The seasonal variation in the sound-scattering layer observed at Fletcher's ice island (T-3) with a 12-kc/s echo sounder

Abstract

FOR NEARLY tWO years, from June 1963 to the present, an echo sounder has been operated continuously at Fletcher's Ice Island (T-3), a drifting research station in the central Arctic Ocean. Diffuse reverberations are frequently seen on the echograms during the summer months. These reverberations are similar in general appearance to the deep scattering layers of other oceans. In non-polar oceans, the deep scattering layer is usually found at depths between 200 and 600 m during the day. At night it rises almost to the surface and weakens or disappears. Two important features distinguish the Arctic Ocean scattering layers from these non-polar scattering layers : (1) the Arctic Ocean scattering layers are relatively shallow, 50-200 m, and (2) the Arctic scattering layers have a pronounced annual, rather than diurnal, cycle. These effects are probably related to light conditions peculiar to polar oceans. First, the light is relatively weak beneath the permanent ice cover. Second, the d a y becomes effectively one year long at these high latitudes so that the Arctic scattering layers are present at moderate depths during the summer light period and then disappear during the winter dark period. In addition to the scattering layers, discrete echoes from shallow depths are frequently recorded. These discrete echoes occur throughout the year but are particularly frequent during the winter. These echoes take a hyperbolic shape indicating relative movement between the ice station and the reflector. Presumably they are caused by nekton such as fish or seals. This is believed to be the first observation of sound scattering layers in the central Arctic Ocean. An examination by DIETZ and SHUMWAY (1961) of echograms taken on the nuclear submarines Nautilus and Skate during their Arctic Ocean cruises revealed no deep scattering layers or discrete echoes. However, the submarines probably cruised within or below the scattering layer. The echo sounder at the ice station was better placed for detecting a shallow scattering layer. T-3 drifted along the track shown in Fig. 1 during the period of echo sounding measurements. The Precision Depth Recorder system consisted of a standard Edo transducer, a spark-type drum recorder and a sonar transceiver manufactured by T. H. Gifft and Associates. The system operated with an electrical power output of 800W at a frequency of 12 kc/s. The outgoing ping was 80/zsec (l cycle) in length and was repeated once each second. The drum rotated once each second