Method of seismic exploration by acoustic well logging

Claim 1

Method of seismic exploration by acoustic well logging of geologic formations penetrated by a drilled well, said method consisting of emitting and receiving various types of acoustic waves by emitting and receiving means arranged in said well, said means defining between them at least two acoustic paths of different length; recording, for each emission of a plurality of emissions, the full waveform received by at least one of the receiving means; marking, at the instant of the emission, and for each recording, the respective positions of the emitter-receiver pair which has given rise to said recorded trace; effecting a velocity analysis of some of said recorded traces to determine an approximate value of the transit time of each type of wave being propagated in said formations, wherein said velocity analysis comprises; adjusting said some of said recorded traces mathematically to account for a range of estimated propogation speeds, said range being estimated based upon the types of geologic formations which might be present; wherein on each trace, the approximate values of the transit times so-obtained are plotted so as to locate the said various types of waves which appear in the traces; selecting a wave of interest from among the said various types of waves and for each type of wave of interest, selecting an event characteristic of the phase of the wave of interest in the trace, and determining the time corresponding to the appearance of this characteristic event in said trace so as to deduce the actual velocity of propogation of said wave of interest in the geologically formed layers inbetween the emitter-receiver pair which give rise to said trace; gathering all the traces in pairs so that in a given pair the two acoustic paths corresponding to the two traces of said pair show a common end located on the same side in the drilled well and a non-common end facing a geologic layer of interest; measuring in each pair of traces the difference of the time of arrival of the characteristic event selected for each type of wave of interest and finally, computing, by means of this time difference and of the thickness of said geologic layer, the velocity of propogation of each type of wave if interest in this layer.