In order to use geophysical methods to monitor production from hydrate accumula­tions, a number of fundamental challenges must be considered. They can be sum­marized as follows:

Suitability of geophysical methods depends on geological setting and expected pro­duction behavior. The spatial and temporal evolution of physical properties in a hydrate accumulation differs dramatically depending on the type of the deposit and the dissociation method, thus requiring different monitoring methods. For example, the study of Moridis and Reagan [114] predicted that depressurization-induced pro­duction from the Tigershark deposit would effect changes in the HBL extending to a radius of 800 m from the production well, and including decreasing thickness of the HBL, increasing gas saturation, decreasing SH, and the formation of free gas lay­ers above and below the HBL. Kowalsky et al. [ 83] showed that such complex changes appear to be amenable to detection with time-lapse VSP measurements col­lected in a well 50 m away from the production well. Another study on production from a permafrost-associated hydrate accumulation in North Slope, Alaska [ 15 [ showed changes that were mostly limited to within 5 m from the production well after 2 years of production, making the system far less ideal for VSP monitoring than in the previous case. Cross-borehole measurements may be more promising in some cases, though the spatial coverage they provide is limited to the inter-borehole region, and there is the risk of non-detection if the changes in the HBL are not pro­nounced [210].

Rock physics models dependence on geological setting and time-varying hydrate configuration. A considerable amount of research has been performed to determine rock physics models (the relationships between sediment properties and geophysi­cal properties) for HBS based on theoretical considerations, laboratory experiments, and field data (e. g., [65, 100, 216]). Depending on the geological setting, GH can be distributed in a variety of ways (e. g., acting as cement between grains, acting as the matrix supporting the grains, or existing mainly in pore space), which dramatically affect the seismic and electrical properties of the sediment mixture [206] . For the purpose of geophysical monitoring at a given site, the rock physics model must be determined in advance using site-specific data such as well logging and core data. However, care must also be taken to account for the geophysical properties changes during production because the hydrate and gas saturations may move to ranges beyond those used to develop the models. It may be necessary to change models in the course of production as the hydrate conditions change, a subject that has not received much attention.

Simultaneously changing physical properties can lead to nonunique interpretations of time-lapse geophysical data. Because geophysical properties are a function of the saturations of all phases in the system, in addition to pore fluid pressures, it is difficult to uniquely attribute the change in a geophysical measurement exclusively to SH changes. For example, during depressurization-induced production from a GH deposit, the P-wave velocity can vary due to changes in both the effective pressure and in phase saturations. As the fluid pressure decreases, the stresses and the frame bulk moduli of the sediment increase [41]. At the same time, the increases in velocity are offset by the effects of the decreasing hydrate saturation and increased gas saturation [89].