LA “COUPE SISMIQUE VECTORIELLE”*

The coefficient of resemblance is defined in the following way:

Consider a group of four traces on a seismic record. For a given time t add the six products that can be formed by multiplication of the signal strengths recorded on two of these four traces. By doing this for all values of t, a new signal is obtained that can be smoothed by having it pass through an appropriate filter. The result of this smoothing procedure is called the smoothed coefficient of resemblance Γ.

Instead of multiplying the signals of two traces at the same time t, the signal of the first trace can be taken at time t‐3θ/2, of the second trace at t‐θ/2, of the third trace at t+θ/2 and finally of the fourth trace at t+ 3θ/2. Thus coefficients of resemblance are obtained that depend on the dip θ.

For each value of θ a plotting system is triggered when Γ has a maximum with respect to time. Then a vector is plotted having a thickness proportional to Γ and a dip determined by θ. If no optical mixing takes place, the vector covers just one trace of the new vectorial registration; in case of optical mixing the vector covers a number of these traces. All vectors belonging to a certain group of traces of the seismic record appear on one trace of the vectorial presentation, so one trace includes all values of θ.

Alternatively the plotting can be triggered when the corresponding sum of the signals of the four traces passes through a maximum. In this latter case the triggering can be conditioned by the maxima of Γ.

The article discusses the influence of:

— the optical mixing,

— the integration time determined by the band width of the smoothing filter,

— the method of triggering,

— the way the seismic traces that have to form a group are selected (electrical mixing).

A consequence of the method is that both phase alignments and energy bursts have an influence on the vectorial registration.