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    Amplitudes of seismic waves on ice sheet in East Antarctica
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    Most seismic surveys are conducted on the surface-of necessity. There are, however, advantages in placing the detector in the subsurface : the seismic response can be related with precision to the subsurface which is known in detail from logging and coring; seismically there is a good environment for recording; and there are also some other benefits.
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    Earthquakes along the mid-Arctic ridge radiate earthborne compressional and shear waves that couple into the Arctic acoustic channel and propagate as T phases. The T-phase energy is transmitted above the epicenter through the seafloor at near vertical angles. Scattering from the ice surface couples some of this energy into waterborne angles. At 5 Hz, the coupled energy is about 40 dB down from the incident levels and at 15 Hz, it is about 30 dB down. Scattering accounts for the characteristic spectral shape of the abyssal T phase that is more energetic at 15 Hz than 5 Hz. Several T-phase arrivals were recorded on hydrophones during the FRAM II experiment. From the measured acoustic levels, a transmission loss prediction between source and receiver, and the scattering conversion losses, the microearthquake acoustic source level in the water column is estimated to be about 250 dB re: 1 μPa/Hz at 5 Hz and 235 dB re: 1 μPa/Hz at 15 Hz. These levels lead to a crustal attenuation estimate of 0.52 dB/km at 5 Hz that is consistent with FRAM II refraction measurements. [Work supported by ONR.]
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