bluuebunny/crossref_metadata_2025 huggingface dataset:
Low‐frequency electric fields induced in spherical cells with gap junctions
Authors: King Ronold W. P.
Published: February 2004

The electric field induced in the interior of a chain of spherical cells joined by gap junctions is determined when the chain is exposed to low‐frequency incident axial or transverse electric fields. When the incident field is axial, the induced electric field increases stepwise as the number of cells in the chain is increased. For cells with the assumed properties the axial field in the interior equals the incident field outside the chain when the number of cells equals or exceeds 200. For cells with different properties this number will change. When the incident field is transverse to the axis of the chain, the electric field induced in each of the spherical cells in the chain is the same as that in a single isolated spherical cell.
Rheological control on the initial geometry of the Raft River detachment fault and shear zone, western United States
Authors: Wells Michael L.
Published: February 2004

The strain, exhumation history, and field orientation of a well‐exposed shear zone and detachment fault in the Raft River Mountains of northwestern Utah, a Cordilleran metamorphic core complex, have been studied to determine the kinematics of ductile shearing and initial orientations of the shear zone and detachment fault. Mapping and strain and kinematic analysis indicate that the top‐to‐the‐east Raft River shear zone initially developed parallel to an unconformity separating Archean rocks from overlying Proterozoic quartzite and schist for at least 24 km in the shear direction. Experimental rock deformation data from lithologies similar to the Archean and Proterozoic rocks suggest the unconformity represented a significant rheological boundary at the deformation temperatures; the base of the shear zone was localized along the boundary between relatively weak quartzite above and stronger monzogranite below. An extensive thermochronological database is used to reconstruct the position of the basement unconformity in temperature‐lateral distance coordinates. The initial average dip of the shear zone and basement unconformity is estimated between 7° and 30°, assuming subhorizontal isotherms and geothermal gradients of 20°–40°C/km. The east dip of the unconformity at the onset of Miocene extension is interpreted to have resulted from late Eocene unroofing and flexure beneath a top‐to‐the‐WNW extensional shear zone in the western Raft River, Grouse Creek, and Albion Mountains. The observations from the Raft River shear zone suggest that the orientation of some midcrustal shear zones may not reflect the predicted orientation for ductile faults according to ductile failure criteria but, rather, the orientation of rheological boundaries along which deformation is localized. Furthermore, detachment faults that are superimposed on mylonite during progressive displacement and footwall unroofing may use an inherited mechanical anisotropy from the mylonite, and their orientations may not reflect the predicted orientation of shear fractures in isotropic rock. The common parallelism between detachment faults and mylonitic foliation may indicate a mechanical and kinematic preference for localization of throughgoing brittle faults parallel to preexisting mylonitic foliation. Because of this preference, studies restricted to detachment faults which lack footwall mylonite or restricted to structural levels between the breakaway and mylonitic front have more bearing on the question of the initial dip of normal‐sense shear fractures (faults) within the seismogenic crust.
Evidence for contemporaneous yet contrasting styles of granite magmatism during extensional collapse of the northeast Greenland Caledonides
Authors: Strachan R. A. , Martin M. W. , Friderichsen J. D.
Published: February 2004

Sectors of orogenic belts that have undergone extension present opportunities for the study of contemporaneous styles of magmatism at contrasting structural levels. The Caledonian (Ordovician‐Devonian) orogenic belt of northeast Greenland experienced widespread extensional collapse either synchronous with or immediately following continental collision and crustal thickening. Extension resulted in juxtaposition of structurally simple upper plate rocks of mainly low metamorphic grade with underlying lower plate rocks that record intense deformation and high‐grade metamorphism. Caledonian intracrustal granitic rocks occur as migmatite complexes, networks of sheets and veins, and as major gently dipping sheeted complexes in the lower plate and as steep‐sided, discordant plutons in the upper plate. Published geochemical and isotopic evidence supports the interpretation of these granitic rocks as a single intrusive suite formed by crustal anatexis that was initiated during crustal thickening and peak metamorphism and continued during decompression and extensional collapse. In this paper we test this hypothesis by the isotopic dating of two granite sheets from the lower plate and three plutons from the upper plate. These yield U‐Pb zircon or monazite ages in the range ∼431–428 Ma that are indistinguishable within error, thus demonstrating that granitic melt emplacement was contemporaneous at the contrasting structural levels. We conclude that at midcrustal levels, processes of magma segregation and transport were controlled by low‐angle, noncoaxial extensional shear. Batches of magma were segregated technically and likely migrated updip as a result of tectonic pumping and buoyancy. These amalgamated to form synkinematic sheets in the lower plate that we interpret as the channelways by which magma was transported to higher crustal levels across zones of detachment to construct the plutons in the upper plate. The major features of this model may be applicable to many orogenic belts that record core complex development during extensional collapse and intracrustal granite magmatism.
Fault kinematics and past plate motions at a convergent plate boundary: Tertiary Shimanto Belt, southwest Japan
Authors: Lewis Jonathan C. , Byrne Timothy B.
Published: February 2004

