Near Surface Geophysics > Acoustic logging at well M20

The transmission of an acoustic wave through geological formations is used for formation characterisation. Monopole-type tools are the most commonly used. Sources and receivers are multidirectional.

  • Step 1: Acoustic tool.
    The acoustic tool used for the field experiment is a flexible monopole tool with two far offset receivers R1 and R2 . The offset between the source and the receiver R1 is 3 meters. The distance between the two receivers is 25 centimeters. The sample interval in time is 5 microseconds and the recording time is 5 milliseconds. The sample interval in depth is 5 centimeters. Sources generate in the fluid a compression wave which creates in the formation a compression wave and a shear wave at the refraction limit angles.

  • Step 2: 3 meters constant offset section.
    On the section, we can observe five propagation modes :
    • The refracted P-Wave between 0.7 and 1.2 millisecond
    • The converted refracted S-wave followed by Rayleigh waves between 1.3 to 2 milliseconds
    • High frequency fluid waves after 2.1 milliseconds
    • The low frequency dispersive Stoneley wave after 2.2 milliseconds

    We can observe two zones with very strong attenuation of the acoustic waves between 82 and 88 meters and between 95 and 100 meters.

  • Step 3: Time - Frequency analysis.
    The Hilbert transform has been used to compute the instantaneous frequency. Figure shows the acoustic section displayed in instantaneous frequency. Above the section, the average amplitude spectrum expressed in dB shows that the frequency bandwidth of the acoustic data is large between 1 and 30 kHz. On the section, we can see the frequency content of the different waves. The fluid waves are high frequency wave with a dominant frequency of 30 kHz.