Method

This method used high frequency signals which are reflected off the ice/rock interface. The data is stored on a video cassette and can be replayed through a printer to obtain an immediate visual record.

The layout of the equipment is similar to that used in seismic surveying. A block diagram of the instrumentation is shown in Figure 17. The transmitter and receiver were mounted on sledges and the sampler-scope and the recording section were situated on a skidoo which was used to tow the sledges. Figure 18 is a plan view of the whole unit. The skidoo was usually driven at a constant speed of 10 kmph and the signals were received at a rate of 4 per second. This gave a good single fold coverage of any reflector. Experiments were made using damped and undamped antennae to determine the best transmitter-receiver separation. All antennae were dipoles which varied in wavelength from 8-16 m.

Common midpoint profiles were also carried out. This method, described below, allows the records to be stacked and thus provides an enhanced signal-to-noise ratio.

For these surveys, the transmitter and receiver were placed 10 m on either side of a point of interest, and then moved away from the midpoint in 5 m intervals. A two minute sweep was recorded at each interval, as well as a single pulse. The maximum separation from the midpoint was 95 m.