[1] Two high-precision gravity surveys were conducted to determine groundwater mass changes at a managed groundwater recharge site in northeastern Colorado. Gravity data were collected during pumping and two months after pumping ceased. During pumping, gravity was lower by as much as 46 mGal near the pumping wells and higher by as much as 90 mGal near the recharge ponds in comparison to data collected after pumping had ceased. These differences are attributed to changes in groundwater mass associated with drawdown and infiltration. Inverse modeling of the gravity data indicates a 5.1 Â 10 5 m 3 decrease in storage beneath the recharge ponds between the two surveys, which we attribute to dissipation of the groundwater mound created by recharge during pumping. This estimate of the change in groundwater storage is made independently of assumptions of physical properties of the aquifer. Dividing the change in water volume per unit area determined from the gravity modeling by the change in water level measured in wells provides an estimate of specific yield (S y ) of 0.21 ± 0.03, which is within the range of specific yield estimates derived from aquifer tests at the site. Water level changes predicted from the gravity data agree on average to within ±0.45 m of those measured, which we take to be an estimate of the uncertainty in water table depth predictions that can be obtained from gravimetric data in unconfined aquifers. The study covers a 3.2 km 2 area, providing a prototype for similar studies at larger scales.
This paper addresses the impact of acquiring a new 3D Broadband seismic survey over an amplitude-supported, discovered gas field containing legacy 3D conventional towed streamer seismic data. The new seismic data were acquired in shallow water depths using Western Geco's dual level streamer technique and processed through PreSDM. Five gas discovery/appraisal wells with reservoirs ranging from approximately 1.2 to 2.5 s TWT (3,500 ft TVDss to 9,500 ft TVDss) existed prior to broadband acquisition and two additional wells were drilled after acquisition was completed. These seven wells serve as control points that provide a valuable link between the seismic and reservoir properties.
Acquisition of new seismic data was primarily motivated by imaging challenges, particularly in the deeper (>1.6 s TWT) section, to which legacy surveys and (re)processing attempts have failed to completely find a solution. These imaging problems stem from: 1) Abundant shallow gas pockets, which contribute to both amplitude and frequency decay in the underlying image; 2) Fault shadow noise resulting from velocity variations which are difficult to capture in the velocity model; and 3) The presence of sub-regional coal layers having high-impedance contrasts that further attenuate the signal and contribute to generation of multiples. A complimentary paper discusses the novel processing techniques applied to overcome these problems. Here, we discuss the comparative benefit of the broadband acquisition versus the legacy conventional acquisition, namely higher signal to noise ratio throughout the entire record. In this context, we demonstrate through interpretation of broadband data that overall, complex geologic layers are better resolved versus conventionally acquired seismic data. We also compare conventional and broadband wavelets and discuss the implications for layer detection and resolution. This is particularly important for imaging and interpreting thin (<20 ft) reservoirs. We compare horizon-based attribute maps with legacy interpretation and address the implications for reservoir model building. We also apply various degrees of de-multiple algorithms and assess the resultant AVO effects. The broadband data has provided significant uplift in terms of reservoir detection and has thus bolstered confidence in our interpretation of thin, gas charged reservoirs.
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