrow beams were formed at low frequencies; half‐power widths at 18° at 7 Hz to 13° at 11 Hz were obtained with an array only 50 m in length. [Work supported by Defense Advanced Research Projects Agency, Maritime Systems Technology Office.]