Creation of a 3D proxy model for hydroge- ological properties

cubes and examining selected plains in arbitrary direction.

5. Creation of a 3D proxy model for hydroge- ological properties

Difference visualization is extremely useful, but limited in that it does not give us informa- tion on the hydrogeological properties of the medium. However, there is obviously informa- tion on hydrogeological properties contained in the difference data. One way to extract this information is by using the change in subsurface reflectivities between different difference cubes. Doing so creates a proxy model for hydrogeo- logical properties, which is an amalgam of a number of different properties, but definitely not a model for one single property. To illus- trate this, the 500 MHz Borden data set is used. The 200 MHz data show essentially the same image, but provide less resolution compared to the 500-MHz data. The difference cubes for the Borden data set allows us to visualize the change in subsurface reflectivity. This change is linked to the migra- tion of the DNAPL. In the Borden experiment, a constant hydraulic head was maintained so that the only driving force for the DNAPL migration is gravity, which makes the DNAPL sink. The parameters influencing the path of the DNAPL are a combination of connectivity and hydraulic conductivity or permeability. Thus, a model of where the DNAPL has gone can be extracted from the GPR data and provide an estimate for these combined parameters. First, the 3D difference between the pre-spill Ž data and the data at repeated acquisition time 5, . 14, . . . 340 h are taken. After that subtraction, two 4D cubes of data for each repeated acquisi- tion time, one for the eight North–South pro- files and one for the eight East–West profiles Ž . Fig. 4a are available. These cubes represented the optimum difference and so the optimum representation of where the DNAPL moved. These two cubes were merged into one cube. Note that this is an inherently unsatisfying effort Ž . as the line spacing 1 m is very large compared Ž . to the trace spacing 5 cm . For certain times, Ž . this 4D cube will have high amplitudes at places where the DNAPL is. However, as the DNAPL migrates, these places shift as a func- tion of time. Our 3D proxy model volume was created by assigning, to each point in the 3D cube, the maximum amplitude value that was associated Ž . with that point in any of the 4D cubes Fig. 6 . This operation creates a map, which indicates ‘‘where and how much of the DNAPL passed through’’. The net result of this effort is a 3D cube Ž which can be either visualized with AVS Fig. . 7 or simply sliced to see the change in hydro- Ž . geological properties Fig. 8 . Note that both figures provide similar information. The red color represents areas with hydraulic properties that provide preferential pathways for the DNAPL. The blue color represents areas of hydraulic properties that are no pathways for the DNAPL during the experiment. Fig. 8a indi- cates the injection point of the DNAPL, which occurs at 70 cm depth. At subsequent depths, the spreading out and the distribution in the channels of the DNAPL can be observed. Fig. 7 Ž . which illustrates the same data shows both the Ž . spreading in the channel Fig. 7a and the fur- ther descent of the DNAPL to the bottom of the Ž cell through a system of vertical channel Fig. . 7b . This information conforms with observa- Ž tions from earlier analysis Sander, 1994; Brew- . ster and Annan, 1994; Brewster et al., 1995 — however, it was arrived at without any manual interpretation of any of the underlying data, and it illustrates the power of our processing and visualization effort in making sense of the data.

6. Conclusions and further work