Anisotropic radio-wave scattering from englacial water regimes, My ´rdalsjo ¨ kull, Iceland Kenichi MATSUOKA, 1 Throstur THORSTEINSSON, 1,2 Helgi BJO ¨ RNSSON, 2 Edwin D. WADDINGTON 1 1 Department of Earth and Space Sciences, Box 351310, University of Washington, Seattle, Washington 98195-1310, USA E-mail: matsuoka@ess.washington.edu 2 Institute of Earth Sciences, University of Iceland, Sturlugata 7, IS-101 Reykjavı ´k, Iceland ABSTRACT. Colinear-polarized 5MHz radar profiling data were obtained on My ´rdalsjo ¨ kull, a temperate glacier in Iceland. Radar transects, and therefore polarization planes, were aligned approximately parallel, transverse and oblique to the ice flow direction. Echoes from the shallower half to two-thirds of the ice were 10–20 dB stronger on the oblique and longitudinal transects than those on the transverse transects. Anisotropy as a function of depth is clearly seen at the sites where the transects cross. Strong scattering on longitudinal transects apparently caused extinction of a radar-reflecting layer that was continuously profiled on the transverse transects. A radio-wave scattering model shows that scattering from a longitudinal water-filled conduit parallel to the glacier surface can explain the observed azimuthal variations of the echo. We conclude that low-frequency (MHz) radio waves can help to characterize englacial water regimes. INTRODUCTION Water passageways introduce heterogeneity to the interior of temperate glaciers. They route rain and surface meltwater efficiently into and beneath the body of the glacier. Basal water, in turn, strongly controls basal motion of the glacier. Macroscopic ice-walled near-circular conduits are generally considered to provide passageways for water through glaciers (Shreve, 1972; Fountain and Walder, 1998). Recently, Fountain and others (2005) proposed that the primary passageways are provided by fractures, rather than pipe-like conduits. Characteristics of these water passage- ways and their seasonal evolution and spatial variations are keys to understanding the hydrology and dynamics of mountain glaciers. Acceleration of polar glaciers following increased surface melting is caused by increased basal motion, which is presumably caused by penetration of surface meltwater to the bed (Zwally and others, 2002). Understanding the geometry of water passageways is crucial if we want to examine spatio-temporal relationships between surface melting and basal lubrication events and, ultimately, assess the response of ice sheets to climate change. Radar is the unique tool to determine the evolving geometry and distribution of water passageways. The strong contrast in dielectric permittivity between ice and water facilitates radio-wave remote sensing as a viable method to investigate spatio-temporal variations of glacier interiors. Frequency dependence of the radio echoes has been used to map thermal regimes of polythermal glaciers (Bamber, 1987; Bjo ¨ rnsson and others, 1996; Moore and others, 1999; Pettersson, 2005) and water cavities in a temperate glacier (Jacobel and Raymond, 1984). In addition, radar polarization can potentially be used to characterize englacial water regimes. As water bodies have curved shapes, regardless of whether they are fractures or conduits, they can depolarize radio waves. At a polythermal glacier, Storglacia ¨ren, Sweden, Walford and others (1986) found that echoes from within temper- ate ice varied with the radar polarization. Using 60 MHz co-polarized waves they made measurements with two polarizations, one aligned approximately parallel to the ice flow, the other approximately perpendicular. Polarization parallel to the ice flow gave, on average, 15% (0.7 dB) stronger echoes at a given depth than polarization per- pendicular to the ice flow. They suggested that this feature was caused by water-filled cavities and, to a lesser extent, air-filled cavities. That study leads us to hypothesize that englacial water passageways make azimuthal variations in the radio echo at low frequencies (MHz) in terms of the radar polarization. Radio waves of 1–10 MHz are more commonly used than those of higher frequencies to examine temperate glaciers because of their greater penetration through heterogeneous temperate ice. Our objective is to test this hypothesis with field observations using 5MHz radar, which we augmented with numerical results for scattering from a cylinder. We established transects at a range of orientations on My ´rdals- jo ¨kull, a temperate glacier in Iceland. We compared the englacial echo intensity as a function of polarization and depth at the crossover sites of these transects to look for polarization dependence. RADAR MEASUREMENTS Radio-echo intensities were recorded along thirteen trans- ects over a 4 km  4 km area (Fig. 1a) on northern My ´rdals- jo ¨kull; six transects were approximately parallel to, and five transects were approximately transverse to, the estimated ice flow direction (i.e. local steepest-descent path on the surface). Two additional transects had intermediate oblique orientations (Fig. 1b). These transects were profiled within a 3 day period in April 2003. Ice thickness in this area, derived from these radar data, is shown in Figure 1c (after Thor- steinsson and others, 2005). The equilibrium-line altitude (ELA) on the northern slope of My ´rdalsjo ¨ kull is estimated to be 1000 m a.s.l. (Brandt and others, 2005). During our measurements, the glacier surface was covered by approxi- mately 7 m of seasonal snow; this depth was estimated while a steam drill was used to install poles for an ice-motion study Journal of Glaciology, Vol. 53, No. 182, 2007 473 Downloaded from https://www.cambridge.org/core. 12 Nov 2021 at 14:08:02, subject to the Cambridge Core terms of use.