Early‐ and late‐stage, outcrop‐scale faults have been documented in Paleogene strata of the Muroto Peninsula of southwest Japan. The former were active early in the consolidation history of the sediment and are crosscut by the latter, which were active after regional‐scale imbrication, folding, and penetrative deformation (i.e., regional, spaced pressure solution cleavage). Orientation distributions of striae and kinematic axes indicate that the two generations of faults reflect distinct kinematic regimes. The orientations and crosscutting relations indicate a counterclockwise (viewed downward) rotation of the principal shortening direction between early‐ and late‐stage faulting. Existing fossil and radiometric age data suggest this transition occurred during Eocene‐Oligocene time. The fault kinematic data are interpreted to reflect a change in relative plate motions between Eurasia and an oceanic plate subducting beneath Eurasia in Paleogene time. These results, coupled with existing paleothermal and paleopressure data, are consistent with the presence of a spreading ridge in the western Pacific basin during this time.
Large‐magnitude extension along metamorphic core complexes of western Arizona and southeastern California: Evaluation with paleomagnetism
Authors: Livaccari Richard F. , Geissman John W.
Published: February 2004

Cordilleran metamorphic core complexes (MCCs) and their associated structures (detachment faults and mylonites) are the products of large‐magnitude Cenozoic extension. Paleomagnetic data from the Harquahala, Harcuvar, Buckskin, and Whipple MCCs of western Arizona and southeastern California (∼1900 samples; 191 of 278 sites accepted for inclusion in group and grand means) allow us to estimate the magnitude of footwall tilting associated with extension. All MCC crystalline rocks sampled are in the footwalls of associated brittle detachment faults. Rocks sampled include synkinematic, Miocene intrusions (dikes and plutons), Late Cretaceous granites, and Proterozoic host rocks. These rocks (with different levels of success at the site level) yield dual polarity, high unblocking temperature, and high to moderate coercivity magnetizations. Demagnetization behavior reveals that those sites with well‐grouped magnetizations and high laboratory unblocking temperatures and/or high to moderate coercivities have characteristic remanent magnetizations carried by either magnetite, with minor hematite, or by hematite, with minor magnetite. Results of baked contact tests show that these footwall rocks possess thermoremanent magnetizations or high‐temperature thermochemical remanent magnetizations acquired early in their cooling history, either before or during extension. Our overall interpretation of the paleomagnetic data is that during Cenozoic extension, MCC footwalls tilted (flattened) only a modest amount (<15°) in the direction of tectonic transport. We also interpret Harcuvar and Whipple MCC paleomagnetic data to indicate tectonic tilting (steepening) of the mylonitic zone to form a back dipping mylonitic front. The Whipple‐Bullard‐Eagle Eye detachment fault system and associated mylonites are interpreted to have originated with a low‐angle dip (10° to 25° NE). We relate the small amount of footwall flattening in the direction of tectonic transport and the steepening of the back dipping mylonitic front to the buoyant rise of exhumed footwalls according to rolling‐hinge/isostatic uplift models. Our interpretation refutes the widespread applicability of models that predict MCCs to represent strongly tilted crustal blocks, originally bounded by high/moderate‐angle normal faults. Additionally, a fold test of Harquahala MCC data yielded “negative” results. This suggests that antiformal footwall arches represent the oldest structures of MCCs that developed prior to, or during mylonitization of footwall rocks (before acquisition of the remanent magnetizations carried by these rocks).
Styles of footwall uplift along the Simplon and Brenner normal fault systems, central and Eastern Alps
Authors: Wawrzyniec Tim F. , Selverstone Jane , Axen Gary J.
Published: February 2004

The Simplon and Brenner extensional shear zones of the central and Eastern Alps, respectively, are low‐angle detachments that accommodated orogen‐parallel extension and unroofed midcrustal rocks in their footwalls. An integrated field and fluid inclusion study of the Simplon low‐angle shear zone reveals several postmylonitic structure types that are consistent with passage of a complex rolling hinge through the Simplon footwall, including synthetic (west down) and antithetic (east down) semibrittle and brittle structures, and strike‐slip, extensional, and contractional structures, all of which are consistent with southwest directed extension along the main detachment zone. Fluid inclusion analysis of kinematically referenced inclusion arrays indicates that west down, semibrittle, and brittle shears were active at conditions above 300°–450°C and 14–26 km. Antithetic deformation along brittle faults occurred at much shallower conditions of <300°C and 3–7 km. Deformation along these structures accommodated footwall uplift by a subvertical simple shear mechanism in the southern part of the Simplon region. These results are strikingly similar to those from the Brenner shear zone. In contrast with the Brenner region, however, the northern Simplon footwall shows strike‐slip dominated postmylonitic deformation related to slip along the Rhône Line. The footwalls of these detachment systems define a unique class of core complex characterized by the development of a midcrustal, synthetic lower hinge that acts to steepen the detachment system and accommodate footwall uplift by a subvertical simple shear mechanism. Both systems have poorly developed upper hinges that flatten the middle to upper crustal ramp by only a few degrees of dip, which is in contrast to detachment fault systems of the western United States, where evidence of a deep hinge is rarely observed, and the upper hinge acts to deactivate the flattened, updip part of the detachment. Our results emphasize the role of tectonic setting buoyancy forces, and synkinematic fluids in controlling the observed style and mechanisms of large‐magnitude extension during collision between continental masses.
Geodynamics of the northern Andes: Subductions and intracontinental deformation (Colombia)
Authors: Taboada Alfredo , Rivera Luis A. , Fuenzalida Andrés , Cisternas Armando , Philip Hervé , Bijwaard Harmen , Olaya José , Rivera Clara
Published: February 2004

