HR: 0830h
AN: C31B-0400    [PDF]
TI: Estimating the ground-penetrating radar velocity distribution in temperate glaciers using common-offset migration velocity analysis
AU: McKinley, A
EM: alex@pacificu.edu
AF: CGISS, Boise State University, Boise, ID 83703
AU: * Bradford, J H
EM: johnb@cgiss.boisestate.edu
AF: CGISS, Boise State University, Boise, ID 83703
AU: Harper, J T
EM: joelh@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071
AB: Most ground-penetrating radar (GPR) surveys are acquired with a constant transmitter-receiver offset and often, little processing is employed in generating the final image. This can provide useful information and is valuable as a rapid reconnaissance tool, but does not take full advantage of the information that can be carried by the radar signal. One of the most valuable material properties that can be extracted from GPR data is electromagnetic (EM) propagation velocity. In cases where there are scattering events distributed throughout a GPR profile, it is possible to compute an accurate velocity distribution from common-offset data using common-offset migration velocity analysis. Migration can be thought of as a focusing operation that attempts to move all scattered energy back to its point of origin. The net effect is that diffractions are collapsed and dipping reflectors are moved to their correct spatial position. Migration is strongly dependent on the accuracy of the velocity field. We can use this dependence and the large number of "point-like" scatterers present in temperate glaciers to our advantage. By applying a series of constant velocity migrations with a range of velocities then picking the velocity at a given depth/horizontal position that yields the most "focused" image, we can estimate a laterally and vertically continuous velocity distribution. We simply choose the velocity distribution that collapses all diffractions. In a field study on Bench Glacier, Chugach Mountains, Alaska, we applied this technique and found significant lateral and vertical velocity heterogeneity which we attribute to primarily to variations in water content. Although simple conceptually, the method requires careful application due to a number of potential problems and pitfalls including out-of-plane reflections in 2D data, pegleg multiples, and precision limited by the distribution of scatterers. In spite of these limitations, with careful application the method can provide a first order estimate of the radar velocity distribution.
DE: 0994 Instruments and techniques
DE: 1827 Glaciology (1863)
SC: Cryosphere [C]
MN: 2003 Fall Meeting