dif3 curve which exceedes a third preselected magnitude reference value such as 0 8 if no 34 dip 34 is located along the portion of the selected interval located between the two peaks a determination of no collar at the selected point will be made at 30 followed by the selection of the next point on data curve dif3 at 31 and a return of the data processing system to step 23 for further processing should a 34 dip 34 be located at 38 the portions of the selected interval outside of the peaks will be examined at 39 for the presence of 34 rises 34 a 34 rise 34 being defined as a third response of polarity opposite to said predetermined polarity on the dif3 curve which exceeds a fourth preselected magnitude reference value such as 0 4 said fourth preselected magnitude reference value typically being of lesser magnitude than said third preselected magnitude reference value should a single 34 rise 34 be identified and located between the outer edge of each peak and the respective boundary of the selected interval the detection of a collar at the selected point will be designated at 40 the absence of two such rises at 39 will result in a.