Simultaneous Borehole and Ocean Bottom Seismometer Recordings of Earthquakes and Explosions: Results from the NGENDEI Experiment at Deep Sea Drilling Project Hole 595B

Abstract

We use data from the 1983 Ngendei Seismic Experiment in the southwest Pacific to compare vertical component seismograms recorded by a borehole seismometer with those from ocean bottom seismometers. The borehole seismometer, the Marine Seismic System (MSS), was emplaced 54 m into the oceanic crust at the Ngendei site. Ocean bottom seismometers (OBSs) from the Scripps Institution of Oceanography were located atop 70 m of sediment and all were within 0.5 km of the borehole. Numerous seismic refraction shots were recorded simultaneously by both the borehole and ocean bottom instruments, as well as a small number of regional earthquakes. The waveforms and instrument-corrected spectra for compressional waves from these events are nearly identical for the two different instruments, while the absolute amplitudes of the arrivals differ by no more than 4 dB. Since borehole noise levels are 10 to 15 dB lower than the seafloor levels, there is a general increase in the signal-to-noise ratio for the buried instrument. The similarity in absolute amplitudes for these observations agrees with results obtained from simple synthetic seismogram calculations.

Numerical modeling of water waves recorded by a subbottom seismometer in the southwest Pacific

“…The OB$s were programmed to record 20-s segments at preselected times aligned to the refraction shooting schedule. The limited dynamic range of these instorments and the short durations preclude meaningful amplitude measurements of waterborne energy, except for a very limited distance range [Shearer et al, 1987a;Orcutt et al, 1987]. However, the MSS data were continuously recorded, except for breaks caused by system maintenance or malfunction, with 21-bit dynamic range frequencies.…”

Detailed Crustal Structure of Northern Yamato Basin

“…The use of OBS's in refraction experiments has been shown to improve the quality and resolution of the seismic structural model as evidenced in the 1985 OBS and multichannel seismic profiling (MCS) experiment in the southwestern Yamato Basin (Hirata et al, 1987(Hirata et al, , 1989. The use of a downhole seismometer in recent refraction experiments has proven to be useful in obtaining higher quality data sets and a high-resolution seismic structure Swift et al, 1988;Little and Stephen, 1985;Stephen and Harding, 1983;Jacobson et al, 1984;Shearer et al, 1986a;Whitmarsh et al, 1986;Duennebier et al, 1987;Byrne et al, 1987;Bee and Bibee, 1989), and in studying of seismic anisotropy and lateral heterogeneity (Stephen, 1985;Swift and Stephen, 1989;Shearer and Orcutt, 1985;Shearer et al, 1986b). A refraction experiment using both a downhole seismometer and OBS's, which promises greater resolution of seismic structures especially in laterally heterogeneous region, can thus contribute important details relevant to understanding the tectonic evolution of a particular region.…”