The depth of investigation of sonic is not very high, it is a tool which measures in the flushed zone. To obtain a measurement of T in the fresh zone, it is necessary to apply a correction. This correction is carried out by adjusting the sonic log time-depth on a calibration point provided by measurement on core sample of VSP.

The T is regarded as the weighted average of T in the matrix Tma and T in the fluid Tfluid. The simplest relation is given by the Wyllie's formula.

Wyllie's formula


and

= porosity
t =Sonic reading
tma = Transit time in the matrix
tf = Transit time in the fluid

This formula which makes it possible to obtain porosity, is experimental and approximate. It is valid only for fresh and consolidated formations.

Acoustic logging at Halle Fosse 2

In the uncemented formations or not compacted sands, the Wyllie's formula gives too high porosity.

In the presence of beds of clay interbedded among sands, the values of porosity read by the sonic will be too high, T clay > T sands.

In the presence of secondary porosity (vacuolar porosity and porosity of fracture) the velocity of sound depending mainly on primary porosity, the porosity obtained by the sonic will be too low.

In this case the use of the neutron or density log will give us an order of magnitude of total porosity, and by substraction with sonic porosity there will be an idea of secondary porosity.