The Usse of SONAR to Measure Ice Thickness

Abstract

The need frequently arises among individuals whose work must be carried out on the ice in rivers, lakes, and reservoirs for an instrument which conveniently measures ice layer thickness. Safety is possibly the most important consideration. In addition scientific studies of the processes involved in ice jam formation or of the effect of the ice layer on the ecology of the river lake or reservoir could be enhanced by an instrument that could conveniently and quickly measure ice thickness. At present ice thickness can be reliably determined by drilling a hole and measuring the thickness by some form of hook gage. This procedure requires, however, that individuals venture out on what is possibly "too thin" ice to make the measurement. For a number of thickness measurements over an area, surface drilling becomes a time consuming process. The various forms of nondestructive measuring devices currently in use employ an echo ranging technique with electromagnetic or acoustic waves. These devices are known by a number of acronyms such as; LASER ranging, RADAR, and SON AR. Their usefulness for ice- layer thickness measurements would depend, in addition to other considerations, on the strength of the reflection from an ice water interface. The expected distance to be measured is estimated from 1 cm to 100 cm (in the Minnesota Region) from the ice surface to the ice-water interface below. The velocity of propagation of electromagnetic waves of nearly 300,000,000 mls indicates echo ranging with electromagnetic waves would require the measurement of very small time intervals to infer distances as small as one centimeter. The time required for distance measurement using ultrasound, with a sound wave velocity of 3200 mls in ice, would be considerably longer, typically a few hundred microseconds. While the technology exists for the measurement of the extremely short transit time of electromagnetic pulses, the cost and complexity compared to the measurement of a considerably longer time for a acoustic pulse to travel the same distance is considerably greater.