Noise Logging - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 123

Figure 9.4 shows a very different spectrum, obtained when 0.3 Mcf/D of gas
expands into a water-fi lled channel. Here the peak noise is at less than 500 Hz.

Fig. 9.4 Noise spectrum for two-phase fl ow (gas/water). Courtesy Baker Hughes

In order to distinguish these spectra from one another, the total signal is fi ltered
through band-pass fi lters that allow the frequencies ≥ 200+, ≥ 600+, ≥ 1,000+, and

≥ 2,000+ Hz to pass. The choice of limits on these bands varies somewhat between

different service companies. Since the 200-Hz fi lter allows all frequencies above
200 Hz to pass, this fi lter normally gives the highest amplitude. The 2,000-Hz fi lter,
which only allows frequencies above 2,000 Hz to pass, normally gives the lowest
amplitude. Figure 9.5 illustrates the band-pass fi lter response.

Fig. 9.5 Filter response in
noise logging. Courtesy
Baker Atlas