Dithered slip-sweep acquisition Claudio Bagaini* and Ying Ji, Schlumberger Summary A new technique for simultaneous vibroseis acquisition and processing, which is denominated dithered slip-sweep (DSS) acquisition, is here introduced. The objective of this technique is the high-productivity acquisition of data whose prestack quality enables applications such as amplitude variation with offset (AVO) and amplitude versus angle (AVA) analyses. This technique, which hinges on features of modern land acquisition systems such as large channel count and continuous recording, leads to acquisition scenarios with favorable conditions for prestack separability of simultaneously acquired data. A two-fold dithered dataset is generated starting from a conventional dataset. It is shown that, after separation of the dithered records using a modeling and inversion techniques, the interference noise due to simultaneous shooting is substantially eliminated and that the prestack and poststack data are comparable with those obtainable with sequential shooting using the same number of shot gathers sequentially acquired. Introduction Several acquisition and processing methods have been proposed in the last three decades to increase productivity in vibroseis acquisition (Bagaini, 2010). The availability of continuous recording systems with a very large channel count and the ability of the source control systems to manage a large number of vibrator fleets have made these methods a viable and often cost-effective solution today. In 2009, peak productivities that exceeded 1000 VPs/hr were reported (Howe et al., 2009; Matheny et al., 2009). These peak productivities are almost an order of magnitude higher than previously published values of average productivity for 3D land crews working in the Middle East (Burger et al., 1999; Al-Ghamdi and Al-Shammery, 2008). The ultimate productivity values can be achieved if all the vibrators sweep independently without any attempt to synchronize their activities (Howe et al., 2009). This technique of independent simultaneous shooting (ISS) generates interference noise that can, depending on the distance between vibrators and the firing times, be extremely severe on individual shot gathers. The challenging attenuation of the interference on prestack data can be attempted either using filtering techniques for random noise attenuation applied in a domain where the interference noise appears incoherent (e.g., common receiver or offset gathers) or using an inversion approach

(Abma and Yan, 2009). Multichannel processing steps such as stack and migration attenuate, to some extent, the residual noise. This abstract describes a new acquisition and processing technique termed dithered slip-sweep (DSS) acquisition that combines, at the acquisition stage:

the dithered acquisition (Stefani et al., 2007; Moore et al., 2008) and

the slip-sweep acquisition technique originally proposed by Rozemond (1996)

and, at the processing stage: the separation for dithered source described by

Moore et al. (2008) and the harmonic noise attenuation method described by

Jeffryes (2006) and Bagaini (2008). The DSS constraints on the firing times, which will be described in this abstract, keeps the interference noise in prestack data under control such that the algorithms for its attenuation can work more effectively than when the vibrators do not synchronize their activities at all. If systems with a very large channel count are used to cover a wide area together with an adequate number of seismic vibrators, distance separation can be used to acquire data with high-productivity rates and minimum contamination of individual records. This idea is at the base of the schemes proposed for marine acquisition by Beasley et al. (1998), and in the case of land, the distance separated simultaneous sweeping (DS3), by Bouska (2010). However, because the minimum separation of vibrators shooting in DSS mode is lower than in DS3, DSS may enable more than two groups of vibrators to shoot at virtually the same time. Although this abstract describes an example of DSS with time dithering, the application of phase dithering is a possible alternative. Theory Dithered slip-sweep acquisition combines at the acquisition stage, the dithered acquisition method (Stefani et al., 2007; Moore et al., 2008) and the slip-sweep acquisition method originally proposed by Rozemond (1996). The interference noise of this acquisition method is extremely predictable and can be attenuated taking into account its specific features. The harmonic noise, always generated by hydraulic vibrators, can be attenuated with the methods described in Jeffryes (2006) and Bagaini (2008). The dithered noise can be addressed with the technique described in Moore et al. (2008).

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Dithered slip-sweep acquisition

Two conditions on the earliest time at which one vibrator (or fleet of vibrators) can start sweeping are cyclically applied. The example of a two-fold dithered slip-sweep acquisition shown in Figure 1 helps to explain these conditions. The first condition is the dithering time condition, and the vibrators to which this condition is applied form a dithering group (or pair in a two-fold dithered acquisition). The dither times (typical values are a few hundred ms) can be planned or determined in real time to generate incoherent events in a domain different than the acquisition domain. The separation will then be carried out in this domain, which can be multidimensional for 3D acquisitions. The two shot points A1 and A2 (here arbitrarily called master and dithered records), which are acquired almost simultaneously, can be extracted from the dithered record by explicitly taking into account the respective firing times. A condition on the minimum distance of the vibrators that compose the dither pair prevents the very energetic short offsets of one vibrator from overwhelming the signals recorded by the medium or large offsets of the other vibrator. In the absence of harmonic noise generated by the vibrators and if the Earth response is of finite length, the second condition (slip-time condition) would simply impose a time delay equal to the practical duration of the Earth’s impulse response, i.e., the listening time. In the presence of harmonic noise, a more restrictive condition, i.e., the slip time, is the second condition applied for the firing times of B1 and B2. The values of the minimum slip time and methods to attenuate harmonic noise are described in Jeffryes (2006) and Bagaini (2008).

