Near Surface Geophysics > Calibration and Synthetic seismograms

The acoustic velocity anomalies have several origins:

  • Errors in velocity measurement due to noises, cycle skipping for picking based on threshold, stretching (difficult to detect),
  • Anomalies linked to formations: poor cohesion between grains (measured velocity too low), vacuoles (measured velocity too high),
  • Anomalies related to the wave path: Invasion, cavities, mud paths (in the case of large diameter holes), alteration of the borehole wall,
  • Anomalies due to bad cementing

The refracted acoustic wave can propagate in the washed or invaded zone of the formation due to the invasion phenomenon and not in the virgin zone. The seismic wave (VSP) emitted at the surface propagates mainly in the virgin formation. In order to obtain acoustic velocities in the uncontaminated zone, it is necessary to correct or calibrate the acoustic velocity log on measurements of the propagation time in a virgin zone provided by the VSP.

The acoustic logs are measured in depth. The velocity log can be used to obtain a time-depth law by integrating the acoustic transit time as a function of depth. The integrated transit time can be used to convert a log in depth into a log in time. The logs in time can be inserted on the seismic sections for calibration in time.

We describe:

  • the depth to time conversion of acoustic logs,
  • the Block shift method, procedure used to compensate acoustic velocity anomalies. In our case, the acoustic velocity anomalies are due to a poor cementing of the borehole,
  • the computation of synthetic seismograms,
  • the calibration in time of the seismic section thanks to synthetic seismograms. The correlation coefficient is used to quantify the calibration.