The Santa Clara Valley is a sedimentary basin whose stratigraphy forms a series of interbedded aquifers and aquitards (Clark, 1924). Land uplift and subsidence in the valley due to the recharge and withdrawal of fluids is well documented by several public agencies such as the Santa Clara Valley Water District (SCVWD) and the USGS (see Poland and Ireland, 1988). An increase in the withdrawal of water from the aquifer and a decrease in rainfall for the first half of the twentieth century resulted in a substantial drop in well levels and a corresponding land subsidence of 4 m. Recovery efforts over the past quarter century, such as the import of water from outside sources and the construction of percolation ponds, have allowed water levels to partially recover. Preservation of the aquifer requires the continual monitoring through extensometers, well water levels, and level line observations.
InSAR is an attractive method for monitoring land subsidence because of its spatial coverage and precision (e.g., Galloway et al., 1998; Hoffmann et al., 2001). However, individual interferograms are often difficult to interpret because the amount of deformation that is observed is highly dependent on the time of the season that the first and second SAR scenes are acquired. A time-series analysis has the potential to overcome this limitation by resolving the temporal variability of the surface deformation. We perform an InSAR time-series analysis using 115 interferograms spanning the time period from April of 1992 to June of 2000. The time-series analysis inverts for the incremental range-change between SAR scene acquisitions, producing a time-dependent signal of land uplift and subsidence.
The InSAR time series reveals an overall pattern of uplift since 1992 (Figure 22.1). The uplift is attributed to an increase in the ground water levels. The increase in pore pressure reduces the vertical effective stress on the skeletal matrix resulting in regional uplift. We observe as much as 4118 mm of uplift centered north of Sunnyvale. Most of this uplift took place between 1992 and 1998 with a mean uplift rate of 6.42.2 mm/yr. Uplift is also observed in the Evergreen Basin located in the eastern portion of the Santa Clara valley. Significant subsidence relative to Oak Hill is also observed along the western margin of the valley. The southwestern portion of the valley shows insignificant uplift with no distinctive pattern. Continual uplift extends along several major tributaries, especially along Garabazas and Saratoga creeks.
The time series also resolves a seasonal uplift pattern with the largest magnitude centered near San Jose (Figure 22.1b). The seasonal signal is sharply bounded on the east by a structure that aligns with the northward extension of the Silver Creek fault. Both ascending and descending interferograms show consistent deformation across this fault suggesting that the relative motion is vertical and not related to strike-slip fault motion. The fault appears as a sharp discontinuity in the deformation field of several interferograms that can be explained by differential subsidence across the structure. A seasonal subsidence pattern is also observed in the northwest portion of the region, however this signal is more complex and difficult to characterize.
The InSAR time-series results suggest that the aquifer can be further subdivided into three domains based on the deformation pattern: two regions experiencing net uplift (1992-2000) centered in Sunnyvale and the Evergreen basin, and a central region dominated by seasonal deformation. The seasonal uplift is bounded on the west where the thickness of unconsolidated sediments decreases along the northwesterly-striking basement high extending from Edenvale Ridge and Oak Hill.
The magnitude of elastic rebound is expected to correlate with the thickness of unconsolidated alluvium, or the depth to basement assuming that a change in pore pressure is translated to the full thickness. Unfortunately, the depth to basement is unconstrained for much of the valley; few of the wells reach consolidated bedrock. At several localities, such as in San Jose and north of Sunnyvale, the depth to basement is believed to be in excess of 420 m (Meade, 1967; California Department of Water Resources (CDWR), 1967). Well data suggests that the depth to basement is highly variable throughout the San Francisco Bay region where faulting and erosion during periods of low sea level have created an irregular bedrock surface.
The thickness of the alluvium is not the only property of the aquifer that may control the elastic response. The heterogeneity of sediment type and the connectivity of permeable beds may also play a significant role. The effective time constants of flow depend on the thickness of interbeds and the vertical hydraulic diffusivity of aquitards. The petrology and grain size of sediments from well cores were cataloged in an effort to characterize the constituents of the aquifer system. Meade (1967) and Johnson et al. (1968) analyzed core samples from the Sunnyvale and San Jose extensometer sites. Fine sands and clays dominate the sediment found at the Sunnyvale site with grain sizes ranging from 0.001 to 0.2 mm. In San Jose, grain sizes were generally larger ranging from 0.004 to 1.5 mm with often abrupt transitions between fine and coarse deposits. Developing geologic cross sections from well logs can be difficult because stratigraphic units may not correlate, especially if the units have an appreciable dip. Johnson (1995) used statistical correlations between well-perforation indicators to determine the lateral extent of water-bearing units. Coarse-grained units were found to be correlated over a wider area near San Jose than in Sunnyvale with an average bed thickness of 2.5 versus 1.3 m, respectively.
Despite the difficulty in correlating units from one well to the next, well log data and hydraulic head levels at different depths identified a sequence of productive water-bearing units identified as the Agnew aquifer (CDWR, 1967; Johnson, 1995). The deposition of these units is associated with the ancestral drainage of the Coyote creek and the Guadalupe river, both of which transport sediment down the valley axis on the way to the bay (Meade, 1967; CDWR, 1967). Seasonal uplift and subsidence shown in Figure 22.1b outlines a tabular region that broadens towards the north and may reflect the seasonal recharge of the Agnew aquifer. The InSAR data is not sufficient to resolve whether recharge has occurred in the shallow or deep sequence of the Agnew aquifer. The southern end of the seasonal uplift signal is co-located with the southernmost limit of the confined zone. Recharge of the basin is accommodated through the flow of subsurface water from the Coyote valley located to the south of the Santa Clara valley. Subsurface water flows through a narrow alluvial channel between Oak Hill and the Edenvale Ridge before cascading into the deeper aquifer (CDWR, 1967).
The long term uplift pattern also suggests that the permeable beds extend along stream channels to the west; however, these regions do not exhibit much seasonal deformation. The subsidence along the west side of the valley observed in Figure 4a outlines the Santa Clara formation which is traditionally characterized as the recharge zone. The coarser sediment found in the San Jose well log implies higher hydraulic conductivities. The large seasonal uplift pattern in San Jose may be explained by a system where the seasonal influx of groundwater is redistributed over short periods of time. In contrast, the aquifer north of Sunnyvale and in the Evergreen basin may be characterized by lower hydraulic conductivities because of a greater percentage of fine sediment and clay layers resulting in the slow influx of groundwater into these parts of the aquifer.
The uplift pattern observed using the InSAR time series highlights the spatial complexity of the aquifer system in the Santa Clara valley. The temporal and spatial pattern of uplift and subsidence afforded by InSAR provides important constraints on the lateral distribution of water bearing units and the time scales over which the groundwater is exchanged. Ultimately, knowing the lateral extent and connectivity of water bearing units will improve numerical studies which attempt to model the mechanics of the aquifer system.
SAR data was provided to the WInSAR Consortium by the European Space Agency (ESA) through their North American distributor, Eurimage. Original SAR data ©ESA (1992-2000). Additional data provided by an ESA (ENVISAT) data grant.
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