Comparison of Swallow Float, Ocean Bottom Seismometer, Sonobuoy Data in the VLF (Very Low Frequency) Band

摘要

VLF data collected concurrently by three sets of instruments--the MPL's freely drifting Swallow floats, an array of OBSs deployed by SIO, and sonobuoys from the NADC are compared. The data were collected in April and May, 1987, near the site of DSDP hole 469, 32.5 deg N, 120.5 W deg. To make the data comparisons quantitative, the OBS and sonobuoy data were resampled to a sampling frequency of 50 Hz and the time bases for the 3 sets of instruments were aligned using coherent arrivals. Spectra, spectral differences, and coherence squared estimates were calculated and plotted. Results indicate that the Swallow float geophone data are consistent i.e., the spectra from different instruments of the same type do not differ within the confidence limits of the spectral estimates as are most of the OBS vertical-axis geophone data. The inter-Swallow-float coherences are significantly different from zero at frequencies thought to be broadcast by the research vessels in the experiment. The inter-vertical-axis-OBS coherences are also significantly different from zero at frequencies below 6 Hz as well as at the ship frequencies. The high coherences below 6 Hz are probably due to the close spacing of the OBSs in the experiment. On the other hand, the OBS horizontal-axis geophone data from different instruments can vary over several tens of decibels in spectral level. The OBS hydrophone time series contain 0.5-second spikes of various amplitudes, and the sonobuoy data appear to suffer from contamination below 10 Hz. (EDC)