Instantaneous frequency-slowness analysis applied to borehole acoustic data

Instantaneous frequency-slowness method (IFS) is based on complex wave form analysis. It delivers the same measures as complex wave form method e.g. slowness and standard deviation, goodness of the data and receiver responses across the neighbouring pairs. Additionally, IFS method computes instantaneous frequency and slowness wave forms as seen between adjacent receivers. Classic semblance method delivers quite limited set of data quality measures. Among them are semblance peak value, semblance projection wave form, and frequency dispersion curves. Semblance method is executed across the entire or the subset of the receiver array. Thus the vertical resolution is limited to the offset between near and far receivers. The implications are that when the receiver array is crossing high acoustic impedance contrast then multiple semblance peaks will smear semblance projection wave form and therefore might make slowness readings erroneous. Furthermore, should it happen that one or more of the receivers are malfunctioning (in amplitude or in phase domain) then semblance method will not be able to flag such a condition. Small drop in semblance peak value might still be present. This effect is typically overlooked by the processing team. Finally, under mixed acoustic modes conditions, as when Stoneley wave contaminates flexural arrival present in the wave train data, semblance method will not able to indicate it.