Michael R. Gorman

Penetration of Antarctic subglacial lakes by VHF electromagnetic pulses: Information on the depth and electrical conductivity of basal water bodies

Owing to the high level of absorption of very high frequency radio waves in water, previous investigators of airborne radio echo sounding (RES) data from Antarctica have assumed that the depth of subglacial lakes cannot be measured directly by this method. However, we have identified a number of RES returns from beneath the ice-water interface at the surface of eight subglacial lakes that we have interpreted as being reflected from the lake floor. The returns allow us to measure the depth of subglacial lakes, since the velocity of radio waves in water (33.4 m μs−1) is relatively unaffected by electrical conductivity. Attenuation of radio waves within water is controlled largely by its electrical conductivity. Consequently, by examining the decay of the radio wave amplitude with depth we can gain information about the conductivity of subglacial water bodies. Our results indicate that the minimum water depths of eight subglacial lakes vary between 8 and 21 m. The lakes from which our depth measurements were taken are distributed widely around the ice sheet. Thus it may be concluded for the first time that Antarctic subglacial water bodies are generally at least several meters in depth. By examining the attenuation of radio waves through subglacial water, the electrical conductivity of the water is estimated to be extremely low (i.e., fresh pure water).

Evidence for Floating Ice Shelves in Franz Josef Land, Russian High Arctic

Examination of digital Landsat TM and MSS imagery of Franz Josef Land, Russian High Arctic, reveals a number of ice caps with apparently very low surface gradients at their seaward margins. The largest of these low gradient areas is 45 km2. The areas are dynamically a part of the parent ice mass, and have a marked break of slope at their inner margins. They generally occur in protected embayments and often have relatively deep water offshore. The presence of deep inter-island channels (up to 600 m) in the archipelago also suggests that deglaciation after the last glaciation may have proceeded rapidly due to enhanced iceberg calving. Tabular icebergs (maximum observed length 2.3 km) are produced from several of the low gradient ice cap margins today. Ice surface profiles, derived from analysis of vertical aerial photographs, show slopes of 0.50 on these features, as compared with 3.5 to 50 on other ice caps. At least some are likely to be floating ice shelves. They have similar ice surface gradients to a known ice shelf on Severnaya Zemlya. There is no requirement for deep water to occur beneath these features, but simply that they become buoyant over a significant part of their base. Glacier thinning, due to reduced mass balance since the termination of the Little Ice Age, may have contributed to the presence of these features. An origin for some of these low gradient margins by deformation of an unlithified substrate cannot be ruled out. Field radio-echo experiments could be used to test the interpretation of these features as ice shelves.

Acoustic Emission in 2-D Carbon-Carbon Coupons in Tension

Carbon-carbon coupons were loaded and unloaded in uniaxial tension to progressively higher loads. Hysteresis in the material was noted especially after the stress-strain curves became nonlinear at about 2000 μ. Each subsequent reloading showed an increase in the linear range similar to work hardening in metals except that the modulus was lower on each cycle. The onset of acoustic emission was monitored on each cycle and the computed felicity ratio correlated with the change in stiffness.