Full Waveform Inversion of Gas Hydrates: Application to estimate of hydrate saturation

Abstract

Through seismic acquisitions there have been discovered layers beneath the seabedthat consist of pores filled with gas hydrates (GH). These reservoirs are potentialenergy reserves and it is therefore desirable to get an estimate over how big they are. The saturation of GH varies horizontally in the layer and research has shown a linear relationship between the saturation and the seismic wave velocity. The seismic acquisition data only shows the GH layer s reflector which is an indicator of where the layer is located, and tells nothing about how the saturation varies in the layer. It is therefore proposed in this report to use the Full Waveform Inversion (FWI) method on synthetic data to see if this method can be used to map the saturation.FWI is an inversion algorithm that iteratively works to minimize the difference between the observed seismic data from a geological model of interest and data froman initial guess of this model. The observed seismic data are in this paper synthetically produced from a geological model with a horizontal and gradually increasing saturation in the GH layer.To test that the FWI were working as expected it was used an initial guess modelwith a constant saturation in the GH layer. This gave promising results and thenext initial model was made more realistic with a gradually increasing velocity vertically through the model. This velocity gradient was determined by the maximal and minimal wave velocity in the geological model, which are values that are assumedpossible to retrieve from seismic acquisition data.The results in this report is limited since it has only been used synthetic and not real data in the study. Despite this obvious limitation the results gives a strong indication in favour of the FWI. It looks like the method would not only be an adequate, but a satisfying approach to the mapping of the GH saturation. The FWI method is capable to retrieve the horizontally increasing saturation of the original model. And this from an initial model were the layers of the model are not even known. This shows that the use of FWI is a promising method for providing a better estimate of gas hydrate saturation.