Monitoring gravel framework dilation using a new digital particle trackingmethod

A digital particle tracking algorithm designed to monitor small (10(-4)-10( -2) m) movements of gravel clasts from digital video imagery is presented. Using the image brightness values as a set of weights, the algorithm identi fies the sub-pixel co-ordinates of black circular transfer markers applied to contrasting tracer clasts. Once identified, marker centroids are tracked from frame to frame at a sampling frequency of 25 Hz to give high resoluti on time-traces df particle trajectories. Theoretical tests on the algorithm using simulated data with additive Gaussian errors suggest centroid detect ion to sub-pixel accuracies of 0.008 pixels (1 sigma). Experimental tests w hich incorporate the effects of camera lens aberrations, CCD defects and su b-optimal illumination degrade the precision by an order of magnitude, alth ough centroid detection to 0.1 pixels at the 99% confidence interval is pos sible. Finally, the algorithm was applied to monitor particle movements in a gravel framework under shear. Preliminary results suggest significant exp ansion of the coarse framework just prior to entrainment, a phenomenon whic h may be important in controlling the pattern of fine sediment ingress. (C) 2000 Elsevier Science Ltd. All rights reserved.