Imaging and characterizing the waste materials inside an underground storage tank using seismic normal modes. 1998 annual progress report

'This work is directed toward finding a way to estimate the properties of wastes in tanks on the Hanford Reservation. Data acquisition work (Turpening et al., 1995) has shown that acoustic measurements can be made inside a Hanford tank. Although that data was collected in a cross-well geometry, other data acquisition methods are now of interest. In particular, it would be very useful if an estimate of the properties of the wastes could be made by lowering a single string of hydrophones into the one liquid observation well (LOW) that exists in every tank while activating a seismic source on the surface outside the tank. Such an endeavor requires an understanding of the normal modes of oscillation of a Hanford tank when it contains various layers of wastes. The objective of this study to gain that understanding. With advent of the cone penetrometer it is now possible to conduct acoustic cross-well measurements in a Hanford tank even though each tank has only one LOW. This in turn makes the study of normal modes even more important since these modes of oscillation constitute a major portion (along with tube waves) of the background noise in any cross-well data set. Knowing where the normal modes exist in the frequency domain, for example, will allow one to plan a data acquisition program that might avoid the especially strong modes.