ctive seismic acquisition is one of the first key steps in the exploration for new oil and gas reserves and is also fundamental in the effort to optimize the development and production of existing oil fields. It uses seismic waves emitted by sources at discrete locations, and some of that energy is reflected back from layers in the subsurface to tens of thousands of recording sensors. The data are then processed to give an image of the subsurface, which is later analyzed by geologists to identify commercial deposits of hydrocarbons.
A typical marine seismic acquisition is conducted using airgun source arrays towed behind a vessel. Such a source typically comprises about 30 single airguns of different volumes, all firing at the same time to produce a sharp acoustic peak. The principle technology of airguns has not evolved much since the 1970s when it replaced the use of dynamite, and airguns stayed inherently inflexible by nature.
It is very challenging to shape the energy spectrum of an airgun source to suit a particular geological application, which means that in most cases the source ends up emitting more energy than what is required for seismic imaging as well as energy at higher frequencies than those used to form the final seismic image. The marine vibrator system has the capability to emit the same energy as an airgun source, but it distributed over time and over a precisely chosen frequency range. Therefore, it is considered less damaging and disturbing to marine fauna.
An alternative approach is to design the sweep to generate just enough energy at each frequency to create the required signal-to-noise ratio for imaging the target area in depth. Therefore, it is a necessity for any marine vibrator operation to not only avoid unused energy transmissions but to be able to move this excess energy to useful lower frequencies. Unlike airguns, vibrators allow the exact definition of which frequencies should be emitted for a specific geological setting and the survey target.
By utilizing advanced processing techniques, combined with a smartly designed source geometry tailored to the survey area, the source productivity is expected to greatly increase. The main enabler of the productivity increase is the directivity of the BASS, making it possible to unlock gradient-source processing capabilities as shown in the case of ocean-bottom nodes (OBN).
Full-azimuth OBN surveys are considered to be the gold standard in terms of image quality and the ability to accurately characterize reservoir properties. Despite their superior quality, less than 40% of all seismic acquisitions are on the ocean bottom, mainly due to cost. One reason for the high cost of seabed acquisition results from the long acquisition times in the field, and consequently the vessel costs. Modeling shows that the BASS marine vibrator system can potentially reduce the acquisition time and the overall survey cost.
The BASS marine vibrator system will enable global E&P companies to operate with lower impact to the environment, which may be particularly important in areas with busy or narrow acquisition windows due to other marine activities, such as commercial fishing. Marine vibrators are a potential game changer because they address three important objectives: more efficiency, less environmental impact and better data.
References available. Contact Brian Walzel at bwalzel@hartenergy.com for more information.