Blanking

A second significant seismic characteristic of hydrate-cementation, called "blanking" is also displayed in Figure 1; blanking is the reduction of the amplitude (weakening) of seismic reflections probably caused by the presence of hydrate. Many observations of blanking in nature have been associated with gas hydrate accumulations (Lee et al. 1993, 1994, 1996; Lee and Dillon, in press; but the reality of blanking was questioned by Holbrook et al., 1996). Blanking is thought to result from preferential accumulation of gas hydrate in the more porous sedimentary strata, where it would increase the velocity of the more porous, initially lower velocity layers by the introduction of high-velocity, gashydrate cement. This would create a more uniform, higher velocity, resulting in reduced acoustic impedance contrasts and thus reduced reflection strengths.

We calculated the amount of hydrate in the most intensively blanked sediments by using known hydrate velocity from published laboratory studies, velocities that we determined for sea floor sediments from multichannel seismic profiles, and porosity in our study area determined in scientific drilling. We assume that such a material is an end member, and, in our computed seismic model, we mathematically "mix" this maximum-concentration, hydrate-bearing material with a similar deposit having no hydrate, in order to model the range of possible blanking effects. The modeled affect on reflection amplitudes of various mixtures of end-member, hydrate-bearing sediment with hydrate-free sediment is shown by computer-generated wiggle traces(Fig. 2).

NO HYDRATE

NO HYDRATE

Figure 2. Synthetic seismograms showing the modeled effect of various amounts of gas hydrate in sedimentary strata on the amplitude of reflections from acoustic impedance changes comparable to those of sedimentary strata.

The reduction in reflection amplitude is the parameter of blanking. Computer-modeled increase of blanking (decrease of reflection strength) with increase of hydrate is apparent. Three classes of blanking have been assigned; the boundaries between classes represent a change in the power of reflections by a factor of two. The class boundaries are indicated on Figure 2, and these classes can be related to overall average amounts of hydrate in bulk sediment of approximately 7 percent (class 3), 12 percent (class 2), and 15 percent (class 1) in this example. To recognize classes in a profile, we measure and plot the amount of blanking (the reduction in reflection amplitude) along the profile in a subbottom window within the hydrate zone. These measured values are used as a calibration, and the extent of the three classes is interpreted along the profile. The total thickness for each class is plotted along each profile and the total volume of each class is estimated by interpolating between adjacent profiles in the area being mapped. Once the volumes for each class are estimated, they are multiplied by the appropriate percentages and a volume estimate of hydrate is made.

It must be emphasized that the approach is constructed from a mathematical model that is based on a logical and carefully thought-out concept of how the presence of gas hydrate in a sedimentary structure affects the acoustics of the sedimentary mass. However, at present we have an essentially inadequate understanding of this relationship. Much more work is required in the laboratory to better define this issue, work that is proceeding at present (see Chapters 24 by Winters et al, and 25 by Stern, et al.). Maps that will be presented give an excellent view of the structure of the gas hydrate stability zone and probably a good indication of the relative distribution of gas hydrate in the sediments, but the absolute amounts of gas hydrate indicated in the map probably will need to be adjusted as new modeling capability is developed.

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