88 Well seismic surveying and acoustic logging 3.4 Processing sequence Conventional processing of an acoustic log enables time-depth relationship and velocity logs to be obtained at the well, as well as certain mechanical parameters such as the Poisson’s ratio. The processing sequence includes: 1. Editing (elimination of poor quality recordings). 2. Calculation of acoustic velocities by picking the arrival times of the different wave trains or by velocities scanning and semblance processing. 3. Quality control of velocities (measurement of the correlation coefficient) and of pickings (for example, by flattening the wave train by applying static corrections equal to the picked times). Comments: • If the picking algorithm uses a threshold, the detection of erroneous picks (spikes and cycle jumps) must be done when editing the velocity logs. This technique is only applicable to compression waves. • If the velocities are measured by semblance, it is recommended to use a tool with a large offset between the transmitter and the first receiver (about 2 to 3 m) and with at least 4 receivers. Measurement is facilitated if the wave trains are well separated in time. Optional: 1. Measurement of the amplitudes of the different wave trains and calculation of the amplitude and attenuation logs. 2. Measurement of the frequencies of the different wave trains and calculation of the frequency logs (attenuation, resolution...) 3. Calculation of the acoustic porosity (Wyllie’s formula) 4. Calculation of synthetic films. It is recommended that tying (block shift and minimum Δt methods) of Δt acoustic measurements on VSP measurements is carried out. 5. Calculation of elastic modules (geomechanical: choice of models used) 6. “Seismic reflection” type processing of reflected waves and obtaining microseismic sections in the vicinity of the well (deviated or horizontal wells) Processing takes from one to several days (or even weeks: micro-seismic processing is equivalent to seismic reflection processing), depending on the processing options required. If the required processing is only the P velocity log, picked by threshold, it can be obtained in real time in the field. Examples of conventional acoustic data processing are presented below. The first example (Mari et al., 2011, Figure 3.6) is an example of acoustic data acquired in a fast formation (formation VS > well fluid VP).
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