leading to maxima in the amplitude at these frequencies. One of the goals in broadband marine seismic acquisition


is to deliver data rich in both low and high frequencies. Te challenge of increasing low-frequency while maintaining high-frequency content is caused by the receiver ghost effect. Towing streamers shallowly favours the higher frequencies at the expense of attenuating the low frequencies, while towing streamers deeper favours the lower frequencies, at the expense of attenuating frequencies within the seismic bandwidth.


5.2.5 Ghost Effect on Z


Today’s most advanced streamer technologies record both particle velocities and pressure. Te hydrophone is omni- directional and measures the sum of upgoing and downgoing pressure waves. Te vertically oriented geophone has directional sensitivity and measures their difference. Terefore, in Z-recordings the ghost has the opposite sign to the one it has in P-recordings: G+ reads |G+ fn


= 1 + exp(iωτ). Its frequency spectrum


(f)|=2cos(2πfzcosθ/c), and has notches at frequencies = (2n+1)c/(4zcosθ), lying mid-between the notches in


the P-recordings. Tus, hydrophones and geophones give complementary information. Where the hydrophone has zero sensitivity due to the ghost, the Z-geophone has its maximum sensitivity, and vice versa (see Figure 5.13). Let R denote the ‘pressure reflection response’ of the


subsurface (including the source ghost). Ten P ∝RG– and Z ∝RG+. It now follows that you can exorcise the receiver ghost


by summing P and Z (when Z is properly scaled to P) since (1/2)(P+Z)=R. Tus, receiver side deghosting, equivalent to


computing the upgoing component of the pressure field, can be done from PZ measurements. Tis is a fundamental basis of both PGS’s GeoStreamer solution and WesternGeco’s IsoMetrix solution.


5.2.6 Deghosting by PZ Summation


However, for low frequencies, below 15–20 Hz, depending on the particular acquisition system and weather, the Z recordings are too noisy to be used in PZ summation. Te solution to this problem was developed and implemented by PGS in the GeoStreamer. Te straightforward combination of the two equations, P ∝R G–


published by Amundsen (1993): Z ∝(G+ /G–)P. Tus, for the low frequencies where Z is noisy, Z can be


and Z ∝R G+


estimated from P by deghosting the pressure (multiplying P by 1/G–) and ghosting the result (multiplying by G+). Ten, this estimate of Z is used at low frequencies whereas the Z measurements are used at higher frequencies in deghosting. Te Z-P model is used also in 3D deghosting of WesternGeco’s IsoMetrix measurements, not to replace low-frequency Z data but rather to further constrain the cross-line reconstruction problem. We observe that P-Z sensor streamers have no direct benefits


for low frequency recording as they use only the hydrophone at low frequencies; the geophones are used to infill the higher frequency ghost notches. To get high-quality pressure measurements at low frequencies, the cables must be towed deeper, where the pressure ghost notch has minimum effect and the S/N ratio is good.


, gives the Z-P relationship, first


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