Seismic and hydroacoustic observations from underwater explosions off the East Coast of Florida

In this study, seismic and hydroacoustic signals from underwater explosions in 2001, 2008, and 2016 near Florida are analyzed. These 10,000 lb chemical explosions were detonated by the United States Navy to validate the ability of new classes of ships to withstand explosions. For many of the explosions, the ground-truth (GT) epicenters are known. These epicenters are used to improve the accuracy of the locations of explosions with no GT data by performing a relative relocation using a Bayesian hierarchical seismic-event locator. Seismic and hydroacoustic signals are also used to characterize the underwater explosion sources. Bubble pulse modulations, characteristic of underwater explosions, are identified at seismic stations in the United States, and the observed bubble pulse frequency is consistent with published GT information. The absence of clear modulations in the spectra caused by reverberations in the water column means that the depth of the explosions in the water and hence the trinitrotoluene (TNT) equivalent charge weight of the explosion cannot be resolved from the frequency of the bubble pulse modulations. Published estimates of the local magnitudes ML and the known charge weights of these explosions are compared with data from previous underwater explosions. A relationship between charge weight and ML from previous well-calibrated explosions detonated in the Dead Sea is shown to provide reasonable estimates of the charge weight once corrected for the salinity of the seawater near Florida. Hydroacoustic signals from the Florida underwater explosions are also observed as H phases on hydrophone sensors near Ascension Island. The bubble pulse is not observed as clearly at the hydrophone sensors at Ascension Island possibly as a result of signal distortion in the shallow water close to Florida. This has implications for event identification using hydrophone stations and demonstrates the importance of combining seismic and hydroacoustic observations.