is varied between successive shots along a sail line (Haavik and Landrø, 2015). Te common factor in these marine source configurations is that they involve sources at more than one depth. We note that Cholet and Fail (1970), Smith (1984) and Lugg (1985) proposed the deployment of sources at different depths and then depth-synchronising the firing such that primary peaks align in phase. Tis design idea has since been adopted and adapted by several authors. Huizer (1988) advocated the use of extended arrays to achieve improved primary-to-bubble ratio and signature shape of the seismic signals. A number of authors have pointed out the importance of
considering not only ghosts but also the hydrostatic pressure in broadband source depth design (e.g., Davies and Hampson, 2007; Parkes and Hegna, 2011; Hopperstad et al., 2012; Landrø and Amundsen, 2014; Haavik and Landrø, 2016). Te bubble time period (representing the fundamental frequency) and the ghost spectrum represent two competing effects. In fact, the reduced source ghost attenuation that can be achieved at low frequencies by increasing source depth is counteracted by the increase in the fundamental frequency (decrease in bubble time period) of the airguns as they are towed deeper. Te hydrostatic pressure varies with depth below the free surface. At shallow depth and low hydrostatic pressure, the bubble time period of the source is longer and consequently richer in low frequencies. At deeper depth and higher hydrostatic pressure, the bubble period is shorter and thus less rich in low frequencies. It may be possible to tune the periods of the gun bubbles from the higher and deeper gun arrays so that they match in order to improve subsequent data processing by firing the deeper sources at different air pressure to the upper sources.
3.3.6 Firing a Big Airgun at Very Shallow Depth
Amundsen et al. (2017b) present experimental findings from airgun source tests in 2010 in the ford of Trondheim, Norway, using a 1,200 in3
gun, where the
focus is on the gun firing at approximately 1.3m. Only one measurement was taken at this depth as the airgun tests were not designed to test shallow firing. We will see that its signature is broadband, has insignificant bubble energy (since the air bubble bursts out in the atmosphere at its first expansion), and interestingly, has higher amplitudes in the spectrum between 0.5 and 2 Hz than guns fired at 3.3m or 5.3m. Te observations we present might be a step in the direction of designing more optimal low- frequency sources for seismic exploration. Te first observation on insignificant bubble energy is
perhaps not very surprising, since this effect for explosives was shown by Lay in 1945. And as we have learnt so far in this chapter, whereas the bubble problem of explosives was overcome by putting the explosive at very shallow depth, the bubble problem of the airgun was handled by the tuned airgun array concept. Further, we note that the signatures of airguns fired at depths of 3m and deeper have been thoroughly investigated. When the airgun releases a volume of air into the water, the air produces a steep-fronted shock wave followed by several oscillations resulting from the repeated collapse and expansion of the air bubble (bubble pulse). We refer the reader to landmark
105
papers by Vaage et al. (1983) and Ziolkowski (1987). However, we are not aware of published studies of airguns where the guns are fired at depths shallower than 2m. We note that shallow-towed big airguns can be used in VSP
or site surveys. Since the signature from a large shallow gun has extremely little bubble energy, de-signature is considerably simplified in the processing of such data. Generally, in site surveys a small volume airgun is used, typically 40 to 400 in3 towed at a depth of 2–3m.
,
3.3.6.1 Fjord Test A source signature field test was done in the Trondheim Fjord in 2010 using a single stationary conventional 1,200-in3
airgun
hanging from A5-buoys. Te water depth at the test location is approximately 390m. Te weather conditions were excellent during the test (calm sea). Ambient noise recordings did not indicate significant ocean noise at low frequencies. A TC4043 Miniature hydrophone offering a wide frequency range was suspended by a wire, and a weight was used to keep it stationary at 80m depth. For each source depth, approximately 30.3, 20.3, 10.3, 7.3, 5.3, 3.3, 2.3, and 1.3m, several shots were fired, and data showed good repeatability for the shots fired at the same depth. At 1.3m depth, however, the gun was fired once only. Figure 3.18 shows two photos of the air bubble venting to the atmosphere. Figure 3.19a shows all the data on which this analysis is
based. To equalise data each trace has been multiplied by the difference between the hydrophone and gun depths. Tis
Figure 3.18: In a source test in the Trondheim Fjord in 2010, a 1,200 in3 airgun was fired
just beneath the sea surface at 1.3m while hanging from A5-buoys. A shallow-fired big airgun source gives a broadband pulse with no bubble effects since the air bubble is vented to the atmosphere during its initial expansion. The downgoing signal is significant and useful in seismic exploration.
Statoil
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