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Measuring Water Content Heterogeneity Using Multifold GPR with Reflection Tomography

Center for Geophysical Investigation of the Shallow Subsurface, Boise State Univ., 1910 University Dr., Boise, ID 83725. Presented at the 75th International meeting of the Society of Exploration Geophysicists, 2004, Houston, TX

View larger version (63K): [in this window] [in a new window]   FIG. 1. Illustration of conventional common-offset radar acquisition (top), where a single source receiver pair is recorded for each sampling location, and multioffset acquisition (bottom), where multiple-source receiver pairs, each with a different offset, are recorded at each sampling location.

View larger version (44K): [in this window] [in a new window]   FIG. 2. Prestack depth migration common image point gathers showing residual moveout (RMO) sensitivity to small velocity (Vel.) errors, for a simple one-dimensional model simulating the water table reflection with a 0.12 m/ns vadose zone and 0.07 m/ns water-saturated zone. The RMO is negative when the velocity is too low, and positive when the velocity is too high.

View larger version (59K): [in this window] [in a new window]   FIG. 3. A synthetic example of imaging lateral aquifer heterogeneity: (A) radar velocity model, where the velocity decreases gradually from the surface to the water table, then drops sharply, simulating full water saturation (a clay layer is present at 16 m; 3- by 3-m-high velocity anomalies are placed in the vadose and saturated zones); (B) simulated common midpoint normal moveout (NMO) stack comparable to a conventional fixed offset ground-penetrating radar survey; (C) unconstrained reflection tomogram and (D) results of prestack depth migration (PSDM) using (C); (E) tomogram with linear gradient and boundary discontinuity constraints and (F) results of PSDM using (E). Unconstrained inversion has good lateral resolution but poor vertical resolution of the anomalies. Allowing velocity discontinuities across horizons and applying a linear gradient constraint produces excellent resolution of the anomalies.

View larger version (48K): [in this window] [in a new window]   FIG. 4. Map showing the position of Lines 1 and 2 relative to the approximate free light nonaqueous-phase liquid plume extent (shown in gold) based on monitoring well data. The monitoring well locations are indicated by black dots. The yellow contour indicates the distribution of the maximum plume thickness under low water table conditions and shows heterogeneity in contaminant transport. A "no data" zone was present near the center of Line 1, preventing continuous imaging from the on-plume to off-plume area. The western section of Line 1 discussed in the text is highlighted in red.

View larger version (31K): [in this window] [in a new window]   FIG. 5. (A) Prestack depth migration (PSDM) image of the western end of Line 1 (Fig. 4). The vertical scale is relative to the Line 2 datum for comparison. (B) Results of velocity tomography. Some minor lateral heterogeneity is evident. The largest vertical contrast occurs at the water table at a depth of about 15 m. (C) Moisture content estimate using the Topp equation. The minimum water content below the water table is about 29% (v/v). Note that the depth scale is from the common site datum to facilitate direct comparison with Line 2.

View larger version (91K): [in this window] [in a new window]   FIG. 6. (A) Common-source gathers along Line 2 (Fig. 4). Steeply dipping reflections are evident from backdipping moveout (e.g., transmitter [TX] = 45 m). A significant decrease in surface velocity past 145 m is evident where the dips of the direct ground arrival steepen, as indicated by the dashed lines (e.g., TX = 198 m). (B) The normal moveout common midpoint (CMP) stack reveals dipping strata in the near surface and a clear reflection from the water table. At distances >145 m, the signal is severely degraded due to increased attenuation. Note the approximately 40-ns push down of the water table reflection at distances >145 m. (C) Prestack depth migration (PSDM) with the laterally variable velocity model places the water table at the correct position in depth across the entire profile.

View larger version (50K): [in this window] [in a new window]   FIG. 7. (A) Reflection tomogram from Line 2 (Fig. 4). Note the abrupt lateral decrease in vadose zone velocity at 145 m, and the anomalously high saturated-zone velocities on the left side of the profile. Horizons used for residual moveout analysis are shown in black. (B) Moisture content estimated from the velocity model in (A). Moisture content increases laterally from 10% (v/v) in Zone 1 to nearly 20% in Zone 2. (C) The moisture content anomaly is computed by subtracting the moisture estimate in (B) from the starting model. The moisture content anomaly just below the water table is consistent with estimated water displacement due to measured amounts of free light nonaqueous-phase liquid.

View larger version (41K): [in this window] [in a new window]   FIG. 8. Location of the survey within the 200 West Area at the Hanford Site and the survey geometry.

View larger version (116K): [in this window] [in a new window]   FIG. 9. Comparison of common midpoint (CMP) gathers from the three-dimensional Hanford Site survey (A) before and (B) after eigenvector filtering to suppress the air velocity scattering noise. The off-end events are horizontal in the CMP domain and are effectively separated from the subsurface reflections. Surface scatter events are indicated with arrows. CDP Y is the inline position as shown in Fig. 8.

View larger version (76K): [in this window] [in a new window]   FIG. 10. (A) Common midpoint stack taken from the center of the three-dimensional volume. Surface scatter (indicated with arrows) obscures subsurface reflections on either end of the profile. (B) The same stack after eigenvector filtering to suppress air velocity noise. (C) Prestack depth migration (PSDM) image after noise suppression.

View larger version (39K): [in this window] [in a new window]   FIG. 11. (A) In-line velocity tomogram from the center of the volume, and (B) moisture content estimate based on (A). There was significantly greater moisture variation than we expected. The velocity low on the left of the profile correlates with a shallow depression of the 4- and 6-m horizons.

View larger version (25K): [in this window] [in a new window]   FIG. 12. (A) Three-dimensional prestack time migrated image at the Hanford Site, showing a detailed stratigraphy of the upper 6 m. (B) Depth slices through the velocity model at 2 and 5.5 m show significant three-dimensional variability. (C) Water content increases toward low in-line positions and high cross-line positions of the volume. A water content high correlates with the stratigraphic low between 4- and 6-m depth.