A short-pulse, long-wavelength radio-echo sounder has successfully measured the ice depth on the South Cascade Glacier. Depths up to 250 m were determined with resolution of about 5%. Bottom returns were clear and almost never ambiguous. Their accuracy was confirmed by comparison with hot-point drilling results. The secret for successful sounding in temperate glaciers is the use of a sufficiently low center frequency. Five megahertz was most successful. Tests at 15 MHz indicated an increase in coherent clutter which rendered the bottom return observable only with prior knowledge of its location. The cause of the clutter is probably water-filled voids in the ice which behave as Rayleigh scatterers.
The sounding system consists of an avalanche-transistor transmitter, which delivers a pulse to an acute-angle crossed-wire antenna. The pulse is shaped and given its center-frequency characteristics by the resonant properties of the antenna. The transmitting and receiving antennas are identical, consisting of wires and lumped resistors. The resistors reduce antennas ringing, thereby maintaining as short a pulse as possible. The receiver consists of an oscilloscope and a Polaroid camera. No preamplification is required for depths up to 250 m, but may be necessary for deeper glaciers.