WP2 - deliverable No. 1: GIS-map of mined data
Deliverable
Jørn B. Jensen and Bo Barker Jørgensen
BALTIC GAS Seismic data collection
During the Baltic Gas project a large seismic dataset was collected through a combination of data mining of existing data and acquisition of new data during the many BALTIC GAS cruises.
Systematic surveys by sediment echo sounder and shallow seismic techniques started in the late 1970’s. The mapping of bathymetry and sediment types was the main ob¬jective of those studies. The early data re¬vealed the occurrence of gassy sediments and indicated the existence of pockmarks in many areas of the Baltic Sea. As part of our data mining process, information from acoustic surveys, often ar¬chived as paper prints with the national geological surveys or other institutions, were analyzed and compared with more recent investigations.
Contributors of existing seismic data have mainly been the Baltic Gas project members, who also collected additional data during the project.
During the EU 5th FP Project METROL (Laier and Jensen 2007), only the free methane distribution in Danish waters was mapped. Therefore, the mapping focus of The BALTIC GAS project has been the rest of the Baltic Sea.
The collected seismic data have been loaded on seismic workstations by the data owners. The distribution of free gas has been digitized and the data have been compiled at GEUS, as a basis for the GIS-mapping carried out by the Alfred Wegener Institute for Polar and Marine Research (AWI). Additional information has been geo-coded and GIS maps have been compiled.
Fig. 1 shows a map of the available seismic lines, either obtained from archived data or sailed during the BALTIC GAS project. As the map shows, the available data cover most regions of the Baltic Sea but with a strong bias towards the central basins. This is due to the special objectives of the BALTIC GAS project which focus on the accumulated Holocene mud in these basins rather than on the coastal regions with harder ground. The hard ground is generally difficult to core and does not contain shallow gas.
Figure 1: Seismo-acoustic lines (i.e. data) complied in a common database by GEUS. Black lines are archive data. Red lines show seismo-acoustic data measured during BALTIC GAS.
The total amount of shallow seismic data is large compared to what has previously been compiled. The total length of seismic lines in the database is now 33,500 km, of which 29,000 km are derived from archived data
The shallow seismic data have been important for locating free methane in the Baltic Sea sediments and, in combination with physical/chemical parameters and methane sampling in sediment cores, it has been possible to establish a model for typical geological settings that lead to free methane. Fig. 2 shows an overview of Holocene mud and clay in the Baltic Sea proper. Also the distribution of shallow gas in the Danish waters is shown. Further GIS maps of methane distribution and methane hot-spots are shown in Deliverables 2.2 and 2.3.
Figure 2. Compiled data input to GIS-mapping.
A main conclusion from the data compilation is that, with very few exceptions, free methane in the Baltic Sea is restricted to Holocene marine mud areas and that a minimum threshold thickness of this mud is required before the appearance of gas bubbles. A combination of detailed key area seismic investigations and regional reconnaissance surveys have shown that in general the Holocene mud deposits are thinner than the required threshold thickness for bubble formation and that the existing areas with free methane may be characterized as geological sediment traps.
Physical parameters in cores
An important part of the characterization of gas-bearing sediments has been done by physical property studies on cores obtained during an extensive coring program of the BALTIC GAS expeditions. Multisensor core logging was used to estimate basic physical properties of gas free and gas charged sediments. The results were used for interpretation of sediment echo-sounder records. From these data the thickness of the Holocene mud (deposits of the Littorina Sea) and of the older deposits from the previous Baltic Sea Stages were estimated.
At the IOW, the physical properties and geo acoustic data provided input to the development of a geo acoustic model based on the BIOT/STOLL theory for gas free and gas charged sediments. Further questions investigated, based on the physical properties data in combination with the acoustics, were the influence of gas bubble on the acoustic properties and on the physical strength of the muddy sediments.
An important part of the data handling has been submission of data for a public database. Due to limitations in data storage capacity, primary data from seismo-acoustic mapping reside with the institutions obtaining the original data. Other data, such as geo-referenced data on gas distribution, are deposited in the data bank, PANGAEA, which cooperates with the World Data Center for Marine Environmental Sciences (WDC-MARE).
Chemical parameters in cores
The mining of geochemical data focused on methane as the key parameter. Particularly useful were high-resolution depth profiles of pore water methane concentrations that enabled flux calculations to be made. However, also a number of other relevant chemical species, both in the pore water and in the solid phase, was drawn from published data or from available databases. In particular the database PANGAEA was a useful source of such data. It was striking that the largest number of useful datasets originated from the earlier EU-funded project, METROL. The data have been compiled and are being prepared for publication in the international scientific literature. At the same time, all the data are publicly available through the database, PANGAEA.
References
Anderson, A.L., Hampton, L.D., 1980. Acoustics of gas-bearing sediments: I. Background. II. Measurements and models. Journal of the Acoustical Society of America 67,1865-1903
Biot,M.A., 1956. Theory of Propagation of Elastic Waves in a Fluid-Saturated Porous Solid. I. Low Frequency Range. II. Higher Frequency Range. J.Acoust.Soc. Am., 28, pp. 168-191
Laier, T. and Jensen, J.B. 2007: Shallow gas depth-contour map of the Skagerrak-western Baltic Sea region. Geo-Mar Lett (2007) 27:127–141.
Stoll, R.D., 1985. Marine sediment acoustics. J.Acoust.Soc. Am., 77(5), pp.1789-1799
Wilkens, R.H., Richardson, M.D., 1998. The influence of gas bubbles on sediment acoustic properties: in situ, laboratory, and theoretical results from Eckernförde Bay, Baltic sea. Cont.Shelf Res. 18, pp.1859-1892