New regional seismological data acquired in Colombia during 1993 to 1996 and tectonic field data from the Eastern Cordillera (EC) permit a reexamination of the complex geodynamics of northwestern South America. The effect of the accretion of the Baudó‐Panama oceanic arc, which began 12 Myr ago, is highlighted in connection with mountain building in the EC. The Istmina and Ibagué faults in the south and the Santa Marta‐Bucaramanga fault to the northeast limit an E‐SE moving continental wedge. Progressive indentation of the wedge is absorbed along reverse faults located in the foothills of the Cordilleras (northward of 5°N) and transpressive deformation in the Santander Massif. Crustal seismicity in Colombia is accurately correlated with active faults showing neotectonic morphological evidences. Intermediate seismicity allows to identify a N‐NE trending subduction segment beneath the EC, which plunges toward the E‐SE. This subduction is interpreted as a remnant of the paleo‐Caribbean plateau (PCP) as suggested by geological and tomographic profiles. The PCP shows a low‐angle subduction northward of 5.2°N and is limited southward by a major E‐W transpressive shear zone. Normal oceanic subduction of the Nazca plate (NP) ends abruptly at the southern limit of the Baudó Range. Northward, the NP subducts beneath the Chocó block, overlapping the southern part of the PCP. Cenozoic shortening in the EC estimated from a balanced section is ∼120 km. Stress analysis of fault slip data in the EC (northward of 4°N), indicates an ∼E‐SE orientation of σ1 in agreement with the PCP subduction direction. Northward, near Bucaramanga, two stress solutions were observed: (1) a late Andean N80°E compression and (2) an early Andean NW‐SE compression.
Geometry and rate of faulting in the North Baikal Rift, Siberia
Authors: San'kov Vladimir , Déverchère Jacques , Gaudemer Yves , Houdry Frédérique , Filippov Andreï
Published: February 2004

We present a detailed morphotectonic analysis of late Quaternary faulting in the North Baikal Rift (NBR), a region characterized by ranges and basins distributed over more than 800 km along strike in eastern Siberia. Remote sensing techniques (SPOT, METEOR scenes, and aerial photographs) are used to map the active fault network which displays a general en échelon distribution from the northern Lake Baikal to the easternmost basin, with ∼30‐km‐spaced overstepping segments of 10–80 km in length. Most faults have a dominant dip‐slip component over their Cenozoic history. The inherited crustal fabric strongly influences the overall geometry of the rifted basins. We use 54 14C ages of postglacial terraces near the foot scarps of the Muya basin to date offsets measured inside alluvial fans. The last main postglacial event in this area appears to be the early Holocene optimum dated at ∼10 ± 2 ka, following the onset of deglaciation at ∼13 ka. Using these time constraints, a detailed leveling across two terraces offset by the Taksimo fault (West Muya basin) shows consistent minimum vertical slip rates of 1.6±0.6 mm yr−1. Using 30 other active scarps analyzed in the field, we find a lower bound for horizontal velocity of 3.2±0.5 mm yr−1 across the NBR, a rate close to the one found in the southern rift from Global Positioning System measurements. We then compare directions of slip vectors from Holocene field data and slip directions from earthquake fault plane solutions: although local discrepancies appear, the mean directions of lesser horizontal stress (σ3) inverted from theses values are ∼N130°E and ∼N155°E, respectively, which are comparable within uncertainties and favor a rifting obliquity of ∼30°–40°. Extrapolating our Holocene rates, we estimate basin ages younger than those generally believed (less than 7 Ma) and propose a spatial and temporal evolution of rifted basins consistent with experimental models of oblique rifting. Total amounts of extension and vertical throw (∼7 and ∼12 km, respectively) across major faults appear rather constant from the central to the northern rift. These results favor a progressive development of asymmetric grabens in a rift zone that widens with times and they indicate a strong rheological control on deformation which seems enhanced by other contributions than the far‐field effects of the Indo‐Eurasian collision.
Structural Setting of Western Monferrato (Alps‐Apennines Junction Zone, NW Italy)
Authors: Piana Fabrizio
Published: February 2004

