Abstract

It is becoming increasingly widely accepted that wide-azimuth 3D acquisition is the best way to improve imaging of complex and subtle traps. It is also claimed that high-density point source/point receiver acquisition constitutes the ultimate seismic acquisition technique. Clearly, both techniques require a considerable increase in equipment and density of both source and receiver points. How much is still a matter of debate. Currently most 3D vibroseis crews use two fleets of vibrators in flip-flop mode. The productivity depends on the sweep length and the time it takes to move from one VP to the next. The slip sweep technique is an attractive way of optimizing productivity to a level that can keep the cost of increasing source density within reasonable limits. The productivity increase with this technique is linked to a new parameter called the slip time (the minimum time interval between two consecutive SPs). In the real world, nothing is free and the price to be paid is that the data can be severely contaminated by harmonic noise due to the use of long sweeps combined with a very short slip-time. To overcome this problem, a method of harmonic noise reduction known as HPVA (High-Productivity Vibroseis Acquisition) has been developed. This method consists in estimating the harmonic noise in the vibroseis signature so that it can be subtracted. Several crews are now routinely using this technique with three to four fleets of vibrators. Recent 3D tests with 12 fleets of single vibrators also show very promising results which will be presented. Combined with a densification of the source grid, single-vibrator acquisition can bring either an improvement in data quality or an increase in productivity and opens the road to affordable dense, wide-azimuth seismic acquisition in desert environments.

Introduction

Going somewhat belatedly down the road opened up by Ongkiehong and Askin in 1988 (1), the seismic industry is attracted by the point-source-point-receiver acquisition technique. The main reason why it takes so long to actually apply this technique lies in the difference between 2D and 3D. Ongkiehong's approach was essentially 2D at a time when 2D group intervals ranged typically between 15 and 30 m. Going one step downward to 5 m cost "only" 3 to 6 times more seismic channels and SPs. Today 3D group intervals typically range between 25 and 50 m. The same step downward would cost 25 to 100 more seismic channels and SPs. This paper relates an experiment conducted in order to evaluate the technical advantage and the economical feasibility of a single vibrator source. We shall first defend the choice of the slip sweep technique, which suffers from its sensitivity to harmonic noise; we will then discuss the design of the experiment and show some results.

Increasing Vibroseis source density with acceptable cost

According to the square root theory, to replace an array of n vibrators by one single vibrator while achieving the same signal to (ambient) noise ratio requires a multiplication by n2 of the vibrating time/km2; n, typically, is between 3 and 5. Therefore, as stated by D. Monk(2) in his address at the 2006 CSEG convention, it is very likely that the road toward the single vibrator will go through some form of simultaneous vibroseis technique. At the 2006 EAGE conference, C. Bagaini(3) compared several of these techniques. He concluded that the choice of a particular method should be dictated by two parameters: source signature repeatability and magnitude of the generated harmonics. At the 2002 EAGE conference, T. Bianchi(4) presented an experiment where various simultaneous acquisition techniques were compared. He concluded that, from a purely operational point of view, the slip sweep method was the most attractive because of its higher flexibility, with all vibrator groups being fully independent. He did recognize however, that harmonic noise and penetration were serious problems to be taken into account. Harmonic noise reduction was indeed proposed in 2002 by Meunier & Bianchi(5). Concerning penetration, Meunier and Bianchi(6) showed at the 2005 EAGE conference that the desired signal to ambient noise ratio could be achieved in slip sweep operations by adjusting the number of source groups to combine adequate sweep length and optimum productivity. This last remark, incidentally, changes the status of the listening time parameter which, being the shortest possible slip time, becomes operationally critical: in slip sweep operations, the shortest possible survey duration is fully defined by the survey area, the source density and the listening time

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