Brine Discharge and Salinization, Concho River Watershed, West Texas     

Alan R. Dutton  Bernd C. Richter  Charles W. Kreitler 《Ground water》1989,27(3):375-383

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Climate change,economic growth and energy policy: A recommended U.S. strategy for the coming decades an editorial     

Alan D. Hecht  Bo R. Döös 《Climatic change》1988,13(1):1-3

Alan D. Hecht Director of the National Climate Program Office, an Office created by Congress in 1978 to coordinate a U.S. program of climate research and services. The office is located in the National Oceanic and Atmospheric Administration within the Department of Commerce; Bo R. Döös Member of the Swedish Academy of Sciences, is a visiting UCAR scientist (University Corporation for Atmospheric Research) in the NCPO. Views expressed in this editorial are of the authors and do not reflect any official views of UCAR or NOAA, DOC, or other federal agencies within the NCP.  相似文献   

A PULSED PLASMA JET ACOUSTIC SOURCE FOR PROFILING THE OCEAN FLOOR1     

P. L. PITT  R. D. SMITH  G. C. SHEFFLER  R. D. WARREN  R. M. CLEMENTS  T. S. HAMILTON 《Geophysical Prospecting》1988,36(5):523-536

A plasma gun source has been successfully used to obtain sub-bottom profiles. The profiles show better penetration than with a 3-5 kHz source and more resolution than with an air gun. The plasma gun source is compact, self-contained, and requires no complex auxiliary equipment. The device was deployed from large research vessels and a small boat. The plasma gun produces sufficient acoustic energy and with its characteristically short pulse length and broad bandwidth it is an attractive sub-bottom profiler, especially in shallow fresh or salt water.  相似文献   

Arsenic in Ground Water of the Western United States   总被引：12，自引：0，他引：12  

Alan H. Welch  Michael S. Lico  Jennifer L. Hughes 《Ground water》1988,26(3):333-347

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Layer-stripping reverse-time migration1     

Ruey-Chyuan Shih  Alan R. Levander 《Geophysical Prospecting》1994,42(3):211-227

We present a layer-stripping method of migration for irregularly layered media in which first-order velocity discontinuities separate regions of constant or smoothly varying velocity. We use the reverse-time method to migrate seismic data layer by layer, from the surface downwards. As part of the migration of a given layer, the bottom boundary of the layer is defined based on power in the migrated signal, and a seismic section is collected along it. This new section serves as the boundary condition for migration in the next layer. This procedure is repeated for each layer, with the final image formed from the individual layer images. Layer-stripping migration consists of three steps: (1) layer definition, (2) wavefield extrapolation and imaging, and (3) boundary determination. The migration scheme when used with reverse-time extrapolation is similar to datuming with an imaging condition. The reverse-time method uses an explicit fourth-order time, tenth-order space, finite-difference approximation to the scalar wave equation. The advantages of layer-stripping reverse-time migration are: (1) it preserves the benefits of the reverse-time method by handling strong velocity contrasts between layers and steeply dipping structures; (2) it reduces computer memory and saves computation time in high-velocity layers, and (3) it allows interpretational control of the image. Post-stack layer-stripping reverse-time migration is illustrated with a synthetic CMP data example. Prestack migration is illustrated with a synthetic data set and with a marine seismic reflection profile across the Santa Maria Basin and the Hosgri Fault in central California.  相似文献   

Strike-angle determination from the magnetotelluric impedance tensor in the presence of noise and local distortion: rotate at your peril!   总被引：2，自引：0，他引：2  

Alan G. Jones  Ross W. Groom 《Geophysical Journal International》1993,113(2):524-534

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Physical,chemical, and biological conditions associated with the early stages of the Lake Michigan vernal thermal front     

Russell A. Moll  Alan Bratkovich  William Y. B. Chang  Peimin Pu 《Estuaries and Coasts》1993,16(1):92-103

