Seismic signals and noise recorded on a seafloor vertical hydrophone array and a colocated OBS during the LFASE experiment

Abstract

During the LFASE experiment conducted in the summer of 1989, a vertical hydrophone array (VHA) was deployed at the site of DSDP Hole 534b in the Blake-Bahama basin. The VHA consisted of 16 hydrophones spaced 30 m apart rising vertically above the seafloor with the bottom of the array anchored to a digital recording package located on the seafloor. The main purpose of the experiment was to record seismic signals and noise above, on, and beneath the seafloor at the same site; the VHA recorded data from above the seafloor, an ocean-bottom seismograph (OBS) was on the seafloor, and a borehole geophone array (BHA) was in a borehole beneath the seafloor. Signal-to-noise measurements of P waves made for airgun shots recorded simultaneously by the VHA and the OBS show that the OBS has a signal-to-noise ratio (SNR) 5 to 10 dB greater than that of a single hydrophone, but P-wave stacks of the VHA data have SNRs for body waves 10 to 15 dB greater than the OBS. This implies that a vertical array is capable of recording distinct body waves out to significantly greater ranges than is a single receiver and so might be a useful tool in seismic refraction experiments. Other stacks of the VHA data revealed S waves not visible on the data from a single hydrophone or geophone. The VHA was also used to construct images of the wavefronts arriving from a distant source and to determine the vertical direction of the source. This array processing capability shows the potential of VHAs for use in 3D seismic reflection surveys, particularly in cases where it is not convenient to use a multi-channel streamer.