New data on the structural setting of the Torino Hill and Western Monferrato domains provide a clearer picture of the kinematic evolution of the Alps‐Apennines junction zone. In the Alps‐Apennines junction, left‐lateral underthrusting of Ligurian units below Alpine crust has often been invoked to explain the underthrusting of the Insubric domain and the NW‐ward movement of the Adriatic indenter. A transpressive fault system (Rio Freddo Fault Zone) developed in the Monferrato epi‐Ligurian succession is here understood as the surficial expression of a deep‐seated thrust along which the Alpine metamorphic basement of the Torino Hill has overridden the Apenninic Ligurian nappes since the Paleogene up to Burdigalian. In the Western Monferrato the regional sinistral transpression differentiated separated crustal blocks during the Oligocene‐early Miocene, and kilometer‐thick successions were steepened by NNW‐SSE faults that acted as reverse and/or left‐lateral faults. Double‐vergent (SE‐NW) low‐angle shear zones then cut the Rio Freddo Fault Zone in post‐Langhian times. They have been here related with the eastward tectonic transport of the Torino Hill and relative Alpine crust toward the Monferrato domain, which occurred before the seismic line scale middle Miocene unconformity was established. This poses the problem of the exact age of this regional unconformity, which in our case should be at least Serravallian. Finally, the Monferrato and Torino Hill domains were coupled, and both were NW‐ward translated by the Padan Thrust Front in the Pliocene. The youngest strike‐slip and reverse faults of the Western Monferrato may be tear or compartmental faults that partitioned the Padan Thrust motion, whose propagation could be controlled by pre‐middle‐Miocene tectonic structures. The structural complexity of the study area is here understood as the inheritance of deep crosscutting regional structures that forced the convergent structural units of the Alpine and Insubric domains to follow different tectonic transport directions since the Oligocene.
Crustal structures of the Rhinegraben and the Massif Central grabens: An experimental approach
Authors: Michon Laurent , Merle Olivier
Published: February 2004

Two of the most important segments of the west European Rift, the Rhinegraben and the Massif Central grabens, show in plan and in cross section a very different crustal structure. The Rhinegraben and the Massif Central grabens are roughly parallel and formed in the same time interval (i.e., Priabonian/Oligocene). The Rhinegraben in the north is a single half graben of ∼35 km wide that resulted from the activity of a major detachment fault running from the surface to a MOHO discontinuity at depth. The Massif Central in the south is composed of two lateral half grabens, similar to the Rhinegraben, and a near‐symmetric central graben. The opposing detachment faults of the two lateral half grabens reveal a striking mirror symmetry on either side of the symmetric central graben. Experiments have been conducted to explain mechanically this contrasted structural evolution in the same rift system. It is shown that this difference may be attributed to (1) the number of ruptures in the brittle part of the mantle lithosphere and (2) variations in the extension rate. A single rupture in the brittle mantle lithosphere results in either a single half graben or a pair of half and symmetric grabens in low and high extension rate experiments, respectively. Two ruptures in the brittle mantle lithosphere result in two half grabens in low rate experiments. The number of grabens is dependent upon the distance between the two ruptures in the brittle mantle lithosphere in high extension rate experiments and may vary from 2 to 4. Estimation of the extension rate in the Rhinegraben and the Massif Central matches experimental results and shows that the strength ratio between lower and brittle crust is the key parameter in determining the graben geometry.
Syncontractional extension and exhumation of deep crustal rocks in the east Greenland Caledonides
Authors: Hartz Ebbe H. , Andresen Arild , Hodges Kip V. , Martin Mark W.
Published: February 2004

Tectonic models for the North Atlantic Caledonides typically show Baltica being subducted below Laurentia prior to late orogenic collapse and exhumation of deep crustal rocks, conveniently explaining the high‐ and ultrahigh‐pressure provinces in the Scandinavian Caledonides. However, these models offer no tectonic explanation for the deformation and overthickening of the overriding Laurentian plate. Here we document major anatectic midcrustal ultraductile top to the (N)W shear zones in the east Greenland Caledonides that may account for this orogenic thickening. U‐Pb dating of monazite and 40Ar/39Ar dating suggests that high‐grade fabrics (∼ 435–424 Ma) and granites (∼427 and 422 Ma) involved in these zones show that these were contemporaneous with syntectonic granites involved in the Fjord Region Detachment Zone (FRDZ), a major top to the east extensional fault. More than 100 km of top to the NW overthrusting has been documented in the foreland of the orogen, yet we show that rocks in the hanging wall of the thrust were thinned by orogen‐parallel extension rather than thickened during emplacement, thereby exhuming the underlying active midcrustal thrust zones. This is documented by Late Silurian (∼419 Ma) to Early Devonian (∼401 Ma) 40Ar/39Ar‐muscovite cooling dates from rocks uplifted along major normal faults and intrusion of mafic dikes (∼ 419 Ma) into extensional faults. Folding and thrusting in the foreland probably continued well beyond this time, and underthrust high‐grade nappes probably were exhumed in the triangular zone between the thrust faults (west) and the structurally higher FRDZ (east) before final NW vergent thrust emplacement over low‐grade sedimentary rocks. Final cooling of the metamorphic core below closure temperatures for Ar diffusion in K‐teldspar occurred in the early Carboniferous. Major folds previously regarded as having formed during the main Caledonian event at ∼425 Ma may in part be up to 100 m.y. younger. Some of these folds also deform sedimentary rocks associated with collapse basins of Middle Devonian to Carboniferous age.
Detrital muscovite 40Ar/39Ar ages from Carboniferous sandstones of the British Isles: Provenance and implications for the uplift history of orogenic belts
Authors: Stuart Finlay M. , Bluck Brian J. , Pringle Malcolm S.
Published: February 2004

