It is a general condition for the approval of seismic surveys that companies use a so-called soft start procedure which gives marine mammals such as porpoises and dolphins a chance to leave the area.
June 8, 2010
By Steen Hartvig Jacobsen, journalist.
Seismic surveys are carried out in order to obtain information on the layers in the subsoil and often form part of the initial phase of an operator’s activities under an exploration licence. Seismic surveys are carried out by sending pressure waves into the subsoil. When the pressure wave encounters different geological layers, part of the pressure wave is reflected back to the surface, where it is recorded by special receivers. This data is then processed using sophisticated computer programmes and can give the companies information about geological structures which could contain oil and gas if the right conditions are present.
Three types of seismic surveyA two-dimensional seismic survey (2D seismics) results in a cross-section of the subsoil. Acquiring the 2D seismic lines in a fine-meshed grid produces a three-dimensional image of the subsoil (3D seismics). When seismic data is acquired from the same area at several-year intervals, a fourth dimension is obtained – time. These 4D seismics can for example provide information on how the oil has flowed towards the production wells. 4D seismics are therefore an important tool for optimizing oil recovery.
Together with the drilling of exploration and appraisal wells, the acquisition of seismic data forms part of the obligations that operators of exploration licences assume under the work programmes that form the basis for the Ministry for Climate and Energy’s award of licences, either through licensing rounds or via the Open Door procedure. As part of the work programmes following the 6th Licensing Round, extensive 3D seismic data has therefore been acquired in the Central Graben in recent years. In addition to oil and gas exploration, seismic surveys can also be used to map the potential for geothermal energy, in the preparation for the establishment of natural gas storage facilities and in analyses concerning the potential for storing CO2 in the subsoil.
On land, seismic data is acquired using pressure waves, which are generated using vibrator vehicles. The reflected pressure waves are recorded via a large number of small, simple microphones called geophones. Offshore, seismic surveys are carried out by specially equipped vessels which generate pressure waves using an air gun which is towed behind the ship. The reflected pressure waves are recorded by hydrophones attached to cables with a length of several kilometres.
Protecting marine mammals
Seismic surveys generate noise which can affect marine fauna. The DEA must approve all seismic surveys in advance before they are commenced. The application must contain information concerning equipment, programme, working methods and what impact the survey will have on the fauna. On the basis of this information, the DEA will then assess the possible impact on the fauna in the area concerned. Against this background, conditions may be imposed to ensure that the surveys are carried out in accordance with the provisions of the Subsoil Act and thus comply with the EU Habitats Directive.
It is a general condition for the approval of seismic surveys that companies use a so-called soft start procedure which gives marine mammals such as porpoises and dolphins a chance to leave the area before the pressure waves reach their operational level. The National Environmental Research Institute (DMU) has formulated the guidelines which the DEA requires companies to follow.
The conditions that the DEA imposes for the approval of seismic surveys are based on the latest data and information concerning marine mammals in the Danish offshore area. At the end of 2009, the DEA received the preliminary results of two monitoring programmes concerning the presence and behaviour of porpoises and other fauna. Mærsk Olie og Gas has carried out these programmes as part of its obligations under the work programmes. Together with DMU and the Agency for Spatial and Environmental Planning, the DEA is currently evaluating the results with the aim of clarifying whether there is a need for additional measures in order to provide marine mammals with the best possible protection.