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101.
Recovering magnetic susceptibility from electromagnetic data over a one-dimensional earth 总被引:1,自引:0,他引:1
While the inversion of electromagnetic data to recover electrical conductivity has received much attention, the inversion of those data to recover magnetic susceptibility has not been fully studied. In this paper we invert frequency-domain electromagnetic (EM) data from a horizontal coplanar system to recover a 1-D distribution of magnetic susceptibility under the assumption that the electrical conductivity is known. The inversion is carried out by dividing the earth into layers of constant susceptibility and minimizing an objective function of the susceptibility subject to fitting the data. An adjoint Green's function solution is used in the calculation of sensitivities, and it is apparent that the sensitivity problem is driven by three sources. One of the sources is the scaled electric field in the layer of interest, and the other two, related to effective magnetic charges, are located at the upper and lower boundaries of the layer. These charges give rise to a frequency-independent term in the sensitivities. Because different frequencies penetrate to different depths in the earth, the EM data contain inherent information about the depth distribution of susceptibility. This contrasts with static field measurements, which can be reproduced by a surface layer of magnetization. We illustrate the effectiveness of the inversion algorithm on synthetic and field data and show also the importance of knowing the background conductivity. In practical circumstances, where there is no a priori information about conductivity distribution, a simultaneous inversion of EM data to recover both electrical conductivity and susceptibility will be required. 相似文献
102.
Martin Connors Alan R. Hildebrand Mark Pilkington Carlos Ortiz-Aleman Rene E. Chavez Jaime Urrutia-Fucugauchi Eduardo Graniel-Castro Alfredo Camara-Zi Juan Vasquez John F. Halpenny 《Geophysical Journal International》1996,127(3):F11-F14
The buried Chicxulub impact structure is marked by a dramatic ring of sinkholes (called cenotes if containing water), and adjacent less prominent partial rings, which have been shown to coincide with maxima in horizontal gravity gradients and a topographic depression. These observations, along with the discreteness and spacing of the features, suggest a formation mechanism involving faulting in the outer slump zone of the crater, which would thus have a diameter of approximately 180 km.
An opposing view, based primarily on the interpretation of gravity data, is that (he crater is much larger than the cenote ring implies. Given the association of the known cenote ring with faults, we here examine northern Yucatan for similar rings in gravity, surface features and elevation, which we might expect to be associated with outer concentric faults in the case of a larger, possibly multiring, structure.
No such outer rings have been found, although definite patterns are seen in the distribution of karst features outside the crater rim. We explain these patterns as resulting mainly from deformation related to the block fault zone that parallels tbe shelf edge of eastern Yucatan. 相似文献
An opposing view, based primarily on the interpretation of gravity data, is that (he crater is much larger than the cenote ring implies. Given the association of the known cenote ring with faults, we here examine northern Yucatan for similar rings in gravity, surface features and elevation, which we might expect to be associated with outer concentric faults in the case of a larger, possibly multiring, structure.
No such outer rings have been found, although definite patterns are seen in the distribution of karst features outside the crater rim. We explain these patterns as resulting mainly from deformation related to the block fault zone that parallels tbe shelf edge of eastern Yucatan. 相似文献
103.
Asymmetric aggregates of fine-grained leucoxene and quartz are reported from siliceous L-S tectonites deformed by progressive simple shear. The leucoxene fish, morphologically similar to mica fish, consistently yield shear senses in agreement with other kinematic criteria. We interpret the leucoxene as a pre- or early syntectonic alteration product of detrital heavy-minerals in a siliciclastic protolith. The leucoxene fish behave as passive markers, and they record kinematics by modification of pre-existing aggregates rather than syndeformational mineral growth. For siliceous metasedimentary rocks otherwise lacking in micro-kinematic indicators, leucoxene fish may provide an alternative to quartz c-axis analysis. 相似文献
104.