Major progradations of clastic sediments are recorded in the sedimentary record of the Famennian and the Visean‐Namurian of the United Kingdom and surrounding waters. We have determined 40Ar/39Ar ages of 162 detrital muscovites from 11 coarse sandstones which were deposited between 370 and 465 Ma, spanning both progradations. Detrital mica ages are dominated by a peak at 415 Ma, with minor peaks at 440 Ma and 390 Ma. The 415 Ma muscovites are derived from the unroofing of the Scandian nappes during the compressional phase of the Caledonian orogeny in Scandinavia. The 440 Ma muscovites record pre‐Scandian orogenic activity, which is rarely preserved in the orogenic record. Thermochronological evidence suggests that episodic postorogenic uplift, and exhumation events kept the Scandian orogen a major topographic feature and likely a sediment source for over 100 million years after nappe emplacement and implicates tectonic rather than climatic control on the clastic sediment progradations. The near total absence of detrital muscovites with ages <415 Ma suggests that the Scandian nappes had not been entirely eroded despite repeated uplift during the postorogenic extension. The river(s) which supplied the sediments probably ran parallel to the strike of the major Scandian thrusts, along the length of the Caledonian orogen, in a manner analogous to the major river systems of contemporary orogenic highlands.
Fate of herbicides in a shallow aerobic aquifer: A continuous field injection experiment (Vejen, Denmark)
Authors: Broholm Mette M. , Rügge Kirsten , Tuxen Nina , Højberg Anker L. , Mosbæk Hans , Bjerg Poul L.
Published: February 2004

A continuous, natural gradient, field injection experiment, involving six herbicides and a tracer, was performed in a shallow aerobic aquifer near Vejen, Denmark. Bentazone, (±)‐2‐(4‐chloro‐2‐methylphenoxy) propanoic acid (MCPP), dichlorprop, isoproturon, and the dichlobenil metabolite 2,6‐dichlor‐benzamide (BAM) were injected along with 2‐methyl‐4,6‐dinitrophenol (not discussed in this paper) and the tracer bromide. The injection lasted for 216 days and created a continuous plume in the aquifer. The plume was monitored in three dimensions in 96 multilevel samplers of 6–9 points each for 230 days, with selected individual points for a longer period. The bromide plume followed a complex path through the monitoring network downgradient of the injection wells. The plume movement was controlled by spatially varied hydraulic conductivities of the sand deposit and influenced by asynchronous seasonal variation in groundwater potentials. An average flow velocity of 0.5 m/d was observed, as depicted by bromide. Bentazone, BAM, MCPP, and dichlorprop retardation was negligible, and only slight retardation of isoproturon was observed in the continuous injection experiment and a preceding pulse experiment. No degradation of bentazone was observed in the aerobic aquifer during the monitoring period. BAM and isoproturon were not degraded within 5 m downgradient of the injection. The two phenoxy acids MCPP and dichlorprop were both degraded in the aerobic aquifer. Near the source a lag phase was observed followed by fast degradation of the phenoxy acids, indicating growth kinetics. The phenoxy acids were completely degraded within l m downgradient of the injection wells, resulting in the plumes being divided into small plumes at the injection wells and pulses farther downgradient. During the lag phase, phenoxy acids had spread beyond the 25 m long monitoring network. However, the mass of the phenoxy acids passing the 10–25 m fences never matched the corresponding bentazone or bromide masses, and the pulse was observed to shrink in size. This indicates that this pulse of phenoxy acids was being partially degraded at a low rate as it traveled through the aquifer.
Historical information in a generalized Maximum Likelihood Framework with partial duration and annual maximum series
Authors: Martins Eduardo S. , Stedinger Jery R.
Published: February 2004

This paper considers use of historical information with partial duration series (PDS) and annual maximum series (AMS) flood risk models. A generalized Pareto distribution for exceedances over a threshold combined with the Poisson arrival model yields a three‐parameter generalized extreme value (GEV) distribution for the AMS. When fitting three‐parameter GEV models using generalized maximum likelihood estimators, the average gains from use of historical information are about the same with both AMS and PDS frameworks, though the exact values depend upon the shape parameter к. The effect of the arrival rate λ is modest. In general, average gains are higher when к = 0.0 as opposed to when −0.3 ≤ к ≤ −0.1. When fitting two‐parameter models (exponential—Poisson and Gumbel), the average gains are less than those observed with the corresponding three‐parameter models with к = 0. Fitting a two‐parameter AMS lognormal distribution to lognormal data yielded higher average gains with use of historical information than were obtained with the two‐parameter AMS/Gumbel distribution.
Analysis of variance method for the equivalent conductivity of rectangular blocks
Authors: Veneziano Daniele , Tabaei Ali
Published: February 2004

