FIELD: well logging. SUBSTANCE: method includes preliminary division of load-carrying logging cable into pieces of different lengths. Pieces of minimum lengths shall correspond to depth of occurrence of producing formations. Each piece is marked by numerated magnetic marks with individual nonrecurrent structure and distributed over length of marked piece. Said magnetic marks present a sequence of parts of cable armoring with different degree of magnetization. Samples of signals correspond to numerated magnetic marks and their number are stored. Downhole instrument is attached to said cable end and lowered into well. In course of lowering or lifting, magnetic mark detected on moving cable by its reading out with help of reader and its number is determined. In this case, signals corresponding to magnetic marks read out from moving cable are compared with their samples. If signal corresponding to read out magnetic mark is congruent with its sample, number of corresponding sample is considered as number n of detected magnetic mark. For this purpose, reader has matched filtration unit consisting of matched filters, one for each signal corresponding to magnetic mark. Inputs of matched filters serve as inputs of matched filtration unit, and outputs of matched filters serve as outputs of matched filtration unit. When magnetic marks are detected on said cable, boundaries of pieces and number of magnetic marks are registered of record of logging curve. Depth of lowering of downhole instrument is calculated by offered mathematical expression. EFFECT: extended functional potentialities due to increase of accuracy of determination of downhole instrument lowering depth. 2 cl, 1 dwg
