- Home
- Conferences
- Conference Proceedings
- Conferences
Marine Acquisition Workshop 2018
- Conference date: 22 Aug 2018 - 24 Aug 2018
- Location: Oslo, Norway
- ISBN: 978-94-6282-259-7
- Published: 22 August 2018
21 - 25 of 25 results
-
-
Seismic Source Strength And Size Versus Seismic Image Quality
Authors H. Westerdahl and L. AmundsenThrough numerous source tests over fields in the Norwegian Sea and the North Sea, we show how source strength and size relate to seismic image quality in the area.
-
-
-
Improving Seismic Data By Using Smaller More Compact Seismic Air-Gun Sources
Authors P.E. Dhelie, V. Danielsen, J.E. Lie, M. Branston, R. Campbell and R. FordThis paper discusses the motivation and development, in the marine seismic industry, to reduce the physical size and the source output energy from a marine seismic air gun source. Generally going down from large multiarray sources to single sub array compact small sources. The idea is to go from using an effective array-source down towards a point-source like output. With the constant desire for ever increasing spatial resolution, it has been deemed necessary to reduce the spatial extent of not just the receivers (from large summed arrays down to single sensor recordings) but also the source. Ideally the source should mimic a singular point source, but as this is practically unachievable, a more compact source of 8x9m (width x length) have been designed, modelled and tested. In addition, both a new compact triple-source as well as several reduced volume sources have been tested, along with an industry first wide-towed dithered hexa-source test. Results from the new compact smaller sources are encouraging and in combination with several smaller autonomous source vessels decoupled from the receivers, the potential for improved imaging through many more sources giving higher spatial density as well as much higher fold is substantial.
-
-
-
Marine Air-Gun Arrays And Ghost Cavitation
Authors M. Landrø and B. KhodabandelooThe major objective of seismic sources is to generate a broadband signal that results in a high signal to noise ratio for seismic exploration. Despite the desire to obtain a broadband signal, we do not normally include ultra-high frequencies (above 5 kHz) in this objective. Hence, we want to avoid generating strong high-frequency signals since such signals might disturb marine mammals. A wellknown mechanism for creation of underwater high frequency signals is cavitation. For marine air-gun arrays it has been suggested (Landrø et al., 2011) that the collective acoustic signal caused by free surface reflections (referred to as “ghost cavitation”) from many air-guns being fired simultaneously might lead to cavity creation. This hypothesis has been strengthen by further studies (Landrø et al., 2013, Landrø et al., 2016, Khodabandeloo et al., 2017 and Khodabandeloo and Landrø, 2017). It is also well documented that single air-guns emit frequencies above 5 kHz, and there has been significant development aiming to attenuate the high-frequency signals generated by single air-guns (Coste et al., 2014). It has also been suggested to exploit the high frequency signals generated by air-gun arrays for leakage detection above a hydrocarbon reservoir or a CO2 storage site (Landrø et al., 2017). In this paper, we will give a short overview of ghost cavitation, and discuss both acoustic and photographic evidence for the phenomenon, and briefly discuss how we can attenuate the high frequency signal by simple means.
-
-
-
The Impact Of Bubbly Water On Airgun Signatures
More LessThe airgun signature, especially the bubble time period, is controlled by the source depth, size of the gun and firing pressure. In addition, several studies are carried out to investigate the impact of heat transfer, viscosity, surface tension or mechanical processes inside the gun on the oscillating bubble. Another aspect that could have an impact on the airgun signature and bubble time period is the presence of small air bubbles in the water which leads to a reduced density and bulk modulus. The water can be saturated with air close to the surface due to waves or due to a leaking airgun located below other sources. We conduct experiments with a small airgun inside a water tank where the source signature is recorded. The near field pressure is measured without air bubbles in the water and for three settings with an increasing amount of air bubbles. The air bubbles are generated from a tube with several holes exactly below the airgun. An increased bubble time period and primary-to-bubble ratio is observed with an increasing amount of air in the water. The amplitude of both, the main peak and bubble peaks, is reduced with an increased amount air saturation.
-
-
-
Source Ghost Generation: Observations From A Dual Near-Field Hydrophone Test
Authors R. Telling, S. Grion, S. Denny and R.G. WilliamsWe evaluate experimental data acquired using a dual-string airgun array instrumented with two near-field hydrophone (NFH) channels per source element, and use these data to estimate far-field signatures without need for assumptions on the reflectivity of the sea-surface (Parkes and Hatton, 1986). The acquired NFH data are consistent with the presence of cavitation in the down-going wavefield and reveal details of the wavefield which are not predicted by a conventional ghost model. Directional de-signature operators are derived and applied to seismic data shot using the test configuration. Results are compared with a reference dataset processed using signatures estimated in the typical way i.e. from just one channel per element of NFH data, and using a frequency-dependent sea-surface reflection coefficient. We discuss the potential benefits of using additional NFH and outstanding issues going forward.
-