We use an analysis of variance (ANOVA) decomposition of the log hydraulic conductivity F(x) = ln K(X) as the sum of an average value F¯ plus main effects and interactions of various orders to evaluate the equivalent conductivity of rectangular blocks in D‐dimensional space. Some of the ANOVA components are dealt with exactly. The effect of the other components is approximated through calibration to the effective conductivity Keff under ergodic conditions. Our analysis applies to both isotropic and anisotropic lognormal K fields. We evaluate the theoretical findings through analytical and numerical comparisons with exact results and previously proposed approximations. We find that the ANOVA formula is nearly unbiased, although its performance is generally inferior to numerical renormalization. Advantages of the ANOVA formula are that it is less computationally demanding and explicitly shows the dependence of the block conductivity Kb on the various components of fluctuation of F inside the block. Such dependence is more complex than previously assumed. Considering the ANOVA formula to be unbiased, we derive the bias of the geometric mean of the Cardwell‐Parsons bounds for three‐dimensional rectangular blocks.
High‐resolution characterization of vadose zone dynamics using cross‐borehole radar
Authors: Binley Andrew , Winship Peter , Middleton Roy , Pokar Magdeline , West Jared
Published: February 2004

Characterization of the dynamics of moisture migration in the unsaturated zone of aquifers is essential if reliable estimates of the transport of pollutants threatening such aquifers are to be made. Electrical geophysical investigation techniques, such as ground‐penetrating radar, offer suitable methods for monitoring moisture content changes in the vadose zone. Moreover, these tools permit relatively large measurement scales, appropriate for hydrological models of unsaturated processes, and thus they offer a distinct advantage over conventional measurement approaches. Ground‐penetrating radar, when applied in transmission mode between boreholes, can provide high‐resolution information on lithological and hydrological features. The technique may be applied in tomographic mode and in a much simpler vertical profile mode. Both modes of measurement have been utilized using two boreholes 5 m apart located at a field site in the UK Sherwood Sandstone aquifer. Radar transmission measurements have been used to characterize the change in moisture content in unsaturated sandstone due to controlled water tracer injection. Continual monitoring of cross‐borehole radar measurements over an 18 month period has also permitted determination of travel times of natural loading to the system and has revealed the impact of subtle contrasts in lithology on changes in moisture content over time. The time series of inferred moisture contents show clearly wetting and drying fronts migrating at a rate of approximately 2 m month−1 throughout the sandstone.
Water retention of prefractal porous media generated with the homogeneous and heterogeneous algorithms
Authors: Sukop Michael C. , Perfect Edmund , Bird Nigel R. A.
Published: February 2004

Fractal models of porous media are of interest in numerous scientific disciplines, including hydrology and soil science. This interest arises in part from the ability of these models to parsimoniously produce highly complex and richly structured geometries. Examination of the soil hydrology literature suggests that there are at least two different ways these models are being constructed. We review the two primary algorithms and compare the computed water retention behavior of the different media. Water retention is computed using a method that explicitly considers the connections between pores and to the atmosphere. The distributions of simulated water saturations (for 1000 realizations) around their median values as a function of applied tension were complex and multimodal for both homogeneous and heterogeneous prefractals. The range between the first and third quartiles for the water retention in heterogeneous prefractal structures was, on average, more than twice as large as that for the homogeneous prefractals. This suggests that relations between fractal parameters used to construct the porous media and the water retention behavior of the media can be more readily determined for homogeneous prefractals.
On modeling the mechanisms that control in‐stream phosphorus, macrophyte, and epiphyte dynamics: An assessment of a new model using general sensitivity analysis
Authors: Wade A. J. , Hornberger G. M. , Whitehead P. G. , Jarvie H. P. , Flynn N.
Published: February 2004

The “Kennet model” is a new model of in‐stream phosphorus (P) and macrophyte dynamics. Based on mass balance equations, the model represents the interactions between P and the suspended and bed sediments, the uptake of P by epiphytes and macrophytes, and the exchange of P between the water column and the pore water. The model simulates the total phosphorus (TP) and the soluble reactive phosphorus (SRP) concentrations observed in a reach of the River Kennet. Furthermore, the model simulates the generalized macrophyte growth patterns and total biomass observed in rivers throughout southern England. A general sensitivity analysis, based on Monte Carlo simulations and parameter values derived from the literature, identifies the key parameters controlling the model behavior when simulating macrophyte growth. The most important parameters are those that directly control macrophyte growth, those that define the epiphyte growth, and those that relate to the storage of P in the streambed
Riparian vegetation controls on braided stream dynamics
Authors: Gran Karen , Paola Chris
Published: February 2004

