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Collaborating Authors
Results
Summary High-resolution shallow seismic reflection (SSR) experiments were conducted during and after a pumping test of an agricultural irrigation well in an effort to image the cone of depression seismically. Despite water table fluctuations of ~0.5 m, we were unable to observe temporal elevation variations in the water table attributable to pumping. However, the seismic data sets showed variations in the frequency of the reflection from the top of the saturated zone (TSZ) and we propose that these changes are caused by changes in the thickness of the partially saturated zone above the water table resulting from applied pumping stresses. A model was developed to test our hypothesis simulating a wedge of increasing thickness that represents the zone of partial saturation. Synthetic seismic traces showed a decrease in reflection frequency with increasing thickness of the partially saturated zone. The results of the modeled data correlate with the differences observed in the field data and support our hypothesis that changes in the partially saturated zone introduce detectable frequency changes in SSR data. Introduction Several attempts have been made to image the cone of depression during a pumping test (Birkelo et al., 1987; Johnson, 2003; Sloan, 2005), but none have been singularly successful. Birkelo et al.’s (1987) experiment was unsuccessful in part due to a previously unknown clay layer that produced a perched water table. However, the authors observed a decrease in the dominant frequency of the reflection from a survey acquired before pumping in comparison to one collected while the water table was at maximum drawdown. Johnson (2003) also had problems imaging the cone of depression because of the subsurface geology and lack of drawdown. These two attempts suggested that an ideal site would have a water table within a thick sand unit that was free of fine-grained materials that would hinder the drawdown of the water table. The site chosen for the experiments presented here is an agricultural field located in north-central Kansas. The water table is located at ~5 m depth in an unconfined aquifer comprised of unconsolidated sands and gravels, coarsening downward below the water table. A hand-augured hole at the site revealed ~5 m of fine-to-coarse sand with a 0.3-m thick silty-sand layer at ~3.1 m depth. An observation well installed ~6 m from the pumping well along the seismic lines showed a water table elevation change of ~0.5 m at maximum drawdown. After data acquisition and processing, we determined that our attempt at imaging the physical manifestation of the cone of depression was unsuccessful; however, a noticeable change in the frequency of the TSZ reflection was observed when comparing data collected during pumping with those collected after the water table had recovered. In light of this, we propose that instead of observing temporal shifts in the TSZ reflection caused by the raising and lowering of the water table, we are seeing changes in the character of the waveform caused by changes in the thickness of the zone of partial saturation above the TSZ.
- Geology > Geological Subdiscipline > Environmental Geology > Hydrogeology (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.34)
An Example of Automated 3D Ultra-shallow Seismic Acquisition
Czarnecki, Gerard P. (The University of Kansas, Department of Geology) | Tsoflias, Georgios P. (The University of Kansas, Department of Geology) | Steeples, Don W. (The University of Kansas, Department of Geology) | Sloan, Steven D. (The University of Kansas, Department of Geology) | Eslick, Robert C. (The University of Kansas, Department of Geology)
Summary We developed instrumentation (3D Autojuggie) capable of automatically deploying a 2D array of geophones for efficient acquisition of 3D shallow seismic-reflection data. The 3D Autojuggie consists of a rigid steel frame for transporting and planting geophones and a hydraulic mechanism for decoupling the geophones from the frame. A seismic survey was collected to determine the effectiveness of the 3D Autojuggie in reducing acquisition time and to evaluate the quality of recorded data. The system proved to be successful in both efficient data acquisition and data quality. An array of 72 geophones was transported and planted in approximately three minutes without any direct human handling of the geophones. Data quality was similar to conventional hand-planted geophone data due to a new design feature that allows the geophones to decouple from the steel frame. The experimental 3D survey was successful at imaging the ultra-shallow top of the saturated zone (~3–4 m in depth) but less effective for deeper targets due to the small area of the prototype geophone array (2.2 m x 1.0 m) and the far offsets needed to image deeper reflections. The ultra-shallow image showed top of saturation elevation variability on the order of one meter, spatially correlating with the surface expression of an abandoned stream channel. In principle, the design of the 3D Autojuggie can be expanded to facilitate faster and more cost-effective deeper subsurface imaging. Introduction Previous research at The University of Kansas has shown that in an effort to reduce the time required for data collection, 2D shallow (~upper 100 m) seismic-reflection data can be acquired using geophones rigidly attached to various media. Vincent (2005) provides a detailed summary of the results of this research. Overall, previous research has established that geophones attached to rigid media can acquire data faster than traditional hand-planted geophones while maintaining a good signal-to-noise ratio (S/N) for 2D seismic reflection surveys. Over the past decade there has been a growing interest in conducting 3D near-surface seismic-reflection surveys in order to better define near-surface characteristics (Lanz et al., 1996; Siahkoohi and West, 1998). Few surveys are collected attempting to image the subsurface at depths as shallow as tens of meters due to the cost of manuallyplanting geophones in a 2D grid pattern at spacing of less than one meter. Bachrach and Mukerji (2001) developed a non-rigid 2D geophone array mount capable of efficient ultra-shallow seismic data acquisition. The mount was used to facilitate manual planting of geophones by establishing precise geophone locations without measuring. With this mount, 72 geophones can be manually planted in less than five minutes, greatly reducing the time required to hand-plant geophones (Bachrach and Mukerji, 2004). We have developed instrumentation that allows for a grid of geophones to be automatically planted without any human contact with the geophones throughout the entire data acquisition process. This new system automatically decouples the geophones from the steel planting device to eliminate any crossfeed through the rigid frame. Tsoflias et al. (2006) provide a detailed description of the 3D Autojuggie design and operation.
- Overview (0.35)
- Research Report (0.34)
- Energy > Oil & Gas > Upstream (1.00)
- Government > Regional Government > North America Government > United States Government (0.70)