Imaging and characterizing the waste materials inside an underground storage tank using seismic normal modes. 1998 annual progress report Page: 2 of 3
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Imaging and Characterizing the Waste Materials
Inside an Underground Storage Tank Using
Seismic Normal Modes
Dr. M. Nafi Toksiz, Massachusetts Institute of Technology
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.
Research Progress and Implications
The initial period of this study has focused on tanks containing non-rigid wastes, i.e. where only
acoustic waves propagate in the waste materials. We have approached the problem both theoretically
and by means of laboratory-sized scale models. The theoretical work has shown, and the lab
measurements have confirmed what was expected, that the spatial complexity of the modes increases
as frequency increases. The low frequency modes show a simple vertical distribution of pressure
while the higher frequency modes (Figure 1) show a more complex distribution of pressure in the
tank. In all of this work we assume that the tank is rigid
The implications of the work to date are twofold. First, the modes are rich in information, especially
when the vertical array of hydrophones is not on the center line of the tank. Fortunately, that will be
the case with many of the Hanford tanks. Second, it is clear that any future cross-well measurements
in a tank should be made at high frequencies to avoid the strong fundamental modes. Again, this
would be the normal action taken by any data acquisition specialist.
Currently, we are treating the case of two liquid layers of waste, both theoretically and in the lab. In
addition, we are including the standing waves in the sides of the steel tank. In the third year we will
conduct a data acquisition program inside a Hanford tank to collect normal mode data. This will be
performed in much the same manner as our successful work performed in 1994.
EMSP Project Summaries
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Toksoez, M.N. Imaging and characterizing the waste materials inside an underground storage tank using seismic normal modes. 1998 annual progress report, report, June 1, 1998; Cambridge, Massachusetts. (digital.library.unt.edu/ark:/67531/metadc619819/m1/2/: accessed January 23, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.