Riparian vegetation can significantly influence the morphology of a river, affecting channel geometry and flow dynamics. To examine the effects of riparian vegetation on gravel bed braided streams, we conducted a series of physical experiments at the St. Anthony Falls Laboratory with varying densities of bar and bank vegetation. Water discharge, sediment discharge, and grain size were held constant between runs. For each run, we allowed a braided system to develop, then seeded the flume with alfalfa (Medicago sativa), allowed the seeds to grow, and then continued the run. We collected data on water depth, surface velocity, and bed elevation throughout each run using image‐based techniques designed to collect data over a large spatial area with minimal disturbance to the flow. Our results show that the influence of vegetation on overall river patterns varied systematically with the spatial density of plant stems. Vegetation reduced the number of active channels and increased bank stability, leading to lower lateral migration rates, narrower and deeper channels, and increased channel relief. These effects increased with vegetation density. Vegetation influenced flow dynamics, increasing the variance of flow direction in vegetated runs and increasing scour depths through strong downwelling where the flow collided with relatively resistant banks. This oblique bank collision also provides a new mechanism for producing secondary flows. We found it to be more important than the classical curvature‐driven mechanism in vegetated runs.
Direct observation of reactive flow in a single fracture
Authors: Durham William B. , Bourcier William L. , Burton Elizabeth A.
Published: February 2004

We carried out a laboratory experiment to examine the relationship between local rate of dissolution and local aperture during flow of a slightly acidic aqueous solution through a rough fracture in Carrara marble under a confining pressure of 0.2 MPa. Fracture surfaces were digitized in three dimensions before the fluid flow tests and after the tests. Digital reconstruction of the aperture then allowed numerical simulation of flow patterns, and digital comparison of surfaces before and after dissolution allowed mapping of patterns of dissolution. We observed that both mean aperture and fracture permeability decreased as a result of dissolution. Despite the low confining pressure, the experiments thus simulate dissolution in deeply buried formations, in contrast to the gaping and karst formation that occur under vanishingly low confining pressure in the shallow crust. We observed that the growth of new pathways for flow changed from stable to unstable as length scale increased. At the millimeter scale the fracture aperture evolved in stable fashion from a strongly heterogeneous arrangement of tortuous flow channels to a smoother topography, while at the scale of the full rock (50 mm), the aperture developed a single, broad flow channel. The scale dependence of the dissolution pattern may be the result of changes with scale in extent of reaction (i.e., the Damköhler number) and in the relative importance of diffusion (the Peclet number). Finally, we also see evidence of a negative relationship between local fluid flux and local rate of dissolution in some locations in the fracture.
Steady state mass transfer from single‐component dense nonaqueous phase liquids in uniform flow fields
Authors: Sale Tom C. , McWhorter David B.
Published: February 2004

In recent years it has become increasingly clear that most remedial technologies fail to completely remove dense nonaqueous phase liquid (DNAPL) from subsurface source zones. Recognition of this limitation leads to the question of what benefit can be achieved through partial removal of DNAPL. To address this issue, a mathematical technique referred to as the multiple analytical source superposition technique (MASST) has been developed. MASST is based on a conceptualization of a DNAPL source zone as a grouping of discrete subzones containing DNAPL (e.g., fingers and/or pools) separated by portions of the aquifer that are entirely free of DNAPL. Using analytical techniques, spatial superposition of responses to multiple sources is used to estimate aqueous mass transfer rates from individual subzones. This procedure accounts for multiple DNAPL subzones with different volumes, geometries, and locations within an overall source zone that is otherwise free of the nonaqueous liquid. The mass transfer rate from a particular subzone is affected by mass transfer from all other subzones in the vicinity. Groundwater flow is assumed to be uniform, and transport processes are considered to be at a steady state. Comparison of MASST results with exact analytical solutions and laboratory data confirms the validity of MASST. Sensitivity analyses indicate that source‐zone architecture is a primary factor governing bulk mass transfer and source longevity. Analysis of rate‐limited mass transfer within DNAPL subzones and advective‐dispersive transport about DNAPL subzones indicates that advective‐dispersive transport is the primary factor controlling mass transfer rates. Finally, results indicate that removal of the vast majority of the DNAPL will likely be necessary to achieve significant near‐term improvements in groundwater quality.
Toward capturing hydrologically significant connectivity in spatial patterns
Authors: Western Andrew W. , Blöschl Günter , Grayson Rodger B.
Published: February 2004

Many spatial fields exhibit connectivity features that have an important influence on hydrologic behavior. Examples include high‐conductivity preferred flow paths in aquifers and saturated source areas in drainage lines. Connected features can be considered as arbitrarily shaped bands or pathways of connected pixels having similar (e.g., high) values. Connectivity is a property that is not captured by standard geostatistical approaches, which assume that spatial variation occurs in the most random possible way that is consistent with the spatial correlation, nor is it captured by indicator geostatistics. An alternative approach is to use connectivity functions. In this paper we apply connectivity functions to 13 observed soil moisture patterns from the Tarrawarra catchment and two synthetic aquifer conductivity patterns. It is shown that the connectivity functions are able to distinguish between connected and disconnected patterns. The importance of the connectivity in determining hydrologic behavior is explored using rainfall‐runoff simulations and groundwater transport simulations. We propose the integral connectivity scale as a measure of the presence of hydrologic connectivity. Links between the connectivity functions and integral connectivity scale and simulated hydrologic behavior are demonstrated and explained from a hydrologic process perspective. Connectivity functions and the integral connectivity scale provide promising means for characterizing features that exist in observed spatial fields and that have an important influence on hydrologic behavior. Previously, this has not been possible within a statistical framework.
Combined effects of heterogeneity, anisotropy, and saturation on steady state flow and transport: A laboratory sand tank experiment
Authors: Ursino Naida , Gimmi Thomas , Flühler Hannes
Published: February 2004

