FIELD: drill-hole research; applicable in determination of round-trip operation velocity. SUBSTANCE: method is based on measurement of velocity of load-carrying logging cable with downhole instrument secured to said cable. Made on cable armor are magnetic marks presenting sequence of sections of different magnetization intensity. Located along moving load-carrying logging cable are two readers which read magnetic marks and convert them into corresponding signals. Distance between readers is fixed and memorized in form of signal. Signal orthogonal to output signal of one of readers is determined and delayed together with output signal of other reader. Correlation function of delayed signals is determined. Duration of delays are set so that value of correlation function of delayed signals is equal to zero. Velocity of downhole instrument are determined using values of distances between readers and duration of delays. Device for claimed method embodiment has besides readers, orthogonal signal computer, two delay units, control unit, successively connected unit for correlation function determination and processing unit, and successively connected memory unit, computer and registration unit. Output of the latter serves as device output. EFFECT: extended functional potentialities and higher accuracy. 2 cl, 1 dwg
