each possible combination defining j max as the index for which c j is maximized then by shifting curve 160 to the right by u03b4t 2 u03b4t time increments and by shifting curve 150 to the left by u03b4t 2 u03b4t time increments the curves are correlated in the time domain this step is illustrated in fig3 d where the shifts of curves 160 and 150 are designated by the numerals 176 and 174 respectively and u03b4t is denoted by the numeral 172 it should be noted that for purposes of illustration the amplitudes of the curves have been normalized at a value in time denoted by the numeral 179 this normalization is only to illustrate the high degree of correlation the lowest values of r 1 and q 1 are then discarded as the subassembly traverses the borehole over the time interval u03b4t and new values of sensors measurements r n 1 and q n 1 are added to the buffer storage forming a new data set of n values for each sensor this procedure is repeated as the subassembly 20 is conveyed along the borehole several observations are appropriate at this point of the data processing sequence for the example of fig3 u03b4t 505 seconds the incremental velocity u03b4v which is defined as is computed for each increment u03b4t since u03b4d is a known design parameter of the subassembly and u03b4t is measured over the correlation interval illustrated in fig3 rop u03b4v 39 505 0 0772 inches second 23 2 feet hour which agrees with the rop measured at the surface to within three significant digits should the rop change significantly the sample intervals u03b4t and the number.