An investigation of the thermal front in southeastern Lake Michigan during April 1988 revealed a dynamic physical, chemical, and biological environment. The front was observed approximately 4 km from the coast as a distinct gradient separating cold open-lake waters from warmer nearshore waters. Surface isotherms near the front were generally parallel to one another but skewed with respect to shore. Comparison between April 22 and 29 showed that the surface isothern pattern was modulated by wind stress. The pattern from April 29 showed signs of flow instabilities with horizontal scales of 1 km to 5 km. Surface drifter trajectories provided estimates of horizontal convergence at the fron which varied from 7×10^?6 s^?1 to 20×10^?6 s^?1. Inferred rates of downwelling, which ranged from 9.5 m d^?1 to 20.7 m d^?1, were sufficient to move a water parcel from the surface to the bottom in 2 to 6 d at the front. Convergent circulation was observed on both sampling dates despite contrasts in wind stress. Concentrations of chloride, soluble silica, and chlorophyll, which were always higher inshore, were 5% to 82% larger than offshore mean values. The aquatic environment just inshore of the thermal front was characterized by chlorphyll concentrations which exceeded 5.0 μg 1^?1 while concentrations offshore were between 1.0 μg 1^?1 and 2.0 μg 1^?1. A relatively uniform vertical structure in chlorophyll concentrations in the frontal zone was consistent with the observed convergence and inferred downwelling near the front.  相似文献   

Determination of part-per-trillion levels of atmospheric dimethyl sulfide by isotope dilution gas chromatography/mass spectrometry     

Donald C. Thornton  Alan R. Bandy  Robert G. Ridgeway  Arthur R. Driedger III  Marija Lalevic 《Journal of Atmospheric Chemistry》1990,11(4):299-308

Stable isotopic dilution was applied to the determination of dimethyl sulfide (DMS) in ambient air at the low part-per-trillion by volume (pptrv) levels. Perdeuterated DMS was used as an internal standard in the gas chromatography/mass spectrometry determination. The isotopically labelled internal standard provided insensitivity to possible losses of DMS in sampling and analysis. The lower limit of detection (LLD) was 1 pptrv with a sample acquisition time of 2 min.  相似文献   

Eolian evidence for spatial variability of late Quaternary climates in tropical Africa     

Edward M. Pokras  Alan C. Mix 《Quaternary Research》1985,24(2):137-149

Study of the eolian fraction of late Quaternary sediments from the tropical Atlantic reveals that two modes of long-term climate variability have existed in tropical Africa during the last 150,000 yr. Tropical northwest Africa (i.e., the southwestern Sahara and Sahel) was driest during glaciations and stades, but wetter than at present during interglaciations and interstades. This may be a response to ice sheets at higher latitudes, via equatorward displacement of the westerlies and the subtropical high. In contrast, central equatorial Africa (southeast of the Sahara) was most arid during interstades and times of ice growth, and most humid during deglaciation. Wet periods in this area correspond to insolation maxima in northern hemisphere summer. A 23,000-yr precessional rhythm is suggested, supporting a direct link between African Monsoon intensity and orbitally modulated insolation. The late Holocene is the only time observed when both areas are arid during an interglacial episode. This may reflect, in part, anthropogenic disturbance of late Holocene climates.  相似文献   

Elemental Concentrations in Japanese Silicate Rock Standards JA-1, JR-1 and JB-2     

N.W. BOWER  E.S. GLADNEY  R.C. HAGAN  P.E. TRUJILLO  R.G. WARREN 《Geostandards and Geoanalytical Research》1985,9(2):199-203

Data on three Japanese geochemical reference samples (JR-1, JA-1, and JB-2) are presented. Ten major and thirty-five trace element concentrations were determined using x-ray fluorescence, instrumental thermal neutron activation, thermal neutron capture prompt gamma-ray spectrometry, delayed neutron assay, automated thermal neutron activation analysis, inductively coupled plasma emission, atomic absorption, and ion selective electrode. Good agreement between the various methods and with recent literature values were obtained using error weighted mean concentrations for the samples.  相似文献