`
`.
`
`. the so-called ghost wave will be the reinforced seismic
`
`wave and will appear prominently on the seismic record.” Id. 8:21-24.
`
`59.
`
`The “ghost wave” referred to by Itria is a well-known phenomenon of
`
`seismic surveying. Whenever sources are located below the surface of the earth or
`
`the water, upgoing portions of the wave energy strike the interface between earth
`
`or water and air. The energy that is reflected back into the survey medium trails
`
`the primary seismic wave and eventually arrives at the receivers. This trailing
`
`wavefront is known as the ghost.
`
`60. A few characteristics of the ghost are important to know in this
`
`proceeding. First, the ghost reflection lags behind the primary signal by the
`
`amount of time it takes the ghost to reach the surface and return to the depth of the
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`source. This delay is known as the ghost delay. Second, due to the difference in
`
`acoustic impedance between air and earth or water, the ghost reflections have
`
`opposite polarity to the primary seismic wave. This is depicted in the figure
`
`below. The left figure depicts the downgoing “direct” raypath and the upgoing
`
`ghost raypath that is reflected at the surface. The right figure depicts the direct and
`
`ghost waveforms as a function of time. Amplitude displacement to the right of the
`
`vertical time axis are positive amplitudes, amplitude displacement to the left are
`
`negative amplitudes.
`
`
`
`28
`
`IPR2016-00407 PGS Exhibit 1002
`
`PGS Exhibit 1020
`PGS v. WG
`
`PGS v. WG
`
`IPR2016-00407
`PGS Exhibit 1020
`PGS v. WG
`
`
`
`
`
`‘II
`
`Direct ray pth
`
`Third, the amplitude of the ghost wave is determined by the various environmental
`
`factors including the acoustic impedance of the two media at the point of
`
`reflection. In the marine context, ghost waves are typically quite strong; the
`
`reflection of seismic waves at the air-water interface in calm seas is almost
`
`perfectly efficient and the ghost has nearly the same amplitude, but opposite
`
`polarity, as the direct wave.
`
`61.
`
`Itria teaches that upward sequential shooting may be used to enhance
`
`the portion of the wavefront that travels upward and reflects off the surface
`
`interface. When this is done, the primary seismic wave is diminished, just as the
`
`ghost would be diminished by downward sequential shooting. Accordingly, the
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`ghost wave, rather than the downward wave, will “appear prominently on the
`
`seismic record and contain the desired intelligence relative to the underlying
`
`subsurface formations.” Id. 8:21-25.
`
`62.
`
`Techniques for using vertically distributed sources continued to
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`develop over the decades following Prescott and Itria. For example, U.S. Patent
`
`29
`
`IPR2016-00407 PGS Exhibit 1002
`
`PGS Exhibit 1020
`PGS v. WG
`
`PGS v. WG
`
`IPR2016-00407
`PGS Exhibit 1020
`PGS v. WG
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