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Time‐lapse seismic surveying has become an accepted tool for reservoir monitoring applications, thus placing a high premium on data repeatability. One factor affecting data repeatability is the influence of the rough sea‐surface on the ghost reflection and the resulting seismic wavelets of the sources and receivers. During data analysis, the sea‐surface is normally assumed to be stationary and, indeed, to be flat. The non‐flatness of the sea‐surface introduces amplitude and phase perturbations to the source and receiver responses and these can affect the time‐lapse image. We simulated the influence of rough sea‐surfaces on seismic data acquisition. For a typical seismic line with a 48‐fold stack, a 2‐m significant‐wave‐height sea introduces RMS errors of about 5–10% into the stacked data. This level of error is probably not important for structural imaging but could be significant for time‐lapse surveying when the expected difference anomaly is small. The errors are distributed differently for sources and receivers because of the different ways they are towed. Furthermore, the source wavelet is determined by the sea shape at the moment the shot is fired, whereas the receiver wavelet is time‐varying because the sea moves significantly during the seismic record.

Seismic interferometry refers to a new range of methods where inter-receiver wavefields (those that would have been recorded if one of each pair of receivers had been a source) can be estimated by cross-correlation of wavefields recorded at each of the receivers. These methods have found many applications in different fields of seismology, including creating "virtual" sources in wells under complex overburdens, computational full-wavefield modelling, and passive construction of surface wave waveforms from background noise in the Earth. Curtis et al. (2006) provide an overview of various applications of seismic interferometry referred to herein, and more in-depth works can be found in the special supplement on Seismic Interferometry in the July-August issue of GEOPHYSICS.One particularly interesting aspect of seismic interferometry is the ability to estimate inter-receiver signals using background noise. This has become popular for crustalscale imaging where crustal and uppermost mantle structure can be constrained using surface wave velocity analysis. However, successful applications of similar methods to higher-frequency data are scarce. One application is presented by Draganov et al. (2007), who use long recordings of noise (around 10 hours) to recover both surface waves and reflected body waves in a hydrocarbon exploration, desert setting. This article will focus on surface-wave construction and isolation and is not restricted to passive noise sources; active sources have also been used to successfully isolate higher frequency, inter-receiver surface waves. For example, direct surface waves determined in this way have been used as part of a predictive ground roll removal algorithm in an exploration setting (Curtis et al., 2006;Dong et al., 2006; Halliday et al., 2007).In the civil engineering community, there are existing methods that extract near-surface information from background noise, known as "micro-tremor analysis." For example, the spatial autocorrelation method (SPAC) of Aki (1957) extracts phase velocities from recordings of background noise, while Louie (2001) uses the refraction micro-tremor technique to resolve velocity structure to depths of 100 m using slowness-frequency analysis of background noise. The advantages of such methods are the low cost and manpower in the data acquisition compared with active source surveys, and the fact that noisy (sub-)urban settings in which the data are often acquired are ideal for the application of methods of noise analysis. Since the same type of data are used for passive interferometry, the same advantages apply. For example, Chávez-García and Luzón (2005) compare and contrast the analysis of micro-tremor measurements using the SPAC method and the passive interferometric method and found that the two methods provide complementary results.In this study, we show results of several different approaches to the interferometric estimation of surface waves in a suburban environment, using both active and passive sources. For the active source case, we illustrate t...

We present a method for receiver ghost correction of towed streamer data that accounts for the rough sea surface. The method explicitly uses the fact that the pressure is zero at the free (sea) surface to estimate the vertical pressure gradient. Continuous elevation measurements of the wave height directly above the hydrophones are required—a measurement which is currently unavailable. The new deghosting method is fundamentally limited to frequencies below the first ghost notch. The lowest‐order implementation requires that the streamer is towed no deeper than approximately 6 m and a receiver spatial sampling interval of about 3 m or less. Using the lowest‐order and simplest implementation of the new method, the rough‐sea error is reduced from 1.5–2.5 dB to about 1–1.5 dB in amplitude and from 20° to 10° in phase, at 50 Hz in a 4‐m significant wave height sea. Higher‐order terms in the approximation promise to further reduce the error.

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