An evaluation of models of apatite compositional variability using apatite from the Middle Banded series of the Stillwater Complex,Montana 总被引:2,自引:0,他引:2
Apatite is a relatively common accessory mineral in the olivine-bearing zones (OB-III and OB-IV) of the Middle Banded series
of the Stillwater complex, occurring interstitial to cumulus grains, as monomineralic inclusions in cumulus grains, and in
polymineralic clusters which may or may not be included in the cumulus grains. Eighty-nine of 185 samples examined were found
to contain some apatite. The F-Cl-OH content of the apatite show distinct stratigraphic variations. XClAp (mole fraction Cl
in apatite) of interstitial apatite increases upsection in both OB-III and OB-IV with average values ranging from 0.15 to
0.85 in OB-III and from 0.03 to 0.60 in OB-IV. XFAp varies inversely with XClApwith average values ranging from 0.00 to 0.70, while XOHAp remains relatively constant near 0.40 or decreases slightly with
height. These variations are remarkable given that no appreciable stratigraphic variations in either the major or trace element
compositions of any of the cumulus minerals are found in the 800 m of section that comprise OB-III and OB-IV. Within-sample
variation of XClAp for samples containing Cl-rich apatite is substantially larger (up to 0.65 XClAp) than for samples with
more F-rich apatite (XClAp varies by approximately 0.15). Although interstitial apatite is found throughout OB-III and OB-IV,
apatite occurring as monomineralic inclusion in cumulus grains or in polymineralic clusters is almost exclusively found in
samples with Cl-rich apatite. The data are best explained by a model involving the degassing of a Cl-rich volatile phase from
the crystallizing interstitial liquid. The up-section migration of this fluid resulted in the crystallization of F-rich apatite
in the lower portion and progressive Cl-enrichment in the apatite with height. The presence of hornblende-bearing dikes, veins
and pegmatoids at the level of maximum Cl-enrichment is consistent with a fluid migration model.
Received: 5 October 1995 / Accepted: 19 March 1996 相似文献
105.
Ryan D. Wilson Douglas M. Mackay John A. Cherry 《Ground Water Monitoring & Remediation》1997,17(3):185-193
Arrays of unpumped wells can be used as discontinuous permeable walls in which each well serves both as a means to focus ground water flow into the well for treatment and as a container either for permeable reactive media which directly destroy dissolved ground water contaminants or for devices or materials which release amendments that support in situ degradation of contaminants within the aquifer downgradient of the wells. This paper addresses the use of wells for amendment delivery, recognizing the potential utility of amendments such as electron acceptors (e.g., oxygen nitrate), electron donors (primary substrates), and microbial nutrients for stimulating bioremediation, and the potential utility of oxidizers, reducers, etc., for controlled abiotic degradation. Depending on its rate and constraints, the remedial reaction may occur within the well and/or downgradient. For complete remediation of ground water passing through the well array, the total flux of amendment released must meet or exceed the total flux demand imposed by the plume. When there are constraints on the released concentration of amendment (relative to the demand), close spacing of the wells may be required. If the flux balance allows wider spacing, it is likely that limited downgradient spreading of the released amendment will then be the primary constraint on interwell spacing. Divergent flow from the wells, roughly two times the well diameter, provides the bulk of downgradient spreading and constrains maximum well spacing in the absence of significant lateral dispersion. Stronger lateral dispersion enhances the spreading of amendment, thereby increasing the lateral impact of each well, which allows for wider well spacing. 相似文献
106.
AVHRR satellite imagery of the southern Mid-Atlantic Bight during May 1993 revealed a large area of cold water over the shelf break and slope that appeared to spin up into a series of southward propagating anticyclonic eddies. The eddies had diameters of 35–45 km at the surface and moved southward at about 20 cm/sec. A radial TOYO CTD (to 50m) and ADCP velocity (to 400m) transect was conducted across the southern-most of these eddies. The upper 50 meters had minimum temperatures of less than 7°C and salinities of about 33 pss, characteristics similar to cold pool waters usually found over the continental shelf. ADCP velocity data from one of the eddies revealed anticyclonic flow extending to a depth of about 250m. The transport of cold pool water by the eddies was estimated to be 0.1 to 0.2 Sv which is of the same order as the annual mean alongshore transport of shelf water in this region. The origin of the deeper water within the eddy is unlikely to be the continental shelf because the shelf break is less than 100 m. The depth and velocity profiles along the TOYO transect were consistent with the constant potential vorticity eddy model of Flierl (1979) although the source of the eddy kinetic energy is uncertain. The cause for the exodus of cold pool water from the shelf, which extended northward to at least 38°N, is unclear but must involve the establishment of an alongshore baroclinic pressure gradient against the usual southwestward shelf flow. It is possible that the intrusion of Gulf Stream waters onto the shelf near Cape Hatteras was a precursor of this off shelf transport. The southern-most eddy was marked by high biological productivity and very high oxygen supersaturation. The phytoplankton bloom detected within the exported cold pool water, located over the continental slope, suggests a mechanism whereby production fueled by nutrients derived from the shelf can be locally exported into deep water. 相似文献
107.
