Estimating the blast sill thickness using changepoint analysis of MWD data

Measurement while drilling (MWD) data are collected during production drilling in the open pit operation in question. On every bench in the open pit, a fractured marble zone (sill) is left on top of the hard and un-fractured marble to facilitate subsequent loading and transportation. The boreholes are drilled through this sill and then continue into the solid marble. The varying sill thicknesses cause challenges in the charging and blasting processes since charging the sill can cause fly-rocks and failing to charge enough can result in big boulders that need to be reduced in size through a cost- and time-consuming hammering process. If the sill thickness in each borehole is known, borehole specific charging depths can be developed. This paper shows how a step-wise changepoint analysis on MWD data can be used to estimate sill thicknesses. The estimated sill thicknesses are compared with optical televiewer (OTV) images to validate the predictions. Principal component analysis is used to create variables which gradually explain a decreasing amount of the variances in the data. The study concludes that a mean and variance-based changepoint analysis on the first principal component (PC1) can detect the sill thicknesses with a correlation coefficient equal to 0.98 and sum of squared deviation 0.12 from actual sill thickness. The PC1 is mainly (46% of the variance) described by rotation pressure, feed pressure and dampening pressure, indicating that the variations in these three variables are most important when studying high contrast physical and mechanical differences in rocks.