Bright spots

The maps “Amplitude anomalies” represent the distribution of gas indicators in the form of amplitude anomalies in seismic data, such as bright spots, gas chimneys, seismic transparent zones and velocity pull-downs. The study area covers the German North Sea sector and the depths of the first 1.000 m below the seabed. Various seismic data were available as data bases: approx. 30.000 km 2D data and approx. 4.000 km² 3D data. The mapping of amplitude anomalies indicating gas serves as a basis for further investigations into near-surface natural gas deposits in the German North Sea sector. The near-surface gas deposits up to 1.000 m depth in neighbouring countries suggest that there could also be near-surface natural gas reserves in the German North Sea sector. Three of the Dutch near-surface natural gas deposits, near the German-Dutch border, are already in production (Muntendam-Bos et al., 2009). This shows that such natural gas reserves are a potential source of energy. If smaller natural gas deposits were to be found near planned wind farms, natural gas could be extracted and burned directly on site, and during windtimes it could propel turbines. This combination could enable a continuous energy supply. Since natural gas in the sediment can negatively affect the building ground properties, knowledge about the distribution of natural gas deposits in the depths of the foundation depth of wind turbines is also important from this point of view. The location of all mapped bright spots is marked green. These are the most widespread amplitude anomalies that can give indications of gas. Bright spots can be created at the top of a gas-carrying layer. Compared to surrounding reflectors, these are characterised by very high, negative amplitudes. These phenomena in seismic data can be explained by the change in impedance (product of seismic velocity and density). Usually, impedance increases with increasing depth and the associated increasing compaction. At a layer boundary there is a positive impedance contrast, which is reflected in the seismic data as a reflector with a positive phase. If the pore filling changes and is located in some areas of gas instead of water, the impedance decreases. A layer boundary with such a negative impedance contrast can be recognised as a reflector with negative phase in seismic data. The negative impedance contrast is also evident at the lateral boundaries from a gas leading to a non-gas-carrying area by a phase change. Since bright spots can also be caused by lithology changes, for example by clay, peat and coal layers, further investigations have to be carried out for the evaluation of the amplitude anomalies. Literature: Muntendam-Bos, A. G., Wassing, B. B. T., Heege, J. H. t., Bergen, F.v., Schavemaker, Y.A., Gessel, S. F. v., Jong, M. L. d., Nelskamp, S., Thienen-Visser, K. v., Guasti, E., Belt, F. J. G. v. d., and Marges, V.C., 2009. Inventory non-conventional gas. TNO, Utrecht.