Normal practice until well into the 1960s was to fire 50 lb
(22.7 kg) charges of nitro-ammonium nitrate 6 ft (1.8m) below the sea surface at intervals of 660 ft (200m). Often 15 tons of this type of explosive (with the trade name Seismex, Imperial Chemical Industries Ltd) could be expended in one day. While the bubble problem was solved by firing the explosive
at shallow depth, it was estimated that approximately one third of the explosive’s energy was used to lift the water spout (Lugg, 1979). Experiments by Lavergne (1970) showed that the reflection amplitudes increased with charge depth to the extent that 30 times less dynamite was required at 30 ft (9m) water depth than at a shallow blowout depth, in order to produce the same level of seismic response. He explained the poor seismic amplitude obtained with shallow small 100g charges by a cutoff effect of the ghost reflection, which reduces the pulse amplitude in the lower seismic frequency band. Te use of dynamite and other high-energy explosives
caused environmental and political concerns as well as safety problems. In 1969, most governments prohibited their use near the surface due to observed high fish kill. Tis led to the development of other viable alternatives to explosive charges. Te 1960s saw a rapid increase in the use of alternative types of energy sources. By the end of the decade the geophysicist could choose from a proliferating spectrum of sources, including dynamite, black powder, gunpowder, ammonium nitrate, PRlMACORD, AQUAFLEX, AQUASEIS, FLEXOTIR, electric sparkers, SSP, WASSP, SONO-PROBE, PINGERS, BOOMERS, gas exploders, DUSS, DINOSEIS, GASSP, AQUAPULSE, airguns, PAR, PNEUMATIC ACOUSTIC ENERGY SOURCE, SEISMOJET, AIRDOX, CARDOX, HYDRO-SEIN, etc (Kramer et al., 1969).
3.3.3 Airgun Developments In the mid-1960s Steve Chelminski began manufacturing and testing airguns for use in seismic surveys, and in 1970 he founded Bolt Technology Inc. Te airgun soon became popular, and it rapidly became the most widely used source for marine surveys. While the airgun won the competition for most popular
marine source, it was not without its problems. It was not yet the ideal marine source that the seismic industry wanted. Tree specific improvements in airguns were sought: more reliability, more power and a wider signal bandwidth. Geophysical Services Inc (GSI) began a development programme to address these needs. A radical new design of airgun based on an external sleeve was produced to replace the conventional internal shuttle valve airguns. Te first production unit was mobilised in 1984. Airgun development continued and improved models of the internal shuttle guns have been introduced so that both sleeve guns and internal shuttle guns are popular today. One other innovation in marine sources needs to be
mentioned. In 1989 Adrien Pascouet and his company Sodera presented the GI airgun, which first generates a bubble and then (delayed by 10–20 ms depending on gun size) injects air into the bubble to prevent unwanted bubble oscillations. Introduced at the 2014 EAGE trade show, Bolt Technology
Corporation demonstrated a new type of airgun designed to reduce the potential impact of seismic acquisition operations on marine life while also delivering optimal bandwidth for
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subsurface imaging. Called eSource™, it is designed to reduce the high-frequency components believed to have most potential for causing disturbance to marine life while retaining the low-frequency components critical to seismic exploration (see Section 3.14).
3.3.4 Early Airgun Arrays
Marine airguns (and other sources) were arranged from an early date into spatial arrays for spatial filtering purposes. Inline arrays were described by Newman et al. (1977), Lofthouse and Bennett (1978) and Ursin (1978, 1983). Source arrays were soon extended in the crossline direction to act as spatial filters in a direction across the survey line (Badger et al., 1978; Parkes et al., 1981; Teer et al., 1982; Tree et al., 1982). Te source concept was extended to arrangements in the vertical direction (Cholet and Fail, 1970; Smith, 1984; Lugg, 1985) to suppress ghost reflections and to improve the peak-to- bubble ratio. Smith (1984) proposed a system of four subarrays deployed at different depths and fired at different times. He noted that the idea of distributing the elements of a seismic source in the vertical direction, with time delays, was “not a new one” and he referred back to the patent by Prescott (1935). Variations on the same theme have been described for seismic surveys on land (Shock, 1950; Van Melle and Weatherburn, 1953; Musgrave et al., 1958; Seabrooke, 1961; Hammond, 1962; Sengbush, 1962; Martner and Silverman, 1962; Fail and Layotte, 1970). Van der Schans and Ziolkowski (1983) proposed applying angular-dependent signature deconvolution in the τ-p domain to correct for the source directivity pattern. Fokkema et al. (1990) suggested a frequency-space directional deconvolutional algorithm for common receiver domain data. Tey observed that the algorithm could be applied in the CMP domain under a plane layered earth assumption.
3.3.5 Developments in Airgun Arrays
Recent advances in marine broadband seismic surveying (see Chapter 5) have spurred new interest in airgun source configurations. Conventionally, airgun arrays have been kept at a constant depth. However, in broadband seismic survey design, the geophysicist is often faced with the challenge of designing surveys that achieve two objectives: enhancement of low frequency energy to compensate for the effects of scattering and attenuation of the primary signal to penetrate the deeper formations, and securing the large bandwidth necessary for imaging of overburden and shallow reservoir targets. Tis challenge in survey design is a Gordian knot: in order to enhance low frequencies the source needs to be deep enough to reduce the effects of ghosting but this will be at the cost of reduced low frequency content in the bubble pulse. In order to improve the bandwidth the source needs to be shallow at the expense of reduced low frequencies due to ghosting. To attack the ghost, source solutions involving multiple
source depths have been proposed, including over/under sources or vertical arrays (Moldoveanu, 2000; Egan et al., 2007; Kerekes, 2011); multi-depth level time-synchronised subarrays (Hopperstad et al., 2008a, b; Cambois et al., 2009; Bunting et al., 2011; Parkes and Hegna, 2011; Sablon et al., 2013); slanted arrays (Shen et al., 2014; Telling et al., 2014); and variable source depth acquisition (VSDA), in which the source depth
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