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After loading the seismic data on the workstation, one of the first questions an interpreter asks the data processor is: “What’s the polarity?” The processor assures the interpreter that the final seismic has true-amplitude processing with a zero-phase wavelet, so that a peak represents a reflection from a high-velocity bed. Knowing the complexities that mother earth can introduce, and having a little bit of processing experience, seasoned interpreters are often heard mumbling quotes such as, “It’s your dream, tell it the way you want to.”

But even if the processor has produced his dream section, there are amplitude and phase anomalies that are often not associated with rock type or pore-fluid changes. Once these anomalies are reconciled, reflection amplitudes both on the stack and within CDP gathers can be investigated for rock type and pore-fluid predictions. These predictions from the amplitude interpretation are normally conducted using simplified versions of the reflection-amplitude equation. Which equation is employed depends on what the interpreter is trying to emphasize.

A variety of geophysical receivers are used by the oil industry to measure the particle motion of seismic waves. Likewise, these receivers can be planted on land, towed as streamers, deployed in vertical cables, or planted on the ocean bottom. With respect to polarity, two issues arise: type of receiver, and direction of wavefront.

Particle-velocity geophones and pressure-sensitive hydrophones are the two receivers commonly employed in seismic. Starting with a displacement wavelet, the amplitude relationship between a particle-velocity phone and a hydrophone is shown in Fig. 3.A.1.

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