RESOLVING RESISTIVE ANOMALIES DUE TO GAS HYDRATE USING ELECTROMAGNETIC IMAGING METHODS

Active marine electromagnetic methods have proven to be a powerful tool to detect resistivity anomalies associated with gas hydrate. However, because the propagation of electromagnetic fields for these methods works in the diffusive regime the spatial resolution of the resistivity structure is limited. So far only bulk electrical properties have been estimated from measured data, although hydrate bearing layers are found to be highly heterogeneous. We computed response curves for synthetic one- and two-dimensional models to investigate the resolution capabilities for various measurement geometries with respect to resistive features. Electric dipole transmitters (TXs) are used as sources. In the marine case, the in-line electric dipole-dipole configuration has proven its capabilities to detect the shallow resistive gas-hydrate. Our model study demonstrates that both the depth to a resistive feature can be resolved nicely using data for multiple TX-RX offsets. However, resolving smaller features of the resistive zone, for example if the zone is split in separate resistive layers, is extremely difficult. The resolution of the target can be improved using electrical downhole transmitters. So far there have been no reports of the detection of permafrost gas hydrate deposits with surface electromagnetic methods. Our calculations show that a similar setup to that used in the marine case is capable of detecting gas hydrate on land. The resolution, however, is lower than for the marine case, because of the significantly greater depths to the target.