Near-field polarization processing of ice fracturing events

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 (∼100×100 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.]

Source bearing estimation in the Arctic Ocean using ice-mounted geophones

“…[1][2][3][4][5]9 This paper will not consider the general theory; however, a basic description of the fundamental seismic wave types is required. Acoustic energy from an underwater source is transmitted to an ice-mounted geophone both as seismic plate waves propagating along the ice ͑due to acoustic energy transmitted into the ice between source and receiver͒, and as acoustic waves that arrive at the underside of the ice directly below the geophone and couple locally into seismic waves.…”

“…While a number of previous studies have been reported that used ice-mounted geophones to investigate sources within the ice layer, e.g., ice cracking and ridging, [1][2][3][4][5] relatively little work has been published considering acoustic sources in the water. The approach to bearing-estimation developed here is based on measuring ice seismic waves for which the direction of wave oscillation ͑particle motion͒ is oriented radially outward from the source.…”

“…To this end, seismic polarization filters, developed in earthquake seismology 10 and previously applied to ice fracturing events, 4 are adapted here for a water-column source and applied in conjunction with the rotational analysis. Polarization filters make use of threedimensional ͑3D͒ measurements of the particle motion, and apply theoretical phase relationships between vertical and horizontal components for the various wave types to selectively suppress or enhance particular waves.…”