Field soils show rather different spreading behavior at different water saturations, frequently caused by layering of the soil material. We performed tracer experiments in a laboratory sand tank. Such experiments complement and help comprehension of field investigations. We estimated, by image analysis, the first two moments of small plumes traveling through a two‐dimensional, heterogeneous medium with strongly anisotropic correlation structure. Three steady state regimes were analyzed. Two main conclusions were drawn. First, low saturation led to very large heterogeneity and to strong preferential flow. Thus the description of the flow paths and the prediction of the solute arrival times require, in this case, more accurate knowledge about the topological structure. Second, saturation‐dependent macroscopic anisotropy is an essential element of transport in unsaturated media. For this reason, small structural soil features should be properly upscaled to give appropriate effective soil parameters to be input in transport models.
Monitoring pumping test response in a fractured aquifer using ground‐penetrating radar
Authors: Tsoflias Georgios P. , Halihan Todd , Sharp John M.
Published: February 2004

Fractured aquifers present a number of problems when attempting to characterize flow on the well scale (less than 100 m). Standard hydraulic testing methods are expensive because of the need for installation of monitoring wells. Geophysical methods may suffer from a lack of resolution and nonunique solutions to data interpretation. We used ground‐penetrating radar (GPR) surveying during a pumping test in a well‐characterized, fractured, carbonate aquifer to monitor the response of a permeable subhorizontal fracture plane. We observed radar signal amplitude and waveform variations along a fracture reflector and correlated the radar signal response to changes in the water saturation of the fracture. Combining hydraulic measurements with GPR data and electromagnetic modeling, we identified an asymmetric fracture drainage pattern, provided accurate spatial information about the saturation of the fracture, and detected the presence of hydraulic boundaries. This study demonstrates that GPR surveying can be used successfully for real‐time monitoring of pumping tests in fractured carbonate aquifers.
Complementary relationship with a convective boundary layer model to estimate regional evaporation
Authors: Sugita Michiaki , Usui Junko , Tamagawa Ichiro , Kaihotsu Ichirow
Published: February 2004

An idea to use a simple convective boundary layer (CBL) model in the complementary relationship to estimate regional evaporation was explored. The CBL model simulated the potential specific humidity deficit D in CBL when the bulk stomatal resistance rst = 0. This value of D was then used in the Penman‐type equation to derive evaporation Epo that would occur with ample soil moisture under the prevailing weather condition. The same equation was also used to produce the potential evaporation Ep with the actual humidity deficit, and these Ep and Epo values allowed the evaluation of the actual evaporation E by applying the complementary relationship E = ηEpo − Ep, where η is assumed as 2.0. This was tested with the data set obtained in Hexi Corridor desert area in northwestern China with a modified version of a simple CBL model developed by Lhomme (1997). It was found that the method produced better estimates of the daytime mean E values with smaller bias than those obtained from a conventional application of the complementary relationship without the CBL model. Also, it was shown that the assumption of η = 2.0 in the complementary relationship was only approximate in most cases. To take this into account, an additional procedure was explored in which η was treated as a variable, and an iteration process with the CBL model determined the final η and E values. It was found that this process produced E values that have smaller systematic error and agree better with the measurements on average, but the unsystematic error got worse than that found with η = 2.0, probably because of use of the CBL model with rst ≠ 0 in the iteration process.
Using precision temperature logs to estimate horizontal and vertical groundwater flow components
Authors: Reiter Marshall
Published: February 2004

A new method of estimating the horizontal and vertical specific discharge components of groundwater flow from precision subsurface temperature measurements is presented. Plotting the vertical temperature gradient as a function of both Z and T and then fitting a plane to the data can provide coefficients from which the vertical and horizontal groundwater specific discharge components may be estimated. Alternatively, by considering the vertical component of groundwater to be zero, it is shown that quadric or cubic fits to temperature data provide coefficients from which horizontal groundwater flow may be estimated. These expressions along with expressions previously derived to estimate only vertical, only horizontal, or both vertical and horizontal flow components are fitted to observational data from four sites and five flow zones. It is learned for these examples (using an available fitting program) that although many of the expressions fit the observational data very well statistically, those expressions which incorporate both the vertical and horizontal flow components of specific discharge must generally be constrained so that resulting parameters will provide flow estimates consistent in direction and heat transfer with available piezometer data and/or the fundamental curvature of the temperature log. Expressions incorporating only νX or only νZ can sometimes yield estimates quite different from the estimates obtained from the expressions incorporating both νX and νZ. It is concluded that an estimate of the horizontal and vertical flow components requires both consideration of the flow calculations along with available piezometric data and the fundamental curvature of the temperature log.