Using Diffractions to Measure Horizontal Motions

A small (meter scale) irregularity on the basal reflector gives rise to a diffraction. The differential traveltime of this diffractor can be used to distinguish horizontal and vertical motions. They key indication of horizontal motion is a change in sign of differential traveltime with range.

Traveltime of the the diffactor (solid) and basal reflector (bold). The transmitter and receiver are colocated on the top surface of the ice, and their range is varied between 0 and 2000 m. The diffractor is on the basal surface (depth of 2600 m) at a range of 1000 m. Note that the diffraction is delayed from the reflection by more than a pulse width (about 100 ns) for ranges greater than a few hunder meters, and should thus be easy to resolve. (Postscript version).

Differential traveltime of the the diffactor (solid) and basal reflector (bold), when the basal reflector (including diffractor) is moved 2 cm horizontally. Note that the differential traveltime of the diffractor is positive for ranges less than 1000 m (the position of the diffractor) and negative for greater ranges. (Postscript version)

Differential traveltime of the the diffactor (solid) and basal reflector (bold), when the basal reflector (including diffractor) is moved 2 cm vertically. Note that the differential traveltime of the diffractor is positive (and approximately constant) at all ranges. (Postscript version)