7.5 km


1.0 Sec


Conventional


Dual-Sensor B


A


C


Acoustic Impedance


Vp/Vs Ratio


Figure 5.24: The two images on the left represent the acoustic impedance and the two on the right represent the Vp/Vs-ratio from the conventional seismic and the dual-sensor streamer acquisition. The acoustic impedance and Vp/Vs-ratio have been estimated in a pre-stack inversion process. For arrows A–C see text. White box denotes area used for cross-plot analysis.


their overall goal: to develop a streamer that measured both pressure and particle velocity with an acceptable noise level. Te journey from the early test experi ments in Houston


in 2000 ended in 2007 with the successful commercial in tro - duction of the new system. Taking the GeoStreamer from idea and proto type to a robust, reliable, seaworthy commercialised product was a dream come true.


5.4.3 Rock ‘n’ Fluid


Te ability to predict reservoir properties from seismic data is crucial in reservoir characterisation. Te accuracy of the reservoir property prediction is improved by having more low frequency seismic information and less a priori (well) in form- ation included in the process. With extended seismic low frequency content, the


dependency on any well information will become increasingly less important. Increased high frequencies provide better vertical seismic resolution throughout the available depth range and more importantly at the reservoir level, allowing reservoir geoscientists to see as much detail as possible. As an example, we have data which were acquired in the


North West Australian Shelf of the Carnarvon Basin with both conventional and dual-sensor streamer technology. Tis area is a world-class gas province with minor oily sweet spots in the Permo-Triassic sediments, overlain by Jurassic to Cenozoic syn and post rift successions. Figure 5.24 presents the result of a relative acoustic inversion


study using both conventional and dual-sensor streamer data. A relative inversion (inversion without any low frequency


208


or a priori model) was chosen to avoid any bias from well information, so the inversion results represent only the seismic contribution. Te dual-sensor streamer provides a clearer acoustic impedance image (left) observed in the following aspects: definition of the flat-spot is greatly improved (arrow A), the top and base of the reservoir is more clearly defined (arrow B) and improved delineation of the geo-bodies (arrow C). From a seismic inversion point of view, one should expect to retrieve the layering of the earth (removal of the wavelet effect) and this is better achieved with the dual-sensor streamer technology as opposed to the conventional streamer which looks more like the seismic. It is always a challenge to estimate the Vp/Vs-ratio directly from streamer data, shown to the right in the figure. Also for the Vp/Vs inversion results we observe a clearer and more precise definition of the reservoir sand as well as of the overlaying shale layer. Both simultaneous inversions were carried out using exactly


the same parameters except for the wave let (narrower frequency band width for the con ventional seismic and broader wavelet for the GeoStreamer dataset), same low frequency model and inversion parameters. Terefore the only differences between the two datasets are simply due to the type of towed streamer. Te dual-sensor seismic inversion results exhibit a clearer definition of the individual geological layers and the fluid contact. To further extend the analysis between the two datasets, a


cross-plot was done over the area enclosed in white on Figure 5.24. Tis area includes the gas reservoir and should therefore have an unambiguous elastic attribute response, with the gas having both lower acoustic impedance and lower Vp/Vs-ratio


PGS TechLink


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