Surface and bedrock topography of ice caps in Iceland, mapped by radio echo-sounding

Since 1977, large areas on western Vatnajokull have been surveyed by ground-based, radio echo-sounding and the whole ice cap, Hofsjokull, was surveyed in 1983. Detai led maps of the glacier-surface elevation and the sub-ice bedrock have been compiled . The instrumentation includes a 2-5 MHz, mono-pulse echo-sounder, for continuous profiling. a satellite geoceiver and Loran-C equipment, for navigation, and a preCISIOn pressure altimeter. The maps of western Vatnajokull cover about 1500 km2 and are compiled from 1500 km-long sounding lines, which yielded about 50000 data points for ice thickness and 20 000 points for ice-surface elevation. The maps of Hofsjokull cover 923 km 2 , the sounding lines were 1350 km long; 42000 points were used for determining ice thickness and 30 000 for surface elevation. The maps obtained from these data are the first ones of the ice caps with surface elevation of known accuracy. The bedrock map of western Vatnaj okull shows details of volcanic ridges and subglacial valleys, running north-east to south-west, as well as the central, volcanic complexes, Hamarinn , Bardarbunga , and Grimsvotn and the related fissure swarms. The map of Hofsjokull reveals a large volcanic complex, with a 650 m deep caldera. The landforms in southern Hofsjokull are predominantly aligned from north to south, but those in the northern ice cap run north by 25 0 east. INTRODUCTION The paper describes detailed mapping of the ice surface and bedrock topography of western Vatnajokull and the whole ice cap , Hofsjokull, in central Iceland (Fig. I ). Both ice caps are situated within the active volcanic zone and cover unexplored landforms, significant geological structures, and volcanic features . Further, some of the largest rivers in Iceland originate at these ice caps. The first maps that outline, with some accuracy, the extent of ice caps in Iceland were surveyed by the Danish Geodetic Institute, in the period 1904 to 1938, and published on a scale of 1:100000 (N0rlund 1944). On these maps , however, the indicated ice-surface elevation was not based on surveying, except along the edge of the glaciers. The contour lines were drawn with the help of oblique aerial photographs and show, according to N0rlund (I 944), the form of the ice surface, rather than its elevation. The Fig.1. The ice caps of Iceland and the location of the acti ve volcanic zones. maps of western Vatnajokull, as well as Hofsjokull, were surveyed in 1937-1938 and the oblique aerial photographs date from 1938 . The position of the glacier edge was later revised by air photographs from 196 I (see Fig .5) . The next maps of the glaciers were compiled by the U.S. Army Map Service, on a scale of 1:50000, on the basis of air photographs from 1945-46 and the triangulation system previously surveyed by the Danish Geodetic Institute. The ice-surface contours, in the marginal areas of the ice caps (some few km up from the edge), were compiled from the air photographs, but, higher up, the contours are identical with those of the Danish Geodetic Survey maps. A recent test of the reliability of the elevation contours on these maps (outside the ice caps, of course) indicates that they are generally accurate to :I: 10 m, but, in some places, they prove to be wrong by up to lOO m (Sigmundur Freysteinsson and Orn Arnar Ing6lfsson, personal communication 1985). In addition to these two sets of maps, ice elevation has repeatedly been geodetically levelled on two profiles on western Vatnajokull (Tungnaarjokull) (see Freysteinsson 1984) and from Grimsvotn north-east, towards K verkfjoll. Further, Landsat images of Vatnajokull (see Fig .3) show the extent of the ice cap and surface forms which reflect sub-ice features (porarinsson et al. 1973). The same applies to Hofsjokull (Fig.4) , which covers prominent, buried , volcanic structures (see e.g. Sremundsson 1979). Exploration of the ice thickness of Vatnajokull and the sub-ice topography started in 1951, when a French-Icelandic expedition carried out seismic soundings at 33 points, spread over the ice cap (Holtzscherer 1954). On the basis of these soundings, Eyth6rsson (1951, 1952) presented a preliminary map of the bedrock beneath Vatnajokull. This map shows the central ice masses resting on a highland plateau , at 700-800 m altitude, and only a few peaks rising above the 1000 m elevation, which marks the present level of the firn line in the southern part of Vatnajokull. In 1955, a second French-Icelandic expedition continued the seismic work on Vatnajokull (Sigurdsson 1970) and on Myrdalsj6kull (Rist 1976). The next step in the exploration of the ice depths was taken on Vatnajokull by gravity surveying . In 1960 and 1961, the average ice thickness was estimated in the Grimsvotn area and along a profile from Grimsvotn to K verkfjoll (Palmason 1964). Two profiles were later surveyed at Tungaarjokull (Sigurdsson 1970) and Thorbergsson (unpublished data) made gravity measurements on several points on Vatnajokull, in 1971. Bjornsson (1974) summarized all the available data. No observations were available on the bedrock topography of glaciers other than Vatnajokull and MYrdalsjokul1. Since 1977, the Science Institute of the University of Iceland has made radio echo-soundings of ice depths . Continuous ice thickness profiling has been carried out by a series of oversnow traverses on large areas of the ice caps Vatnajokull, Hofsjokull, and Myrdalsjokull, as well as on some valley glaciers (Bj6rnsson 1977, 1981, Bjornsson et. al. 1977, Sverrisson et. al. 1980, and Bjornsson (various internal reports» . The total length of the sounding lines is about 4000 km so far. Maps of the glacier surface and the sub-ice bedrock have been compiled. Since 1980, the main work has been funded by the Iceland Power Company.