Zhengtong Xie Peter R. Voke Paul Hayden Alan G. Robins 《Boundary-Layer Meteorology》2004,111(3):417-440
A family of wall models is proposed that exhibits moresatisfactory performance than previousmodels for the large-eddy simulation (LES) of the turbulentboundary layer over a rough surface.The time and horizontally averaged statistics such asmean vertical profiles of windvelocity, Reynolds stress, turbulent intensities, turbulentkinetic energy and alsospectra are compared with wind-tunnel experimental data.The purpose of the present study is to obtain simulatedturbulent flows that are comparable with wind-tunnelmeasurements for use as the wind environment for thenumerical prediction by LES of source dispersion in theneutral atmospheric boundary layer. 相似文献
108.
Mitigating the Effects of Climate Change on the Water Resources of the Columbia River Basin 总被引:5,自引:3,他引:5
Jeffrey T. Payne Andrew W. Wood Alan F. Hamlet Richard N. Palmer Dennis P. Lettenmaier 《Climatic change》2004,62(1-3):233-256
The potential effects of climate change on the hydrology and water resources of the Columbia River Basin (CRB) were evaluated using simulations from the U.S. Department of Energy and National Center for Atmospheric Research Parallel Climate Model (DOE/NCAR PCM). This study focuses on three climate projections for the 21st century based on a `business as usual' (BAU) global emissions scenario, evaluated with respect to a control climate scenario based on static 1995 emissions. Time-varying monthly PCM temperature and precipitation changes were statistically downscaled and temporally disaggregated to produce daily forcings that drove a macro-scale hydrologic simulation model of the Columbia River basin at 1/4-degree spatial resolution. For comparison with the direct statistical downscaling approach, a dynamical downscaling approach using a regional climate model (RCM) was also used to derive hydrologic model forcings for 20-year subsets from the PCM control climate (1995–2015) scenario and from the three BAU climate(2040–2060) projections. The statistically downscaled PCM scenario results were assessed for three analysis periods (denoted Periods 1–3: 2010–2039,2040–2069, 2070–2098) in which changes in annual average temperature were +0.5,+1.3 and +2.1 °C, respectively, while critical winter season precipitation changes were –3, +5 and +1 percent. For RCM, the predicted temperature change for the 2040–2060 period was +1.2 °C and the average winter precipitation change was –3 percent, relative to the RCM controlclimate. Due to the modest changes in winter precipitation, temperature changes dominated the simulated hydrologic effects by reducing winter snow accumulation, thus shifting summer streamflow to the winter. The hydrologic changes caused increased competition for reservoir storage between firm hydropower and instream flow targets developed pursuant to the Endangered Species Act listing of Columbia River salmonids. We examined several alternative reservoir operating policies designed to mitigate reservoir system performance losses. In general, the combination of earlier reservoir refill with greater storage allocations for instream flow targets mitigated some of the negative impacts to flow, but only with significant losses in firm hydropower production (ranging from –9 percent in Period1 to –35 percent for RCM). Simulated hydropower revenue changes were lessthan 5 percent for all scenarios, however, primarily due to small changes inannual runoff. 相似文献
109.
David W. Ostendorf Paula L. S. Rees Shawn P. Kelley Alan J. Lutenegger 《Journal of Hydrology》2004,290(3-4):259-274
We consider the response of a deep unconfined horizontal aquifer to steady, annual, and monthly recharge. A groundwater divide and a zero head reservoir constrain the aquifer, so that sinusoidal monthly and aperiodic annual recharge fluctuations create transient specific discharge near the reservoir and an unsteady water table elevation inland. One existing and two new long-term data sets from the Plymouth-Carver Aquifer in southeastern Massachusetts calibrate and confirm hydraulic properties in a set of analytical models. [Geohydrology and simulated groundwater flow, 1992] data and a new power law for tritiugenic helium to tritium ratios calibrate the steady recharge that drives the classical parabolic model of steady hydraulics [Applied Hydrogeology, 2001]. Observed water table and gradient fluctuations calibrate the transient recharge models. In the latter regard, monitoring wells within 1 km of Buttermilk Bay exhibit appreciable specific discharge and reduced water table fluctuations. We apply [Trans Am Geophys Union 32(1951)238] periodic model to the monthly hydraulics and a recharge convolution integral [J Hydrol 126(1991)315] to annual flow. An infiltration fraction of 0.79 and a consumptive use coefficient of 1.08×10−8 m/s °C relate recharge to precipitation and daylight weighted temperature across all three time scales. Errors associated with this recharge relation decrease with increasing time scale. 相似文献
110.
Recent results and current plans for redshift surveys were the subject of an RAS discussion meeting on 10 October 2003, organized by Ofer Lahav and Jon Loveday. Sarah Bridle and Alan Heavens report. 相似文献