Azimuthal dependence of hydroacoustic blockage at Diego Garcia and implications for discrimination: Army Space and Missile Defense Command

An understanding of hydroacoustic blockage around bathymetric features serves many purposes in ocean monitoring. These include the planning of station placement, estimation of network detection thresholds, and the evaluation of evasion scenarios. Our early estimates of hydroacoustic blockage in the Indian Ocean (Pulli and Upton, 2001) utilized ray based propagation models and binary blockage estimates based on a variety of environmental criteria. These models predicted, for example, that T-waves from events along the Sumatran Arch would not be seen at the north array of Diego Garcia. However, once actual data were accumulated for the Diego Garcia arrays, observations showed that paths that were predicted to be blocked were actually attenuated, often by approximately 30 dB. Possible mechanisms for this process include diffraction around the island, refraction, and perhaps acoustic-to-seismic-to-acoustic conversion. In order to better under the azimuthal and frequency dependence of blockage around Diego Garcia, we have accumulated a dataset of earthquake sources located around the atoll and recorded by both the north and south arrays. The goal is to assemble a virtual array of sources for high-resolution studies. For example, events to the west of Diego Garcia include those occurring along the Carlsberg Ridge and Chagos Archipelago. A typical blockage value from the north to the south arrays is approximately 30 db. To the east of Diego Garcia, the recent sequence of events along Sumatra provides a vast amount of data for analysis. Blockage estimates here are about 25 dB. To the south of Diego, events along the Mid-Indian Ridge are used. For each event, the spectrum of the T-wave recorded on the north and south arrays is measured and compared. To understand the implications of these measurements for detection and discrimination, we combine these measurements with our studies of T-wave amplitudes (sound pressure levels) versus seismic magnitude (Pulli et al., 2005). These studies indicate a near-linear trend of 15 dB/Mw. Broadband background noise levels at Diego North are approximately 90_95 dB and at Diego South they are 95_100 dB. Combining these measurements with an average blockage value of 30 dB, we estimate that blocked signals from events along the Sumatran arch can still be detected if the event is at least magnitude 4.5. Spectral decay of T-waves averages 1.5 dB/Hz above 1 Hz. Hence, by 30 Hz, the high frequencies of the blocked signals at the minimum detection level will be below the noise. Discriminants based on the ratio of high-frequency to low-frequency energy will thus not work for these small events. However, bubble-pulse frequencies are typically lower than 10 Hz, so cepstral parameters may still be able to separate underwater explosions from nonexplosions using these low amplitude blocked signals.
Pulli J , Upton Z .
hydroacoustic blockage, Army Space and Missile Defense Command, Diego Garcia