Figure 1. Example of a two-fold dithered slip-sweep acquisition. The admissible firing times according to the dithered slip-sweep rules are shown for four VPs beneath the corresponding vibrator. The sweep starting times are, therefore, partially determined by the dither times, which in general, may be positive or negative, and represent the start time of the second sweep in the dithered pair relative to the start time of the master sweep in the same pair. The start times of both sweeps B1 and B2 of the dither pair 2 must be more

than the slip time ST, after the later of the start times of sweeps A1 and A2. For the sake of simplicity, Figure 1 shows the sweep start times in the case when the dither times are all positive. Obviously, the acquisition system selects the vibrators that will constitute the dither pairs in real time among those vibrator that satisfy the distance and slip-time conditions and not at the planning stage. Figure 2 shows a schematic representation in time-frequency domain of a trace recorded from four shot points during a dithered slip-sweep acquisition as the one depicted in Figure 1. The Earth’s response to the fundamental and the second-order harmonic are the grey shaded beneath the green and red lines, respectively. If the slip-sweep time is set such that the slip-sweep component of the dithered slip-sweep acquisition is non-aggressive, the most significant interference noise is due to the dithered vibrator. In the examples in Figure 1 and Figure 2, this is the contamination of A1 due to A2 (and vice versa) and the contamination of B1 due to B2 (and vice versa).

Figure 2. Schematic time-frequency representation of a trace acquired by a single receiver illuminated from four vibration points in a dithered slip-sweep sequence. The responses to the fundamental component of the ground force and its 2nd harmonic are represented as the shaded (grey) parallelograms beneath the green and red straight lines, respectively. The duration of Earth’s response is schematically shown as frequency independent. Data example To assess the effectiveness of the modeling and inversion algorithm proposed by Moore at al. (2008) for the separation of dithered slip-sweep vibroseis data, a dithered slip-sweep dataset was generated starting from a conventionally acquired 2D vibroseis line. A 5.6-km 2D line was acquired with a linear sweep (5-80 Hz, 16 s) and a shot-receiver group interval of 25 m. The receiver spread, whose length was also approximately 5.6 km, was fixed. The dither pairs were simulated by summing the shot gathers acquired by one vibrator that started at the

92SEG Denver 2010 Annual Meeting© 2010 SEG

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Dithered slip-sweep acquisition

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Figure 4. Common-offset gather extracted from the dithered slip-sweep acquisition. (a) Original gather. (b) After separation for the master source in the master time. (c) After separation for the dithered source in the master time.

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94SEG Denver 2010 Annual Meeting© 2010 SEG

EDITED REFERENCES Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2010 SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web. REFERENCES

Abma, R. L., and J. Yan, 2009, Separating simultaneous sources by inversion: 71st Annual Meeting, EAGE, Expanded Abstracts, V002.

Al-Ghamdi, T., and H. Al-Shammery, 2008, Optimizing vibroseis productivity in Saudi Aramco: EAGE Vibroseis Workshop, Extended Abstracts, EarthDoc-15993.

Bagaini, C., 2008, Vibroseis acquisition method: US Patent application WO2008110743 A2.

Bagaini, C., 2010, Acquisition and processing of simultaneous vibroseis data: Geophysical Prospecting, 58, no. 1, 81–100, doi:10.1111/j.1365-2478.2009.00842.x.

Beasley, C., Chambers, R,E. and Zerong Jiang, 1998, A new look at simultaneous sources, 68th Annual Meeting, SEG, Expanded Abstracts, 133-135.

Bouska, J., 2010, Distance separated simultaneous sweeping, for fast, clean, vibroseis acquisition: Geophysical Prospecting, 58, no. 1, 123–153, doi:10.1111/j.1365-2478.2009.00843.x.

Burger, P., B. Duijndam, and D. Wasmuth, 1999, Marine production levels in land 3-D seismic : The Leading Edge , 18, no. 10, 1170–1175, doi:10.1190/1.1438176.

Howe, D., M. Foster, T. Allen, I. Jack, D. Buddery, A. Choi, R. Abma, T. Manning, and M. Pfister, 2009, Independent simultaneous sweeping in Libya-full scale implementation and new developments: 79th Annual Meeting, SEG, Expanded Abstracts, 109-111.

Jeffryes, B. P., 2006, Method of seismic surveying: US Patent 7,050,356, Priority date 6 Apr 2002.

Matheny, P., R. Sambell, S. Mahrooqi, S. Yarubi, and S. Abri, 2009, Evolution of the land seismic super crew: 79th Annual Meeting, SEG, Expanded Abstracts, 81-85.

Moore, I., W. Dragoset, T. Ommundsen, D. Wilson, C. Ward, and D. Eke, 2008, Simultaneous source separation using dithered sources: 78th Annual Meeting, SEG, Expanded Abstracts, 2806-2810.

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