volumes of each gun, and is typically in the range of 3,000– 8,000 in3
(49.2–131.6 l). When the source is fired (i.e. the air is
released), an acoustic signal is sent through the water and into the earth. In the 1960s Conoco developed the first marine vibrator
(Proffitt, 1991) and for a number of years there was competition between gas exploding sources, vibroseis and airguns. According to Proffitt, it was the safety, simplicity and the reliability of the airgun which made it the winner of this competition. During the eighties, improved design and new developments for hydraulic marine vibrators were undertaken. In 2005, PGS launched a new marine seismic vibrator. Tis
has a flextensional shell (Tenghamn, 2005) which consists of two independent vibrators, one covering the frequency range from 6–20 Hz (Subtone) and the other covering 20–200 Hz (Triton). At the time of writing, several initiatives are ongoing to
develop commercially and technically viable marine vibrators as an alternative sound source for seismic surveys. A primary motivation is to use vibrators in environmentally sensitive areas where the airgun sources are considered potentially harmful. In addition, vibrators could be used in shallow water operations where airgun arrays cannot be towed, to improve bandwidth control, and in simultaneous source operations – standard today in l and seismic surveying as a way to increase efficiency. A recent development for marine seismic streamers has
been the introduction of sensors measuring particle motion in addition to the conventional hydrophone pressure sensors. Tis type of streamer enables a broader frequency data spectrum, again leading to higher seismic data quality. In some cases, the streamer depth is varied along the streamer. We refer the reader to Chapter 5 on Broadband Seismic Technology and Beyond for a discussion of this topic. Another trend on the horizon is simultaneous shooting with
the potential to increase the rate at which seismic data can be acquired while improving subsurface sampling by increased shot density, a technique already established for land data (see next section). Te idea is to trigger two or more sources sufficiently close together in time so that the recorded signal energy interferes. Tis interference of signals is handled in data processing to separate the information provided from each source.
1.3.2.2 Land Seismic
To acquire high-quality 3D seismic data on land is much more expensive than it is at sea because of the infrastructure, time and manpower required. Furthermore, acquisition is even more costly in complex terrain and vegetation. In land seismic surveys, trucks with hydraulic vibratory
plates stop at regular intervals and shake the ground to send sound waves into it. Tese are reflected off the rock layers below, sensed by grids of receivers, called geophones, laid on the surface of the earth, and turned into an electrical signal which passes along the receiver cables to a central recording unit. In strongly vegetated and mountainous areas where large
vibrators are impractical, the best choice for source is dynamite, with a small charge being dug into the ground. Te explosion provides a sharp, short signal that is well suited to seismic. Te
Figure 1.28: High-speed photos of one sub-array of airguns just after firing in the sea. Top: Prior to firing; middle: 8 ms after firing; bottom: 32 ms after firing.
downside is the risk of injury to personnel working with the collection of seismic data, and environmental effects. In densely populated areas, seismic activity with the use of dynamite is considered neither environmentally friendly nor suitable. Simple weight drop sources and airguns are also used in
land acquisition, but much less frequently than vibrators and dynamite, which each have a similar portion of the total land acquisition market. Conventional land seismic surveys use an array of vibrators
together with grids of long cables. Each cable can be up to 30 km in length, with geophones spaced every 50m. Today, land recording systems with 150,000 channels are available and it is predicted that 1 million channel systems will be available by 2020. Tis is a lengthy data acquisition process and it can take
several years to survey large areas, so to speed acquisition, new techniques have been developed. One of these is simultaneous shooting, which can speed
conventional shooting by a factor of 10 or more. Trucks are allowed to operate independently of each other during
21
Courtesy of CGG/Statoi
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