WP2 - deliverable no. 3: GIS-map of hot-spots of present and future CH4-emmission
Deliverable
Michael Schlüter, Torben Gentz, Roi Martinez.
Potential hot-spots for present and future CH4-emmision are regions characterized by the intense formation of biogenic methane or the transport of thermogenic methane from depths via fracture zones or other conduits. In such regions features like (1) pockmarks (morphological depression at the seafloor), (2) gas flares, where gaseous methane is released from the seabed, (3) shallow gas regions, where gas bubbles are detected in close vicinity to the sediment-water interface as well as (4) chemo-autotrophic communities nourished by upward fluxes of e.g. methane were observed.
An objective of WP2-3 was the data mining and mapping offree gas layers in the sediment, pockmarks, gas flares etc. known for the Baltic Sea. This allows to characterize regions in terms of drivers for the formation of methane. By application of GIS techniques to such data sets the occurrence of free gas regions can be predicted for the entire Baltic Sea.
In close cooperation with the partners of the BalticGas project, we compiled data about the spatial distribution of Pockmarks, free gas regions etc. (Fig. 1). These geodata where integrated into the Geo-Information-System (GIS) and combined with geodata about sedimentology, bottom morphology, mass accumulation of particulate organic matter as well as bottom water concentration of oxygen or sulfate, for example.
- FIGURE 1A and 1B
- Figure 1C
Figure 1a: Seismic line crossing a pockmark area in Eckernförde Bay. Morphological depressions as well as the occurrence of free gas in the subsurface are visible. Figure 1b: The depth of free gas layer was mapped during AUV dives. Figure 1c: Example for the spatial distribution of pockmarks as well as free gas regions in the Baltic Sea.
Pockmarks and free gas areas were observed especially in the western part of the Baltic Sea. In the eastern part of the Baltic Sea pockmarks were detected in Gdansk Bay or the Gulf of Finland. For some of these sites, there seems to be indications for thermogenic methane.
As a first step towards a predictive mapping, weighting factors were assigned for each parameter which is considered as indicative for the occurrence of free gas. Applying Map Algebra the different weighting factors were applied to each information layer to calculate a new layer containing at each raster cell a probability value for the occurrence of free gas.
The derived predictive map was compared with the known spatial distribution of free gas. By an iterative procedure the weighting factors were adjust to produce an optimize accordance between observed and predicted free gas regions. Figure 2 provides an example for the predictive map of free gas in surface sediments of the Baltic Sea.
Figure 2: Example for a predictive map about the spatial distribution of free gas in surface sediments of the Baltic Sea derived by spatial modeling. The prediction is based on the analysis of parameters like POC-accumulation, O2 and SO4 concentration in bottom water as well sediment type, observed within known free gas areas. The data set was factorized to obtain a prediction for the occurrence of free gas areas within the entire Baltic Sea. This procedure was optimized by comparison of the similarity of the spatial distribution of known free gas and predicted free gas areas. Based on this on this comparison predicted probability levels of gas occurrence (low, medium-low, medium, high) were assigned.
References
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