Anomalous Values (anomalous + value)

Distribution by Scientific Domains

Selected Abstracts

Geophysical indicators of culturally emplaced soils and sediments

Rinita A. Dalan
Archaeologists dealing with earthen forms must distinguish those constructed by humans from those with a natural origin. Geophysical techniques can help identify culturally loaded soils and sediments. We suggest that intrinsic changes in geophysical properties, due to cultural loading, can serve as fingerprints in determining whether a mound or other earthen form is natural or culturally constructed. Culturally emplaced soils might be identified through anomalous values in geophysical properties or through unusual spatial or stratigraphic complexity. The identification of this "lumpiness" in geophysical properties may involve geophysical techniques quite different from those employed in traditional archaeogeophysical surveys. Experiments at three prehistoric mound sites (the Cahokia Mounds State Historic Site, Effigy Mounds National Monument, and the Hopeton Earthwork) illustrate a number of these techniques including studies of the anisotropy (directionality) of geophysical properties, seismic Rayleigh (surface) waves, and magnetic susceptibility. © 2002 Wiley Periodicals, Inc. [source]

The effects of near-surface conditions on anisotropy parameter estimations from 4C seismic data

Bärbel Traub
ABSTRACT We present a study of anisotropic parameter estimation in the near-surface layers for P-wave and converted-wave (C-wave) data. Near-surface data is affected by apparent anisotropy due to a vertical velocity compaction gradient. We have carried out a modelling study, which showed that a velocity gradient introduces apparent anisotropy into an isotropic medium. Thus, parameter estimation will give anomalous values that affect the imaging of the target area. The parameter estimation technique is also influenced by phase reversals with diminishing amplitude, leading to erroneous parameters. In a modelling study using a near-surface model, we have observed phase reversals in near-surface PP reflections. The values of the P-wave anisotropy parameter , estimated from these events are about an order of magnitude larger than the model values. Next, we use C-wave data to estimate the effect of anisotropy (,) and compute , from these values. These calculated ,-values are closer to the model values, and NMO correction with both ,-values shows a better correction for the calculated value. Hence, we believe that calculating , from , gives a better representation of the anisotropy than picked , from the P-wave. Finally, we extract the anisotropy parameters , and , from real data from the Alba Field in the North Sea. Comparing the results with reference values from a model built according to well-log, VSP and surface data, we find that the parameters show differences of up to an order of magnitude. The ,-values calculated from the C-wave anisotropy parameter , fit the reference values much better and show values of the same order of magnitude. [source]

Process Considerations for Trolling Borehole Flow Logs

Phil L. Oberlander
Horizontal hydraulic conductivity with depth is often understood only as a depth-integrated property based on pumping tests or estimated from geophysical logs and the lithology. A more explicit method exists for determining hydraulic conductivity over small vertical intervals by collecting borehole flow measurements while the well is being pumped. Borehole flow rates were collected from 15 deep monitoring wells on the Nevada Test Site and the Nevada Test and Training Range while continuously raising and lowering a high-precision impeller borehole flowmeter. Repeated logging passes at different logging speeds and pumping rates typically provided nine unique flow logs for each well. Over 60 km of borehole flow logs were collected at a 6.1-cm vertical resolution. Processing these data necessitated developing a methodology to delete anomalous values, smooth small-scale flow variations, combine multiple borehole flow logs, characterize measurement uncertainty, and determine the interval-specific lower limit to flow rate quantification. There are decision points in the data processing where judgment and ancillary analyses are needed to extract subtle hydrogeologic information. The analysis methodology indicates that processed measurements from a high-precision trolling impeller flowmeter in a screened well can confidently detect changes in borehole flow rate of ,0.7% of the combined trolling and borehole flow rate. An advantage of trolling the flowmeter is that the impeller is nearly always spinning as it is raised and lowered in the well and borehole flow rates can be measured at lower values than if measurements were taken while the flowmeter was held at a fixed depth. [source]

Comparison of 850-hPa relative humidity between ERA-40 and NCEP/NCAR re-analyses: detection of suspicious data in ERA-40

Aurélien Ben Daoud
Abstract An exploratory study is performed on the 850-hPa relative humidity data (RH850) extracted from the Re-Analysis Project (ERA-40) and National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) re-analyses covering a domain centred on western Europe (60°W,60°E, 15°N,75°N). The largest deviations between the two data archives are observed over the North Atlantic Ocean. In addition, unrealistic values of RH850 are detected in the ERA-40 re-analysis at resolutions of both 2.5° and 1.125°. There is no strong correlation between RH850 provided by ERA-40 and observations from radio sounding stations, thus ruling out a straightforward correction of the detected anomalous values. Copyright © 2009 Royal Meteorological Society [source]