Abstract:

As part of the ONR Sea-Ice Mechanics Initiative (SIMI) experiments in the spring of 1994, arrays of high-resolution seismic sensors were deployed in areas of high seismicity identified by real-time processing of acoustic emission events recorded by a wide, horizontal aperture hydrophone array. One of the areas most rich in ice events was located on a small ice floe (~100x100 m) 4 km northeast of the main camp, where the seismic array consisting of five 3-component geophones in a 70-m aperture pentagon was deployed. One of the methods employed for analysis of the near-field data from this deployment was motion-product seismograms introduced by J. E. White [Geophysics 24, 288--298 (1964)]. In this method each of the horizontal components of ice motion was multiplied by the vertical component, with or without phase shift, and after integration, the two resulting products identified a vector pointing to the source of the seismic waves. The ability of this method to separate different polarizations of the seismic waves and to determine the direction to the source was especially useful for geophone array data, because other analysis methods for such data occasionally failed due to the overlapping of waves generated by different ice events. Using the polarization processing method the development of ice fractures in the array near field was successfully tracked in the time and spatial domains. One result of the polarization analysis was that these fractures seemed to mostly generate vertically polarized shear (SV) waves. [Research supported by ONR.]