Extending Vibroseis Bandwidth Using Customised Non-Linear Sweeps: A Case History in Oman
Mahrooqi, Said 2; Bianchi, Thomas *1; Postel, Jean-Jacques 1
(1) CGGVeritas, Massy, France. (2) PDO, Muscat, Oman.
There is interest in increasing the bandwidth of Vibroseis sources, not just at the high end, but at the low end of the spectrum. The motivation is to provide better seismic imaging at depth and more accurate estimates of rock properties from seismic inversion.
The generation of low-frequency energy with the Vibroseis method is challenging due to the mechanical and hydraulic limitations of the vibrator systems. The mass displacement limit and hydraulic pump flow limitations are the main constraints for vibrator output at low frequencies (Bagaini, 2008, Sallas, 2010). These can be accompanied by a wide variety of additional factors such as failure of control electronics to maintain amplitude- and phase-lock, vehicle isolation and baseplate flexure.
To this end a low-frequency Vibroseis test was performed in 2009, (Baeten et al, 2010). The test demonstrated that industry standard vibrators could emit signals as low as 2Hz and that these signals could be recorded by borehole and surface arrays of receivers.
These tests lead to the further development of customised, non-linear Vibroseis sweeps designed to work within the mass displacement, hydraulic flow and servo-valve limit constraints for a given vibrator model. The concept is to deliver the desired bandwidth by varying the force and sweep rate with frequency to stay within the envelope defined by the constraints. At low frequencies this is achieved by tapering the force and increasing the dwell time. It is also possible to increase the drive to higher levels than conventional sweeps over the mid range of frequencies and therefore same time during this segment of the sweep. As well as increasing bandwidth, the technique has some additional benefits including increased overall energy output thanks to increased drive levels and reduced harmonic interference during slip-sweep operations.
Since the tests in 2009, several projects have been recorded successfully all over the world using this technique. In Oman, after several tests, PDO selected a customised sweep starting at 1.5 Hz for production in 2011. The test results and examples of production data will be presented.
AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain